A cientific Review

of
Dust Collection
IO N
D EDIT
C ON
SE

“The Real Dirt on Dust”

by
Scientific Dust Collectors
 2nd Edition

INTRODUCTION

Welcome to the world of Scientific Dust Collectors. We live in a time of continual change and rapid
development. This advancement has led us to a point where pollution control considerations and
environmental concerns are a real part of our everyday lives. We, as well as future generations, want
the air that we breathe to be free from pollution. We want our employees to lead safe and healthy lives.
We live in an age where management and employees are working together in teams. This team effort
demands improved and cleaner working conditions so that manufacturing efficiencies are achievable in
a marketplace that is becoming more global and competitive with each passing year.

Individuals involved in specifying, purchasing and operating dust collection equipment should be aware
of the various types of equipment available. The move towards higher and higher collection efficiency
requires a good understanding of the process and the equipment involved. This booklet was written in
order to provide a basic overview of pollution control equipment. It is meant to capsulize the various
types of products that most people are familiar with in manufacturing environments.

Scientific Dust Collectors is an autonomous division of Venturedyne, Ltd., a large diversified industrial
manufacturing corporation with divisions specializing in dust collection, indoor air quality, environmental
test chambers and sub-micron particle counting for clean rooms. All dust collector design,
manufacturing, applications and sales support are done in one location providing close control over all
key aspects of our business.

Scientific Dust Collectors began business in 1981 when our first patented improvement for cleaning a
filtering media was issued. A number of additional patents that relate to further improvements in dust
collector cleaning technologies have been issued since that time.

The trend toward high ratio products, which cost less to install and maintain, is continuing. This comes
at a time when increasing requirements for more effective equipment is mandated by law or by company
goals. Scientific Dust Collectors is committed to the ongoing promotion and advancement of this
technology. Let us help you to “DISCOVER THE DIFFERENCE”.

This second edition provides additional information on system design, filter media, and explosive dust
control.

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 TABLE OF CONTENTS

Chapter 1 – Cyclones and Inertial Separators 3

Chapter 2 – Airwashers (Scrubbers) 9

Chapter 3 – Electrostatic Collectors 13

Chapter 4 – System Design 21

Chapter 5 – Mechanical Cleaning Collectors (Shaker Collectors) 41

Chapter 6 – High Pressure Reverse Fan Cleaning Collectors 46

Chapter 7 – Pulse Jet Baghouse Collectors 52

Chapter 8 – Cartridge Collectors 64

Chapter 9 – Using Pleated Bags in Dust Collectors 74

Chapter 10 – Filter Media 78

Chapter 11 – Fires, Explosions, Hazards 85

Chapter 12 – Impact of Moisture in Dust Collectors 103

Chapter 13 – Future Trends in Dust Collecting 113

Appendices 116

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 Chapter 1

CYCLONES AND INERTIAL SEPARATORS

The simplest type of collector is an inertial separator.

This design depends on slowing the flow through the system so that the air velocity is not sufficient to hold the
particles in suspension in the air stream. Figure 1-1 illustrates this design which utilizes both inertial and gravity
forces upon the dust particles.

As the dirty air enters the inlet of the collector,

the air immediately reacts to an internal baffle
that causes the dirty air to take a downward
direction which is followed by a 180° upward
turn. The inertia and gravity forces drive the
particles toward the open hopper.

The hopper is shaped such that it intercepts

the particles. The particles will often agglom-
erate and slide toward the hopper outlet. This
agglomeration will allow the collection of
smaller particles than those particles that might
be captured by only the action of gravity and
inertia forces.

A common application of this type of collector

is as a pre-filter to separate the large particles
that might harm some collector models. On
process venting hot applications, it will remove
large sized hot particles that are not cooled by
the process gas. This design also has limited
application as a Spark Trap since sparks often
have buoyancy and are little affected by gravity
or inertial forces.

FIGURE 1-1

It has become increasingly more common for this type of separator to be used upstream from a fabric dust
collector when the dust being collected contains large chips, heavy particles, and/or is abrasive. The separator,
often referred to as a “Drop Out Box”, slows the air down to drop out the larger and heavier particles out of the air
stream and remove them before reaching the fabric dust collector. This method protects the fabric filters from
possible abrasion issues and reduces the dust load on the dust collector allowing for longer filter life as shown in
Figure 1-2.
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 FIGURE 1-2

Centrifugal collectors are more commonly known as

“cyclones” and depend on centrifugal force to move the
dust particles toward the wall of the collection chamber.

The dust laden air enters the collector tangentially at the

top and the flow forms a vortex pattern as it travels down
the inside vertical wall or barrel of the cyclone (see Figure
1-3). The tangential forces propel the particles toward the
wall. In the whirling air stream, these particles are held
against the wall by the centrifugal forces, agglomerate,
and slide downward toward the cone of the hopper. The
acceleration exerted on the particle is according to the
centrifugal equation:
a = r Ȧð
where Ȧ is the rotation in radians per second, r the radius
of rotation, and a is the acceleration on the dust particles.
If we assume that the inlet velocity to the cyclone is a
fixed velocity V, then:
Ȧ = V/r
Therefore:
a = Vð/r

Then using Newton’s Second Law to find the force F:

F=ma
where m is the mass of the particle. Force then becomes:
F = m Vð/r

FIGURE 1-3

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From this we realize two things. First, that the forces on the larger particles are greater than the smaller particles
since the larger particles have more mass. Second, a smaller diameter cyclone has higher forces than a large
diameter cyclone since the radius is smaller.

To get an idea of just how large the forces are, see the following example:
Example 1.1
Determine acceleration on a particle for an 8-foot diameter cyclone and 4-foot diameter cyclone using an inlet
velocity of 3000FPM.
Use equation #1A: a = Vð/r
Where V = velocity rate is given at 3000 feet per minute or 50 feet per second (used for this example).
r = Cyclone radius equal to half of the diameter.
a8-foot = (50 feet/second)ð = 625 ft/sð
4ft
a4-foot = (50 feet/second)ð = 1,250 ft/sð
2ft
It is interesting to note that the acceleration due to gravity, g = 32.17 ft/s2. For the 8-foot cyclone the acceleration is
almost 20 times greater than gravity and the 4-foot cyclones acceleration is over 38 times greater than gravity.

As we can see in Figure 1-3, the air can take multiple revolutions as it travels down the barrel of the cyclone. Also
we see that at first as the air enters the cyclone it begins spinning in the Clockwise (CW) direction until reaching
the bottom of the vortex where it changes to Counter-Clockwise direction (CCW). If the air stream had started off
spinning in the CCW from the inlet, it would then change to CW as it leaves the outlet. This is very similar to using
the right-hand-rule in physics for dealing with vectors, as the flow spins one direction the movement is down and
once the flow reverses direction the flow is up as seen in Figure 1-4.

FIGURE 1-4

The efficiency of the collector depends on the size of the particle, the exerted force, and the time that the force is
exerted on the dust particles. When the force brings the dust to the cyclone barrel and it is agglomerated, the dust
will slide down the wall. The designer has a choice of designing a cyclone with a small diameter and a shorter
barrel or a larger diameter with a longer barrel to get the same performance.

High narrow inlets reduce the distance that the dust must travel to reach the wall. In designing ducts for carrying
these air streams, the transitions must be smooth to get the maximum performance from the cyclone.

As far as the dust carrying capacities, there are two opposite characteristics. In general, small diameter cyclones
will collect dust efficiently even at relatively low loads (0.1 to 6 grains per actual cubic foot), but the pressure drop
will range from 6 to 10 inches w.c. (water column). However, at high dust loads, some of the dust outlets may have
a tendency to plug. Large diameter cyclones can handle dust loads in the 50-100 grains per cubic foot range with
low pressure drops (1½" to 3" w.c.), but the collector efficiency will be lower at the low dust loads because the dust
particles may be swept from the walls of the collector before the dust particles can agglomerate.

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The first generation cyclones (Figure 1-5) had low
pressure drops (1½" to 2" w.c.) and relatively large
diameters. These collectors were usually arranged
so that a fan would blow the dust laden air stream
into the inlet. The bottoms of these collectors were at
atmospheric pressure and the collected dust would
drop into a bin or truck.

FIGURE 1-5

Dust Discharge Considerations. In high performance,

high pressure drop cyclones (Figure 1-6), a very
intense vortex is formed inside the main swirling stream
at the discharge point. If this dust is allowed to collect
at this junction, it will re-entrain and be swept upward
into the outlet tube. Expansion hoppers or vortex
breakers are necessary to allow the dust to be
discharged through an airtight feeder. Vortex breakers
increase the efficiency and lower the pressure drop in
the cyclone. Also, in some heavy moisture applications,
they can be effective in “wringing” out moisture before
moving onto the baghouse.

FIGURE 1-6

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Multiple Cyclone Collectors with vane spinners are a
very effective compromise. These are illustrated in
Figure 1-7. The sloped dirty air plenum allows for
effective air and dust distribution on the dirty side and
even distribution on the clean air side. The most
prevalent design uses 6 inch diameter barrels. These
multiple cyclones were often applied in boilers as the
only acceptable dust collectors. More recently, they
are used as the preliminary cyclones and followed by
more efficient fabric collectors to meet discharge
codes.

FIGURE 1-7

There are other unique methods of

designing inertial separators. Figure 1-8 is
a rotary dry centrifugal unit which has
specially designed blades that serve the
dual function of a fan and the acceleration
of the dust particles which are thrown
against the scroll of the inertial separator.
The housing is fabricated of cast iron for
maximum abrasion resistance. These
were commonly applied in venting grinding
applications and were limited to relatively
small volume flows.

FIGURE 1-8
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Louver type collectors are a rather specialized form of centrifugal or inertial collectors. The louvers have very
narrow spacing which causes the dust laden air to make a very abrupt change in direction. The dust particles are
thrown against the flat surfaces, agglomerate, and fall into the lower part of the collector. These are effective in
collecting very light loads of fine dust. Heavier loads would quickly plug the collector. There is a portion of the air
stream that is separated in order to remove the dust from the dirty side of the collector. This side stream is usually
vented into a small diameter cyclonic centrifugal collector. One of the common applications of a louver collector is
to reduce the load entering the replaceable panel filters. Figure 1-9 outlines the construction and design of this
louver design. These louver designs are limited to inlet loads of less than 0.5 grains per cubic foot load.

FIGURE 1-9

Mechanical collectors are mostly used as a preliminary filter in front of other filters or dust collection devices. They
can increase the overall efficiency of a solids separation process, especially when the final collector is a water
scrubber or an electrostatic precipitator. Also, they are sometimes used for capturing the larger particulates from
an air stream where this separation fits into process requirements.

The collection efficiency of these mechanical “cyclone” or inertial separators have some limitations and will not
perform as well as cartridge or baghouse collectors. The fact that these mechanisms have few internal parts is a
definite advantage; however, ongoing and future requirements for higher filtration efficiency are causing these
devices to take a “back seat” to other more sophisticated methods.

-8-
 Chapter 2

AIRWASHERS (SCRUBBERS)

Most air scrubber designs were developed as attempts to improve the performance of inertial collectors. The
limitations on inertial separators were that the dust particles as they reached the collecting surface did not
agglomerate sufficiently. The finer dust particles did not stay on the collection surfaces and were swept back into
the air stream.

Modification of Cyclone Collectors

The first modification came when the standard design cyclones were modified. Water was sprayed on the interior
walls of the cyclone. This improved the collection efficiency, but the difficulties came with keeping the surfaces
coated and getting the water distribution on the interior of the barrel and the cone. Any surface that was not kept
wet would form mud and sludge, which resulted in frequently cleaning the collector interior. The next evolution of
the design was to spray water into the inlet of the wet cyclone. The slurry that was formed had a long distance to
travel inside the collector. Also the inner vortex was frequently a problem that interfered with the water dropping
into the expansion chamber. These slurry droplets were typically swept upward into the outlet.

The collection efficiencies of these modified cyclones were much higher than the dry units. Two applications that
compare efficiencies between clay and wet cyclones are listed below:

Application Cyclone Efficiency Wet Cyclone Efficiency

Material Handling (Rock) 80-85% 90-93%

Dryer 75-80% 92-96%

In order to have an efficient scrubber, the gas velocities in

the scrubber had to be sufficient for the dust to be driven
through the surface tension of the water coated surfaces
and/or water droplets. For a good design, the scrubbing or
washing action also produced a secondary generation of
water droplets and induced a mist collection section. See
Figure 2-1.

FIGURE 2-1

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The Dynamic Wet Precipitator consists of
adding water sprays to a centrifugal type
dry collector which is shown in Figure 2-2.
The blade design of the centrifugal
collector is modified to handle dust and a
flow of water. A spray is centered in the
inlet and the blades are coated with water.
As the air hits the water surfaces at a
moderate velocity, the slurry is thrown into
the outer walls and into the drain. The
liquid water enters the centrifugal
separator and the mist enters the drain.
This design is limited in the load it carries
because the wear on the blades is high
due to the solids content.

FIGURE 2-2

Orifice Scrubbers

These scrubbers are

sometimes called orifice
scrubbers as illustrated in
Figure 2-3. It is essentially
an inertial trap/inertial
separator except that the air
impinges against a water
surface. Spray nozzles,
however, offer a greater
degree of spray dispersion.
All of these scrubbers
produce coarse water
droplets and separate the
droplets from the air by
changing the flow directions
at least once or twice which
results in a pressure drop
range of 3-6" w.c. These
units are generally shorter
than other types of wet
collectors and they can be
installed inside the plant.
FIGURE 2-3

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Collection Efficiency Options for Low Pressure Scrubber
Chapter
Designs
2

In order to increase the collection efficiency while maintaining a low to moderate power load, there are several
design approaches that may be taken by scrubber suppliers:
AIRWASHERS (SCRUBBERS)
1) The velocity of the blades is increased so that the dust impacts the water surface at a faster velocity rate.

Most2)air The gas streams

scrubber designs are wereseparated
developed intoassmall individual
attempts jets so that
to improve the the dust staysofininertial
performance contactcollectors.
with the waterThe
limitationssurfaces for aseparators
on inertial longer time.were Somethatcollectors
the dustare designed
particles as with
theyorifice plates.
reached These orifices
the collecting range
surface didfrom
not
agglomerate 1/10sufficiently.
to 1/4 inch Thein diameter.
finer dustAlso, theredid
particles arenot
other
stayorifices
on the that are designed
collection surfaceswith
andsmooth spheres
were swept backoninto
a
coarse
the air stream. grid. In this case, the air bubbles would travel upward to the water surface while accomplishing a
very effective scrubbing action.
Modification of Cyclone Collectors
3) The velocity of the water sprays is increased in an effort to collect finer particles.
The first modification came when the standard design cyclones were modified. Water was sprayed on the interior
walls of the cyclone. This improved the collection efficiency, but the difficulties came with keeping the surfaces
Basic
coatedLimitations of Scrubbing
and getting Action
the water distribution on the interior of the barrel and the cone. Any surface that was not kept
wet would form mud and sludge, which resulted in frequently cleaning the collector interior. The next evolution of
Inthealldesign
of these designs,
was to spraythewater
collection of the
into the inletfinest
of thedust
wetand powderThe
cyclone. fractions are limited
slurry that by onehad
was formed main factor
a long which isto
distance
the deflection
travel of the
inside the fine particles
collector. Also theaway from
inner the water
vortex surface due
was frequently to the water
a problem surface tension.
that interfered with theTo increase
water the
dropping
penetration and collection
into the expansion chamber. efficiency
Theseofslurry
the fine dust,were
droplets the venturi scrubber
typically (Figure into
swept upward 2-4)the
is introduced.
outlet.

The collection efficiencies of these modified cyclones were much higher than the dry units. Two applications that
compare efficiencies between clay and wet cyclones are listed below:

Application Cyclone Efficiency Wet Cyclone Efficiency

Material Handling (Rock) 80-85% 90-93%

Dryer 75-80% 92-96%

In order to have an efficient scrubber, the gas velocities in

the scrubber had to be sufficient for the dust to be driven
through the surface tension of the water coated surfaces
and/or water droplets. For a good design, the scrubbing or
washing action also produced a secondary generation of
water droplets and induced a mist collection section. See
Figure 2-1.

FIGURE 2-1
FIGURE 2-4

-11-
-9-
Venturi (High Pressure Scrubbers)

By increasing the air velocities to between 15,000 and 20,000 feet per minute at the venturi throat and by adding 4
to 6 gallons of water per 1,000 CFM of cleaned air at the venturi throat, very fine water droplets are formed. The
impact of these very fine water droplets at the high air velocity allows for the efficient collection of the fine particles.
The pressure drop ranges from 15 to 60 inches water column. After the dust is entrapped in the liquid slurry, a mist
eliminator is needed to separate the mist from the air stream. Mist eliminator designs are similar to inertial
separator designs where the mist from air separation is either accomplished by the change in air flow direction or
by the spin in the air stream which creates the centrifugal forces.

Also, as the slurry impinges against the collecting surface, the slurry is directed to the scrubber outlet. This
invariably requires that the water must flow through a hydraulic trap. Typically, the leakage around these traps
causes the dirty droplets to exit the scrubber outlet.

Humidification is required in the scrubbing process. If the air stream is not close to the saturation point, the
entrapped dust may again be liberated as the slurry evaporates. In most applications, the exhaust air is seldom
returned to the work environment.

Exhaust Plumes. When a warm humid air stream is mixed with colder air, a white plume will usually be formed due
to condensed water vapor. Even though the air may be buoyant, the droplets may increase until the density of the
air causes it to descend towards the ground. In some cases, the plume may reach ground level miles away as the
plume becomes invisible.

Application of Scrubbers

Scrubbers are most often applied to separate from process air streams the solids that are explosive. They are also
applied where the slurry is used in other parts of the process or where the mixture is sold in a slurry form. Some
scrubbers are applied so that chemical reactions will be generated within the scrubbing action. In other applica-
tions they are even applied as air absorbers.

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 Chapter 3

ELECTROSTATIC COLLECTORS

Electrostatic collectors operate by the forces generated by electrostatic charges which draw the dust particles to
the collection plates. These particles lose their charges and agglomerate when they reach the grounded plates.

In general, the main advantages of an electrostatic precipitator are:

1) The efficiency can exceed 99 percent in some applications.

2) The size of the particles collected can be very small.

3) The precipitator can function at temperatures of 700ºF and with special designs as high as 1300ºF.

4) The pressure and temperature changes through the collector are small, usually less than 0.5 inches
water column.

5) The collected dust is dry, an advantage for the recovery of loss product.

6) Large flow rates are possible.

7) Difficult acid and tars can be collected.

8) Collectors can tolerate extremely corrosive materials.

9) The electrical power requirement is low to clean the dirty gas.

As there are advantages to using electrostatic precipitation, there are also disadvantages:

1) The initial cost is generally more costly than other approaches to solve the pollution problem.

2) Some materials are extremely difficult to collect in an electrical precipitator due to very high or low
resistivity.

3) Variable condition of airflow causes the precipitator to become very inefficient. Automatic voltage control
improves the collector efficiency somewhat.

4) Space requirements for the equipment can be greater than those for other approaches such as
baghouses and/or cartridge units.

5) Electrical precipitation is not applicable for the removal of materials in the gaseous phase.

6) A cyclonic pre-cleaner may be needed to reduce the dust load before the precipitator.
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Single Stage Precipitator

Figure 3-1 is a typical plate type precipitator. It consists of a rectangular shell or casing in which a number of
grounded plates are suspended parallel to each other and has equal spacing between plates to form channels
through which the gas flows. High voltage discharge electrodes are suspended vertically between the plates from
an insulated mounting frame. The distance between the grounded plates are in the 4 to 6 inch range, and the
voltage on the electrodes is between 40,000 and 60,000 volts. This voltage causes the gases to ionize and when
this occurs the dust particle becomes negatively charged. The strength of this charge is a function of the dielectric
characteristics of the dust. Some dusts will have a high charge and the forces to attract it to the grounded collect-
ing plates will be high. The time interval is determined by the distance the dust particle has traveled to the
grounded collector plate and the magnitude of the charged dust particle. Some dust particles (or liquid droplets)
have higher forces that attract them to the collection plates at a greater efficiency rate than others. Other factors
include the other gases in the process stream. For instance, some sulfur compounds in boiler gas will increase
collection efficiency.

FIGURE 3-1

The velocity of the gas passing through the plates will also affect the efficiency of collection. For instance, at a 50
fpm gas velocity, only half of the particles will reach the collecting plates with an associated collection efficiency of
50%. At 25 fpm, the efficiency might be 95% and at 12 fpm, it might be 99%. The pressure drop across the
precipitator collection section will usually stay in the range of 0.2 to 0.5 inches of water.

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From the above analysis, it is important to have a very even velocity distribution through the precipitator from side
to side and from top to bottom in the collection compartment. If the velocity varies, the efficiency will be lower
across the sections with higher velocity (and higher flow), and the collection efficiency might be much lower than
might be predicted based on the average velocity. In designing these electrostatic dust collectors of the single
stage high voltage design, it is necessary to design the distribution baffles very carefully. This is accomplished with
computer programs followed by modeling in a test laboratory.

In some precipitators, the high voltage electrodes are in the form of hanging wires with weights on the bottom of
the wires to keep them straight. This is an economical approach, but many of the premium designs have fixed
frames. The charging wires and/or electrodes can be viewed as “lightning rods”, rods that drain charges from
buildings. The closer the electrodes are placed to the grounding plates, the more effective the charging force
becomes. With smaller electrode to plate distances, the voltage becomes lower and smoother to ionize the gas
stream. Under the circumstances, smaller diameter wires are more effective. The more costly framed electrodes
are built with points sticking out from the electrode frames.

Dust Removal from Plates

The collecting plates are cleaned by rapping with an air powered anvil. The power supply is shut off during the
rapping and the dust falls into the collection hopper.

Once the particles get a charge, they will migrate to any grounded (or uncharged surface), even a surface at a
lower potential. The collection surface may include the high voltage insulators. If dust collects on the insulators, a
path for the high voltage to ground is formed. Eventually, this will cause failure of the high voltage power supply.
In order to reduce or eliminate this effect, the insulators are pressurized with a blower and a flow of outside air is
maintained in the collecting compartment. Then the charged particles will not have enough attraction to rest on the
insulator surfaces.

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The tubular precipitator consists of pipes with the electrodes in the center of the pipes. These designs have much
more rigidity and are often employed with wet electrostatic precipitators. These designs either keep the walls
continuously wet or use a washing system to clean the grounded electrodes. The construction and a schematic of
the insulator supports are shown in Figure 3-2. The pipe collection electrodes provide unusually effective gas
distribution within the precipitator.

FIGURE 3-2

These types of precipitators are able to adjust to the expansion and contraction of parts as they are heated and are
widely applied to higher temperature gas streams, especially boiler exhausts in power plants. They are sometimes
subject to corrosive gases, and the life of the collectors and the frequency of maintenance depends on the thick-
ness and ruggedness of the electrodes and the grounded collecting plates.

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The Two Stage Precipitator

A schematic is shown in Figure 3-3. The grounded plates are about an inch apart and have an intermediate plate
that is also charged. Instead of the 40,000-60,000 volt D.C. supply, the two stage precipitator has a 13,000-15,000
volt supply with the intermediate supply at 7,500 volts.

FIGURE 3-3

This collector was developed for HVAC (heating and ventilating service). It provides very efficient dust collection
and is designed with a self-cleaning washing system. The dust load in this service is between 0.01 to 0.1 grains
per 1,000 cubic feet. The washing system is a light duty unit designed for 250 cycles. Since the usual cleaning is
only required monthly, this unit exceeds the life of other components of the HVAC systems.

The high voltage electrodes consist of very fine wire stretched across springs. At 15,000 volts, a finer wire is
required for ionization. The plates have to be maintained at more precise distances and to manufacture these
components requires very special tooling.

In this kind of service, the air distribution is usually very even since the dust collecting filtering device operates at
the same velocities as the heating and cooling coils.

Industrial Dust Venting with Two Stage Precipitators

In the early seventies, Two Stage units were supplied as general ventilation modules in industrial plants where
welding, burning, and grinding operations were performed. The units had integral fans and drew air from the plant
at one end and blew it out the opposite end. Because the load to these precipitators was 10 to 50 times as high,
these units typically required cleaning 2 to 7 times a week.

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Since the precipitators were designed for 250 cleaning cycles, major maintenance was required within months.
The required maintenance consisted of removing the precipitator frames and manually cleaning them. These
assemblies were delicate and often the electrode wires were broken and the collection efficiency suffered. The
washing mechanisms would also require replacement or an overhaul.

Soon these two stage electrostatic filters were being applied to hooded and ducted automatic welding machines or
to welding booths. In these applications the dust loading was increased to 30-50 grains per 1,000 cubic feet per
minute.

Insulator Deterioration

As discussed above, the charged dust particle will be attracted to a grounded element, or any element at a lower
potential than the charge carried by the particle. Some of these particles will be collected on the intermediate
charged plate while others will be attracted to the insulators. However, an electrical charge inherently cannot be
bled to ground so it adheres to the insulator. The particle sometimes can be washed off during the cleaning cycle,
but some of it will paint the insulator. Soon a leakage path forms from the high voltage charging wires to the inter-
mediate plate which results in not maintaining enough voltage to the power supply to perform the function of ioniz-
ing in the precipitator. The normal maintenance in this case would be to install new insulators. This requires some
specialized abilities from the maintenance personnel and is presently performed by specialized maintenance
organizations.

Pressurized Insulators

Single stage precipitators have the insulators installed in compartments through which air from outside the precipi-
tator is drawn or blown into the insulator compartment. The charged particle must overcome the velocity vector of
the air that is flowing towards the precipitator so that few, if any, particles will reach the insulators. This allows
insulators in very heavy dust load service to operate for many years.

The same approach was taken on two stage precipitators. This allowed their application to become more wide-
spread and to be applied on industrial processes as severe as asphalt saturators.

Plating

Most of the two stage precipitator collectors were applied on processes like welding. It was especially effective
since it could tolerate condensed hydrocarbons as well as the particulate fume. The cleaned gas was discharged
into the room instead of outside. The electrostatic is very sensitive like all precipitators to even flow distribution.
When applied to industrial hooded processes, it is difficult and expensive to get even flow across the collection
plates.

With even distribution, a correctly selected lower velocity collector can achieve a collection efficiency of 99%. But if
an improperly designed distribution component is installed in front of the collector, the efficiency may drop to 90%
or lower.

The charged particles leaving a properly designed precipitator will quickly lose their static charge. Normally this will
occur within a few inches of the discharge into the room. Under some atmospheric conditions, notably low
humidity, this zone may extend to a couple of feet (Figure 3-4).

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 FIGURE 3-4

If the velocity distribution is poor, the distance required to dissipate the charge may be several feet (Figure 3-5).
Under certain conditions of low humidity, this distance may extend indefinitely, even up to or more than a hundred
feet. In that occurrence, all the surfaces in the room become collecting plates. This includes the walls, machines
and operator eyeglasses, etc.

This phenomenon is called plating.

FIGURE 3-5

Competitive pressures have led many suppliers to offer precipitators that operate at higher velocities. Many times,
even on welding fume collectors, these units would only achieve efficiencies in the 90-95% range. From a design
viewpoint, this seemed sufficient since it was quite effective in eliminating the haze in the work area. Unfortunately,
they did not always consider the plating phenomenon. This gave the two stage precipitators a bad reputation and
contributed to the rapid rise of pulse jet cartridge collectors for welding fume collection.

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De-ionizing Sections

The designers came up with an effective remedy to remove the charges from the dust particles. They applied an
alternating current to the high voltage power supply and this effectively removed the charge from the particles that
were coming through the collector. This de-ionizing could be accomplished even at fairly high velocities.

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 Chapter 4

SYSTEM DESIGN

Description of Mathematical Variables

Air is a compressible gas. This means that the density of air varies in relation to the change in pressure. Liquids
like water are considered incompressible because very large changes in pressure cause very small changes in
density. For example, if a pressure increase of 725psi were applied to water, the density of water would change by
less than 0.5%; which is negligible. In contrast, if the same increase in pressure was applied to air, the density
would change by roughly 5000%.
To calculate density of air, the familiar equation of the Ideal Gas Law must be used: p V = n Ru T where:
p = Absolute Pressure
V = Volume
n = Number of moles
Ru = Universal Gas Constant
T = Absolute Temperature

Multiplying both sides by the molecular weight of the gas “M” to get: p V M = n M Ru T
n ˜ 0 Ru
Then, further rearranging the equation to get: p T
V 0
n0 R
Now is mass per unit of volume or density (ȡ) (Greek letter Rho) and u is the gas constant R.
V 0
p
This simplifies now to p = ȡ R T to solve for pressure or U to solve for density. (Equation #4a)
R˜T

When using the English unit system, the density of air (ȡ) is expressed as pound mass (lbm) per cubic foot (lbm/ft3).
By using equation #4a, the density of air at the following standard ambient conditions (SATP): ambient temperature
of 70°F, zero water content, and standard atmospheric pressure of 14.7 pounds per square inch absolute (psia)
can be calculated as shown in Example 4.1.
F
Note: Pressure is defined as force per unit area or p
A
When pressure is expressed in pounds per square inch (psi), it is pound force (lbf) per square
inch.

Example 4.1 Calculate the air density at standard ambient temperature and pressure
(SATP) of 70°F and 14.7 psia.

p
Use equation #4a: U
R˜T
-21-
 Where T= Absolute Temperature, expressed in Rankine (°R) for the English unit system.
= 70°F + 460° = 530°R

R = Gas constant for air

ft ˜ lb f
= 53.35
lb m ˜ qR
p = Absolute Pressure, expressed in psia (lbf/inð). Convert to pounds per square foot (psf):
lb f 144in 2
= 14.7 ˜ = 2116.8 lbf /ftð
in 2 1ft 2
ȡ = air density expressed in pound mass units per cubic foot
lb f
2116.8
U ft 2
ft ˜ lb f
53.35 ˜ 530qR
lb m ˜ qR
ȡ = 0.075 lbm/ft³ = air density at (SATP) conditions

Derivation of System Pressures

There is an important relationship between stagnant or static pressure and moving or kinetic pressure that is used
in order to define the flow of a moving air stream. Total Pressure (TP) is the mathematical sum of Static Pressure
(SP) and Velocity Pressure (VP) or:

TP = SP + VP (Equation #4b)

Static Pressure (SP) is defined as either a positive or negative pressure that is uniformly applied to surfaces which
causes the surface to either expand or contract. For example, in a stationary positive pressurized container such
as an inflated balloon, the internal static pressure keeps the balloon inflated since the internal static pressure is
greater than the local atmospheric pressure outside as shown in Figure 4-1.

FIGURE 4-1

On the other hand, in a confined moving air stream such as a length of duct, the internal static pressure tends to
either burst or collapse the duct and can be greater than or less than local atmospheric pressure. The orientation of
the pressure in a duct is shown in Figure 4-2.
-22-
 FIGURE 4-2

The relationship between the velocity and the velocity pressure is very useful in determining the critical pressure
requirements to move the process air stream from point to point through the ductwork and through the fan itself.
The velocity pressure is always a positive value and is proportional to the kinetic energy of the system. This
relationship is given by the equation below:
Equation #4c: * VP  U ˜9 

*Note: Equation uses International System of Units (SI Units)

To utilize the Standard English Units, conversion and unit factors must be applied to yield the following equation:

§ V ·
Equation #4d: VP U ˜¨ ¸
©  ¹
Where VP = velocity pressure in inches of water gauge (inWG)
Ref: 27.68 inWG = 1 PSI

ɪ = mass density is expressed in lbm/ft3

V = velocity expressed in feet per minute (fpm)

At SATP conditions, the density of air will be p = .075 lbm/ft3. Using this value in equation #4d produces equation
#4e below: (Note: When not at standard ambient temperature and pressure conditions, use Equation #4d.)

Equation #4e: § V ·
VP ¨ ¸
©  ¹
Measuring of System Pressures

Before measuring any system pressures, the differences and relationships between the different types of pressures
must be recognized. There are three different types of pressures that can be measured: Absolute, Gauge, and
Vacuum Pressure.

-23-
In a vacuum such as in a laboratory or outer space, the pressure is basically zero. When the pressure is measured
relative to the absolute zero pressure, it is designated as Absolute Pressure. Our local atmospheric pressure at
70°F and 0ft above sea-level is 1 atm (atmosphere) or 14.7 psia. Most pressure measuring devices, or gauges, do
not measure absolute pressure but rather differential pressures. The differential pressure gauge uses the local
atmospheric pressure as its reference; while the atmospheric pressure is still 14.7 psia, the gauge will provide a
reading of 0 psig. When Gauge Pressure is being measured, the possibility exists for the pressure to be positive or
negative; negative pressures are referred to as Vacuum Pressure, see Example 4.2.

Example 4.2 Determine the Gauge and Vacuum pressure of air at an altitude of 35,000ft
when the Absolute Pressure p = 3.3 psia
Absolute Pressure Gauge Pressure Vacuum Pressure
3.3 psia (3.3 – 14.7) = -11.4 psig 11.4 psi vacuum

Figure 4-3 shows a graphical representation of the following pressure relations: Absolute Zero, Absolute Pressure
of 3.3 psia, Local Atmospheric Pressure, and a Pressurized Tank filled to 20psig.

Pressurized Tank p = 34.7 psia

p = 20 psig
Local Atmosphere
p = 14.7psia

p = -11.4 psig
p = 11.4 psig vacuum

p = 0 psig Altitude of 35,000ft

p = 3.3 psia

Absolute Zero
(Absolute Ref.) p = 0 psia
Figure 4-3

Note: It is very important to keep units in check when both measuring and calculating Absolute Pressure (psia) and
Gauge Pressure (psig).

To measure pressure in an enclosed vessel, a simple dial or electronic pressure gauge can be utilized. To visually
measure and represent pressure, there is a type of gauge called a manometer that uses water on a graduated
scale to measure static and velocity pressure. The manometer measures pressure in the units of “Inches of Water
Column” denoted as inWC or “Inches of Water Gauge” denoted as inWG; inWC and inWG are used
interchangeably in industrial ventilation. If the balloon from the previous figure is resolved down to a simple vessel
and a simple pressure gauge and manometer are used to measure the internal pressure, it would look like
Figure 4-4.

-24-
 FIGURE 4-4
Note: The conversion is 27.68 inWG = 1 psig; the 0.2psig would convert to 5.54 inWG

This same manometer can be used to measure moving air in a duct as shown in Figure 4-5.

FIGURE 4-5

Example 4.3: What is the velocity pressure of air at standard conditions (SATP) when it is
traveling at velocity rate of 4000 fpm through a duct.

Use equation #4e: § V ·
VP ¨ ¸
©  ¹

Where V = velocity is given at 4000 fpm

VP = velocity pressure is expressed in inches of water gauge


§  · VP = 1 inWG
VP ¨ ¸
©  ¹

-25-
This measuring can be completed anywhere within the system. As shown in Figure 4-6, even though the total and
static pressures change from negative to positive, the velocity pressure remains constant.

FIGURE 4-6

There is a measuring device that combines two manometers into a solitary unit; it is called a pitot tube. As shown in
Figure 4-7, the pitot tube can be inserted into a duct and will provide both the Total Pressure and the Static
Pressure of the system at the measuring location.

FIGURE 4-7

This is accomplished by the simple design of the pitot tube and can be seen in the close up view in Figure 4-8.
There is a solid inner tube that runs the entire length down to the exit which provides the Total Pressure
measurement. The outer tube has tiny holes on the exterior to measure and transmit the Static Pressure to the
horizontal pressure tap on the pitot tube.

-26-
 FIGURE 4-8

By taking the pressure differential between the two pressure ports, the Velocity Pressure can be measured. As can
be seen in Equation #4f., by rearranging the pressure terms and taking the differential pressure between total and
static pressure, the result is velocity pressure.
VP = TP - SP (Equation #4f)

The relationship among the variables of flow rate, velocity, and cross-sectional area is given by the equation below:

Q = (V) (A) (Equation #4g)

Where Q = volumetric flow rate given in cubic feet per minute (CFM)

V = average velocity expressed in feet per minute

A = area expressed in square feet

Example 4.4 Determine the flow rate (CFM) of air at standard conditions (SATP) through
an 8 inch diameter duct and at a velocity rate of 4500 feet per minute.

Use equation #4g: Q = (V) (A)

Where V = velocity rate is given at 4500 feet per minute

A = cross-sectional area of 8 inch diameter duct that is expressed in square feet

2
ʌ ʌ 1ft
A = × d2 = ×( 8")2 = 50.27in2 × 2 = 0.349 ft2
4 4 144in

Q = (V) (A)

Q = (4500 ft/min) (.349 ftð)

Q = 1570 CFM
Note: This equation is used in many flow/pipe applications.

-27-
Figure 4-9 shows the conversion of Pressure, Velocity, and Flow in standard and common units.

Standard
Standard Metric OTHER UNITS
English
Inches of
Pascals
inWG psig Mercury
(Pa)
(inHg)
PRESSURE inWG 1 248.9 0.036 0.0735
Pa 0.004 1 0.00014 0.0003
psig 27.68 6894.8 1 2.036
inHg 13.60 3384.5 0.491 1
ft/min (fpm) m/s m/min m/hr
fpm 1 0.0051 0.305 18.29
VELOCITY m/s 196.85 1 60 3600
m/min 3.28 0.0167 1 60
m/hr 0.055 0.00028 0.0167 1
CFM m3/s m3/min m3/hr
CFM 1 0.00047 0.028 1.70
FLOW m3/s 2118.8 1 60 3600
m3/min 35.3 0.0167 1 60
m3/hr 0.589 0.00028 0.0167 1
Figure 4-9

Ductwork can be made to any diameter that is required, to save costs it is recommended to use local common
sizes. Figure 4-10 shows common sizes available in the United States in inches. Figure 4-11 uses equation #4g to
show the air flow in CFM in common duct sizes.

Diameter Area Diameter Area Diameter Area Diameter Area

(inches) (sq. ft) (inches) (sq. ft) (inches) (sq. ft) (inches) (sq. ft)
1 0.005 11 0.660 26 3.687 48 12.566
2 0.022 12 0.785 28 4.276 52 14.748
3 0.049 13 0.922 30 4.909 56 17.104
4 0.087 14 1.069 32 5.585 60 19.635
5 0.136 15 1.227 34 6.305 64 22.340
6 0.196 16 1.396 36 7.069 68 25.220
7 0.267 18 1.767 38 7.876 72 28.274
8 0.349 20 2.182 40 8.727 78 33.183
9 0.442 22 2.640 42 9.621 84 38.485
10 0.545 24 3.142 44 10.559 90 44.179
Figure 4-10. Cross Sectional Area for Round Duct.

-28-
'LDPHWHU Velocity in fpm
        
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         
Figure 4-11. Air Flow Rates in common duct sizes in CFM.

Calculating System Performance

As air travels through a duct, energy is lost due to friction of the duct and the resistance through fittings such as
flanges and elbows. In order for the system to function properly, these losses must be accounted for and
calculated. Calculation methods exist to determine required flow rates, determine duct diameters, and estimate
these friction losses (in the form of static pressure) required for fan sizing. It should be noted that the methods are
based on years of field experiments and laboratory testing. Estimated losses will vary compared to measured
losses due to random influences that cannot be accounted for, from fabrication issues such as poor welding to field
issues such as material buildup. In any case, the calculated losses tend to be conservative and allow for some
unanticipated irregularities.

In order to calculate duct losses in a dust collection system, it is necessary to have a foundational understanding of
the information discussed up to this point. The following section will be a basic introduction into calculating simple
duct and pressure losses in a dust collection system. For further information, consult the current edition of
Industrial Ventilation: A Manual of Recommended Practice for Design by ACGIH®.

-29-
Air will always take the path of least resistance. This fact can cause frustration when not properly accounted for in
dust collector design and duct design. In a dust collection system with more than one pick-up point, there are
junction points where merging airflows meet. At these points, a natural balance will occur based upon the friction
and resistance losses of each of the air flows up to that point. It is critical that the static pressure requirements at
these points are in balance to prevent airflow and velocity issues at each pick-up point. To achieve this balance, it
is highly recommended that a blast gate be used to get the desired airflow at each pick-up point and maintain the
required transport velocities. A common blast gate is shown in Figure 4-12.

Figure 4-12

In duct systems where the air is carrying particulate (dust), there are minimum transport velocities that must be
maintained to ensure that the dust being carried does not settle in the ductwork. The transport velocity is
dependent upon density, size, moisture content, and hardness of the dust being collected as well as dust load. The
table below, Figure 4-13, shows a range of minimum recommended duct velocities along with some examples.

Design Velocity Contaminant Density Examples

1000-2000 fpm Gases, Smoke, Vapor
2000-2500 fpm Fume, Metal Smoke Welding
2500-3000 fpm Very Fine Light Dust < 10 lb/ft3 Cotton lint, Paper (grinding)
10-30 lb/ft3 Polishing, Fine Rubber Dust,
3000-3500 fpm Dry Dust and Powders Pharmaceutical Powders,
Wood Powders
30-60 lb/ft3 Grinding, Buffing, Wood Dust,
3500-4000 fpm Average Industrial Dust Limestone, Plastics, Clay,
General Material Handling
60-90 lb/ft3 Sawdust (heavy, large chips, or
4000-4500 fpm Heavy Dusts
wet), Foundry, Lead Dust, Coal
4500 fpm and > 90 lb/ft3 Cement, Lead dust with chips,
Heavy or Moist Dusts
up Iron Oxide, Lime
FIGURE 4-13
-30-
Before the air can flow through a duct, it first has to be captured and accelerated from free atmosphere to the
transport velocity. Just as there are recommended transport velocities, there are also recommended capture
velocities. Figure 4-14 provides a range of recommended capture velocities and examples of each application. The
ranges tend to be broad and dependent upon the application and the surrounding environmental conditions. It
should also be noted that there can be conditions where the capture velocity can be excessive and cause pick-up
of product or material, not just the nuisance dust.

Velocity Environmental Conditions Examples

75-100 fpm Little Motion Degreasing, Tank Evaporation
Welding, Occasional Container Filling, Low Speed
100-200 fpm Average Motion
Conveyor Transfers (belt speed less than 200 FPM)
Barrel Filling, Crushers, Truck or Rail Loading, High
200-500 fpm High Motion
Speed Conveyor Transfers (belt speed over 200 fpm)
Blasting, Grinding, Tumbling
500-2000* fpm Very High Motion
*Recommend 2000 fpm for Slotted Hoods
FIGURE 4-14
There are many ways to capture the air from a plain edge duct to a slotted hood. Of course, energy is required to
accelerate air to the transport velocity and the design of the capturing source impacts the amount of energy
required. The energy required to accelerate the air and also overcome any friction or resistance losses is referred
to as Velocity Pressure loss. In the calculation process, these losses are measured in terms of Static Pressure and
are divided up into two categories: Hood Static Pressure Loss and Duct Static Pressure Loss.

Hood Static Pressure Equation (SPh)

SPh = SPsl + SPhe
Hood Static Pressure is divided up into two different types of losses, losses due to Slotted Entry or SPsl and losses
due to Hoods Entry SPhe

Slot Static Pressure Equation

SPsl = -[(Hsl)(VPs)]
Where Hsl is the Static Pressure Loss due to a slot entry per VP.
VPs is the Velocity Pressure through the slot
Hsl = cSL + cA
cSL is the Slot Loss Coefficient.
cSL = 1.78.
cA is the Slot Loss Acceleration Coefficient. cA = 0 or 1 *

Hood Static Pressure Equation

SPhe = -[(Hhe)(VPd)]
Hhe is the Static Pressure Loss due to hood entry per VP.
VPd is the Velocity Pressure through the duct
Hhe = cHE + cA
cHE is the Hood Entry Loss Coefficient.
See Figure 4-15
cA is the Slot Loss Acceleration Coefficient. cA = 0 or 1 *
*In a compound hood (slotted hood combined with a tapered hood), cA, the Acceleration Coefficient is applied only
at the point of highest velocity; either the slot or the hood, not both.
-31-
 Duct Static Pressure Equation (SPd)
SPd = -[(Hfl + Hel + Hbr)(VPd)]
Hfl is the Static Pressure Loss due to friction per VP.
Hfl = Ld x f’d
Ld is the length of duct in ft.
f’d is the coefficient of friction per ft. of duct. See Figure 4-16
f’d (rigid) = 0.0307*(Vd 0.533 / Q 0.612 )
f’d (flexible) = 0.0311*(Vd 0.604 / Q 0.639 )
Hel is the Static Pressure Loss due to elbows per VP.
Hel = cEL x xEL (See Figure 4-15)
cEL is the elbow loss coefficient.
xEL is the quantity of 90°eblows
Hbr is the Static Pressure Loss due to branch entry per VP. See Figure 4-15

In a system of ductwork, the system is broken down into the largest possible sections. Section starting points are
typically pick-up points while the end points are branches where multiple flows come together. When flows come
together at a branch, there will be a Governing Static Pressure (SPgov) where the higher absolute value static
pressure up to that point will be the controlling static pressure for that section. The other flows will naturally balance
and adjust to the SPgov. It is ideal to have the static pressures at each balance point as close to equal as possible;
however, in the real world this is not easy. If the ratio of the SPgov to the lower static pressure is less than 1.2, the
balance can be achieved by increasing or correcting the airflow (Qcor) in the section of lower resistance using the
following equation:
Qcor =Q x ( SPgov / SPtot )0.5 (Equation #4h)

If the ratio is greater than 1.2, either the sections must be redesigned to be closer together, or a mechanical blast
gate must be used to balance the flows.

At each section, the static pressure losses will accumulate to yield a Cumulative Static Pressure (SP cum). The
Cumulative Static Pressure can be positive or negative, depending on which side of the fan the section is on.
After all of the sections have been calculated, the total system pressure loss can be determined:

SPSYSTEM = SPout – SPin – VPin (Equation #4i)

Where SPout is the Static Pressure at the Outlet of the Fan (Positive)
Where SPin is the Static Pressure at the Inlet of the Fan (Negative)
Where VPin is the Velocity Pressure at the Inlet of the Fan (Positive)

To make the calculation of SPSYSTEM easier, a step-by-step worksheet has been developed. It can be seen in
Figure 4-17.

-32-
 HOOD ENTRY LOSS COEFFICIENTS
PLAIN BELL
FLANGED SLOT OR SHARP EDGE
EDGE MOUTH

cHE = 0.93 cHE = 0.49 cHE = 0.04 cHE = 1.78

TAPERED HOOD

SQUARE OR RECTANGULAR ROUND OR CONICAL

cHE ș cHE
0.35 30Σ 0.24
0.25 45Σ 0.14
0.17 60Σ 0.065
BRANCH ENTRY LOSSES ELBOW LOSSES

ș Hbr R/D
cEL
15Σ 0.09 1.0 1.5 2.0 2.5
30Σ 0.18 3-Piece 0.42 0.34 0.33 0.33
45Σ 0.28 4-Piece 0.37 0.27 0.24 0.24
60Σ 0.44 5-Piece 0.33 0.24 0.19 0.19
90° (Tee) 1.00 Stamped 0.22 0.15 0.13 0.12
Figure 4-15 Loss Coefficients

-33-
 4” 5” 6” 7” 8”
V FL CFM FL CFM FL CFM FL CFM FL CFM
2000 1.9 175 1.4 273 1.1 393 0.9 535 0.8 698
2500 2.9 218 2.2 341 1.7 491 1.4 668 1.2 873
3000 4.1 262 3.1 409 2.5 589 2.1 802 1.7 1047
3500 5.5 305 4.2 477 3.3 687 2.8 935 2.3 1222
4000 7.1 349 5.4 545 4.3 785 3.6 1069 3.0 1396
4500 8.9 393 6.8 614 5.4 884 4.5 1203 3.8 1571
5000 10.9 436 8.3 682 6.6 982 5.5 1336 4.6 1745
5500 13.0 480 9.9 750 7.9 1080 6.6 1470 5.6 1920
6000 15.4 524 11.7 818 9.4 1178 7.8 1604 6.6 2094

10” 12” 14” 16” 18”

V FL CFM FL CFM FL CFM FL CFM FL CFM
2000 0.6 1091 0.5 1571 0.4 2138 0.3 2793 0.3 3534
2500 0.9 1364 0.7 1963 0.6 2673 0.5 3491 0.5 4418
3000 1.3 1636 1.1 2356 0.9 3207 0.7 4189 0.6 5301
3500 1.8 1909 1.4 2749 1.2 3742 1.0 4887 0.9 6185
4000 2.3 2182 1.8 3142 1.5 4276 1.3 5585 1.1 7069
4500 2.9 2454 2.3 3534 1.9 4811 1.6 6283 1.4 7952
5000 3.5 2727 2.8 3927 2.3 5345 2.0 6981 1.7 8836
5500 4.2 3000 3.4 4320 2.8 5880 2.4 7679 2.1 9719
6000 5.0 3272 4.0 4712 3.3 6414 2.8 8378 2.4 10603

20” 22” 24” 30” 32”

V FL CFM FL CFM FL CFM FL CFM FL CFM
2000 0.3 4363 0.2 5280 0.2 6283 0.2 9817 0.1 11170
2500 0.4 5454 0.4 6600 0.3 7854 0.2 12272 0.2 13963
3000 0.6 6545 0.5 7919 0.5 9425 0.3 14726 0.3 16755
3500 0.8 7636 0.7 9239 0.6 10996 0.5 17181 0.4 19548
4000 1.0 8727 0.9 10559 0.8 12566 0.6 19635 0.6 22340
4500 1.2 9817 1.1 11879 1.0 14137 0.8 22089 0.7 25133
5000 1.5 10908 1.3 13199 1.2 15708 0.9 24544 0.9 27925
5500 1.8 11999 1.6 14519 1.5 17279 1.1 26998 1.0 30718
6000 2.1 13090 1.9 15839 1.7 18850 1.3 29452 1.2 33510
Figure 4-16. Friction Loss per 100’ of Rigid Duct. (Ld x f’d x Vd)

As shown earlier, the equation for friction loss is slightly complicated: f’d (rigid) = 0.0307*(Vd 0.533 / Q 0.612 )
This can be simplified to f’d (rigid) = 0.4937/(Q 0.612 x d 1.066 ). This simplification implies that as the diameter
increases, the friction loss decreases when the flow stays the same.

-34-
1 Duct Section I.D. FAN INLET FAN OUTLET
2 Qd Duct Flowrate CFM
3 d Duct Diameter inches
4 Ad Duct Area sq. ft. See Fig. 4-10
5 Vd Velocity of flow through duct fpm ( #2 / #4 )
6 VPd Velocity Pressure of flow through the duct inWG Eqn. 1.
7 cSL Slot Loss Coefficient. 1.78
8 cA Acceleration Coefficient 0 or 1
S L
9 As Slot Area sq. ft.
L O
10 Vs L Slot Velocity fpm ( #2 / #9 )
H O S
11 VPs O Slot Velocity Pressure inWG Eqn. 1.
O T S
12 Hsl S Slot Loss per VP #7 + #8
13 SPsl O Slot Loss #10 x #12
S
D
14 cHE E Hood Entry Loss Coefficient See Fig. 4-15
15 cA S Acceleration Coefficient 0 or 1
16 Hhe Hood Entry Loss per VP #14 + #15
17 SPhe Hood Entry Loss inWG #6 x #16
18 SPh Hood Static Pressure Loss inWG #13 + #17
19 Ld Length of Straight Duct ft
20 f'd Duct Friction Loss Factor
21 hFL L Duct Friction Loss = f’d · Ld #17 x #18
D
22 cEL O Elbow Loss Coefficient Table 6a
U
23 xEL S Quantity of 90° Elbows. Table 6b
C
24 Hel S Elbow Loss = fel · xel #20 x #21
T
E
25 Hbr S Branch Entry Loss Coefficient See Fig. 4-15
26 Hd Duct Loss per VP #19+#22+#23
27 SPd Duct Static Pressure Loss inWG #24 x #6
28 SPother Other Losses (Blast Gates, Collectors, etc.) inWG
29 SPtot Total Static Pressure for Section inWG #18+#27+#28
30 SPgov Governing Static Pressure NEGATIVE POSITIVE
31 SPcum Cumulative Static Pressure - +
32 Qcor Corrected Flowrate CFM Equation 3
33 Vcor Corrected Velocity of flow fpm #32 / #4
34 VPcor Corrected Velocity Pressure inWG Equation 1
Equation 1 ( V / 4005 )2 R/D
Table 6a
f’d (rigid) = 0.0307*(Vd 0.533 / Q 0.612 ) 1.0 1.5 2.0 2.5
Equation 2
f’d (flexible) = 0.0311*(Vd 0.604 / Q 0.639 ) 3-Piece 0.42 0.34 0.33 0.33
Equation 3 Qcor =Qd x ( SPgov / SPtot )0.5 4-Piece 0.37 0.27 0.24 0.24
Table 4 15° 30° 45° 60° 5-Piece 0.33 0.24 0.19 0.19
Branch Loss 0.09 0.18 0.28 0.44 Stamped 0.22 0.15 0.13 0.12
TAPERED HOODS ș 30° 45° 60°
Table 5 Table 6b
Rect. ș/90° 0.33 0.50 0.67
ș Round
PLAIN EDGE 0.93 /Square
FLANGED 0.49 0.35 30° 0.24
Equation 7 SPSYSTEM = SPout – SPin - VPin
BELL MOUTH 0.04 0.25 45° 0.14
SLOT/SHARP EDGE 1.78 0.17 60° 0.065

System Static Pressure inWG -- - =

SPout SPin VPin SPSYSTEM
Figure 4-17
** NOTE THESE CALCULATIONS ARE BASED ON “STANDARD AIR”

-35-
 Figure 4-18 and 4-19 show a common dust collection system setup, a graphical representation of the
pressures throughout, and the calculated pressure losses.

Figure 4-18
-36-
Figure 4-19 Completed Worksheet for System

-37-
There are some items that should be noted in the calculations:

SLOTTED HOOD
The acceleration factor is applied to the hood itself not the slotted hood because 2000fpm < 4000fpm

BLAST GATES & DAMPERS

It is highly recommended that blast gates be utilized at ALL pick-up points to control and balance the airflow.
It is highly recommended that fans have dampers or VFDs to control flow.

In a system, the fan damper/VFD acts as the coarse adjustment of airflow and the individual blast gates at each
pick-up point act as the fine adjustment of airflow.

DUST COLLECTOR PRESSURE DROP

When a dust collector manufacturer provides a pressure drop, ǻP, or dP; it will typically be the pressure drop
across the filters. In a generic dust collector with a venturi, this can range from 4inWG to 6inWG from clean filters
to dirty filters. This is lower in a system when no venturi is used.

BAGHOUSE ACCELERATION
After the air has slowed down to enter the dust collector and pass through the filters, it must be re-accelerated to
duct velocity again. The “flange-to-flange” loss across the dust collector is typically 6inWG when venturis are used
and 4inWG when venturis are removed. This acceleration factor can vary depending on the outlet velocity and
resulting VP. Once the air is clean, a higher transport velocity is no longer required and the velocity can be much
lower. To achieve the lower velocity, the duct size will need to increase which will increase overall costs.

However, it should be noted that anywhere Static Pressure can be lowered, there will be significant energy and
cost savings as shown by Equations #4j & #4k and in Examples 4.6 and 4.7.

NO LOSS STACK
The system is shown with a “No-Loss Stack”. The stack does not have any loss because there is an internal
expansion as shown in Fig 4-18. This expansion creates a regain of static pressure. When there is a contraction in
a duct (change in diameters from larger to smaller), the air is forced to accelerate and this acceleration creates a
loss. When there is an expansion in a duct (change in diameters from smaller to larger), the air naturally slows
down and this creates a “regain” in static pressure. Thus, a “No-Loss Stack” does not require any additional static
pressure of the system. The “No-Loss Stack” has another benefit in that it provides superior rain protection
compared to deflecting caps.

-38-
Based on the information from the velocity pressure worksheet, a blower fan can be sized for 10,000 CFM at
9.5inWG. This information can also be used to estimate the energy required in BHP (horsepower) as shown in
Equation
WFAN
CFM ˜ SP (Equation #4j)
˜inWG
6356 CFMBHP ˜ Ș fan

Where Șfan is the efficiency of the fan. For general applications Șfan=80% or 0.80

This equation can also be used to estimate energy costs for fan operation:

CFM ˜ SP ˜ 0.746 BHP

kW
˜ kW$˜hr ˜# hr
COST (Equation #4k)
˜inWG
6356 CFMBHP ˜ Ș fan

Example 4.6 Determine the energy required (BHP) for the fan in Figure 4-19. Then
determine the cost to operate this fan annually, assume 8000 hours and
the price of electricity to be $0.08 kW/hr.

Use equation #4j: WFAN

CFM ˜ SP
˜inWG
6356 CFMBHP ˜ Ș fan

WFAN
10000CFM ˜ 9.5inWG
˜inWG
6356 CFMBHP ˜ 0.8

WFAN = 18.7 BHP

CFM ˜ SP ˜ 0.746 BHP

kW
˜ kW$˜hr ˜# hr
Use equation #4k: COST ˜inWG
6356 CFMBHP ˜ Ș fan

10000 ˜ 9.5inWG ˜ 0.746 BHPkW

˜ kW ˜hr ˜ 8000hr
0.08$
COST
6356 BHP ˜ 0.8
CFM˜inWG

The annual cost to operate this fan is $8,920.00

-39-
As stated earlier, anywhere Static Pressure can be conserved or lowered, a significant energy and cost savings
potential exists.

Example 4.7 Determine the energy and cost savings if the static pressure in
Example 4.6 was reduced by 2inWG.

New SP = 7.5

WFAN
10000CFM ˜ 7.5inWG
˜inWG
6356 CFMBHP ˜ 0.8

WFAN = 14.8 BHP for a 3.9 BHP energy savings

10000 ˜ 7.5inWG ˜ 0.746 BHPkW

˜ kW ˜hr ˜ 8000hr
0.08$
COST
6356 BHP ˜ 0.8
CFM˜inWG

The annual cost to operate this fan is $7,043.00 or savings of $1,877.00.

This reduction in Static Pressure accounts for a 15% energy and cost savings per year.

It is important for proper system operation that a fan is sized correctly and has enough static pressure to overcome
the system losses. It is also highly recommended that a fan be slightly oversized and employ a control damper to
ensure proper suction and flow. This is because it is always possible to “damper down” the air flow; however, it is
impossible to “damper up” the air flow.

-40-
 Chapter 5

MECHANICAL CLEANING COLLECTORS (SHAKER COLLECTORS)

Basic Unit
Some industrial processes emit large quantities of dust and the capture of these dusts is costly with non-cleanable
types of filters; therefore, in most applications, cleanable filter arrangements are required. The simplest and oldest
version of this type of filter (Figure 5-1) consists of a bag with the same diameter as a 55 gallon drum and an inlet
sewn into the bottom of the bag. Typically, the fan’s paddle wheel is the air mover and receives the incoming dust.
The filter bag is suspended from a rope that is convenient for agitation. These collectors have the fan on the dust
laden side and were the first collectors to be adapted to manual paper trim operations.

FIGURE 5-1

The most common filter material used in these designs is a sateen weave cloth which is a very tight weave and
very flexible. The dust collects on the inside of the bag with velocities ranging from 0.5 to 8.0 fpm. The virgin
media’s initial permeability is between 30-50 CFM per square foot at ½ inch of water column which indicates an
initial pressure drop of 0.1 to 0.2 inches water column across the media. As the dust collects on the inside of the
bag, the pressure drop rises and, at some point between 2 and 3.5 inches of w.c., the filter media is cleaned. The
bag is usually cleaned by a person actually pulling up and down on the rope. After this cleaning process, the
pressure drop generally readjusts to a lower value in the range of 0.5 to 1.0 inch water column.

-41-
Filter Cake/Operating Characteristics

This difference between the initial pressure drop and the pressure drop after cleaning is due to what is referred to
as a filter cake. Let us look at the operating parameters of this basic collector as shown in Table 5-1:

Table 5-1

Air Volume Flow: 500 to 1000 CFM

Bag Size: 24 inch diameter
Bag Length: 10 to 14 feet
Bag Area: 88 sq. ft. @ 14 feet
Filtering Velocity: 5.7 fpm at 500 CFM
11.4 fpm at 1000 CFM
Initial Pressure Drop: 0.1 inches w.c.
Average dust holding capacity at 3” w.g.: 8-16 oz.
Dust holding capacity after cleaning at 1” w.g.: 2-4 oz.
Inlet load: 5 grains/cu.ft.
Maximum time between cleanings: 10-20 hrs. at 500 CFM
5-10 hrs. at 1000 CFM

This collector selection is affected by three basic parameters:

x Filtering Velocity
x Dust Load
x Residual Pressure Drop

There is usually a time between cleanings that may be tolerated. If the requirement is for cleaning twice in an eight
hour shift, the capacity selection would be 1000 CFM, but if the requirement is once per shift, the collector should
be selected to operate at 500 CFM. If the inlet load is doubled, the time would be reduced to one-half between the
cleaning cycle. The residual pressure drop is related to the dust holding capability of the filter media which, in turn,
is related to the dust characteristics. Generally, the filter ratio is based on past experiences with the type of dust
collection or the industrial process.

-42-
Envelope Filter Bags

In order to provide more filter media area in a given volume or floor space, other configurations are used. One of
the earliest types is the unit dust collector (Figure 5-2) with envelope bags. The filter bags are sewn into envelopes
with spacers to keep the media from collapsing on itself. The earliest spacers used wire inserts and some of the
later designs use open foam. The collectors either shake manually with the cleaning operation being similar to the
single bag collector or automatically by motor power.

FIGURE 5-2

Since the typical filter media stretches after the cleaning cycle, the filter bags are held taut by springs that are
attached to the closed end of the envelope. In fact, the stretch is enough that after a few weeks of operation, it is
desirable to adjust the spring tension. These types of designs are effective on most mechanically generated dusts
such as transfer points on belt conveyors, screening, clamping stations, grinding, and abrasive blast cabinets.
Conversely, they are less effective in collecting dust from processes such as dryers and furnaces. These types of
dust particles seem to imbed themselves more deeply into the filter media. There is not enough energy in the
cleaning mechanism to provide suitable cleaning to give reasonable filter element life and low residual pressure
drops.
-43-
Tubular Shakers
To handle the difficult embedded dust particles, the
tubular shaker collectors were developed. These
collectors (Figure 5-3) have the opening to the bag
at the bottom of the collector, gathers the dust on
the inside of the bags, contains some form of
tensioning device to keep the cloth tight on the
bag, and uses a variation of some type of shaker
mechanism. Typically, each specific design can be
evaluated by its mechanism designs. Other
parameters include the relatively small bag
opening diameters as compared to the overall
length. Usually the bag diameter varies from three
to twelve inches in diameter and has an overall
length that corresponds to a length to diameter
ratio of 20 to 35. This ratio gives the cleaning
motion a better action and is able to remove the
dust more successfully than other shaker types.

FIGURE 5-3
Many of these processes require continuous cleaning and cannot tolerate stopping the process for cleaning. To
accommodate this requirement, continuous cleaning compartmental collector systems (Figure 5-4) were developed
and consist of dividing the unit into multiple modules that have separate dampers.

FIGURE 5-4

-44-
By closing a damper in one compartment and diverting the flow into the other compartments, the isolated module
can be cleaned. These effective filter units have at least two compartments and are produced with as many as 20
compartments. First, the fan flow is stopped momentarily by a damper in one compartment and the isolated
compartment unit is effectively now “off-line”. Then, the cleaning action occurs with no fan flow moving through the
compartment.

The main drawback of this type of collector is the off-line cleaning process as well as higher maintenance due to its
internal moving components. Since it is required to operate at a low air to cloth ratio compared to other designs,
this type of collector is usually larger and more costly than other models. Its main advantage is in low volume
applications or in environments where compressed air is not available for filter cleaning requirements.

-45-
 Chapter 6

HIGH PRESSURE REVERSE FAN CLEANING COLLECTORS

Development of a Continuous Cleaning Collector with High Pressure Reverse Air

The next step in the evolution of the equipment was a type of collector that was able to continuously clean during
the ongoing industrial process and without any special compartments. Also, this collector style utilized either a
large pyramid hopper or trough hopper, and because of the capability to clean during the industrial process, this
type of collector became known as an “on-line” cleaning collector as illustrated in Figure 6-1. This fan pulse
collector utilizes a traveling manifold that traverses back and forth across the mouth of the envelope filter bags.
The reverse air circuit only cleans one bag or one row of bags at a time.

FIGURE 6-1

-46-
There are some important elements to this design when compared to mechanical shaker collectors. For example,
there are no requirements to keep the bags stretched during the cleaning cycle and the filter bags are pressurized
from the cleaning manifold. The first important reverse air cleaning principle is developed in this collector design.
This principle is the filtering capacity of the bag in the cleaning mode is related to the cleaning flow. Also, on larger
collectors, they are able to operate at much higher air to cloth ratios than the shaker collectors that they replaced in
identical processes. These collectors were very effective on a wide range of applications.

Cleaning Process Analysis

If we take a collector with sixteen bags (regardless of cloth area and maintain some limits of filter media
permeability) and size it to filter 100 cu. ft. per minute (CFM) per bag, the total flow would be:

100 cu. ft. /min. x 16 bags = 1600 cu. ft./min.

To clean a single bag, we need a reverse flow:

100 cu. ft./min. x 3 = 300 cu. ft./min.

To maintain 1600 cu. ft. per minute through the system, the exhaust fan must draw:

1600 + 300 = 1900 cu. ft./min.

In selecting a fan, the reverse airflow is treated as from another source in the system. If the negative pressure
drop in the collector is less than 10 inches w.c., an auxiliary reverse air blower is required for the cleaning air
circuit. On systems with a negative pressure greater than 10 inches w.c., an adjusted slide gate is placed in the
reverse air cleaning circuit.

On these types of continuous cleaning designs, the bags that are next to those being cleaned must be blocked to
prevent the cleaning manifold from propelling the dust to the adjoining bag. When the flow is blocked for cleaning,
the agglomerated dust falls vertically from the targeted bag into the collection hopper. The traversing manifold is
powered by a chain driven gear motor which is located in the clean air plenum. In a typical collector, 3% of the
collector is cleaned so the cleaning or reverse airflow must be provided at about three times the pressure drop.
The power requirements are as follows:

3% x 3 times pressure drop = 9% power consumption

-47-
A variation of this continuous cleaning collector (Figure 6-2) consists of a cylindrical housing and filter bags that are
arranged in a radial pattern. This arrangement features a rotating arm with traveling manifold as compared with
the back and forth motion of the rectangular manifolds. Also, the rotating arm extends to the adjoining bags in
order to also block their airflow during the cleaning phase.

FIGURE 6-2

Application Details

A common application is venting wood floor and general wood dust. The dust loads in the vent exhaust stream are
about 10-15 grains per cubic foot. The vent stream is normally positive and the addition of several positive
pressure blowers can also vent into one collector. The positive blowers are less efficient with their paddle fan
wheels than the backward inclined designs. However, the backward inclined designs must be mounted on the
clean air side of the collector. In general, the positive pressure blowers have some advantages:

1) Multiple fans can divide the branches of the vent system with low and high pressure drops to reduce the
power draw between the various branches. The system fans energize only when the specific branches
are active while back draft dampers prevent the dust from entering the inactive branches.

-48-
 Modifications are easier to make to the branches as demonstrated by the change in airflow which is
accomplished by only changing the fan drive (belts and sheave) for the individual branch.

2) The dust is collected at one point for ease of disposal.

3) The conical hopper makes the flow of product to the hopper outlet smoother than the trough or pyramid
hopper when comparing the same width of collector.

4) Often these collectors mount to the same structures that support the low pressure cyclone collectors.

5) When upgrading to more efficient fabric collectors, the previous system ductwork connects with very little
changes.

Low pressure cyclones operate with no rotating seal on the cyclone hopper outlet. The hopper outlets are at
atmospheric pressure on positive pressure systems; therefore, the dust falls freely into a collector container.
Generally, the hopper openings are between 16 and 24 inches in diameter so that the wood dust falls easily into a
container and does not bridge across the opening of the hopper outlet.

When fabric dust collectors were initially introduced and applied to the woodworking applications, the fan air unit
was located on the clean air side of the collector. An airlock device was required to maintain a vacuum inside the
collector.

Conversely, with a positive pressure venting system, a less expensive welded housing is needed and wear on the
airlock’s wiper blade is low since the fan’s air pressure is less than four inches w.c. The flow capacity of the
collector can be increased by adding more flow through the reverse air fan, which made the positive pressure
systems very versatile.

These collectors are especially effective in applications such as grain collection and other similar processes. In
many of these systems, the process gas is close to the dew point, and to help spread the difference between the
dew point and the dry bulb temperature, both the system and cleaning fans contribute a drying effect on the dust
that is collected. Also, more heat can be added to the reverse air circuit when an additional temperature spread is
needed between dry bulb and dew point temperature.

Fan Pulse Collectors

From these reverse air fan collectors, the first major modification was to pulse the cleaning airflow. It was found
that almost all the dust would be ejected from the bags during the first tenth of a second that the bag was being
cleaned. During the cleaning air pulse flow, the opening and closing of the dampers were usually accomplished by
the rotation of the arm on the cylindrical collectors. In Figure 6-3, the reverse air pressure blower is mounted
outside on the shell of the collector.

-49-
 FIGURE 6-3

To start and stop the cleaning airflow, some rectangular designs had solenoids that opened and closed the
dampers. The typical damper opening was for 1/2 second, and the total collector could be cleaned in 3 to 6
minutes. For instance, if the collector had 30 radial rows, the power consumption is calculated to clean the bags as
follows:

The reverse airflow requires an instantaneous cleaning flow of nine percent which is the same as the continuous
fan cleaning system. The new power requirement can be calculated:

(30 radial rows) (9%) x (0.5 seconds)__ = .75%

(3 min.) x (60 sec./min.)

The cleaning airflow from the reverse air fan is trying to flow continuously even though the dampers are closed
most of the time except during the cleaning cycle. It is a mistake to estimate the cleaning air power consumption
by merely noting the horsepower of the motor. The reverse fan pulse cleaning collectors used less cleaning air to
clean the bags than the earlier collectors. The fan pulse units had the same advantages as the continuous
cleaning (fan) collectors with the additional feature of much lower power consumption.

-50-
Advantages of Reverse Air Cleaning Collectors
The cleaning flow gradually increases with the reverse fan air cleaning collectors so that the dust leaves the bag at
low velocity and gradually increases to a velocity of approximately 10,000 feet per minute. Because the fine dust
leaves the bags at the lowest possible velocity, the dust is not subject to the higher cleaning forces. For grain and
other food applications, this is preferable.

Disadvantages of Reverse Air Cleaning Collectors

The main disadvantages of reverse air fan pulsing are:
x Practical capability and manufacturing costs limit the use of fans with both high positive air pressure and
at high air flow rates.
x Damper cleaning arrangements are inherently slow in operation and can be expensive to purchase and
maintain.
x The reversing air fan motor is operating continuously in order to provide instantaneous pulsed air for
cleaning.

Air Pump Fan Pulse Variations

The next major improvement in fan pulse collectors consists of increasing the pressure of the cleaning arm to
approximately 7½ psig and the use of a positive displacement air pump. This design has different design features
(Figure 6-4):
1) The reverse airflow pulsed into 8 to 12 inch diameter diaphragm valves which are able to open and close
faster than the dampers.
2) The exhaust velocity exits fast enough from the openings on the rotating arms so that the openings could
be placed inches from the clean air plenum. The extra flexibility in the location of the openings allowed
versatility in mounting various types of bags and provided easier top access with the clean air plenums.
3) The clean air pumps could be placed on the ground next to the collector with little pressure drop losses
because of the high pressure of the cleaning system.

FIGURE 6-4

-51-
 Chapter 7

PULSE JET BAGHOUSE COLLECTORS

Blow Ring Collector

The first continuous cleaning jet collector was a blow ring collector as illustrated in Figure 7-1. In this type of
collector, the dust collected on the inside of tubular bags which were typically 14 to 16 inches in diameter and 6 to
20 feet long. For cleaning, each bag had a blow ring that traveled up and down the outside of the bag. The dust
laden air entered the inlet at the top of the dirty air chamber of the collector and flowed from the inside to the
outside of the bag. For this type of collector, the filtering velocity (filter ratio) was commonly in the range of 18 to
22 feet per minute.

FIGURE 7-1

Let us further analyze a typical collector with the following specifications:

Bag Length 8 feet

Bag Diameter 18 inches
Bag Area 38 square feet
Filter Flow per Bag 750 CFM per bag
Number of Bags 4
Total Flow per Collector 3,000 CFM
Average Pressure Drop 2 inches water column
Average Dust Penetration at 10 gr./cu.ft. load 0.0002 gr./cu.ft.
-52-
The air entered the bag from the top and flowed downward at 425 feet per minute. The air traveled slow enough
so there was minimum bag abrasion and formed an effective “drop out” section at the bottom of the open bag.
Also, while the dusty air was traveling inside the bag, it was also traveling towards the inner circumference of the
bag at a velocity of 18 to 22 feet per minute.

The cleaning of each bag was accomplished by traveling blow rings which consisted of a tubular duct with holes
that faced the outside of the dust collector bag. The blow ring traveled up and down the side of the bag by
mechanical power, usually chain and sprockets.

Since most collectors were built in multiples of four bags, the blower fan was able to connect to each of the four
blow tubes by using flexible hoses.

The exit velocity of the blower air was generally 14,000 to 15,000 feet per minute while the cleaned width of the
bag was less than half an inch. The cleaning blower flow was typically about 50 ACFM or about six percent of the
filter flow which resulted in only one percent of the filter media being continuously cleaned during the operation of
the collector.

The advantages of this collector were:

x It operated at low pressure drops which were usually in the 1-1/2 to 3 inch water column pressure drop
range.

x It operated at very high dust loads with a limit of 150 grains per cubic foot.

x It was suitable for air recirculation on most operations.

x Bag life was only limited by abrasion which gave an average bag life of five years or greater.

x The dust in the bag formed a very stable cake.

x The system was inherently able to operate under a wide range of dust loadings without adjusting the
speed of the blow rings.

x Adjustment for dust loads was accomplished by either shortening the bags for heavy dust loading or
lengthening the bags for the lighter dust loading.

The disadvantages of this collector were:

x It was not suitable for operations at high temperature or in corrosive atmospheres.

x The mechanical drives for operating the blow tubes generally required frequent maintenance.

-53-
Fabric Pulse Jet Collector Early Designs (Circa 1963)

To expand in the application area for process streams that operate at higher temperatures and corrosive condi-
tions, an improved fabric pulse jet collector was developed. The early design is illustrated in Figure 7-2. The main
changes in the collector include the collecting of dust on the outside of the bag, the grouping of bags into rows, and
the cleaning of the bags by rows. Each bag was typically 4 to 6 inches in diameter, 6 feet long, and arranged with
6 to 10 bags in a row. The cleaning sequence was accomplished by cleaning each row of bags individually. The
cleaning energy consisted of a compressed air powered eductor or reverse jet that ejected compressed air into
each bag in the row. A pipe or purge tube was common to all of the bags in a given row and it was located over
the center of each bag in the row. Orifice holes were positioned in the purge tube at the center of the bags which
directed the compressed air jet into the throat of the bag.

FIGURE 7-2

When the compressed air travels through the orifice, it becomes an air jet that expands by the Law of Conservation
of Momentum until it is stopped by one of the following:

x The opening of the bag itself.

x A tube which is inserted into the center throat of the bag and the tube diameter is calculated to generate
the proper jet velocity in relation to the size of the orifice in the purge tube.

x A so-called venturi, which serves the same purpose as the tube described above, is basically a tube with
smooth transitions attempting to reduce the pressure drop as the fan air flows from the bag into the clean
air chamber.

x An orifice plate that is centered in the throat at the top of the bag and it has the same purpose as the
tubular insert or venturi.
-54-
The characteristics of these early cleaning jets were as follows:

Table 7-1

Average velocity at throat of the tube, venturi or orifice 15,000 feet per minute
(It should be noted that this was the same velocity as the blow ring outlet.)
Venturi throat opening 1 7/8 inches diameter
Jet flow 290 CFM
Bag diameter and length 4½ inches x 72 inches
Bag area 7 sq. ft.
Filter flow rating per bag 100 CFM
Nominal filter ratio 14 FPM*
Average pressure drop 3 1/2 inches water column
Average Air Consumption ¾ SCFM/1000 CFM of filtered air
Average dust penetration at 10 gr./cu.ft. load 0.0005 gr./cu.ft.

* Actual filter ratio or filtering velocity was lowered by various dust and process characteristics, primarily
because of the dust laden air entering into the hopper. Average filter ratios were approximately 10:1
or 10 FPM filtering velocity through the bags.

Fabric Pulse Jet Collector Later Designs (Circa 1971)

The original design was later modified by the original patent holder and the characteristics of the cleaning jet were
altered, presumably to accommodate ten foot long bags. This “generic” cleaning design was then copied by the
whole industry. The new characteristics were:

Table 7-2

Average jet velocity at the throat 25,000 feet per minute

Venturi throat opening 1 7/8 inches diameter
Jet flow 500 CFM
Bag diameter and length** 4½ inches x 120 inches
Bag area 12 sq. ft.
Filter flow rating per bag 90 CFM
Nominal filter ratio 8 FPM
Average pressure drop 6 inches water column
Average Air Consumption 1 ¼ SCFM/1000 CFM air flow
Average dust penetration at 10 gr./cu.ft. load 0.008 gr./cu.ft.

** Over time there were a variety of bag diameters and lengths introduced by different suppliers.
However, the jet characteristics and performance were similar.

-55-
The new design was expected to operate at the same nominal filter ratio as the early designs. However, field
experience showed that the nominal filter rate actually dropped from the designed 14:1 ratio to an actual ratio of
10:1. The true reason for this reduction in performance will not be understood until much later. In reality, the
nominal filter ratio for the new design was 8:1, however, most collectors actually operated between 5:1 to 6:1
ratios.

In the new 8:1 ratio design, the air consumption and pressure drop increased dramatically. Unfortunately, in the
general selection of dust collectors, the air-to-cloth ratio became the dominant specification in selecting the pulse
jet collectors. In time it was generally accepted that the pressure drop, air consumption, and dust penetration
would be at the new higher levels. In addition, the average bag life went from 5 to 6 years for the 1963 design to 2
to 3 years for the 1971 design. In the rapidly expanding market of the early 70’s, this deterioration of performance
was accepted by the engineers. In fact, to solve any operational or application problems, the cure was to lower the
filter ratios even further.

It is important to understand the reason for this deterioration of performance. There were two main factors: 1)
upward velocity of dust entering the filter compartment from the prevalent hopper inlets (sometimes referred to as
“can” velocity), and 2) the change in the velocity characteristics of the cleaning jet.

Changes in Jet Characteristics (“Generic” Baghouses)

The obvious change was that the jet velocity for cleaning had increased from 15,000 FPM to 25,000 FPM. It has
been well documented that on the 1971 design, the bag inflated and formed a cylindrical shape during cleaning.
This change from a concave shape between the vertical wires on the cage during cleaning has led many to believe
that the primary cleaning mechanism was this flexing of the bag during the cleaning cycle. Like all engineering
determinations, there was a certain underlying truth to these studies. The fact was that when the collectors were
compartmentalized and cleaned off line, this so-called “flexing” of the bag allowed the application of the pulse jet
collectors to be used in many processes where no other collector, including the continuous cleaning pulse jet, was
effective. However, with the development of the cartridge collector, this type of flexing could not happen during the
cleaning of the media; therefore, these theories seemed to be discarded with the passing of time.

It is important to note that if the aggregate open area in the filter cake is larger than the venturi or jet area, suitable
pressure will not develop and the bag will not leave the wires. Therefore, no flexing of the bag or media will
develop from the velocity of the cleaning air. Typically, when collectors are running below 2 inches water column,
whether cartridge or fabric, this indicates that the effective area of the cake and media combination is very large
and the flexing of the bag does not occur. When the pressure drop is over 3-1/2 inches water column, the flexing
of the bags will occur on generic venturi-based fabric collectors. After the cleaning cycle, the aggregate area of the
opening in the bag/cake is increased. It is in this newly opened area that the dust collects and the pressure drop is
lowered until an overall pressure balance is reached.

-56-
Velocity of Dust Ejected During the Cleaning Mode

It can be concluded that the dust leaves the bag during the cleaning cycle at the velocity of the cleaning jet. The
change from the 1963 design increased from 15,000 fpm to 25,000 fpm. If these velocities are converted to
velocity pressure, we get 14 inches w.c., and 38 inches w.c. respectively. This indicates that the propelling force of
dust from the bag has increased by 2.7 times during the cleaning mode. Refer to Figure 7-3. At the higher velocity,
the dust is thrown from one row of bags in the cleaning mode towards the adjoining row of bags.

FIGURE 7-3

This dust at the higher velocity drives itself through the adjoining bag and its cake. The dust cake becomes
increasingly denser and develops a more resistant barrier until equilibrium conditions are reached.

When examining the dust collected from the clean side of the collector during performance testing, a wide range of
dust particles are noted which includes those that are in the 20 micron range and smaller.

On many applications, “puffing” can be observed from the exhaust of collectors immediately after the pulsing of
each cleaning valve. This phenomenon is dependent on the effective density of the dust. The lower density dusts
tend to penetrate the adjoining bags more than the higher density dusts. Very low density dust such as paper and
many fibrous dusts can also operate at low pressure drops, low air consumption, and extremely low penetrations.

-57-
Effect on Media Selection

The phenomenon of driving dust through adjoining bags has led bag suppliers to offer a wide array of bag media
formulations. If we ignore the requirements imposed by temperature and chemical attack, the main consideration
in selecting filter media is its ability to resist the penetration of the propelled dust that traveled through the bag and
its associated cake. There are several approaches.

The most effective approach is to use bags with laminated construction where PTFE media is laminated to the
felted or woven bag. This laminate has such fine openings that the coating can hold water, yet allows air to pass
through the laminate freely. Its original application was to make waterproof fabrics that prevent water from entering
the fabrics yet allows the vapor and air to pass through unimpeded. Unfortunately, PTFE bags are expensive
when compared to the standard media and therefore are usually used only in special applications.

Another approach is to fabricate the filter cloth with finer threads, especially near the filter surface, to provide a
more complex serpentine path so that the dust penetration is reduced. Dual dernier felts and woven felts are
examples of materials that have a layer of fine threads on the filter surface and coarser threads below the surface.

Bag Modifications

Use pleated filter elements. When a pleated filter is cleaned, the dust can be driven against adjoining elements at
high jet velocities, but since the dust is directed at another dust collecting surface that is also blowing dust in the
opposite direction, penetration does not occur. This will be explained further in later chapters. There are some
limitations and principles that must be applied to selecting and applying pleated filter elements that are beyond the
scope of this discussion.

Insert bag diffusers. These proprietary inserts reduce the velocity of the jet cleaning forces as the bags are
cleaned. The inserts consist of perforated cylinders that fit into the cage but around the outside of the venturi.

Baffles. Baffles have been inserted between the rows of bags to prevent the dust from impacting the adjoining
rows.

-58-
Pulse Jet Collector Technological Breakthrough (1979) by Scientific Dust Collectors Company

Noting that the blow ring collector was able to operate at very low pressure drops and filter ratios of 18 to 22:1, the
engineers at Scientific Dust Collectors launched a research and development effort to determine if they could
develop a pulse jet collector that had the same characteristics. They made some important discoveries and a
number of patents were issued.

A key principle was identified to be that filter flow of air depends on the cleaning capability which in turn depends
on the flow of reverse air in the cleaning jet, whether the filter element is a bag, cylindrical, pleated or envelope
configuration. In other words, the better the media can be cleaned, the more airflow can be tolerated.

By reducing the jet velocity, the operating pressure drop is reduced even to the equivalent of the blow ring
collector. This is actually 50 percent below the old technology designs. In addition, the reduction of the jet velocity
reduces the dust penetration by over 80 percent and accomplishes a gain in bag life in the 200 percent range.

Since its introduction, a great many “high ratio” collectors of this design have been installed with the following
operating characteristics:

Table 7-3

Average velocity at bag opening 10,000 feet per minute

Bag opening (no venturi) 4½” diameter
Jet flow 740 CFM
Bag diameter and length 4½ inches x 96 inches
Bag area 10 sq. ft.
Filter flow rating per bag 190 CFM
Nominal filter ratio 20 FPM
Average pressure drop 2½ inches water column
Average air consumption ½ SCFM/1000 CFM of flow
Average dust penetration at 10 gr./cu.ft. load 0.0005 gr./cu.ft.

In achieving the high performance of these “High Ratio” collectors (see Figure 7-4), there were some additional
modifications that had to be developed:

Special Inlet Configurations. The inlets were moved from the hoppers to the upper section of the baghouse. This
“high side inlet” created a naturally downward air flow pattern. The new cleaning system can now collect very fine
dust that previously was driven out of the exhaust. Typically, these fine dust particles do not agglomerate as well
and will not fall into the collection hopper, especially if high upward air flows are present which is usually the case
with the use of hopper inlets. These inlets also changed the direction of the airflow which caused larger particulate
to simply drop out of the airstream.

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Special Baffles. The use of perforated vertical baffles directs the horizontal air and dust distribution into predeter-
mined dust flow patterns in the filter compartment. In addition, a wider bag spacing was introduced.

FIGURE 7-4

Applications. These fabric collectors can be applied everywhere other old technology collector designs were
applied whether it consisted of fabric or cartridge filters. This includes the collection of submicron fume dusts such
as in smelting, welding, or combustion processes.

Advantages of Scientific’s High Technology (“High Ratio”) Fabric Collectors

x Most compact collector available. Normal operation at 14 to 18:1 filter ratios or typically twice the filter
ratio of “generic” baghouses.

x Bag life increased by over 200% using fewer bags.

x Compressed air usage decreased by at least 50%.

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Compressed Air Actuated Pulse Jet Considerations

When compressed air leaves an orifice drilled in a pipe, the air increases in velocity to the speed of sound. This
sonic velocity is developed when the pressure in front of the orifice is approximately 13 psig. If this pressure is
further increased, more air will flow through the orifice but the velocity will stay the same. The pressure in the
orifice throat remains at 0.528 times the absolute pressure in the pipe. The difference between the throat pressure
and atmospheric pressure is wasted. Table 7-4 shows the conversion efficiency for the orifices and advanced
nozzle designs at various pressures in the compressed air pulse pipe.

Table 7-4

Col. 1 Col. 2 Col. 3 Col. 4 Col. 5

Pulse Pipe Air Orifice Exit Conversion Nozzle* Exit Nozzle*
Pressure in PSIG Pressure Efficiency Pressure Efficiency

13 psig 0 psig 100% N/A N/A

25 psig 6.5 psig 74% 0 psig 95%
50 psig 19.3 psig 61% 0 psig 95%
75 psig 32.5 psig 57% 0 psig 95%
90 psig 40.0 psig 55% 0 psig 95%

* Converging Diverging Nozzle used in Scientific Dust Collectors. When comparing these results, one can see that
the efficiency of the nozzle (Col. 5) is much greater than the orifice (Col. 3)

Converging Diverging Nozzles

Nozzles mounted on pulse pipes were developed as part of a proprietary cleaning system by Scientific Dust
Collectors. Nozzles process the air at orifice pressures to allow further conversion of pressure energy to velocity
energy. In the orifice throat, the velocity is sonic or nominally 1,000 feet per second (60,000 ft./min). When
converging diverging nozzles are mounted on the pulse pipe, the exit velocity from the nozzle will increase to 1,750
ft./sec. (105,000 fpm), or 1.7 times sonic velocity with 90 psig in the pulse pipe. Refer to Figure 7-5. The throat
profile is computer developed for optimum conversion.

FIGURE 7-5

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For the dust collector’s throat orifice in both the nozzle and the standard diameter hole orifice, Scientific takes
advantage of the higher velocity to induce more air into the cleaning jet as determined from the momentum
equation. This key accomplishment results in better bag cleaning during everyday operation.

Like the generic cleaning system, Scientific Dust Collectors also limits the expansion of the air jet by stopping the
induction of the induced air. However, instead of using a flow restricting venturi, Scientific Dust Collectors uses the
whole open area of the bag mouth to limit the secondary air induction. A lower air jet velocity can be used because
the filtered fan air velocity through the bag opening is also lower.

Generic Pulse Jet Cleaning

The 1¾" diameter venturi stops the expansion of the induced secondary jet air. Refer to Figure 7-6. However, the
jet velocity stays high, thereby allowing the air jet to overcome the filtering fan air and reach the bottom of the bag.
Then, the jet cleaning air reflects off the bottom of the bag and expands to fill the interior of the bag with cleaning
air. Also, the filtered fan air flow is reversed by the oncoming jet air, and the built up layer of dust cake is blown off
the outside surface of the bag material by the jet air so that it can fall into the hopper.

FIGURE 7-6

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Comparison of Generic vs. Scientific Dust Collectors Cleaning Systems

The comparison which follows assumes the only difference between dust collectors is the method of pulse jet
cleaning and the air-to-cloth ratio used. A Scientific Dust Collector is operating at twice the air-to-cloth of the
generic system which is typical of actual field practice.

Generic System Scientific Dust Collectors

Bag Length 8' 8'
Bag Diameter 4½" 4½"
Bag Fabric Area 9.46 ftð 9.46 ftð
Air-To-Cloth Ratio 5:1 10:1
Filtered Air Volume per Bag (5)(9.46) = 47.3 CFM (10)(9.46) = 94.6 CFM
Bag/Venturi Throat Diameter 1¾" at venturi 4½" at bag opening
S(1-3/4)ð = 0.0167 ftð S(4-1/2)ð = 0.1104 ftð
Bag/Venturi Throat Area (4)(144) (4)(144)
Filtered Air Velocity at 47.3_ = 2,832 fpm fan air 94.6_ = 857 fpm fan air
Bag/Venturi Throat Opening 0.0167 0.1104
Cleaning Air Jet Velocity at
Bag/Venturi Throat Opening Higher Lower

As these calculations indicate, in the generic system the cleaning air jet must overcome 2,832 fpm, a much higher
filtered air velocity, even though the air volume flow per bag is only half that of the volume flow run through the
Scientific Dust Collector bag. The energy required to overcome the high filtered air velocity in the generic system
is not available to effectively clean the dust cake from the fabric bags.

Special Configurations Are Available on Pulse Jet Baghouse Collectors:

¾ Walk-In Plenum with Top Bag Access

¾ Roof Doors with Top Bag Access

¾ Bottom Door Bag Access

¾ Horizontal Bag Configuration with End Door Bag Access

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 Chapter 8

CARTRIDGE COLLECTORS

Cartridge dust collectors were introduced around the 1970’s. They promised to give more powder collection in a
smaller space. To take advantage of the existing filters in the marketplace, manufacturers selected intake filters
like those used on tractor trailer engines. Since these collectors were designed empirically, it was concluded that
they would operate at low filtering velocities. It was not known at that time that the capacity of the self-cleaning
pulse jet collector was a function of the reverse air flow. The first major successful application came in applying
them to powder coating booth vent applications.

For many years these operations were vented by shaker collectors, but as the booths were placed into continuous
duty service, fabric pulse jets were also applied with mixed success. When the fabric pulse jet units were collecting
very dense powder pigments, they operated at high pressure drops and consumed large amounts of compressed
air. In fact, the high dust penetration into adjoining bags limited the recirculation of the exhaust air into the work
areas. When the cartridge collectors were applied to venting powder paint booths, these problems disappeared.

Operating and Test Results

Soon it was found that these cartridge designs were more effective in many other applications where the
conventional (old technology) pulse jet collectors were marginal. These included collecting products that had
characteristics of high bulk densities or very fine particulate size fractions.

These new cartridge filters were able to maintain a filter cake that combines both a low pressure drop and higher
filtration efficiency which was a large improvement over any other self-cleaning collector at that time. In 1978, the
American Foundry Society conducted some tests on venting very dense and fine dust from a typical foundry
operation and discovered some startling results as shown in Table 8-1 below:

Table 8-1

Inlet Load 1-2 Grains/Cubic Foot

Type of Collector Outlet Dust Flow

Pulse Jet Fabric 0.00660 grains/cu. ft.

Shaker Collector 0.00035 grains/cu. ft.
Cartridge Collector 0.00005 grains/cu. ft.

For comparison, the blow ring collector would have similar dust penetration to the adjoining bags with its felted
media as the shaker collector.

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Typically these cartridge filters were constructed as shown in Figure 8-1. They were constructed of rugged inner
perforated or expanded metal cores and the end cap material was made from steel. A potting adhesive joined
each end cap with the cellulose media and inner and outer perforated cores. The cellulose media has filtering and
pressure drop characteristics that are similar to the felted media in conventional pulse jet collectors. For many
years, a spiral groove of hot melt adhesive was applied to the outside core of the cartridge which was developed
for intake air filter cartridges used by the truck industry. The spiral groove of adhesive prevented the pleats from
resonating at detrimental frequencies which are encountered in truck engine operation.

FIGURE 8-1

These filter media elements were designed for truck service and consisted of corrugations 0.014 inch deep that
were pressed into the media surface. Even though the pleats were very tightly spaced, the dirty air could penetrate
to the bottom of the pleat. In this kind of intake truck filter service, the inlet loads were in the 0.0002 grains per
cubic foot range, and these filters could operate for many months before any change was required. In this non-
cleaning truck filter application, more media results in a longer lasting filter. Since the dust was very fine, it
penetrated into the upper layer of the media and a smaller amount collected on the other media surfaces. Often
the filters were manufactured with 16 pleats per inch and conventional filter media as shown in Figure 8-2.

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 FIGURE 8-2

However, there are some important differences between the operation of an intake filter on a truck engine and a
dust collector:

x Typically, in comparison, the dust loads in dust or powder collectors are 1-20 grains per cubic foot which
is 5,000 to 100,000 times more than the engine intake filters.

x The dust collector or powder collector relies on a filter cake that is on the surface of the media as well as
within the media while an intake cartridge filter relies on the cake that is formed within the media.

x The average operating pressure drop across the filter cake is two or three times larger on a typical dust
collector as compared to the intake filter.

Some powders and dusts can operate with a wide range of dust filter cake thicknesses which significantly affects
the cleaning frequency or the collection efficiency while other dusts can only operate at cake thickness of over 0.05
inches.

The pleated cartridge filter elements are suitable for applications where the dust filter cake can stabilize at cake
thicknesses of less than 0.02 inches. Most very fine dust falls into this category. It is obvious that streams
containing particles with average dimensions over 0.03 inches would not be suitable for pleated cartridge elements
with narrow pleat spacing. Narrow pleat spacing is defined by more than 8 pleats per inch which is based on the
inner diameter of the filter elements.

Initiation of the Cleaning Pulse Cycle

Even when the dust can be collected by using very thin filter cakes, thicker filter cakes can usually be tolerated. In
cylindrical filter elements (not pleated such as filter bags), thicker filter cakes can save compressed air in many
applications. In fact, it can be effective and is becoming popular on many coarse dusts to initiate the pulse
cleaning cycle with a pressure switch (photohelic gage). If more dust can be ejected during each cleaning cycle,

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then the frequency of the cleaning cycles can be reduced. A common pressure switch setting is 3 1/2 inches water
gage. The best way to set a pressure actuated cleaning system is to operate the collector at a setting of 1/4 inch
water column above the initial pressure drop. This is accomplished by reducing the “off” time cleaning interval
which will allow the pressure drop to stabilize.

This lower pulsing frequency has one other advantage in the operation of cylindrical bag collectors. For reasons
beyond the scope of this discussion, the overwhelming dust penetration occurs during each cleaning cycle and the
dust penetration into the adjoining filter media is directly related to the pulsing frequency. This relationship is not
the same with the pleated filter elements. While pleated filter elements in pulse cleaning collectors can operate
with pressure switch actuation, the pressure drop setting is normally limited by the pleat spacing. In fume dusts
and welding operations, the dust loads approach the rate of intake truck filters and may require only infrequent
cleaning such as one or two cleaning cycles per day.

Figure 8-3 demonstrates the effect of different pressure switch actuation settings for a dust that can effectively filter
at a wide range of pressure settings.

FIGURE 8-3

The initial pressure drop across the filter media is 0.1 inches water gage in the example that is being considered. If
the dust bridges across the pleat, the cleaning air will not flow below the dust bridge. The cleaning air takes the
path of least resistance and all the media below the bridge is rendered ineffective when the bridge forms. In the
example in Figure 8-3, the dust load is assumed at 2 grains per cubic foot. The dust is mineral dust from a mining
application. Starting from the right side of Figure 8-3, the pressure switch can be set to start at 1 1/2, 2 1/2 or 3 1/2
inches W.C.

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The shaded portion between the pleats is the cleanable filter cake. The darkened section between the pleats is the
location and size of the dust bridge. The results are taken from Figure 8-3 and tabulated in Table 8-2 below.

Table 8-2

Cleaning Typical Compressed

Pressure Switch Cycle Time Air Usage at 85 psig

1 1/2 inch w.c. 5 minutes 0.7 scfm per 1000 acfm filtered air
2 inch w.c. 5 minutes 0.6 scfm per 1000 acfm filtered air
2 1/2 inch w.c. 4 minutes 0.9 scfm per 1000 acfm filtered air
3 1/2 inch w.c. 3 minutes 1.2 scfm per 1000 acfm filtered air

In the above example, if the pleats are placed closer together, an increase in compressed air consumption would
also be expected. Two effects must be considered. A deeper filter cake can allow more powder to be collected
between each cleaning pulse, however, a deeper cake will form a higher dust bridge in the valley of the pleat. The
result is that there is less filter media to be cleaned since the air travels the path of least resistance and tends to go
around the dust bridge. In the table, the air consumption starts with larger air usage due to the very shallow cake
and then the air consumption is reduced even though some bridging occurs. Soon the effect of the bridging
becomes more pronounced and the cleaning frequency increases and the compressed air consumption also
increases.

Cartridge Pulse Jet Cleaning

It is important to note that the cleaning configuration for a cartridge collector is identical to a fabric jet collector.
Figure 8-4 shows a standard cartridge arrangement and Figure 8-5 illustrates a standard fabric bag arrangement.
At “A”, the compressed air orifice is energized and, as the air leaves the orifice, the flow of air drags or “induces”
more clean air from the clean air plenum. The jet will grow with a cone angle from 14-16 degrees and the air flow
will follow the Law of Conservation of Momentum. As it enters the opening in the top of the filter element, the filter
media and the existing filter cake have enough resistance so that no more fan air can be drawn into the cleaning
air jet. The air jet then compresses all of the fan air that is inside the filter element. The force of the air jet is
transmitted through the opening at the top of the cartridge by compressing the column of fan air that is inside the
filter. The pressure wave on the column of fan air beneath the jet travels toward the bottom of the filter element at
the speed of sound since forces travel through any material solid, liquid or gas at the speed of sound. When this
pressure wave reaches the bottom of the cylindrical column, the cleaning air jet displaces out the fan air that is
inside the filter element and generates a positive pressure inside the filter element. This positive pressure causes
a uniform outward velocity that ejects the agglomerated dust that rests on the outside surface of the filter media in
the dirty air chamber of the collector.

The dust is ejected perpendicularly from the surface of the media and it is propelled at the velocity of the cleaning
air jet. The ejected dust does not strike the adjoining cartridges/filter cakes since the direction of the ejected dust
is perpendicular to the media surface and towards an adjacent pleat that is also angled and ejecting dust. This
pleated configuration eliminates the entrainment of dust from active cleaning cartridge rows to the passive filter
rows. In cylindrical and envelop bag pulse collectors, the dust can get re-entrained from the active cleaning row to
the adjoining passive rows. In fact, the improvement in collection efficiency in Table 8-1 of the cartridge collector is

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explained over the other types of collectors by this phenomenon of lack of dust being transferred onto neighboring
cartridges during the cleaning process.

FIGURE 8-4 FIGURE 8-5

During each cleaning cycle, some dust bleeds through the filter media until the dust cake develops to its most
effective configuration. In a short time period, the bleeding of dust through the media is not noticeable, but in a
long time period, the dust on the clean side causes problems by imbedding itself into the clean side of the media
during the cleaning pulse cycle. This causes a high pressure drop, increases the need to clean more frequently,
and lowers the life of the filter element. To combat this tendency, some suppliers have provided pleated filter
elements with surface treatments that minimize but do not eliminate the filter cake damage that is caused by this
dust bleeding phenomenon.

Filter Mounting Arrangements

The original cartridge filter element pulse jet collectors had vertically mounted filter elements that were mounted in
the dirty air section of the collectors. Changing the filter elements exposed the maintenance and operating
personnel to the dust when the cartridges were replaced. Then collectors were introduced to the industry that
allowed the replacement of filters from the outside of the collector. These types of collectors became popular
because of the ease of cartridge changeover. The most common of these arrangements is the horizontal mounted
filter collectors as shown in Figure 8-6. These pleated cartridge filter elements were arranged in vertical rows.
Most often the cartridges were cleaned so that the top filter was cleaned first, then the one below it was cleaned
next in sequence until the bottom one was cleaned.

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The general steps for the mechanism of collection of dust and powders are as follows:

x The fine dust collects on the surface of the filter media.

x The fine dust agglomerates with other fine dust particles on the surface of the filter media.

x The agglomerated dust is removed by the cleaning process and is collected in the hopper only if the
agglomerated dust is large enough to fall into the hopper while it is being unaffected by the uplifting air
currents.

FIGURE 8-6

In some collectors with horizontal cartridge filter elements where the cartridges are cleaned from the top row to the
bottom row, some impediment can occur to prevent the cleaning process from being as effective as vertically
mounted cartridge filters.

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When a cartridge is cleaned in the top row #1, the dust is propelled radially outward from the horizontal center of
the cylindrical axis of cartridge. The ejected dust from the bottom of the cartridge filter in row #1 falls radially
downward and towards the upward portion of the top of the cartridge filter in row #2 which will tend to catch the
dust. Soon these pleats on the top of filter #2 can bridge and that bridged portion of the filter will be rendered
ineffective. A similar action results when the filter in row #2 is cleaning which causes the filter in row #3 to be
affected by the cleaning of the cartridge that is directly above it. Figure 8-7 illustrates a configuration that will
reduce or eliminate the dust deposits on the top of the filters in the lower rows by the use of horizontal deflectors
that are placed at the inlet of the collector.

Internal Inlet Baffles

When the dust laden air enters through the inlet of a collector, it is channeled through an initial opening such as a
rectangular or round inlet. Upstream from the inlet, the dusty air has traveled through various arrangements of
elbows, transitions, or other abrupt obstructions. As a result, the dirty air enters the collector with existing
turbulence and unbalanced air flows that reduce the efficiency and capacity of the collector. The goal inside the
dirty air chamber is to provide an equal distribution of fan air to all of the cartridges, to promote a downward air flow
with little or no updrafts, and to reduce the inlet velocity of the incoming air stream into the dirty air chamber.

Scientific Dust Collectors has addressed these issues and has a patented baffle arrangement as illustrated in
Figure 8-7. In general, the process consists of the dirty air entering the inside of the collector at the inlet and
encountering the baffle arrangement. The baffles cause the dust laden air to be divided into multiple flows in order
to evenly redistribute the dirty air throughout the dirty air chamber and to promote a general downflow of process
air. In fact, some of the air flows are redirected enough to cause counter flows that hit into each other so that their
initial velocities are greatly reduced. This process tends to reduce the internal swirling around the walls of the
collector by the process dirty air and to reduce the updrafts that can cause dust re-entrainment on the cartridges.

FIGURE 8-7

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Filter Ratio Considerations

It is important to note that when using a cartridge with a significant amount of media, a low face velocity in a
cartridge will not be acceptable if the proper amount of cleaning air is not achieved. Likewise, a higher face
velocity in a cartridge such as that obtained when using a cartridge with a minimal amount of media can still be
effective, providing a good cleaning system is used and the correct amount of media is chosen for the particular
application. In order to clean the filter element, the cleaning air-to-process air ratio should be between 4 and 6.
Therefore, a cartridge filter rated for 400 CFM of process air would require a cleaning flow between 1600 and 2400
CFM. Even at this high cleaning air flow rate, a portion of the cartridge remains uncleaned but collects some dust.
In time, the active filter media becomes plugged and the previously uncleaned filter media becomes active.

As long as there is excessive dormant filter media, the operating pressure drop across the filter media will remain
constant. Over time, when the active effective media area drops due to excessive plugging and when there is no
more dormant filter media available to be converted into an active filter media, the pressure drop at the magnehelic
pressure gage will start to rise. It is important to note that the cleaning frequency must be increased to stabilize the
collector since there is less effective filter media available to clean the dust from the cartridge. Eventually, the
pressure drop becomes too high to maintain the required fan air flow through the system and the filter cartridges
must be removed and replaced. In a cartridge filter element, the uncleaned pleats collect dust and the dust quickly
bridges across the pleats, which renders the bridged pleats useless. As the amount of dust builds in the uncleaned
portions of the cartridge, the mechanical stresses are also increasing on the seals and mounting structure.

Cartridge Cleaning System Improvements by Scientific Dust Collectors

Scientific Dust Collectors’ cartridge cleaning system optimizes the use of dry compressed air and nozzle
technology. The orifice of the nozzle is sized to supply the necessary amount of compressed air, and the nozzle
section efficiently increases the velocity of the compressed air from a standard orifice velocity of 1030 feet per
minute to a nozzle velocity of approximately 1750 feet per minute. The extra velocity allows more induced air to be
merged into Scientific Dust Collectors’ cleaning air jet than the generic clean air jet which helps to clean more filter
media. As the expanding cleaning air jet enlarges and travels outwardly from the exit of the nozzle, its shape
remains conical and the approximate angle between the diverging elements is 14° to 15°. When the cleaning air
jet reaches the venturi/evase, the velocity of the cleaning air jet has slowed considerably so that the cleaning air jet
action on the filter media is both robust yet gentle.

The venturi/evase is located before the entrance into the filter media in the clean air plenum in order to maximize
the total cleaning of the cartridge. In addition, it provides the structure to seal the fan air from leaving the cartridge
when the cleaning air jet reaches the orifice of the venturi/evase. Also, when the cleaning air jet is not flowing, the
venturi/evase helps the fan air to exit the cartridge area with less turbulence and, therefore, at a lower loss in
velocity pressure. In addition, to help provide the best opportunity to clean the cartridge and to not have the dust
re-entrained back onto the cartridges, the distance between the cartridges is generous and controlled to give an
optimum downward flow rate.

It is an ongoing common and prevalent misunderstanding throughout the industry that the more media we can put
into a cartridge, the better the system will function and the longer the cartridge will last. In fact, this is totally and
solely dependent upon how well you can clean the cartridge. If the cleaning system is unable to remove the dust
particles captured in the pleats, any original system gain in airflow and pressure drop is short lived and ineffective
due to the plugging of the cartridges over time. It is not always in the end users best interest to install cartridges

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with the greatest amount of media. The correct amount and type of media used is really dependent upon the
nature and size of the dust particles being filtered.

Proper Seal Design

Seals play an important role in separating air spaces and relative pressures such as:

x Isolation of process air from the cleaned filter air.

x Maintain separate differential working pressures such as isolating the outside atmospheric air from the
clean air plenum and the dirty air chamber.

In many generic cartridge collectors, seal failure is frequently the cause of plugged cartridges, dusty effluent air,
and overall loss of collection efficiency. Scientific Dust Collectors’ patented cartridge door design has multiple
positive advantages that ensure long lasting sealing service. Refer to Figure 8-8. The main advantage is the use
of a compression spring that automatically adjusts for seal deflection on both inner cover door as well as front and
back cartridges. Each seal is generously sized and made from quality gasket material so that the seals do not
crush with time. The door assembly consists of an inner door cover that seals the back cartridge opening, front
outer door plate that seals the internal dusty air chamber from the outside atmospheric pressure environment, and
washers to seal the outside locking door handle from the internal dirty air chamber. Installation and/or removal of
the door assembly is easy since all of the interconnecting parts are attached to each other.

FIGURE 8-8

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 Chapter 9

USING PLEATED BAGS IN DUST COLLECTORS

A pleated bag consists of a length of multiple folds of filter media that are formed into the shape of a cylinder. In
general, the outside diameter of the pleats typically matches the outside diameter of a standard cylindrically
shaped filter bag that it sometimes replaces. The filter media is encapsulated between the bottom end cap and the
top sleeve arrangement which allows for the attachment to the bag cup. It then is fastened to the tube sheet. The
pleat depth varies with the outside diameter of the filter element. Typically for the family of sizes of pleated bags,
the smallest outside diameter has the shortest pleat depth. The spacing of the pleats can vary so that two to three
times the standard circumferential media area is achieved when comparing it to the standard cylindrical shaped
bag. The inside diameter of the pleated bag is less than the standard felted polyester bag due to the pleat depth.
Generally, for most collector designs, the smaller inside diameter of the pleated bag has a negligible effect on the
filtering or cleaning of the bag. This is especially true if the manufacturer continues to use a venturi in the mouth of
the pleated bag.

PLEATED BAG

History

Pleated bag elements were originally used in retrofitting existing baghouses where existing fabric bag media was
unacceptable. As the manufacturing prices reduced due to volume production and competition, the dust collection
industry has seen an increase in the use of these types of bags.

Benefits/Advantages

Some of the key benefits are listed below.

1) Able to retrofit pleated bag into an existing baghouse.

2) Able to triple the amount of filter media in each bag due to the pleating process, but each type of filter
media has its unique Frasier Permeability rating which has an effect on the pressure drop through the
filter media. For instance, a lower Frasier Permeability in the filter media (reference media A) than the
original filter media (reference media B) indicates that less CFM fan air is able to pass through a one
square foot of filter media at the given ½” WG. Therefore, to retain the ½” WG and the original CFM
(reference media A) more area of the filter media B will be needed in order to compensate for the reduc-
tion of CFM @ ½” WG as shown in Figure 9-1. The following example will help clarify the principle.

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 FIGURE 9-1

Clean filter media “A” passes Clean filter media “B” passes
24 CFM fan air per square foot and 12 CFM fan air per square foot and
causes a ½” WG pressure rise. causes a ½” WG pressure rise.

For clean filter media B to pass 24 CFM through the filter media and to have a maximum limit of ½” WG
pressure rise, then an area factor needs to be calculated and applied to the reference media B as shown
below:

Step 1 original filter media CFM/ftð = area factor

replacement filter media CFM/ftð

Step 2 multiply (area factor) times (1 ftð area of media B) =

2 ftð is the area required of media “B” to be

equivalent to media “A” at (24 CFM at ½” WG).

3) Able to diffuse the cleaning air due to the pleating arrangement. Therefore, the cleaning velocity is less
direct and causes less dust cake compression on the neighboring pleated bags which helps the pulse
cleaning system to clean at a lower differential pressure. It should be noted that in a typical retrofit appli-
cation, the cleaning capacity is fixed in the original design of the collector. Therefore, a large addition of
more pleated filter media will not be cleaned and will eventually become plugged.

4) May be able to reduce the operating pressure differential if the available cleaning air can clean the addi-
tional area of the pleated filter media.

5) Able to reduce the size of a new collector as compared with a standard baghouse due to the increase in
filter media area per unit length of the pleated bag.
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Disadvantages of Pleated Bags

Some disadvantages are listed below:

1) The pleated bag, much like a cartridge filter, is a rigid element. Therefore, it is cleaned solely by the
amount of compressed air and induced air able to pass through the narrow opening of the filter.

2) The pleated bag collector is not practical to convert to standard bags since the air flow and cleaning
design is adapted to the larger filter media areas. The reduction in filter media area could be from 1/2 to
2/3.

3) A fixed cleaning system is a limitation in a retrofitted baghouse where straight bags are replaced by
pleated bags. The cleaning system is designed to clean a given area of media. The extra media will not
be cleaned and it will eventually plug.

4) The pleated media may have a lower Frasier Permeability number which in this case requires more filter
media area in order to have equivalent fan air CFM at the same pressure rise in inches of WG.

Physical Characteristics

The pleated bags are manufactured in both top and bottom removal styles. See Figures 9-2 and 9-3, respectively.
There is generally an inner core that gives rigidity and strength to the pleated bag. Also, various manufacturers
supply bands that are placed circumferentially to prevent ballooning of the media during the cleaning pulse.
Multiple pleat depths are manufactured which typically range from .6 inches to 1.875 inches in correlation with bag
outside diameters that range from 4½ inches to 8 inches in diameter. Also, a wide range of overall bag heights are
manufactured from 24 inches to 90 inches.

FIGURE 9-2 FIGURE 9-3

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Various media options are also available in pleated bags. In addition to standard spun bonded polyester material,
specialty material options can be obtained. Some of these are hydro and oleophobic finishes which help to repel
water and oil; static dissipating membranes; and PTFE membranes for higher efficiency and release.

Some design features of pleated bags are:

x Molded urethane top and bottom.

x Positive interlocking seal.
x Integrated internal pleat retainers.
x High chemical tolerance.
x Shallow pleats with open spacing.
x 200°F standard operating temperature (higher temperatures available).
x Positive sonic welded seaming

The installation of the bottom removal pleated bags is very similar to the mounting of standard fabric bags. The
mounting of the bottom removal pleated bags requires a compatible bag cup that is mounted onto the tube sheet
and a band clamp.

The band clamp is loosely placed over the urethane neck of the pleated bag, and then the pleated bag is located
on the bag cup. When the pleated bag is in place, the bag is secured to the bag cup by tightening the band clamp.

For the installation of the top removal pleated bag, a bag cup is not required. Instead, the pleated bag manufac-
turer provides a snap band or insert that secures the pleated bag to the tube sheet.

A good retrofit application is when the baghouse air jet cleaning system has ample capacity to clean more filter
media, but the present amount of media is insufficient to prevent the operating pressure from rising significantly. A
high differential pressure at the magnehelic gage indicates a compaction of the dust cake, loss of fan air flow for
the process application, reduced bag life, and higher fan energy costs. In this case, a pleated bag with two to three
times more filter media allows the process air to flow at a lower operating pressure since it has the capacity to
clean more media. The lower pressure flow equates to more fan air flow as predicted by the manufacturer’s fan
curve.

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 Chapter 10

FILTER MEDIA

Purpose of Filter Media

The main purpose of the filter media is to separate the gaseous air from the solid dust particles in the process air
stream by using a membrane material or more commonly referred to as “filter media”. The filter media forms a
support surface that allows the gaseous air molecules to pass through, while the larger dust particles are captured.
A second vitally important capability is for the filter material to easily release the captured dust particles when, for
example in an air pulse arrangement, a separate burst of clean air temporarily reverses the flow of the process air
stream. The clean air burst has a higher velocity and a greater velocity pressure potential than the process air
stream so that the cleaning air is able to overcome the process air flow and thereby release a large percentage of
the captured dust particles. A third important capability is for the filter media to prevent a high percentage of the
dust particles from passing through the filter media. To assist the filter media in capturing 99.99 percent of the dust
particles, a layer of dust or “dust cake” is generated on the incoming surface of the filter media. As more dust
particles arrive at the dust cake which rests on the surface of the filter media, the thickness of the filter cake
increases and filter efficiency also rises. During the burst of cleaning air, most of the dust cake will be separated
from the surface of the filter media and drop downward into the hopper area.

In all dust collection applications temperature and humidity always must be taken into account when selecting the
most efficient media for your dust collector. The ideal media should be chosen based on the highest process
temperature and the highest moisture situations possible that may occur over time. Another key system
requirement for best operation and efficiency is to pay close attention to the dust characteristics. Is the dust particle
granular in shape and “free flowing” or is the particle irregular in shape and agglomerative in nature. Dusts that
exhibit the tendency to clump together and grow in size require a different media and filter specification than those
dusts that flow freely. This is especially important for cleaning of the media and maintaining a suitable stabilized
pressure drop across the filter. Efficient filtration is achieved when the dust cake that is formed is maintained using
a cleaning system that is reliable and allows sufficient cleaning air to be introduced into the filter media to properly
clean the media. Extra time and attention must be given to this important item in any dust collector since this
cleaning process will most definitely affect the performance of any media. By doing this, the media as well as the
entire collector will be the most efficient system.

There are other important capabilities of the filter media for specific application needs that will be briefly listed here
and will be discussed in more detail later in this chapter.

1) Temperature considerations of the process air stream and the filter media with normal upper limits of
200°F for cellulose to 500°F for fiberglass material.

2) Fire retardant coatings which will retard combustion. (Note: It is not fireproof.)

3) Static dissipation properties:

a) Carbon Impregnation – Applies to wet-laid media (cellulose) and gives excellent static dissipation
properties.

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 b) Metallized Finish – Applies to polyester media (spun-bonded) and gives an improved dust cake
release and excellent static dissipation properties.

4) Hydro and Oleophobic Finish – Applied into the polyester/media resulting in excellent moisture and mild
oil mist tolerance, dust collection efficiency, and material strength.

Process of Collecting Dust on the Surface of the Filter Media and Some General Mathematical Relationships

For illustration purposes, process air contains both dust particles and gaseous air. The goal is to stop the dust
particles at the incoming surface of the filter media while the air molecules are able to travel through the existing
pores or openings in the filter media. In Figure 10-1, a one square foot area of filter media is represented with
varying sizes of holes/openings.

FIGURE 10-1

There is another important relationship called the Frasier Permeability rating for the filter media. It states that the
volumetric air flow rate number is determined at a ½ inch of water gage pressure and through an area of one
square foot of media. For an area of one square foot of standard filter bag media, the Frasier Permeability number
ranges from 20 to 40 CFM at ½ inch water gage velocity pressure. At the same area and velocity pressure, the
cartridge filter media has a Frasier Permeability range of 4 to 30 CFM. In most cartridge and baghouse collectors,
a magnehelic differential pressure gage measures the pressure in inches of water gage between a port that is
inserted into the dirty air chamber (where the filter bags or cartridges are housed) and a port that is inserted into
the clean air plenum (where the cleaning purge tubes are housed) inside the collector. The value of this static
pressure differential measurement gives an indication of the working status of the filter cartridge or bag. Typically,
a low number such as 1½ to 2” WG indicates a good balance between the collecting of dust and the cleaning of the
filter bag or cartridge. Conversely, a number from 5 to 7” WG indicates an “out-of-balance” system between the
filtering of dust on the media and removal of dust by the air pulse from the cleaning system. Some individuals
mistakenly relate the differential pressure reading directly to the original Frasier Permeability rating. However,
there are other variables that are combined into the pressure reading of the magnehelic gage which include dust
cake, orifice in venturi or other openings in the mouth of the bag or cartridge.

The dust particles collect around and in the openings of the filter media to form a dust cake which is helpful in
raising the filtration efficiency of the collector. A fan provides the energy to either pull or push the air through the
existing media openings.

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Selection of Fabric Materials for Dust Collectors

There are many types of fabric materials available that have been developed in order to satisfy the specific
requirements of a given application. The basic criteria for selecting a specific material are listed below:

x Temperature of the process air stream in the collector

x Moisture level inside the collector and/or hydroscopic nature of the dust
x Electrostatic characteristics of the dust
x Abrasion of the dust particles on the filter media
x Acid chemical resistance
x Alkali chemical resistance
x Ease of release of the captured dust particles from the media
x Permeability of the fabric to allow only air to pass through the media
x Cost of fabric materials
x Size of the dust particles to be collected

There are a wide variety of media types used in dust collection filters. The most common are:

Baghouse Filters:

x Polyester: the standard and most widely used baghouse material in the industry.
x Singed Polyester: used for improved dust cake release and with explosive dusts.
x Polyester with Hydro-Oleophobic treatment: ideal for moisture resistance.
x PTFE Membrane Polyester: used for capture of fine particulate where an artificial dust cake is required.
x Aramid: used for high temperature applications.
x Fiberglass: has very good performance in acid or alkaline environments where high temperatures are
present.
x Polypropylene/PPS: has superior chemical resistance.
x P-84: specialty fabric used in high temperature applications where fiberglass is not required.

Cartridge Filters:

x Cellulose: the standard and most widely used cartridge material in the industry.
x Cellulose/Polyester: synthetic fibers blended with cellulose to create a high durability media with very
good abrasion resistance.
x Nano Fiber: special media that provides a higher starting efficiency and dust release.
x Spun Bonded Polyester (SB): media having good release characteristics with moisture tolerance and
excellent abrasion resistance.
x Spun Bonded Polyester Hydro-Oleophobic (SB-HO): treated media for moisture resistance.

Pleated Bag Filters:

x Spun Bonded Polyester: the standard material in this application.

x Spun Bonded Polyester Hydro-Oleophobic (SB-HO): treated media for moisture resistance.

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Two general filter material selection tables are presented below which correlates key application parameters with
the various strengths and limitations of the filter media.

Table 10-1 gives properties of textiles for dry filtration for low to medium temperature filtration:

TABLE 10-1
Generic Name Cotton Polyamid Polypropylene Polyester
Fiber
Trade Name Nylon 66 ® Herculon® Dacron®
Recommended continuous operation 180°F 200°F 200°F 270°F
temperature (dry heat) 82°C 94°C 94°C 132°C

Water vapor saturated condition 180°F 200°F 200°F 200°F*

(moist heat) 82°C 94°C 94°C 94°C

Maximum (short time) operation 200°F 250°F 225°F 300°F

temperature (dry heat) 94°C 121°C 107°C 150°C

Specific density 1.50 1.14 0.9 1.38

Relative moisture regain in % (at

8.5 4.0 – 4.5 0.1 0.4
68°F and 65% relative moisture)

Supports combustion Yes Yes Yes Yes

Biological resistance (bacteria, No, if not No No

Excellent
mildew) treated Effect Effect

*Resistance to alkalis Good Good Excellent Fair

*Resistance to mineral acids Poor Poor Excellent Fair +

*Resistance to organic acids Poor Poor Excellent Fair

*Resistance to oxidizing agents Fair Fair Good Good

*Resistance to organic solvents Very Good Very Good Excellent Good

* At operating temperatures. *Not

Recommended
Comments: Based on typical fiber
manufacturers published specifications.

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Table 10-2 gives properties of textiles for dry filtration for high temperature filtration:

TABLE 10-2

Generic Name Aramid Glass PTFE Polyphenylene

Fiber Sulfide (PPS)
Trade Name Nomex® Fiberglas® Teflon® Ryton®
Recommended continuous operation 400°F 500°F 500°F* 375°F
temperature (dry heat) 204°C 260°C 260°C 190°C

Water vapor saturated condition 350°F 500°F 500°F* 375°F

(moist heat) 177°C 260°C 260°C 190°C

Maximum (short time) operation 450°F 550°F 550°F* 450°F

temperature (dry heat) 232°C 290°C 290°C 232°C

Specific density 1.38 2.54 2.3 1.38

Relative moisture regain in % (at

4.5 0 0 0.6
68°F and 65% relative moisture)

Supports combustion No No No No

Biological resistance (bacteria, No No No No

mildew) Effect Effect Effect Effect

*Resistance to alkalis Good Fair Excellent Excellent

*Resistance to mineral acids Fair Very Good Excellent Excellent

*Resistance to organic acids Fair + Very Good Excellent Excellent

*Resistance to oxidizing agents Poor Excellent Excellent *

*Resistance to organic solvents Very Good Very Good Excellent Excellent

* At operating temperatures. *475°F for *PPS fiber is

reverse air attacked by strong
Comments: Based on typical fiber & shaker oxidizing agents.
manufacturers published specifications. collector For example, at
200°F for 7 days.

Since there are many possible options for some applications, price and availability will also need to be considered
before the final filter media material is chosen.
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Also, there are some different ways of making the filter material and optional surface treatments that enhance the
properties of the material. The filters can be either natural or manmade and are combined together in various ways
as briefly mentioned below:

1) Woven or interlacing of fibers is a process to construct a fibrous material which generally consists of the
following types of weaves:

a) Plain weave is the most basic and consists of the filtering yarn having an over and under construc-
tion. By controlling the counts per inch of the interlacing yarn, the weave may be made porous or
tight.

b) Twill weave consists of having the warp yarn pass over two or more filling yarns. Typically, the twill
weave has fewer interlacings than the plain weave; therefore, the twill weave tends to have a
greater porosity and is more flexible than the plain weave.

c) Sateen weave has even more distance between the filling yarns than the twill or plain weave which
makes it more porous, flexible, and smoother than the other weaves.

2) Needled-felt material has the short felt fibers pressed together and mechanically fixed by needle punch
machine. The main advantage is the low pressure operation that is coupled with excellent dust collection
efficiency and a higher flow rate.

3) Singed material is made by a heat process that slightly burns or singes the material surface in order to
enhance the surface of the bag material.

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Filtration Aid

In some applications where the dust collector contains some moisture, oil and/or very small dust particle sizes, the
addition of an inert material or “pre-coat” may be helpful. See Figure 10-2.

FIGURE 10-2

Preferably, the pre-coat material is initially applied onto the new, clean surface of the filter media which forms a
protective dust cake layer. The benefits are:

x Aids in dust cake release.

x Pre-coat material is porous which helps to prevent blinding.

x Helps to capture the small particles and limits their ability to penetrate the filter media.

x Increases initial dust collection efficiency to over 99.99 percent.

There are some limitations to the use of pre-coat materials:

x Frequent applications of pre-coat may be required in order to replenish the protective coating that was
partially removed during the pulse cleaning.

x Recycling dust from the hopper is made more difficult due to the mixing of pre-coat material with product
dust.

In making a decision to apply or not apply the pre-coat material, the end user must evaluate his unique application
and determine whether the benefits are worthy enough to incur the extra material and labor expense of incorporat-
ing the pre-coat material into his system.

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 Chapter 11

FIRES, EXPLOSIONS, HAZARDS

Combustible and Non-Combustible Dust

Dust can be separated into two categories: combustible and non-combustible. Non-combustible dust as its name
implies will not under predictable conditions ignite, burn, release flammable gases, or support combustion when
subjected to fire or heat. Combustible dust on the other hand, when suspended in an oxidizing medium will pose a
fire or deflagration hazard, regardless of size or shape of the dust. Particle size however, does indeed have an
effect on the hazard level of dusts. When it comes to size of dust, the largest size that is classified as “dust” is any
material that can sift through a 0.420mm screen mesh as shown in Table 11-1.

Particle Size Distribution

Pellets > 2mm diameter
Granules 0.42mm – 2mm
Dust Particles < 0.42mm or 420ȝm (micron)
Table 11-1

So, a dust particle is generally smaller than 420ȝm (micron). In Table 11-2 are sizes of common dust.

Corn Starch 7ȝm

Charcoal (Wood) 14ȝm
Magnesium 28ȝm
Sugar 30ȝm
PVC (Poly Vinyl Chloride) 107ȝm
Human Hair 40-300ȝm
Table 11-2

In general, as the micron size of the dust decreases, the hazard increases. This is due to finer particles having a
larger total surface area, which allows for quicker ignition. To compare this, think of a wood burning fire. To start a
fire, one would use smaller kindling to start a fire because kindling ignites easier than a larger log. The same is
true with dust, a smaller particle reacts quicker with oxygen and requires less energy to ignite.

Mechanics of Fires and Explosions

A critical characteristic about a dust explosion or deflagration is that it is a rare event. A process handling
combustible dust could have been operating for many years with no problems. Then unexpectedly, all of the
conditions that trigger an explosion are present and it happens.

A general fire or explosion requires three things: fuel, oxygen, and an ignition source. These make up the
explosion triangle as shown in Figure 11-1:

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 FIGURE 11-1

A dust explosion is a little more complex, it still has the requirements of the explosion triangle: fuel (combustible
dust), oxygen, and ignition source; however, there are two more requirements. First, the dust must be in a state of
dispersion in air or another oxidant. Second, the dust cloud must be confined such that its concentration is at or
above the minimum explosive concentration (MEC). These five requirements make up the Combustible Dust
Explosion Pentagon as shown below in Figure 11-2:

FIGURE 11-2

Primary & Secondary Explosions

There are different types of explosions as well: an initial or primary explosion and one or more secondary
explosions. Often, these secondary explosions are far more destructive than the primary explosion. Figure 11-3
illustrates a general scenario to how a primary explosion in a dust collector could occur and level a building with a
secondary explosion in less than one second.

In a ventilation system, all equipment is connected by ductwork. This ductwork is a very efficient means of not only
distributing air; but it can also provide a means with enough fuel and oxygen to allow a flame front to travel to either
end. If a piece of process equipment has an explosive concentration of dust along with an ignition source; that
could trigger a primary explosion that could travel through the ductwork to the dust collector and cause a
secondary explosion in the collector. The same is true in the opposite direction; if an explosion were to occur in a
dust collector and it was contained within the dust collector, the resulting flame front could travel through the
ductwork back to the process equipment.

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 1. Primary Deflagration inside 2. Shockwaves caused by Primary
Dust Collector. Deflagration

3. Shockwaves reflected 4. Dust clouds thrown in the

by surfaces within the building. air by the shockwaves

5. Primary Deflagration breaks out 6. Secondary Deflagration

of the Dust Collector. ignited.

7. Secondary Deflagration propagates 8. Secondary Deflagration

through the Dust Cloud. bursts from the building.

9. Collapsed building with remaining fires.

Total Elapsed Time: 325 milliseconds (0.325s)

Figure 11-3.

Hazard Prevention and Protection

The National Fire Protection Association (NFPA) is an international nonprofit organization based out of the U.S.
that is responsible for creating and maintaining the minimum standards for fire and other hazard prevention. The
NFPA was established in 1896 by a group of insurance underwriters focused initially on establishing codes for
automated fire sprinklers. Over the years, this organization has created and maintained over 300 consensus codes
and standards that influence our lives and safety on a daily basis.

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The following is a top ten list of Guidelines and Standards from the NFPA that are related to dust collection:

NFPA 61 Agricultural and Food Processing

NFPA 68 Explosion Protection by Deflagration Venting
NFPA 69 Explosion Prevention Systems
NFPA 70 National Electrical Code, Articles 501, 502, 503
NFPA 91 Exhaust Systems for Air Conveying
NFPA 484 Combustible Metals
NFPA 499 Classification of Combustible Dusts and of Hazardous (Classified) Locations
NFPA 654 Manufacturing, Processing, and Handling of Combustible Particulate Solids
NFPA 655 Sulfur Fires and Explosions
NFPA 664 Wood Processing and Woodworking Facilities

NFPA 70

Looking briefly at NFPA 70, this standard defines Classes, Groups, and Divisions of Hazardous matter and
locations and is important to take note of:
Class I: Gases, Vapors, and Liquids
Class II: Dusts
Group E: Metal Dusts (conductive and explosive)
Group F: Carbon Dusts (some conductive, all explosive)
Group G: Flour, Starch, Grain, Combustible Plastic, or Chemical Dusts (explosive)
Division I: Ignitable quantities of dust that is normally in suspension or conductive.
Division II: Dust that is abnormally suspended in an ignitable concentration (dust layers)
Class III: Ignitable Fibers and Flyings

When dealing with combustible dust, it is required by the Occupational Safety and Health Administration (OSHA)
that all necessary safeguards be put in place to collect and handle dust. This includes using electrically powered
cleaning devices approved for the hazard classification of the dust. Something as simple as a sweeper or vacuum
cleaner used in dusty areas must be approved to operate in a Class II Division 1 Group EFG Hazard Location.

NFPA 68: Introduction

NFPA 68 is known as the Standard on Explosion Protection by Deflagration Venting. It is the standard used to
design and use devices that vent the combustion gases and pressures of a resulting deflagration within an
enclosure to protect the structural integrity of the structure. NFPA 68 is used to design explosion vents as well as
any supporting systems to not only protect the enclosure; but to also guarantee safety for the surrounding area.

The NFPA 68 Standard, as it is currently known as in the latest edition, has changed in both understanding and
legality. In 1945, The Guide for Explosion Venting was adopted as a temporary standard until it was replaced in
1954 by the Guide for Explosion Venting. The guide was a collection of the best experimental data and information
available at that time and gave way to the “rules of thumb” of vent ratios that were used for many years. Up until
the 1970s, little to no extensive experimentation was performed in the United States. However, significant
advancements were made in Great Britain and Germany that provided a means to more accurately calculate vent
areas than the “rules of thumb”. The 1974 and 1978 editions of the Guide for Explosion Venting were published
based upon these advancements. From 1978 through 2002, the Guide for Explosion Venting underwent changes
and revisions to keep up with the current developments and contributions from a variety of sources including

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Factory Mutual Research Corporation. Notably, NFPA 68 changed from a Guide in the 2002 edition to a Standard
with the publishing of the 2007 edition. This change also included words like “should” being replaced with “shall” to
show intent for mandatory conformance.

NFPA 68: Deflagration Control by Venting

Explosion Venting is a function of both the dust collection system design and the combustible dust being collected.
Table 11-3 below shows the variables associated with the design of the dust collection system

Variable Units Description

Pes bar or psig The enclosure or vessel strength.
Pred bar or psig The maximum pressure developed in a vented
enclosure during a deflagration and determined by
Pes. It must not exceed two-thirds of Pes.
Pstat bar or psig The minimum pressure required to activate an
explosion vent. It must be lower than Pred.
V m3 Volume of vessel.
Dhe m Hydraulic Diameter of Vessel determined by the
general shape relative to the central axis.
H m The maximum flame length inside the vessel.
L/D dimensionless Where L = H and D = Dhe. The L/D ratio is used to
determine if an adjustment calculation is required.
Av m2 Explosion Vent area required per the calculations.
Table 11-3

It should be noted that the NPFA 68 Standard employs the use of the Metric system for units. To see conversion
factors between the standard units used in NFPA 68 and common English units see Table 11-4.

Standard English Standard Metric

psig bar
PRESSURE psig 1 
bar  1
ft m
LENGTH ft 1 0.3048
m 3.2808 1
ft2 m2
AREA ft2 1 0.0929
m2 10.7639 1
ft3 m3
VOLUME ft3 1 0.0283
m3 35.3147 1
Table 11-4

When sizing explosion vents, the majority of the variables focus on attributes of the dust collection system;
however, there are two critical variables that are entirely dependent on the combustible dust being collected.

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These are Pmax and Kst. Pmax is the maximum pressure in bar, developed in a contained deflagration at an optimum
concentration. Kst is the deflagration index; or rate of pressure rise in bar/sec. Based on Kst values, dusts have
been classified into three hazard classes: St-1, St-2, St-3 as shown in Table 11-5.

Hazard Class Kst (bar-m/s) Pmax (bar)

St-1 < 200 10
St-2 201-300 10
St-3 > 300 12
Table 11-5

Kst and Pmax are both determined experimentally by a spherical test chamber, either 20L (0.2 m3) or 1 m3 in volume,
that use a chemical igniter to produce a deflagration similar to the design shown in Figure 11-4.

Figure 11-4

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The dust is tested in various concentrations, generally starting at 250 g/m3 and increasing 250 g/m3 until the
maximum Kst is found at the optimum concentration. Figure 11-5 shows a typical chart for Pmax and Kst, in this
case the Pmax = 8.5 and the Kst = 138.

Figure 11-5

The NFPA 68 does provide a list of some common dusts and their respective Kst and Pmax values. However, these
values are intended as reference only. The best way to find out the explosivity of a combustible dust is to have it
tested in a lab. In Table 11-1, the Kst, Pmax, and Dust Hazard Class values of the dusts compared in size in Table
11-6 are shown.

Material Particle Size Kst (bar-m/s) Pmax (bar) Class

Corn Starch 7ȝm 202 10.3 2
Charcoal (Wood) 14ȝm 10 9.0 1
Magnesium 28ȝm 508 17.5 3
Sugar 30ȝm 138 8.5 1
PVC (Plastic) 107ȝm 46 7.6 1
Table 11-6

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Once the explosion vent area is calculated based upon all of the system and dust variables, the explosion vent
must be oriented in such a fashion to ensure that an explosive event is directed in a safe manner. Explosion Vents
cannot freely vent indoors! As shown in the previous drawing, an explosion creates a pressure wave that can
loosen and throw settled dust into the air. If the explosion vent were to vent indoors, it could have catastrophic
consequences. There are three options to orient and locate dust collectors and vessels with explosion vents as
shown in Figure 11-6.

Figure 11-6

As shown, the safest but most expensive option is to use a flameless or flame-quenching explosion vent. Placing
the dust collector outside or venting through an external wall eliminates the exposure inside the building; however it
adds risk external to the building. When an explosion vent does rupture from an explosion, a flame front or fireball
is projected from the vessel. The distance and size of the fireball can be calculated and depends on the Kst of the
dust, volume of the vessel, and number of vents as shown in Equation #11a.

1/ 3
§V· (Equation #11a)
D K ˜¨ ¸
©n¹
Source: NFPA 68 - 2013
Where D = Projected distance the fireball will travel in all directions about the central axis (in m)
(D = Length = Width = Height) (Note: Distance shall be limited to 60m maximum)
K = Flame Length Factor. K=8 for Chemical and Agricultural Dusts, K=10 for Metal Dusts
V = Volume of vessel (in m3)
n = Number of vents with separate discharge directions along the central axis. If multiple vent panels
cover a single direction, they should not be treated as multiple vents

The distance calculated by this fireball equation shall be assumed to be equally distributed about the centerline of
the vent discharge as shown in Figure 11-7

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 Figure 11-7

This distance can range significantly depending on the dust as shown in Example 11.1

Example 11.1 Determine the projected fireball for a dust collector with a volume of 24m3
with two (2) explosion vents, adjacently installed, collecting a chemical dust.

1/ 3
Use equation #11a: §V·
D K ˜¨ ¸
©n¹

1/ 3
§ 24m 3 ·
D 8 ˜ ¨¨ ¸¸
© 2 ¹

D = 18.3m or 60ft

As the example shows, the resulting fireball from a deflagration presents a hazard to personnel and equipment in
the vicinity of the dust collector. Within the fireball area, obvious risks are due to the fireball itself; however outside
the fireball area there are risks resulting from thermal radiation effects. To reduce the risk to personnel, structures,
and equipment from a resulting deflagration, a proper assessment of the local environment should be completed
that will allow for sufficient area within and around the explosion vent fireball.

NFPA 69: Introduction

NFPA 69 is known as the Standard on Explosion Prevention Systems. The standard contains a variety of options
and methods to suppress and isolate systems from explosions. NFPA 69 can be used in tandem with NFPA 68 or
exclusively to provide complete protection for the system. Contained within NFPA 69 are three different types of
protection systems: Prevention, Suppression, and Isolation. It should be noted that these different types of systems
do overlap and often combine together (with NFPA 68 as well) to provide a complete explosion protection system.

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The NFPA 69 Standard has continued to advance in understanding and keep pace as technology provides
different and more accurate means to protect processes against explosions. Unlike NFPA 68, NFPA 69 has always
been a Standard rather than a guide full of “rules of thumb”.

NFPA 69: Prevention

A simple and effective way to protect against explosions is to prevent them from occurring. This method involves
focusing on controlling ignition sources. A spark or flaming ember presents the greatest risk at being the ignition
source of a deflagration. A spark consists of a dense particle that surrounds itself with an envelope of hot air, which
allows the solid particle to be buoyant in the surrounding cooler airstream as shown in Figure 11-8.

Figure 11-8

As the spark travels in the duct, one would assume that the spark would be cooled or drop out and prevent any fire
or deflagration. However, this is not often the case because ducts are designed to have a smooth or laminar flow.
So as the spark travels in the duct, the smooth flow continues to provide the spark and envelope of hot air with
oxygen to fuel the burning ember. To prevent a spark from reaching the dust collector and creating a fire or causing
an explosion, the spark must be extinguished. This can be accomplished through a variety of methods that can be
classified as either passive or active. Passive methods for explosion protection rely on mechanics and physics in
their design and do not require power to provide protection; however, they may be powered and will still provide
protection if power is lost. Active methods on the other hand do require power for their operation and by design
must have safeguards and fail-safes should the power be lost.

NFPA 69: Passive Prevention

Passive spark extinguishment methods tend to be simple and mechanical in nature. In order for the spark to be
extinguished, the envelope of hot air needs to be disrupted. This can be accomplished by changing the airflow from
laminar to turbulent. A very common approach to a spark extinguishment is the use of Inertial Separators like the
ones discussed in Chapter 1. “Drop Out Boxes” are often called “Spark Boxes” when the purpose for the separator
is to extinguish sparks rather than separate heavier material. It must be noted here that cyclones are not good at
spark extinguishment. The airflow inside a cyclone stays fairly laminar and it is very common for the sparks to pass
through a cyclone without any change. The Spark Boxes cause the air to strike a wall or plate, creating turbulence
and separating the spark from the envelope.

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An alternative to a Spark Box is a Spark Arrestor that can be mounted in the ductwork. These spark arrestors
resemble a turbine in that they separate the airflow and create a churning, highly turbulent airflow as shown in
Figure 11-9.

Figure 11-9

This churning turbulence allows the spark to separate from the bubble and continue to cool rapidly as it moves
along the duct in the turbulent airstream. The in-line spark arrestor varies in size depending on the airflow through
the duct. The velocity through the spark arrestor can range from 2,500fpm-4,500fpm and vary depending on
application. Depending on the velocity, the pressure drop of these spark arrestors can range from 0.5inWG to
1.0inWG and should be accounted for when designing for system pressure losses.

Both of these methods of spark extinguishment tend to be very efficient and cost effective. However, 100%
complete spark elimination cannot be guaranteed. These methods tend to be a part of a total solution that includes
other fire and explosion protection.

NFPA 69: Active Prevention

Active spark extinguishment systems on the other hand tend to be complex. The active spark extinguishment
systems revolve around two key components: sensing and suppression. First, the spark(s) must be sensed by
means of high efficiency infrared sensors. The sensors are designed to operate in the worst possible environments
from heavy dust load, high static pressure, and high air stream temperatures. Second, once the spark is detected
by the sensor, it must be suppressed or extinguished with water. Figure 11-10 shows the typical system setup and
operation of a spark extinguishment system.

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 Figure 11-10

Once the spark is picked up by the infrared sensor, a signal is sent to a control panel. The control panel processes
the incoming signal and sends an outgoing signal to a quick acting solenoid valve. The solenoid valve opens,
releasing water that is sprayed into the duct via an array of nozzles determined by the size duct. Typically, the
system will have a response time of 300ms to 400ms from sensing of the spark to activation of the solenoid valve.
The distance between the sensor and suppression becomes very critical with respect to the velocity through the
duct. At a velocity of 3000fpm or 50ft/s, a spark will travel 15ft in 300ms and 20ft in 400ms.

NFPA 69: Control by Suppression

At the onset of an explosion, pressure waves expand out at the speed of sound ahead of the fireball. These
pressure waves can be detected by sensors that can then trigger an extinguishing agent that will quench the
developing fireball before it reaches a destructive magnitude. A typical system consists of a pressure sensor,
monitoring control panel, power supply with battery back-up, and suppressant delivery device as shown in
Figure 11-11.

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 Figure 11-11

The pressure sensors available are able to sense the pressure increase in less than 1 millisecond. Once the
pressure is sensed, the suppressant agent can be delivered to quench the explosion within several milliseconds.
This is very important as it only takes a matter of 50 milliseconds for the pressure created by an explosion to reach
a destructive magnitude. The location, quantity, and size of the sensors and suppression devices are determined
for each application based upon the dust characteristics (Kst and Pmax) and vessel characteristics (size and
strength).

NFPA 69: Isolation

Even though a deflagration may be controlled by suppression or venting, the vessel must be isolated to prevent the
deflagration from traveling back upstream to the process. For most applications, this requires that an isolation
device be installed on the inlet of the dust collector. In the cases where a dust collector is employed and the filtered
air is exhausted back into the building; an isolation device must also be installed on the outlet of the dust collector.

NFPA 69: Control by Passive Isolation

Similar to passive prevention methods, there are passive isolation methods that are widely used to contain a
deflagration. To prevent a deflagration from returning to the process via the interconnecting ductwork, a back blast
damper is employed. A back blast damper is essentially a one-way valve that only allows air to flow through one
direction as shown in Figure 11-12 below.

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 Figure 11-12

When a deflagration occurs in the dust collector, the pressure waves travel back up the interconnecting ductwork
which causes the internal flap to shut and seals off the upstream system from the approaching flame front. These
back blast dampers can come equipped with a pressure relief panel (explosion vent) as shown in Figure 11-13.
The pressure relief panel will burst once the flap shuts to provide further alleviation of pressure from the
deflagration event.

Figure 11-13

To prevent the deflagration from exiting the vessel via the hopper, a material choke valve or rotary air lock (RAL) is
used. The rotary air lock must meet three main requirements in order to adequately provide isolation. First, the
body, rotor, and vanes must have the necessary strength to withstand the maximum anticipated pressure during
the explosion, Pred. Second, to prevent flame passage through the valve, a close clearance of < 0.2 mm (0.0079”)
must be maintained between the body and the tip of the vanes. The vane clearance must be monitored for wear
during normal preventative maintenance activities to ensure that the design clearance is not exceeded. Last, the
rotary air lock must have two vanes per side area always in contact with the housing at all times. To accomplish
this, the minimum number of vanes recommended is eight vanes as shown in Figure 11-14.

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 Figure 11-14

NFPA 69: Control by Active Isolation

Like the active prevention methods, active isolation methods require devices to sense the deflagration event and
controls to activate a means to either divert or suppress the travelling flame front. A high speed abort gate isolates
the airflow from either the inlet or outlet of the dust collector and diverts the deflagration event into the open
atmosphere as shown in Figure 11-15 below. These high speed abort gates are triggered by a control panel that
receives an explosion confirmation from a pressure monitor on the dust collector.

Figure 11-15
Another means to isolate a deflagration is by chemical isolation. The chemical isolation is triggered by the same
means that the high speed abort gate is triggered. Frequently, the chemical isolation is part of a complete chemical
suppression of the dust collection system. The same pressure sensor that triggers the dust collector chemical
suppression device will also trigger the inlet and outlet chemical isolation devices as shown in Figure 11-16.

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 Figure 11-16
Summary of Protection
There are many options to choose from when it comes to explosion protection; the application and the severity of
the dust will determine what options are available. Each option has its own associated costs; generally speaking
the Passive options tend to be less expensive while the Active options tend to allow for greater flexibility in design
and application. The process and the preference of the end user will determine whether an active system, passive
system, or system that has a combination of active and passive methods is used. Ultimately, it will be the
responsibility of the end user, in accordance with the Authority Having Jurisdiction, to select the most effective
solution for the application.
Examples of complete Passive and Active methods are shown in Figure 11-17, 11-18, and 11-19.

Figure 11-17

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Figure 11-18

Figure 11-19

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Authority Having Jurisdiction

The Authority Having Jurisdiction is the party, generally an individual, office, or organization, that is responsible for
enforcing the standards and for approving equipment, installation, materials, or procedure covered within the
standards.

Throughout the NFPA documents, the phrase “Authority Having Jurisdiction” or the acronym AHJ is used
commonly and broadly due to the variation in both jurisdiction and responsibility for different authorizing agencies.
Depending on the scope of the application, the authority having jurisdiction may be a federal, state, or local
department or an individual such as a fire marshal.

On the other hand, an insurance inspection department or company representative may be the authority having
jurisdiction for insurance purposes. However, often it is the property owner or an agent designated by the property
owner that assumes the role of the authority having jurisdiction.

In summary, it is important for any facilities that have a potentially combustible dust that particular attention be paid
to good housekeeping and the elimination of dust accumulation on any horizontal surfaces.

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 Chapter 12

IMPACT OF MOISTURE IN DUST COLLECTORS

Moisture is water in the liquid vapor or solid state and it is very disruptive to the dust collection process. Basically,
the moisture tends to cause the pores of the filter media to become plugged and, if the dust is hygroscopic, absorb
water. The dust-liquid mixture can make an impermeable coating of “muck” that can resist most types of cleaning
systems. In cartridge collectors, if the dust is not hygroscopic in nature, the moisture alone can ruin the cellulose
media since it loses its permeability when it becomes wet and does not recover when it is dried. In a typical fabric
filter in a baghouse, an intermittent moisture occurrence can sometimes be resolved when the moisture is removed
and the bag is allowed to be dried. In many applications, the discovery of moisture in the dust collection system
comes as a surprise and the damage has already occurred. In many cases, it results in plugged bags or
cartridges, high pressure reading on the magnehelic differential pressure gage, and low flow rates of the process
air stream resulting in poor vacuum.

Planning for Moisture

It is important to understand the conditions that can contribute to producing moisture and in having equipment or
processes in place that can control it. Usually, the solution is to provide a mechanism to keep the moisture in a
vapor form or to remove the excess vapor by using equipment to collect it before the water vapor can condense
into its liquid state. There are some key factors that cause the moisture to be present:

x The dust itself may be hygroscopic and/or contain moisture. Example: Sawdust usually has a moisture
content of 19%.

x Temperature differentials in the process air stream. Example: The air temperature at the collection
source may be considerably hotter than in the dust collector.

x Temperature differentials caused by seasonal changes. Example: The air temperature at the collection
source inside the manufacturing building may be hotter than the outside temperature where the dust
collector is located and subject to the local weather conditions.

x Humidity changes. Example: The process air can change its humidity due to wind directional change
and other weather changes.

x Wet compressed air for pulse cleaning may contain a high percent of moisture. Example: No upstream
dryer in high humidity environment.

x Mists or aerosol sprays that are intermittently or continuously added into the process air stream.
Example: Coolant sprays for machine cutting tools.

Actually, it takes only one of these factors to cause a moisture problem in the dust collector.

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How to Recognize a Moisture Problem in a Baghouse or Cartridge Collector System

Since moisture can arrive at the dust collector from a number of sources, the procedure is to identify the reasons
for moisture and to divide the investigation into multiple steps. The first and most obvious step is to identify the
presence of moisture in the collector which may occur during various times of the day, seasons of the year, or
special events in the process air stream. At an opportune time when the possibility of moisture is high and the dust
collector is not operating, check inside the dirty air chamber where the bags or cartridges are housed for signs of
moisture past or present. A simple moisture test consists of inspecting the inside walls of the dust chamber for
drops of moisture, wet dust clumps, and/or areas of rust formation. Also, for non-hazardous dusts and while
wearing appropriate protective clothing, carefully remove some dust and place the dust onto a paper towel. By
squeezing the paper towel with the dust sample inside it, see if any moisture or oils may have transferred to the
paper towel. If any noticeable amount of moisture was transferred to the paper towel, a moisture problem is
present and needs to be corrected.

If the dust collector is located outside and is subject to the seasons of the year, the inside of the collector may tend
to gather moisture at the same time as outside dewpoint phenomena is occurring such as the formation of heavy
water droplets on blades of grass or the actual time period of frost on the windshields of automobiles. Follow the
same safety precautions as mentioned in the previous paragraph and inspect the inside of the dirty air chamber for
the presence of moisture. Again, check a sample of the collected dust for moisture content by removing a small
amount of dust, placing it carefully in a clean paper towel, and by squeezing the paper towel with the dust sample
inside it. If moisture has been transferred to the paper towel, a moisture problem is present and it needs to be
corrected. It is important to note that the presence of moisture may be intermittent and disappear before anyone
has noticed it. However, the damage, especially to cellulose cartridge filters, results in plugged cartridges which
can cause them to be replaced frequently.

Process Air Stream Moisture

The quantity of water vapor is affected by the change in temperature in the process air stream. Hot air can hold
more water in the form of water vapor than cooler air. If the dust collection point is over the heat source and the
process air stream travels far enough to cause significant cooling of the air stream, the water vapor in the air
stream can condense when the temperature passes through the dewpoint temperature. The dewpoint temperature
is a key concept and is defined as the temperature at which the air vapor mixture is saturated. As the temperature
is lowered, some moisture will be released as droplets of water. There are many common examples of these dew-
point phenomena. A few examples are 100% humidity (fog), first formation of frost on the windows and surfaces of
cars, or dew on blades of grass in the early morning.

For every given temperature, there is a corresponding dewpoint temperature which varies according to amount of
water vapor that is present in the process air stream at that moment in time. The process air stream needs to be
monitored for times or events that cause sudden changes in the process air stream. For example, a second
factory machine starts to add a cool air stream to the warmer main machine air flow. At these moments of change,
temperature measurements at different points in the duct and in the collector should be taken in order to
understand the effect of each part of the collection process. Large temperature differences of approximately 15°F
or more between the collection source and the dust collector can indicate a potential condensation of moisture.
This temperature differential can vary depending on the relative humidity (amount of moisture vapor in the air) of
the process air stream.

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Moisture Creation During Filter Cleaning

There is a another seldom considered aspect of the reverse jet characteristics in all pulse jet dust collector designs
that may be important in some special applications. This characteristic is that the compressed air expands and
cools as it leaves the orifice of the pulse pipe. The cleaning jet air draws in process air which moderates the
temperature of the overall cleaning jet. Inside the pulse pipe during the initial part of the pulse, we will assume the
following conditions:
Compressed air pressure = 100 psia or 85 psig.
Temperature, Ti = 530°R or 70°F.

As the air expands through the orifice, it cools. This cooler temperature, To, can be calculated by the following
equation:
To = (Ti) (0.528) (k-1)/k where k is a gas constant for air, k = 1.4
To = (530)(0.833) = 441°R = -19°F.
Since the jet grows by drawing in at least 4 times the air by inducing cleaned process air into the jet, the mixture
temperature is:
Heat Lost = Heat Gained
1 CFM (Tm – 441) = 4 CFM (530 – Tm)
Tm = 512°F or 52°F.

There is some heat regained because of turbulence in the air jet and the net effect is that the jet cools off by 5°F to
10°F. If the cooling effect drops the jet below the dew point, plugging of the bags from mud forming may occur.

There is another important moisture situation that can develop which involves both the warm process air stream
and the clean, dry, compressed air pulsing. The mixture of the warm process air stream with the cooler, clean, dry
air can potentially lower the overall temperature of this mixture. If the warm process air stream is carrying signifi-
cant moisture (the air is almost saturated with moisture or closer to its dewpoint), then any significant cooling by the
clean air jet could lower the temperature of the mixture and cause condensation to appear inside the collector. For
example, in some parts of the North America, winter can mean temperatures of below 0°F. The cleaning air
manifold will tend to cool the compressed air to almost equal to the outside atmospheric temperature. Also it is
likely that the warm process air stream temperature will remain fairly constant throughout the seasons. The cold,
clean air and dry, compressed air differential is greater in the winter months and this greater temperature
differential may cause more moisture to condensate. It is important to take periodic temperature readings between
the collection source and the dirty side chamber of the dust collector during different times of the day and seasons
of the year. These temperature differentials will help to anticipate potential condensation problems.

Compressed Air Moisture

Wet compressed air can cause moisture problems for both bags and cartridge filter media. Typically, when the
cleaning air pulses moist air in the inside of the cartridge or bag, the filter media becomes soaked from the inside
and the wetness extends to the outside surfaces of the filter media. Also, moisture wets the layer of dust cake that
clings to the filter media and acts like an impermeable coating. Furthermore, the process air or fan air flows with
more difficulty through the filter media and causes the pressure drop across the filter media to rise with a
corresponding reduction in the processed air flow rate.

It must be emphasized that clean, dry, compressed air is required for successful long term clean air pulsing of the
filter media and will extend filter life. The standard method to achieve this is by using air dryers and air filters.

-105-
Filters offer some protection but are not nearly as effective as air dryers. There are two main types of dryers.
These are refrigerant dryers and desiccant dryers.

Refrigerant Dryers

Refrigerant dryers are a relatively low cost solution to clean and dry the air from an air compressor. This type of
dryer uses a combination of heat exchangers and filters to remove moisture from the airstream. As Figure 12-1
shows, the Refrigerant Dryer has four phases:

Phase 1. Hot, saturated air from the compressor travels through an air-to-air heat exchanger where it is
cooled by the colder dry air leaving the heat exchanger.

Phase 2. The cooler air travels through an air-to-refrigerant heat exchanger where it is cooled to the dew
point by the refrigerant constantly recirculating through the exchanger at a constant flow via a
refrigeration system.

Phase 3. The refrigerated air passes through a multi stage filter that removes water droplets resulting
from condensation during the refrigeration cycle. Commonly, a high efficiency cold coalescing
element immediately following the moisture separator is used to remove oils and any solid
particulates larger than 3 microns.

Phase 4. The cold, dry air is directed back through the air-to-air heat exchanger where it is warmed by
the hot, moist air from the compressor. This re-heating increases the volume of the air and also
prevents the downstream piping from sweating.

Figure 12-1

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Desiccant Dryers

Desiccant dryers operate differently than the previous refrigerant dryers. The desiccant dryer uses a combination of
filters and adsorbent to remove moisture from the airstream. There are three different types of desiccant dryers:
Heatless, Heated, and Heated Blower. A Heatless Desiccant Dryer, as shown in Figure 12-2, has four steps:

Step 1. Pre-Filter
Hot, saturated air from the compressor travels through a coalescing filter where oil and some
water are removed. To prolong the life of the adsorbent media, a two-stage coalescing filter
system is highly recommended.

Step 2. Drying
The saturated air flows through an automatic diverter valve that diverts the air into one of the
drying towers, in Figure 12-2 it is Tower #2. The drying towers are filled with a desiccant or
adsorbent material such as activated alumina, molecular sieve, or silica gel. The adsorbents
are very hygroscopic and attract moisture away from the compressed air as the air flows
through the tower. Over a short period of time (5 minutes) the adsorbent in the tower or “bed”
as it is commonly referred to, becomes saturated with moisture and needs to be purged.

Step 3. Regeneration
While one of the drying towers is drying, the other tower is purging or regenerating. During the
regeneration process, an exhaust valve opens and the clean air diverter valve will open. The
dry air diverter valve will allow roughly 10% to 15% by pass into the regeneration tower to
purge the desiccant bed, in Figure 12-2 it is Tower #1. As the compressed air passes through
the bed, the moisture is purged and exits the exhaust valve. After a few minutes, the exhaust
valve will close and the pressure will begin to build back up in the tower. Once the pressure is
built up, the diverter valves will simultaneously change position and the tanks will switch
operations.

Step 4. After Filter

The dried air travels through a particulate after filter which removes particulates from the air
stream.

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 Figure 12-2

The typical cycle of the Heatless Desiccant Dryer is drying for 5 minutes, regeneration for 4 minutes, and re-
pressurizing for 1 minute. These systems are the simplest of the desiccant dryers and tend to have a lower initial
cost but higher operation cost.

A Heated Desiccant Dryer, as shown in Figure 12-3, has the same amount of steps as the Heatless Desiccant
Dryer. The only difference is in the Regeneration step. During regeneration, the dried compressed air is diverted
through an external heater before entering the tower being purged. Heated air can hold a significantly more
moisture than unheated air and consequently only about half the amount of dried compressed air is required for the
regeneration. These systems are more complex and tend to have a higher initial cost than the heatless system;
however, the operation cost is lower.

-108-
 Figure 12-3

A Heated Blower Desiccant Dryer, as shown in Figure 12-4, uses the same heated air principle as the Heated
Desiccant Dryer. The difference is that the Heated Blower Desiccant Dryer uses no compressed air for the purging,
instead a separate pressure blower is used to direct air through a heater before entering the purging tower. This
Heated Blower Desiccant Dryer has the highest initial cost than the other two; however, it also has the lowest
operation cost.

Figure 12-4

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 When it comes to compressors, each system has its own advantages and disadvantages. The refrigerant dryers
The
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that frequently needs.
drains the compressed air manifold
on the dust collector such as illustrated in Figure 12-5, the moisture problem can usually be eliminated.
Depending upon the application, a simple point of use compressed air filter will provide an advantage. Preferably, a
combination refrigerated & desiccant dryer would provide the best conditioning of the compressed air system. If the
compressed air is perfectly clean and dry, the problems associated with valve and solenoid freezing will be
eliminated.

FIGURE 12-5
The second alternative which actually removes the moisture down to approximately -40°F is a desiccant dryer.
The advantage with this system is that no secondary system is needed to remove the moisture from the
compressed air system.

There is a third alternative where both a refrigerant and a desiccant dryer arrangement are combined together as
shown in Figure 12-6. The refrigerated is employed first to remove the moisture down to a 35°F dewpoint and then
a smaller desiccant dryer system is added to remove the remaining moisture down to an approximate -40°F.

-110-
-112-
Figure 12-6. Combined Refrigerated & Heatless Desiccant System
-111-
The benefit to this system is that the desiccant dryer can be smaller with lower operating costs because the
refrigerated dryer does about 85% of the work. Another benefit is that with the combined system, the desiccant
dryer can be bypassed during the warmer months when the lower dewpoint is not required. These combination
systems tend to have the highest initial cost of all the systems; however, it is the most efficient system and has the
lowest operating cost. All of these dryer systems function very well, but it is important to review your requirements
with qualified personnel who can assist you in specifying your present and future needs.

Depending upon the application, a simple point of use compressed air filter will provide an advantage. Preferably, a
combination refrigerated & desiccant dryer would provide the best conditioning of the compressed air system. If the
compressed air is perfectly clean and dry, the problems associated with valve and solenoid freezing will be
eliminated.

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 Chapter 13

FUTURE TRENDS IN DUST COLLECTING

By understanding both the collecting mechanisms and the related mathematics for explaining the phenomena, new
equipment designs will continue to “break through” traditional limitations. Listed below are key future trends which
will improve all aspects of dust collecting:

x Greater collection efficiency and cleaning capability of all particle sizes, especially the finer particles two
microns and smaller through the use of refined cleaning systems and better filter media options.

x Higher air to cloth ratios that are beneficial for achieving high process air stream flow rates while reduc-
ing the number of bags or cartridges. The end user benefits from lower costs for purchasing the initial
collector, less space occupied by the collector, and reduced yearly maintenance and replacement filter
costs.

x Explosive dusts are gaining more attention from governing agencies.

x User friendly designs of both collectors and controls are becoming more commonplace which include
easier installation and removal of cartridges from the collector and more electrical options that are
prewired at the factory.

x Application specific devices are more available such as devices for reducing the moisture in the
compressed air manifold, apparatus for freeing dust blockages in the hopper, and electronic control
options for customizing the timing of the cleaning cycle.

x Specialized collector designs for low headroom and other difficult physical location configurations are
becoming more commonplace.

Collecting Dust with Greater Efficiency

Scientific Dust Collectors is a leader in providing greater dust collection efficiency for both baghouse and cartridge
media design. For cartridge collectors, a patented downflow direction is used to redirect the inlet air around the
cartridges and to the hopper below. By using a combination of inlet baffles, the process air stream is separated
into multiple downward air streams that work together to minimize the frequent upward turbulent air flows inside the
dirty air chamber where the cartridges are housed. For baghouses, a different approach is used which is
accomplished by locating the process air inlet in the upper portion of the dirty air chamber. The process air stream
does not flow directly onto the bags but rather enters the collector horizontally to the vertically mounted bags. As
the process air enters into the bag zone, its velocity is reduced and the large particles of dust will simply drop to the
hopper below.

In addition, a vertical perforated baffle is provided so that the inlet can distribute the dust laden air into the area
where the bags are located. As the dust laden air is being pulled towards and through the various bags, there is a
continuous downward flow that allows the bags to capture the fine dust particles. Also, since the process air flow is
-113-
downward, the cleaning air releases the finer particles into a downward air stream. Both the baghouse and
cartridge collectors have a generous distance between each of the bags or cartridges which further assists in the
cleaning and collection process by helping to prevent re-entrainment of the dust particles.

Equally important to easily collect dust is the ability to clean the dust from the bag or cartridge media without
driving the just removed dust into and through the neighboring bag or cartridge. Scientific Dust Collectors’
approach is to maximize the air from the compressed air valve and corresponding purge tube by the use of a
nozzle. Instead of achieving the 1030 feet per minute orifice velocity as found in many generic collectors, Scientific
Dust Collectors’ nozzle approaches 1750 feet per minute air velocity. The benefit from the higher speed comes
with the greater amount of induced air into the mouth of the cartridge or bag.

It is important to note that the initial cleaning air velocity of 1750 feet per minute is momentary and is greatly
reduced as it enters into the mouth of the cartridge or bag so that it will have the best cleaning effect on the filter
media. The end result is less compressed air that is used per cubic foot per minute of process air stream
compared to other collectors, and an ideal velocity control of the cleaning air when it reaches the filter media.

Higher Air to Cloth Ratios

The push towards higher air to cloth ratios has accelerated in recent years. The dust collectors of tomorrow will
continue to strive towards having higher and higher air to cloth ratios. The benefits to the end user are both initial
cost reductions for the purchase of the dust collectors and for the long term maintenance due to less filter media
(cartridges or bags) and less compressed air cleaning valves. Also, more process air stream flow rates will be able
to be utilized in smaller dust collectors which gives more flexibility to the end user.

Scientific Dust Collectors’ designs are current and successful because of its use of advanced nozzle technology
with induced air benefits in cleaning more filter media. In addition, a high inlet configuration for providing a natural
“drop-out” zone and process air velocity reduction, and generous spacing between the bags or cartridges in the
dust collector results in less dust re-entrainment. With the proper selection of filter media for the application, this
total combination of design capabilities ensures success in capturing dust efficiently and with minimal costs.

Specialized Collector Designs for Welding Fumes and Other Chemical Dusts

Since the dust collection industry encompasses such a wide range of chemicals and substance mixtures, the use
of specialized collector designs are expected. Dust collector manufacturers are continuously adapting their equip-
ment to accommodate these special needs. For example, welding fumes contain a mixture of very small
submicron and larger dust particles, moisture droplets, and hot sparks that need to be successfully treated in the
process air stream, captured in the filter media, and then released to the hopper section.

Scientific Dust Collectors provides a fume dust collector that cools the hot sparks in a spark trap, provides filter
media that is water repellent, and a downward process air stream flow environment that quickly captures the fine
particles. Also, a cleaning air jet with nozzle technology is sized to properly clean the dust from the filter media and
to allow the agglomerated dust to settle into the hopper or drawer below. The unit is user friendly in that the
cartridges are easily installed by use of foot pedals so that no hand tools are needed.

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Scientific Dust Collectors Products

Dust collecting technology and equipment is continuously evolving and end users are demanding that the dust
collectors be more user friendly to their specific needs and environments. Dust collectors can be located both
indoors and outdoors. They can be located close to people or installed remotely on top of buildings or silos. When
the collector is located indoors, headroom is usually limited. Therefore, Scientific Dust Collectors provides user
friendly designs that give the end user choices on the type of collector that will best suit his space and budget.

With some size limitations as to the number of bags for some styles, four styles of baghouse are currently available
from Scientific Dust Collectors as listed below:

x Top Removal: Bags are removed and/or replaced from inside the walk-in upper bin area which is above
the tube sheet. Also, changing the bags is contained inside the upper bin. The advantage is the bag
changes are done from the clean side and inside the upper bin.

x Top Railing Top: Provides the capability to have the bags removed/replaced through the roof of the
collector. The advantage is less collector height than top removal style.

x Bottom Removal: Bags are removed and/or replaced from inside the dirty air chamber where the bags
are housed. The main advantages are a lower headroom height compared to top removal styles and a
lower cost for the collector.

x Horizontal Baghouse: Bags lay horizontally and are removed horizontally from the compressed air
manifold end. Main advantage is low headroom.

For cartridge collectors, Scientific Dust Collectors offers two styles of collectors:

x Vertical Style Cartridge Collector: Single vertical mounted cartridge. Main advantage is gravity aided
cleaning of cartridge due to its vertical location.

x Horizontal Style Collector: Horizontally and tandem mounted cartridge. Main advantage is ease of
release of cartridge without entering the collector.

We hope the information provided to you in this booklet will help you make an informed decision at the time of your
next dust collector purchase. Working with experienced individuals who can assist you in analyzing your require-
ments and understand the unique characteristics of your application will ensure a successful installation.

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 APPENDIX A

DUST COLLECTOR SELECTION DATA FORM

Customer: Date:
Submitted By:

Installation:
Jobsite: Outside:
Inside:
Application: Headroom Limit:
Process: Discharge Height:
Source: Options: 55 Gallon Drum
ACFM: Rotary Air Lock
Duty: Screw Conveyor
Dust Type: 3 Cubic Yd. Hopper
Dust Load: Baghouse Unit:
Particle Size: Cartridge Unit:
Density: Media:
Moisture: Model:
Temperature: Media Access: Top:
Abrasive: Side:
Corrosive: Top Access: Walk-in Plenum:
Hygroscopic: Railing Top:
Explosive: Fan Mounting Options: Ground:
Sprinklers: Top:
Side:
Construction: Fan Static:
Carbon Steel: Fan Damper:
Stainless Steel: Fan Silencer:
Motor Starter(s):
NOTES: Inlet Transitions:


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 APPENDIX B

START-UP CHECKLIST

Check the following items in sequence:

Check electrical supply and connections per wiring diagram.

Apply caulk to inside and outside connectors on timer board/solenoid boxes installed
outside to make sure no moisture can get inside box(es).
Inspect ductwork and gates for proper positions.

Check bags and cages for proper assembly.

Check housing joints assembly and foundation anchors.

See if compressed air supply is regulated to 90-100 psig at the manifold.

Activate the timer. Check sequence of valve operation.

Check for air pressure recovery between pulses at the manifold (90-100 psig).

Check for air pressure drop at the manifold, during the pulses. Not to drop below 45 psig.

Close all access doors. Check for seal integrity.

Check discharge configuration for seal integrity (conveyor/airlock).

Adjust filtering air fan damper to between 1/2 and 3/4 open.

Activate the filtering air fan, without introducing the dust load.

Record the start-up differential pressure gauge reading, before introducing the dust load.
Slowly introduce the dust load. When the differential pressure gauge indicates a change,
sufficient “cake” will have been accumulated to resist media blinding. Readjust the fan
damper to design specifications.
Monitor the differential pressure gauge readings. Adjust “off-time” timer settings as
required.

NOTE: TROUBLESHOOTING BEGINS WITH A REVIEW OF THE RECORDED DIFFERENTIAL

PRESSURE GAUGE READINGS. IT IS IMPORTANT THAT CLEAN-START VALUES BE
RECORDED AS WELL AS THE OPERATING DATA!

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 APPENDIX C

MAINTENANCE

Once properly adjusted, the collector requires little maintenance. The units should be inspected at 3 month
intervals as follows:

Note the differential pressure gauge reading and add to your record. Severe applications may
require more frequent checks (weekly or daily).

Check the cleaning air supply for cleanliness and moisture.

Drain any accumulated moisture from the manifold tank.

Empty the dust trap.

Note the operating air pressure gauge readings at the manifold. Must recover to 90-100 psig
between pulses.

Check the air valves for sequential operation.

Check the door and cover seals for leaks.

Refer to Assembly and Start Up Sections for filter replacement.

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 APPENDIX D

TROUBLESHOOTING

Problems with the collector usually appear soon after installation, often the result of improper assembly or
operating techniques. The dust collector has been sized for specific conditions, such as moisture and dust
characteristics. Occasionally deviations from the original design parameters will be introduced. In some instances,
the new conditions can be compensated for by substituting special media. This should only be considered after a
thorough analysis of the problem has been made.

Troubleshooting should begin with a review of the start-up checklist. Normally, the cause of the problem will be
within the scope of that list.

The following list is provided as a supplement to the start-up checklist. It covers a wider range of symptoms and
their causes.

SYMPTOM CAUSE ACTION

Fan stopped Overload Switch Electrical supply/rating
Drive sheaves reversed
Improperly adjusted sheaves
Improperly adjusted dampers
System differential pressure low
Discharge equipment failure
Poor hood control Fan Malfunction Sheaves reversed
Belts slipping
Defective fan wheels
Incorrect sizing – fan
Duct System Incorrect sizing – ducts
Incorrect damper position setting
Incorrect blast gate setting
Pressure Differential High See excessive pressure differential
Excessive pressure differential Cleaning System Gauge lines plugged
Insufficient cleaning air
Timer – fuse/board malfunction
Valve malfunction
Solenoid malfunction
Plugged Media Free moisture in air stream
Liquid hydrocarbons
Overloading
Hygroscopic dust
Hopper bridging
Cleaning system malfunction
Compressed air constantly Stuck Valve Check for solenoid valve malfunctions (turn
running power off and on again to see if solenoid
resets).
Check for diaphragm valve failure.
Undersized Air Compressor Check with compressor manufacturer

-119-
Items to consider in a dust collection project:

x Dust Characteristics
x Dust Collector Location
x Ductwork Layout
x System Pressure Drop Requirements
x Dust Removal from Hopper Discharge
x Electrical Requirements
x Compressed Air Requirements
x Foundation Requirements
x Ongoing Maintenance Considerations

DISCLAIMER: This booklet was prepared as an overview of industrial dust

collection equipment. The information provided here is for reference only.
Scientific Dust Collectors, or its affiliates, make no warranties, express or
implied, concerning the application or use of this information.

For assistance in solving dust problems and in equipment selection,

contact Scientific Dust Collectors at:

Phone: 708-597-7090
Fax: 708-597-0313
www.scientificdustcollectors.com

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©2014,2002, Scientific Dust Collectors
A Venturedyne Ltd. Company

Scientific Dust Collectors

4101 W. 126th Street
Alsip, IL 60803
P 708.597.7090
F 708.597.0313
www.scientificdustcollectors.com
Bulletin#202