What is industrial ventilation?

1. Ventilation is the mechanical system in a building that brings in

"fresh" outdoor air and removes the "contaminated" indoor air.
2. In a workplace, ventilation is used to control exposure to
airborne contaminants. It is commonly used to remove
contaminants such as fumes, dusts, and vapours, in order to
provide a healthy and safe working environment.
3. Ventilation can be accomplished by natural means (e.g.,
opening a window) or mechanical means (e.g., fans or
blowers).
4. Industrial systems are designed to move a specific amount of
air at a specific speed (velocity), which results in the removal
(or "exhaust") of undesirable contaminants. While all ventilation
systems follow the same basic principles, each system is
designed specifically to match to the type of work and the rate
of contaminant release at that workplace.
What is the purpose of a ventilation
system?
There are four purposes of ventilation:
1. Provide a continuous supply of fresh
outside air.
2. Maintain temperature and humidity at
comfortable levels.
3. Reduce potential fire or explosion
hazards.
4. Remove or dilute airborne contaminants.
Why have an industrial ventilation system?
1. Ventilation is considered an "engineering
control" to remove or control contaminants
released in indoor work environments.
2. It is one of the preferred ways to control
employee exposure to air contaminants.
3. Other ways to control contaminants include:
a. Eliminate the use of the hazardous
chemical or material,
b. Substitute with less toxic chemicals,
c. Process change, or
d. Work practice change.
Pressure In A Ventilation System

1. Air movement in the ventilation system is a

result of differences in pressure.
2. In a supply system, the pressure created by the
system is in addition to the atmospheric
pressure in the work place.
3. In an exhaust system, the objective is to lower
the pressure in the system below the
atmospheric pressure.
Types Of Pressures In A Ventilation Systems

Three types of pressures are of

importance in ventilation work. They are:

1. Static pressure
2. Velocity pressure
3. Total pressure
 Velocity Pressure

1. It is defined as that pressure required to accelerate

air from rest to some velocity (V) and is
proportional to the kinetic energy of the air stream.
2. VP acts in the direction of flow and is measured in
the direction of flow.
3. VP represents kinetic energy within a system.
4. VP is always positive.
 Static Pressure

1. It is defined as the pressure in the duct that tends

to burst or collapse the duct and is expressed in
millimeter of water gauge (mmwg).
2. Static Pressure acts equally in all directions.
3. Static Pressure can be negative or positive
Static pressure can be positive or negative

Positive static pressure results in the tendency of the air to

expand. Negative static pressure results in the tendency of
the air to contract.
For example, take a common soda straw, and put it in your
mouth. Close one end with your finger and blow very hard.
You have created a positive static pressure. However, as
soon as you remove your finger from the end of the straw, the
air begins to move outward away from the straw. The static
pressure has been transformed into velocity pressure, which
is positive.
 Total Pressure

TP = SP + VP
1. It can be defined as the algebraic sum of the static
as well as the velocity pressures
2. Static Pressure (SP) represents the pressure energy
of a system and velocity Pressure(VP) the kinetic
energy of the system, the sum of which gives the
total energy of the system
3. Total Pressure(TP) is measured in the direction of
flow and can be positive or negative
How do you measure the Pressures in a
ventilation system?
The manometer, which is a simple graduated
U-shaped tube open, at both ends, an inclined
manometer or a Pitot tube can be used to
measure Static pressure.
The impact tube can be used to measure Total
pressure.
The measurement of Static and Total
pressures using manometer and impact tube,
will also indirectly result in measurement of the
Velocity pressure of the system.
What are the parts of an industrial
ventilation system?
Systems are composed of many parts
including:
1. An "air intake" area such as a hood or an
enclosure,
2. Ducts to move air from one area to
another,
3. Air cleaning device(s), and
4. Fan(s) to bring in outside air and exhaust
the indoor contaminated air.
What are the basic types of ventilation
systems?
There are two types of mechanical ventilation
systems used in industrial settings:
Dilution (or general) ventilation reduces the
concentration of the contaminant by mixing
the contaminated air with clean,
uncontaminated air.
Local exhaust ventilation captures
contaminates at or very near the source and
exhausts them outside.
What are main features of dilution ventilation?
1. Dilution, or "general", ventilation supplies and exhausts
large amounts of air to and from an area or building. It
usually involves large exhaust fans placed in the walls or
roof of a room or building.
2. Dilution ventilation controls pollutants generated at a
worksite by ventilating the entire workplace. The use of
general ventilation distributes pollutants, to some degree,
throughout the entire worksite and could therefore affect
persons who are far from the source of contamination.
3. Dilution ventilation can be made more effective if the
exhaust fan is located close to exposed workers and the
makeup air is located behind the worker so that
contaminated air is drawn away from the worker's
breathing zone.
When used to control chemical pollutants,
dilution must be limited to only situations
where:
1. The amounts of pollutants generated are not very
high,
2. Their toxicity is relatively moderate, and
3. Workers do not carry out their tasks in the
immediate vicinity of the source of contamination.
4. It is therefore unusual to recommend the use of
general ventilation for the control of chemical
substances except in the case of solvents which
have admissible concentrations of more than
100ppm.
What are the limitations of dilution ventilation?
1. As a method for protecting workers, it is important to know that
dilution ventilation:
2. Does not completely remove contaminants.
3. Cannot be used for highly toxic chemicals.
4. Is not effective for dusts or metal fumes or large amounts of gases or
vapour.
5. Requires large amounts of makeup air to be heated or cooled.
6. Is not effective for handling surges of gases or vapour or irregular
emissions.
7. Regular "floor" or "desk" fans are also sometimes used as a method
of ventilation, but these fans typically blow the contaminant around the
work area without effectively controlling it. Opening doors or windows
can be used as dilution ventilation, but again, this method is not
reliable since air movement is not controlled.
8. As a general note, the air or "volumetric" flow rate of dilution
ventilation depends largely on the how fast the contaminant enters the
air as well as the efficiency that fresh air mixes with workroom air.
What is local exhaust ventilation?
Local exhaust system is used to control air
contaminants by trapping them at or near the
source, in contrast to dilution ventilation which lets
the contaminant spread throughout the workplace.
Local exhaust is generally a far more effective way
of controlling highly toxic contaminants before they
reach the workers' breathing zones.
This type of system is usually the preferred
control method if:
1. Air contaminants pose serious health risk.
2. Large amounts of dusts or fumes are generated.
3. Increased heating costs from ventilation in cold weather
are a concern.
4. Emission sources are few in number.
5. Emission sources are near the workers' breathing zones.
6. In a general way, a local exhaust system operates similar
to a household vacuum cleaner with the hose as close as
possible to the place where dirt would be created.
What are the components of local exhaust
ventilation?
A local exhaust system has six basic elements:
1. A "hood" or opening that captures the
contaminant at the source.
2. Ducts that transport the airborne chemicals
through the system.
3. An air cleaning device that removes the
contaminant from the moving air in the system
(not always required).
4. Fans that move the air through the system and
discharges the exhaust air outdoors.
5. An exhaust stack through which the
contaminated air is discharged.
6. Make up air that replaces the exhausted air.
How do I know which type of ventilation
system is best for my workplace?
All industrial ventilation systems, when
designed properly, should be able to provide
long-term worker protection. The two types of
ventilation, dilution and local exhaust, are
compared in the following table.
 Comparison of Ventilation Systems
Dilution Ventilation Local Exhaust Ventilation
Advantages Disadvantages Advantages Disadvantages
Usually lower Does not completely Captures contaminant at Higher cost for
equipment and remove contaminants . source and removes it design, installation
installation costs . from the workplace. and equipment.
Requires less Cannot be used for Only choice for highly Requires regular
maintenance. highly toxic chemicals. toxic airborne chemicals. cleaning,
inspection and
maintenance.
Effective control for Ineffective for dusts or Can handle many types of
small amounts of metal fumes or large contaminants including
low toxicity amounts of gases or dusts and metal fumes.
chemicals. vapours.
Effective control for Requires large Requires smaller amount
flammable or amounts of heated or of makeup air since
combustible gases cooled makeup air. smaller amounts of air are
or vapours. being exhausted .
Best ventilation for Ineffective for handling Less energy costs since
mobile or dispersed surges of gases or there is less makeup air to
contaminant vapours or irregular heat or cool .
sources . emissions .
In general, what are limitations of any
ventilation system?
Some limitations include:
1. The systems deteriorate over the years because
of to contaminant build-up within the system,
especially filters.
2. Require ongoing maintenance.
3. Regular and routine testing is needed to identify
problems early and implement corrective
measures.
4. Only qualified persons should make modifications
to a ventilation system to make sure the system
continues to work effectively.
5. The following is an example of changes that can
affect how a system works.
What should I know about make-up air?
1. An important and sometimes overlooked aspect of local ventilation is
the need to provide enough air to replace the air that is exhausted from
the workplace. If enough make-up air is not provided when large
volumes of air are exhausted, the workplace becomes "starved" for air
and negative pressure is created.
2. Negative pressure in the workplace increases resistance on the
ventilation system causing it to move less air.
3. Air will also enter a building through cracks around doors or windows
or other small openings to try to "equal" the rate of air being removed.
4. The result is that workers may be exposed to cold air in the winter, and
additional heating costs may occur. One simple way to judge if a
building is under an excessive negative pressure is if you have
difficulty opening a door that pushed into the room or building (the air
wants to force the door closed).
5. A separate intake fan, located away from the exhaust fans, should be
used to bring in fresh, uncontaminated air from outside. This air must
be clean and heated in winter or cooled in summer, as needed.
What is a duct system?
The ventilation system in a building
consists of air moving devices such as
fans and blowers and a network of ducts
to exhaust the contaminated indoor air
and to bring in air from the outside of the
building.
What are some basic principles of duct
design?
1. Duct systems should be designed to have air flow
through the ducts with as little friction or resistance
as possible.
2. The amount of air that flows through a duct depends
on the cross section area (duct opening area) of the
duct and the air speed. Air moving too slowly will
allow contaminants such as dusts to settle and
accumulate and these particles will eventually clog
the duct.
3. Air moving too fast wastes power, can create noise
problems, and may cause excessive abrasion by
dust particles hitting the ducts.
What is a hood?
A hood - correctly called a local exhaust
hood - is the point where contaminated air is
drawn into the ventilation system. The sizes
and shapes of hoods are designed for
specific tasks or situations. The air speed
(velocity) at the hood opening and inside the
hood must be enough to catch or capture
and carry the air contaminants. To be most
effective, the hood should surround or
enclose the source of contaminant or be
placed as close to the source as possible.
What are the common types of hood?
The three common classes of hoods are:

1. Enclosing

2. Receiving

3. Capturing.
Enclosing Hood

Enclosing hoods, or "fume" hoods, are hoods

surrounding the process or point where the
contaminants are generated. Examples of
completely enclosed hoods (all sides enclosed)
are glove boxes and grinder hoods. Examples of
partially enclosed (two or three sides enclosed)
hoods are laboratory hoods or paint spray
booths. The enclosing hood is preferred
whenever possible.
Enclosing Hood
Receiving Hood

These hoods are designed to "receive" or

catch the emissions from a source that has
some initial velocity or movement. For
example, a type of receiving hood called a
canopy hood receives hot rising air and gases.
An example is a canopy hood located over a
melting furnace.
Receiving Hood
Capturing Hood

These hoods are located next to an

emission source without surrounding
(enclosing) it. Examples are a rectangular
hood along the edge of a tank or a hood on
a welding or grinding bench table or a
downdraft hood for hand grinding bench.
Capturing Hood
What is meant by "capture velocity"?
1. The ventilation system removes contaminants by
"pulling" the air (and the contaminant) into the exhaust
hood and away from the worker or the source.
2. Airflow toward the hood opening must be fast or high
enough to "catch and transport" the contaminant until it
reaches the hood and ducts. The required air speed is
called the "capture velocity".
3. Any air motion outside of the hood and surrounding
area may affect how the air flows into the hood.
4. The ventilation system will require a higher airflow
speed to overcome air disturbances.
5. As much as possible, the other sources of air motion
should be minimized or eliminated effectively
Common sources of external air movement
include:
1. Thermal air currents, especially from hot processes or
heat-generating operations, motion of machinery such as
by a grinding wheel, belt conveyor, etc.
2. Material motion such as dumping or filling,
3. Movements of the operator,
4. Room air currents (which are usually taken at 50 fpm
(feet per minute) minimum and may be much higher),
5. Rapid air movement caused by spot cooling and heating
equipment.
6. Most of the capture velocities are around 100 feet per
minute (fpm). How fast is 100 fpm? Blowing lightly on
your hand so that you can just barely feel air movement is
about 100 fpm. It is easy to see how it will take very little
air movement from other sources to affect how well a
hood can capture contaminants.
What are general rules for hood design?
1. The shape of the hood and its size, location, and rate of airflow each
play an important role in design considerations. Each type of hood also
has specific design requirements, but several general principles apply to
all hoods:
2. The hood should be placed as close as possible to the source of
contamination, preferably enclosing it. The more completely enclosed
the source is, the less air will be required for control. The required
volume varies with the square of the distance from the source.
3. The air should travel from source of the contaminant and into the hood
with enough velocity (speed) to adequately the contaminant.
4. The hood should be located in a way that the operator is never
between the contaminant source and the hood.
5. The natural movement of contaminants should be taken into
consideration. For example, a hood should be placed above hot
processes to trap rising gases and heat. A grinding wheel or
woodworking machine should be equipped with a partial enclosure to
trap the flying particles where they spin off.
6. The flanges or baffles should be used around the hood opening to
increase the capture effectiveness and reduce ventilation air
requirements.
Air Cleaning
Devices
What are air-cleaning devices?

In a ventilation system, an air-cleaning device removes or

captures the contaminants that are present in the air.

The type of air cleaner used will depend on:

1. Type of air contaminant to be removed
2. Concentration of the contaminant in the air
3. How much contaminant must be removed to meet
any regulations or standards
4. Type and concentration of toxic chemical
contaminants,
5. Type and size of dust particles,
6. Temperature, humidity, etc.
7. fire safety and explosion control, and air pollution
control regulations.
What are air-cleaning devices for gases and vapours?

Gases and vapours can be removed by using the following processes:

1. Adsorption
The removal of a contaminant by contact with other materials such as
activated alumina, activated charcoal and silica gel (referred to as adsorbers).
2. Absorption
Absorbers remove soluble or chemically reactive gases from the gas stream
by close contact with an appropriate liquid so that one or more of the air
contaminants will dissolve in the liquid.
3. Catalytic conversion
In this process, a catalyst converts a contaminant to a chemical form not
considered to be hazardous. Catalysts are substances that alter the rate of a
chemical reaction without being affected by the chemical reaction.
4. Thermal Oxidation (Combustion)
The combustion process (also called incineration) converts volatile organic
compounds (VOCs) to carbon dioxide and water vapour by burning them. It is
a very effective means of eliminating VOCs. Typical applications for
incineration devices include odour control, reduction in reactive hydrocarbon
emissions, and reduction of explosion hazards.
What should be considered when selecting an air
cleaning device?
Following are some tips for selecting an air-cleaning device in your
workplace. Remember that a qualified professional should make final
decisions regarding the suitability of an air-cleaning device.

1. Before the air cleaning device is selected, it is very important to know

maintenance and access requirements, the physical size of the equipment
and how it will be installed in the plant as well as the methods of removing
the collected contaminants.
2. The air cleaner must be reliable. Many installations require monitoring or
proof of continual operations by measuring conditions in the system.
3. Maintenance and operating costs must be considered. The air cleaner
must operate in stable conditions as well as variations such as plant start-
up and shut down. Considerations also include if it must be accessible for
maintenance or if the air cleaner must continue to operate while
maintenance or repairs are being done.
4. The device must meet local and national regulations (at start-up and over
time)