Baghouse Selection
TYPES OF BAGHOUSES, FILTER BAGS, HOOD DESIGN
you want. There are three basic types
that are widely used in the grain and SECOND IN A SERIES
feed industries, and each are very simi-
lar in the way they filter dust-laden
air. Air-to-Cloth Ratio
1. Pulse jet. This system works on In the grain and feed industries, an
compressed air, usually in the range of air-to-cloth ratio of 10:1 is considered
90 to 120 psi. the maximum desirable in a baghouse
2. Medium pressure. This system system.
makes use of a positive displacement To calculate the air-to-cloth ratio,
pump, usually rated at 10 to 15 psi. divide the system cfm by the square
3. Low pressure. This type of sys- feet of the filtering media inside the
tem uses a backwardly inclined cen- baghouse filter.
trifugal fan, usually generating pres- The air-to-cloth ratio is expressed
sure equivalent to 3 to 6 inches WC. as a velocity in fpm. It is the average
In all three types of systems, dirty velocity of air moving through the
air enters the baghouse in the lower surface of the filter bags. An air-to-
part of the structure. The air then is cloth ratio higher than 10:1 will cause
filtered as it goes to the inside of the a higher pressure drop across the bags,
filter bags, leaving the dust on the making it harder to remove the dust
outside. Clean air then is discharged that is clinging to the outside of the
out of the top of the unit, and the dust bags.
falls to the bottom of a hopper and is To determine interstitial velocity,
discharged, generally through an air take the filter’s horizontal cross-sec-
lock. tional area in square feet, then sub-
Inevitably, some of the dust will tract the total cross-sectional area of
cling to the outside of the bag and all the filter bags, also in square feet.
will need to be removed periodically. Then divide the result into the system
The three types of baghouses de- cfm.
scribed above are designed so that For most applications, an intersti-
filtration and bag cleaning can go on tial velocity of 500 fpm or less is a
simultaneously and so that bags last good target.
a long time before they need to be
changed. Filter Bags
Baghouse system cross-section showing loca-
tion of a typical bag filter. Another inevitable phenomenon is Filter bags are called a number of
“re-entrainment,” which is dust mov- names in the grain industry, includ-
ing from one bag to a nearby bag ing bags, filters, socks, or sleeves.
This series is based on a pre-confer- during the bag-cleaning process. While The material for filter bags most
ence workshop held prior to the start of this almost always happens, an exces- commonly used in the grain and feed
GEAPS Exchange 2001 in March in sive amount of re-entrainment can industries is 12- or 16-oz. polyester
Phoenix, AZ. This part of the series is cause premature bag blinding and felt cloth. The outer surface of the
based on remarks by Delmar Mains, high pressure drops across the bags. bags usually is singed.
dust control projects engineer for The Factors affecting the amount of re- When a filter bag goes into service,
Boone Group, Boone, IA (800-263- entrainment include: not all of the dust will be cleaned from
2010/www.boonegroup.com). • High air-to-cloth ratios. the bag. A certain amount of the dust
• Low-density dust. will implant itself between the fibers
A major consideration in selecting • Closely spaced bags. of the bags, forming a “cake.” While
the right baghouse for your operation • Extra-long bags. an excessive amount of caking is bad,
is the type of bag cleaning method • High interstitial velocities. you actually want a certain amount of
Reprinted from May/June 2001 GRAIN JOURNAL
it in order for the system to operate Two factors that affect hood perfor- ity for shelled corn is higher than for
efficiently. mance are face velocity and capture bees wings.
For a new bag, the recorded pres- velocity. • Shelled corn 3 inches from the
sure drop will be 1 to 2 inches WC. Face velocity is the cfm for the hood face will have a lower capture
Pressure drop will rise over time as hood, divided by the cross sectional velocity than bees wings that are 20
the filter bag is in use. When it reaches area of the hood at the point where it feet from the hood face.
4 to 6 inches WC, the bag will need to mounts onto the equipment. • Shelled corn moving toward the
be either laundered or replaced. For example, consider a hood mea- hood will have a lower capture veloc-
Proper system startup and shut- suring 18-inches-x-18-inches at the ity than shelled corn that is station-
down can extend bag life. Manufac- point where it mounts on a drag ary.
turers often recommend that the bag- conveyor. To calculate face velocity: • Consider the situation of laminar
cleaning mechanism be allowed to flow hoods mounted on the up side of
continue operating for 10 minutes to (1,750 cfm) x (144 sq. in.) a bucket elevator in the first section
one hour after the main system fan has (18 inches) x (18 inches) x (1 sq. ft.) above the boot assembly. Because of
been shut off for the day. The amount what is happening inside the bucket
of time depends on the severity of dust Face velocity = 778 fpm. elevator at the point where the hood is
loading. Capture velocity is the velocity in mounted, the capture velocity for this
Bags with too much caking are said fpm required to pick up the product. application will be significantly lower
to be “blinded.” That means that Capture velocity varies according to than for the same product in a drag
sufficient air will no longer pass the density of the product, the prox- conveyor where the hood is mounted
through the bags to provide adequate imity of the product to the dust hood, on the top cover.
aspiration to the system. and the movement of the product in In the next issue of Grain Journal,
relationship to the airflow at the hood we’ll cover duct design considerations,
Hood Design face. airlocks, and pneumatic conveying sys-
The design of the hood is another For example: tems.
item to consider in selecting a • Shelled corn has a higher density
baghouse system for your facility. than bees wings, so the capture veloc- Ed Zdrojewski, editor
