Working of Bag Type Fabric Filters

In pneumatic conveying systems handling fine or dusty material, the method of filtration that
has become almost universally adopted is a bag type fabric filter. These filters are commonly
called baghouses. Most baghouses use long, cylindrical bags (or tubes) made of woven or
felted fabric as a filter medium.

Dust laden gas or air enters the baghouse through hoppers by suction (normally) or positive
pressure and is directed into the baghouse compartment. The heavier dust particles fall off
at the entry itself, while the lighter dust particles along with gas get carried upward to the
bags. The gas is drawn through the bags, either on the inside or the outside depending on
cleaning method, and dust accumulates on the filter media which increases the resistance to
gas flow. Due to this, the filter must be cleaned periodically when sufficient pressure drop
occurs.

During cleaning, dust that has accumulated on the bags is removed from the fabric surface
and deposited in the hopper for subsequent disposal. Depending on the type/construction of
baghouse, cleaning can be carried out while the baghouse is on-line (filtering) or is off-line
(in isolation).

If gas enters into the baghouse tangentially at the bottom of the casing, it gives the dust
laden gas a circular motion which helps in removing the heavy and coarser particles that are
present in the gas stream in a manner similar to a cyclonic collector. These collected
particles are directly discharged into the hopper. It is only the very fine particles that get
carried to and collected by the bags. Thus the total dust load on bags is reduced.

Baghouses are very efficient particulate collectors. They collect particles with sizes ranging
from submicron to several hundred microns in diameter at efficiency of 99 percent or better.
The layer of dust, called dust cake or cake, collected on the fabric is primarily responsible for
such high efficiency. The cake is a barrier with tortuous pores that trap particles as they
travel through the cake.

Typically, inlet concentrations of pollutant to baghouses are 1 to 23 grams per cubic meter
(g/m3) [0.5 to 10 grains per cubic foot (gr/ft3)], but in extreme cases, inlet conditions may vary
between 0.1 to more than 230 g/m3(0.05 to more than 100 gr/ft3).

Standard fabric filters can be used in pressure or vacuum service, but only within the range
of about ± 640 millimeters of water column (25 inches of water column). Well-designed and
operated baghouses have been shown to be capable of reducing overall particulate
emissions to less than 0.05 g/m3(0.020 gr/ft3), and in a number of cases, to as low as 0.002
to 0.011 g/m3(0.001 to 0.005 gr/ft3).

Differential Pressure (ΔP)