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Fabric Filtration 73

Table 6
Comparison of Fabric-Filter-Bag Cleaning Methods
Pulse-jet
Mechanical Reverse Pulse-jet compartmented
Parameter shake airflow individual bags bags
Cleaning on-line
or off-line Off-line Off-line On-line Off-line
Cleaning time High High Low Low
Cleaning uniformity Average Good Average Good
Bag attrition Average Low Average Low
Equipment ruggedness Average Good Good Good
Fabric type a Woven Woven Felt/woven a Felt/woven a
Filter velocity Average Average High High
Power cost Low Low to medium High to medium Medium
Dust loading Average Average Very high High
Maximum temperature b High High Medium Medium
Collection efficiency Good Good Good c Good c
a With suitable backing, woven fabrics can perform similarly to felted.
b Fabric limited.
c For a properly operated system with moderate to low pressures, the collection efficiency may rival
other methods.
Source: US EPA.

vary greatly from those reported. Fabric-filter size and cost will vary with A/C ratio.
Lower A/C ratios, for example, require a larger and thus more expensive fabric filter.

4.4. Baghouse Configuration
Baghouses have two basic configurations, with gases either pushed through the system
by a fan located on the upstream side (forced draft fan) or pulled through by a fan on
the downstream side (induced draft fan). The former is called a positive-pressure bag-
house; the latter, is called a negative-pressure or suction baghouse. Positive-pressure
baghouses may be either open to the atmosphere or closed (sealed and pressure-isolated
from the atmosphere). Negative-pressure baghouses can only be of the closed type.
Only the closed suction design should be selected for a hazardous air pollutant applica-
tion to prevent accidental release of captured pollutants. At temperatures near the gas
stream dew point, greater care must be taken to prevent condensation, which can moisten
the filter cake, plug the cloth, and promote corrosion of the housing and hoppers. In a
suction-type fabric filter, infiltration of ambient air can occur, lowering the temperature
below design levels (8).
4.5. Construction Materials
The most common material used in fabric-filter construction is carbon steel. In cases
where the gas stream contains high concentrations of SO or where liquid–gas contact
3
areas are involved, stainless steel may be required. Stainless steel will increase the cost
of the fabric filter significantly when compared to carbon steel. However, keeping the