Page 91 - Air Pollution Control Engineering

02_chap_wang.qxd 05/05/2004 12:40 pm Page 71
Fabric Filtration 71

Table 5
Factors to Obtain Gross Cloth Area from Net Cloth Area
2
2
Net cloth area, A (ft ) Factor to obtain gross cloth area, A (ft )
nc tc
1–4,000 Multiply by 2
4,001–12,000 Multiply by 1.5
12,001–24,000 Multiply by 1.25
24,001–36,000 Multiply by 1.17
36,001–48,000 Multiply by 1.125
48,001–60,000 Multiply by 1.11
60,001–72,000 Multiply by 1.10
72,001–84,000 Multiply by 1.09
84,001–96,000 Multiply by 1.08
96,001–108,000 Multiply by 1.07
108,001–132,000 Multiply by 1.06
132,001–180,000 Multiply by 1.05
Source: ref. 8.

Computer software provides rigorous design. However, the purpose of this section is to
provide the reader with some qualitative insight concerning the design and operation of
fabric filters. Therefore, these programs are not discussed.
In addition to evaluating a particular fabric filter application, the A/C ratio and the
emission stream flow rate (Q ) are used to calculate net cloth area (A ):
e,a nc
Q
,
ea = A (8)
AC ratio nc
where Q is the emission stream flow rate at actual conditions (acfm), A/C ratio is the
e,a
2
2
air-to-cloth ratio, (acfm/ft or ft/min) (from Table 4), and A is the net cloth area (ft ).
nc
The net cloth area is the cloth area in active use at any point in time. Gross or total
cloth area (A ), by comparison, is the total cloth area contained in a fabric filter, including
tc
that which is out of service at any point in time for cleaning or maintenance. In this text,
costing of the fabric-filter structure and fabric filter bags uses gross cloth area. Table 5
presents factors to obtain gross cloth area from net cloth area:
A × Factor = A (9)
nc tc
2
where Factor is the value from Table 5 (dimensionless) and, A is the gross cloth area (ft ).
tc
Fabric filters with higher A/C ratios require fewer bags and less space, and may be less
expensive. However, the costs of more expensive (felted) bags, bag framework structure,
increased power requirements, etc., may reduce the savings of high-A/C-ratio systems.
4.3. Fabric Cleaning Design
One removes the cake from the fabric by mechanically disturbing the system. This
can be done by physically scraping the fabric, mechanically shaking it, or pneumatically
or hydraulically reversing the flow of fluid through the fabric to clean the pores. For gas
cleaning systems, the common cleaning methods include mechanical shaking, pulse
cleaning, and reverse flow.