Page 97 - Air Pollution Control Engineering

02_chap_wang.qxd 05/05/2004 12:40 pm Page 77
Fabric Filtration 77

6. Are the values provided for the gas flow rate, A/C ratio, and net flow area consistent?
The values can be checked with the following equation:

Q
AC ratio = ea (8)
,
A nc
where the variables are as described earlier.
7. Is the baghouse configuration appropriate; that is, is it a negative-pressure baghouse?

6.2. Economics
Fabric filtration systems are attractive in that they are highly efficient collection
devices that can be operated at low-energy requirements. In addition, they usually have
no water requirements so that the solid-waste-disposal problem may be significantly
less than that for wet systems. On the other hand, fabric filtration systems are expensive
2
2
in that they require a large amount of space for installation [about 1 ft (0.1 m ) of floor
3
3
space per each 5 ft /min (0.14 m /min)] and have a large capital investment.
The highest maintenance component of fabric-filter systems is the fabric itself. In
baghouses, the bags have an average life of 18–36 mo and account for 20–40% of the
equipment cost. If the system is expected to have a 10-yr life, this means that the bags
must be replaced anywhere from three to seven times during this lifetime. Causes of bag
failure include blinding (mudding), caking, burning, abrasion, chemical attack, and
aging. Prior discussion in this chapter indicated how these problems can be reduced by
proper operating and maintenance procedures.
The Industrial Gas Cleaning Institute (IGCI), representing about 90% of all fabric-filter
gas cleaning device manufacturers, estimated that about half of the filter systems in the
United States are low energy and half are high energy.
This chapter mentions factors affecting the economics of filter systems. These factors
include the composition of both the solids and the gas, the type of filter system desired,
requirements for gas conditioning, and proper operating and maintenance procedures.
Other factors that also influence the cost of fabric filtration systems are, for example, spe-
cial properties of the gas stream (toxic, explosive, corrosive, and/or abrasive), space
restrictions in the installing facility, and the nature of ancillary equipment, such as hoods,
ducts, fans, motors, material-handling conveyors, airlocks, stacks, controls, and valves.
These costs (Tables 8–10) are averages of all industries, and actual operating and rela-
tive costs would depend on the specific application. Abrasive, corrosive, hot applications
may have greater total costs plus proportionally greater replacement and labor costs.
Equipment costs for a fabric-filter system can be estimated by either obtaining quota-
tions from vendors, or using generalized cost correlations from the literature. Total capital
costs (see Table 9) include costs for the baghouse structure, the initial complement of
the bags, auxiliary equipment, and the usual direct and indirect costs associated with
installing or erecting new structures. The price per square foot of bags by type of fab-
ric and cleaning system appears in Table 8 (3rd quarter 1986 dollars). The prices repre-
sent a 10 % range and should be escalated using the index provided in Chemical
Engineering (27). The annual costs (see Table 11) for a fabric-filter system consist of the
direct and indirect operating costs. Direct costs include utilities (electricity, replacement