842 PARTICULATE REMOVAL
The dust loading per se does not affect the efficiency
significantly; however in the case of very low dust loadings
(0.5 gr/ft^3 ), a low efficiency will be observed for a time
because of the low dust buildup on the fabric. To cirumvent
this problem, a precoat is often used.
The relative humidity of the gas affects the efficiency
somewhat, depending on the material being filtered. For
some particles, an increase in relative humidity results in
increased particle adhesion force, which tends to increase the
rate at which the openings in the filter medium are bridged,
resulting in an increase in the collection efficiency. The fil-
tering velocity does not appear to affect efficiency signifi-
cantly, except that higher velocities would tend to force more
dust through defects in the cloth.
The pressure drop of the gas through the filter is a func-
tion mainly of the gas velocity, the thickness of the dust
cake, and of the particle characteristics which determine the
porosity of the cake which has built up. There have been
a number of studies on pressure drop through fabric filters
and the reader is referred to Billings and Wilder^37 for a more
complete treatment of this subject.Economics of Fabric FiltrationA detailed survey of fabric filtration costs was done by
Billings and Wilder.^37 There was a relatively large range of
reported costs. Capital investment was found to vary from less
than $l per cfm to greater than $10 per cfm. operating costs
varied from about $17 per cfm per year Values of about $2.50/
cfm capital and $1.10/cfm annual operating can be consideredtypical. These costs are broken down into component parts
into Table 3. It is apparent from an examination of the items
contribution to the total economic picture that the costs for an
installation will depend a great deal on many factors specific
to the installation and gas being handled.GUARANTEE SPECIFICATIONSEquipment vendors usually make a guarantee of overall
particle collection efficiency as a part of bid proposals, but
historically such guarantees have been more an indication
of anticipated performance than an enforced or enforceable
contract for several reasons. The isokinetic sampling always
necessary for a proper efficiency test is difficult and time
consuming and in some cases, e.g. for internal cyclones
above fluid beds, it is a practical impossibility. Secondly, in
the days of less stringent emission statutes the mere presence
of control equipment often served to satisfy the spirit if not
the letter of the law. Most guarantees in the past were written
for a fixed set of operating conditions and were not legally
enforceable at the deviant conditions that usually obtained
during a test. Finally a legal recourse to poor performance
is seldom spelled out in a contract. In the present climate of
stringent emission limits and enforcement thereof, of sub-
stantial penalties for non-compliance, and of long equipment
delivery times, purchasers are obliged to become more insis-
tent that equipment perform according to a strict guarantee.
Accordingly, the following suggestions are made for the
writing of a performance specification.
A guarantee can only deal with the cleaning efficiency of
a piece of equipment on the dusty gas stream actually enter-
ing it. Vendors quite understandably will refuse to guaran-
tee an outlet loading, a Ringlemann number, or a dustfall,
because these depend directly on the amount and particle
size of dust entering the equipment as well as on the inherent
capabilities of that equipment. In addition, optical properties
of plumes are too little understood for generally acceptable
prediction of Ringlemann numbers, even when the concen-
tration and particle size of the emissions are known.
Performance guarantees are often written for a single set
of design conditions without any provision for adjustment
if, as usually happens, the design and actual test conditions
are different. In such a situation the likelihood of proving
and receiving compensation for substandard performance is
small. It is therefore desirable that a range of operating con-
ditions be specified and a procedure be defined for adjusting
the guarantee performance at design conditions to a consis-
tent performance at other conditions. The adjustment should
be based on generalized efficiency relationships, e.g. Eq. (1)
for cyclones and (4) for precipitators, or else on vendors
design procedures. For instance Eq. (1) predicts that for a
given cyclone, the response to changes in operating condi-
tions is given byd(^50) U
∝
m
r
.
TABLE 3
Breakdown of capital and operating costs for fabric filtration
A. Capital investment
Item Percentage of total cost
Planning and design 4.2
Baghouse, FOB 33.6
Freight 2.0
Fan and motor 10.5
Ducting 27.5
Dust disposal 4.2
Instrumentation 2.0
Installation labor 11.8
Start-up 4.2
B. Operating cost
Item Percentage of total cost
Electric power 11.4
Labor 28.6
Plant overhead 23.8
Cloth 9.5
Depreciation 15.2
Interest 11.5
C016_002_r03.indd 842C016_002_r03.indd 842 11/18/2005 1:06:47 PM11/18/2005 1:06:47 P