Page 95 - Air Pollution Control Engineering

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Fabric Filtration 75

5.3. System Pressure Drop
The pressure drop across the fabric-filter system depends on the resistance to the gas
stream flow through the filter bags and accumulating dust cake, amount of dust deposit
prior to bag cleaning, efficiency of cleaning, and plugging or blinding of the filter bags.
Normally, the design pressure drop is set between 5 and 20 in. of water. In practice,
variations in pressure drop outside the design range may indicate problems within the
fabric-filter system. Excessive pressure differentials may indicate (1) an increase in gas
stream volume, (2) blinding of the filter fabric, (3) hoppers full of dust, thus blocking
the bags, and/or (4) inoperative cleaning mechanism. Subpar pressure differentials may
indicate (1) fan or motor problems, (2) broken or unclamped bags, (3) plugged inlet
ducting or closed damper; and/or (d) leakage between sections of the baghouse. For
these reasons, continuous pressure-drop monitoring is recommended.
As the dust cake builds up during filtration, both the collection efficiency and system
pressure drop increase. As the pressure drop increases toward a maximum, the filter bags
(or at least a group of the bags contained in one isolated compartment) must be cleaned
to reduce the dust cake resistance. This cleaning must be timed and performed to (1) main-
tain the pressure drop and thus operating costs within reasonable limits, (2) clean bags as
gently and/or infrequently as possible to minimize bag wear and to maximize efficiency,
and (3) leave a sufficient dust layer on the bags to maintain filter efficiency and to keep
the instantaneous A/C ratio immediately after cleaning from reaching excessive levels, if
woven fabric with no backing is used. In practice, these various considerations are bal-
anced using engineering judgment and field trial experience to optimize the total system
operation. Changes in the process or in fabric condition through fabric aging will shift in
the cleaning requirements of the system. This shift may require more frequent manual
adjustments to the automatic control to achieve the minimum cleaning requirements.
5.4. Power Requirements
The cost of electricity depends largely on the fan power requirement. Equation (10) can
estimate this requirement, assuming a 65 % fan motor efficiency and a fluid specific grav-
ity of 1.00:

×
.
F = 181 10 −4 Q ( e a P )( )(HRS ) (10)
,
p
where F is the fan power requirement (kWh/yr), Q is the emission stream flow rate (acfm),
p e,a
P is the system pressure drop (in. H O), and HRS is the operating hours (h/yr). For mechani-
2
cal shaking, Eq. (11) provides an estimate of the additional power:
P ms = 605 × 10 −6 (HRS )( A ) (11)
.
tc
where P is the mechanical shaking power requirement (kWh/yr) and A is the gross cloth area
ms tc
2
(ft ). The annual electricity cost is calculated as the sum of F and P , multiplied by the
p ms
cost of electricity given in Table 10.
A pulse-jet system uses about 2 scfm of compressed air per 1000 scfm of emission
stream. Thus, a 100,000 scfm stream will consume about 200 scfm. Multiplying by both
60 and HRS gives the total yearly consumption. Multiplying this value by the cost of
compressed air given in Table 10 gives annual costs. For other cleaning mechanisms,
this consumption is assumed to be zero.