Geoscience Reference
In-Depth Information
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EX AMPLE 17.12
Problem:
A proposal to install a pulse jet fabric filter system to clean an air stream containing
particulate matter must be evaluated. Select the most appropriate filter bag considering the perfor-
mance and cost (USEPA, 1984b, p. 84).
Given:
Volumetric flow rate of polluted air stream = 10,000 scfm (60°F, 1 atm)
Operating temperature = 250°F
Concentration of pollutants = 4 grains/ft
3
Average air-to-cloth ratio (A/C ratio) = 2.5 cfm/ft
2
cloth
Collection efficiency requirement = 99%
Table 17.8 lists information given by filter bag manufacturers. Assume no bag has an advantage
from the standpoint of durability under the operating conditions for which the bag is to be designed.
Solution:
Eliminate from consideration bags that, on the basis of given characteristics, are unsatis-
factory. Considering operating temperature and bag tensile strength required for a pulse jet system,
• Bag D is eliminated because its recommended maximum temperature (220°F) is below the
operating temperature of 250°F.
• Bag C is also eliminated because a pulse jet fabric ilter system requires the tensile strength
of the bag to be at least above average.
Determine comparative costs of the remaining bags. Total cost for each bag type is the number of
bags × cost per bag. No one type of bag is more durable than the other. Establish the cost per bag.
From the information given in Table 17.8, the cost per bag is $26 for bag A and $38 for bag B. Now
determine the number of bags (
N
) for each type. The number of bags required (
N
) is the total filter-
ing area required divided by the filtering area per bag. Calculate the total filter area (
A
t
) and the
given flow rate to acfm (
Q
a
):
Q
a
= (10,000)(250 + 460)/(60 + 460) = 13,654 acfm
The
A
/
C
ratio, expressed in cfm/ft
2
, is the same as the filtering velocity, which is given above as 2.5
cfm/ft
2
cloth. From the information given in Table 17.8, the filtering velocity is
v
f
= 2.5 cfm/ft
2
= 2.5 ft/min
TABLE 17.8
Filter Bag Properties
Filter Bag
Property
A
B
C
D
Tensile strength
Excellent
Above
average
Fair
Excellent
Recommended maximum temperature
260°F
275°F
260°F
220°F
Resistance factor
0.9
1
0.5
0.9
Cost per bag
$26
$38
$10
$20
Standard size
8 in. × 16 ft
10 in. × 16 ft
1 ft × 16 ft
1 ft × 20 ft
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