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INTRODUCTION TO ENVIRONMENTAL ENGINEERING
Cleaned gas
Dirty gas
Dust
FIGURE 9-37
Reverse flow cyclone. (Source: Crawford, 1976.)
Particulate Pollutants
Cyclones. For particle sizes greater than about 10m m in diameter, the collector of
choice is the cyclone (Figure 9-37). This is an inertial collector with no moving parts.
The particulate-laden gas is accelerated through a spiral motion, which imparts a centrifugal force to the particles. The particles are hurled out of the spinning gas and impact on the cylinder wall of the cyclone. They then slide to the bottom of the cone.
Here they are removed through an airtight valving system. The standard single-barrel
cyclone will have dimensions proportioned as shown in Figure 9-38.
The efficiency of collection of various particle sizes (h) can be determined from
an empirical expression and graph (Figure 9-39) developed by Lapple (1951):
The formula taught is :- [(9*u*W)/(2*pi*Vi*N*(Pp-Pg)]
d0.5 5 c
9mB2 H 1/2
d
rpQg u
(9-64)
where d0.5 5 cut diameter, the particle size for which the collection efficiency is
50 percent
Here it's given Q (volum 5 dynamic viscosity of gas, Pa ? s
metric flow) instead of Vi
B 5 width of entrance, m
Vi*B*H=Q so here in this
H 5 height of entrance, m
formula Numerator contains
rp 5 particle density, kg/m3s
(B^2*H) instead of only B
Qg 5 gas flow rate, m3/s
u 5 effective number of turns made in traversing the cyclone as defined in
Equation 9-65.
The value of u may be determined approximately by the following:
u5
p
(2L1 1 L2 )
H
where L1 and L2 are the length of the cylinder and cone, respectively.
(9-65)
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AIR POLLUTION
FIGURE 9-38
Standard reverse flow cyclone proportions.
Note: Standard cyclone proportions are as follows:
Length of cylinder, L1 5 2D2
Length of cone, L2 5 2D2
Diameter of exit, De 5 0.5D2
Height of entrance, H 5 0.5D2
Width of entrance, B 5 0.25D2
Diameter of dust exit, Dd 5 0.25D2
Length of exit duct, L3 5 0.125D2
Fractional efficiency
(Source: Crawford, 1976.)
FIGURE 9-39
d/d0.5
Empirical cyclone collection efficiency.
(Source: Lapple, 1951.)
Example 9-13. Determine the efficiency of a “standard” cyclone having the following
characteristics for particles 10 mm in diameter with a density of 800 kg/m3:
Cyclone barrel diameter 5 0.50 m
Gas flow rate 5 4.0 m3/s
Gas temperature 5 258C
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Solution.
From the standard cyclone dimensions we can calculate the following:
B 5 (0.25)(0.50 m) 5 0.13 m
H 5 (0.50)(0.50 m) 5 0.25 m
L1 5 L2 5 (2.00)(0.50 m) 5 1.0 m
The number of turns u is then
p
[2(1.0) 1 1.0]
0.25
5 37.7
u5
From the gas temperature and Table A-4 of Appendix A, we find the dynamic viscosity is 18.5 mPa ? s. The cut diameter is then
9(18.5 3 1026 Pa ? s) (0.13 m) 2 (0.25 m) 1/2
d
(800 kg/m3 ) (4.0 m3/s) (37.7)
5 2.41 3 1026 m 5 2.41 mm
d0.5 5 c
The ratio of particle sizes is
10 mm
d
5
5 4.15
d0.5
2.41 mm
From Figure 9-39 we find that the collection efficiency is about 95 percent.
As the diameter of the cyclone is reduced, the efficiency of collection is increased.
However, the pressure drop also increases. This increases the power requirements for
moving the gas through the collector. Because an efficiency increase will result, even
if the tangential velocity remains constant, the efficiency may be increased without
increasing the power consumption by using multiple cyclones in parallel (multiclones).
From the example, you can see that cyclones are quite efficient for particles larger
than 10 mm. Conversely, you should note that cyclones are not very efficient for particles 1 mm or less in diameter. Thus, they are employed only for coarse dusts. Some
applications include controlling emissions of wood dust, paper fibers, and buffing
fibers. Multiclones are frequently used as precleaners for fly-ash control devices in
power plants.
Filters. When high efficiency control of particles smaller than 5 mm is desired, a
filter may be selected as the control method. Two types are in use: (1) the deep bed
filter, and (2) the baghouse (Figure 9-40). The deep bed filter resembles a furnace
filter. A packing of fibers is used to intercept particles in the gas stream. For relatively
clean gases and low volumes, such as air conditioning systems, these are quite effective.
For dirty industrial gas with high volumes, the baghouse is preferable.
The fundamental mechanisms of collection include screening or sieving (where
the particles are larger than the openings between the fibers), interception by the fibers
themselves, and electrostatic attraction (because of the difference in static charge on
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AIR POLLUTION
Exhaust outlet
665
Pressurized air
cleaning jets
Outlet
Clean gas
Pulse-jet
cleaning
gas flow
Collars
Venturi
nozzles
Clean gas
Wire retainers
Filter
bags
Inlet
Dusty air
side
Dustladen
air
Hopper
(a)
Inlet
Hopper
(b)
FIGURE 9-40
Mechanically cleaned (shaker) baghouse (a) and pulse-jet-cleaned baghouse (b). Pulse-jet baghouse shows
normal operation for three left-hand-side bags and pulse-jet cleaning for the bag on the right-hand side. (Source:
Walsh, 1967.)
the particle and fiber). Once a dust cake begins to form on the fabric, sieving is probably the dominant mechanism. As particulate matter collects on the bag, the collection
efficiency increases. The buildup of the dust cake also increases the resistance to
gas flow.
At some point the pressure drop across the filter bags reduces the gas flow rate to
an unacceptable level and the filter bags must be cleaned. The three methods used to
clean the bags are mechanical shaking, reverse air flow, and pulse-jet cleaning.
Mechanically cleaned baghouses operate by directing the dirty gas into the inside
of the bag. The particulate matter is collected on the inside of the bag much in the
same manner as a vacuum cleaner bag. The bags are hung on a frame that oscillates.
They are shaken at periodic intervals, ranging from 30 minutes to more than 2 hours.
The bags are arranged in groups in separate compartments that are taken off line
during cleaning.
In reverse air flow cleaning, a compartment is isolated and a large volume of gas
flow is forced countercurrent to normal operation. The dust cake is removed by collapsing or flexing the bag. The reverse flow combined with the inward collapse of the
bag causes the collected dust cake to fall into the hopper below.
Pulse-jet baghouses are designed with frame structures, called cages, that support the bags. In contrast to the other two cleaning methods, the particulate matter is
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collected on the outside of the bag instead of the inside of the bag. The dust cake is
removed by directing a pulsed jet of compressed air into the bag. This causes a sudden expansion of the bag. Dust is removed primarily by inertial forces as the bag
reaches maximum expansion. The pulse of cleaning air is at such a high pressure
drop and short duration that cleaning is normally accomplished with the baghouse on
line. Cleaning occurs at 2- to 15-minute intervals. Extra bags, which are normally
provided to compensate for the bags that are required in the other cleaning schemes,
are not required in pulse-jet baghouses (Noll, 1999).
Bag diameters for shaker and reverse air flow baghouses range from 15 to 45 cm.
The bags may be up to 12 m in length. Pulse-jet baghouses use bags that are 10 to
15 cm in diameter with lengths less than 5 m (Noll, 1999; Wark, Warner, and Davis,
1998). The bags are made of either natural or synthetic fibers. Synthetic fibers are
widely used as filtration fabrics because of their low cost, better temperature- and
chemical-resistance characteristics, and small fiber diameter. Cotton fiber bags
cannot be used for sustained temperatures above 908C. Glass fiber bags, however,
can be used at temperatures up to 2608C (McKenna et al., 2000). Because of the
stress produced in cleaning, only woven fibers are used when the bags are cleaned
mechanically or by reverse air flow. Felted fabrics are used in pulse-jet-cleaned
baghouse (Noll, 1999). Bag life varies between 1 and 5 years. Two years is considered normal.
The fundamental design parameter for baghouses is the ratio of the volumetric
flow rate of the gas to be cleaned to the area of filter fabric. This ratio is termed the
air-to-cloth ratio.* It has units of m3/s ? m2 or m/s. Typical air-to-cloth ratios are shown
in Tables 9-18 and 9-19.
Baghouses have found a wide variety of applications. Examples include the
carbon black and gypsum industries, cement crushing, feed and grain handling,
limestone crushing, sanding machines, and coal-fired utility boilers. Of all of the
particulate control devices, filtration is the only technology that has the potential to
include the addition of adsorption media to facilitate concurrent removal of gas
phase contaminants.
TABLE 9-18
Typical air-to-cloth ratiosa
Baghouse
cleaning method
Shaking
Reverse air
Pulse jet
Air-to-cloth
ratio, m/s
0.010 to 0.017
0.010 to 0.020
0.033 to 0.083
a
Compiled from Davis, 2000; Noll, 1999; Wark, Warner,
and Davis, 1998.
*It may also be called the gas-to-cloth ratio, filtration velocity, or the face velocity.
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AIR POLLUTION
TABLE 9-19
Air-to-cloth ratios for bag filters
Bag system
Dust
Shaker/woven, reverse,
m3/min ? m2
Pulse jet,
m3/min ? m2
0.8
0.6
0.8
1.1
0.9
0.8
0.8
1.1
0.8
2.4
2.4
1.5
4.3
3.4
2.1
3.0
3.0
3.0
Coal
Cement
Fly ash
Grain
Iron ore
Lead oxide
Lime
Paper
Sand
Source: U.S. EPA, 1990.
Example 9-14. An aggregate plant at Lime Ridge has been found to be in violation
of particulate discharge standards. A mechanical shaker baghouse has been selected
for particulate control. Estimate the number of bags required for a gas flow rate of
20 m3/s if each bag is 15 cm in diameter and 12 m in length. One-eighth of the bags
are taken off line for cleaning. The manufacturer’s recommended air-to-cloth ratio
for aggregate plants is 0.010 m/s.
Solution.
Noting that the air-to-cloth ratio units of m/s are equivalent to m3/s ? m2, calculate
the net cloth area required with one compartment off line for cleaning:
A5
Q
20 m3/s
2
5
3
2 5 2,000 m
v
0.010 m /s ? m
The net number of bags is the total area divided by the area of one bag:
2,000 m2
5 353.67 or 354 bags
(p) (0.15 m) (12 m) per bag
With one-eighth of the bags off line, an additional one-eighth of the net number is
required:
354 bags
5 44.25 or 44 bags
8
The total number of bags is 354 1 44 5 398.
Comment: In order to have an equal number of bags in each compartment, the total
number of bags will have to be slightly larger (398 bags/8 compartments 5 49.75
bags per compartment). With 50 bags per compartment, the total will be 400 bags.
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Venturi “throat”
Cyclonic
collector
Pollutant-laden
gas
Liquid
injection
FIGURE 9-41
Venturi scrubber.
Liquid Scrubbing. When the particulate matter to be collected is wet, corrosive, or
very hot, the fabric filter may not work. Liquid scrubbing might. Typical scrubbing
applications include control of emission of talc dust, phosphoric acid mist, foundry
cupola dust, and open hearth steel furnace fumes.
Liquid scrubbers vary in complexity. Simple spray chambers are used for relatively
coarse particle sizes. For high efficiency removal of fine particles, the combination of a
venturi scrubber followed by a cyclone would be selected (Figure 9-41). The underlying
principle of operation of the liquid scrubbers is that a differential velocity between the
droplets of collecting liquid and the particulate pollutant allows the particle to impinge
onto the droplet. Because the droplet-particle combination is still suspended in the gas
stream, an inertial collection device is placed downstream to remove it. Because the
droplet enhances the size of the particle, the collection efficiency of the inertial device
is higher than it would be for the original particle without the liquid drop.
The most popular collection efficiency equation is that proposed by Johnstone,
Field, and Tassler (1954):
h 5 1 2 exp (2kR1c)
where
(9-66)
h 5 efficiency
exp 5 exponential to base e
k 5 correlation coefficient, m3 of gas/m3 of liquid
R 5 liquid flow rate, m3/m3 of gas
c 5 inertial impaction parameter defined by Equation 9-67
The inertial impaction parameter (c) relates the particle and droplet sizes and relative
velocities:
c5
Crpyg (dp ) 2
18dd m
where C 5 Cunningham correction factor defined by Equation 9-68, unitless
rp 5 particle density, kg/m3
yg 5 speed of gas at throat, m/s
(9-67)