Particulate Control-2
Fabric Filters
Particulate Scrubbers
Lecture notes adapted from
Prof. Dr. Dentel Notes and Prof. Dr. Chang-Yu Wu
Fabric Filters
• Well known and accepted method for separating
dry particles from a gas stream
• Many different types of fabrics, different ways of
configuring bags in a baghouse and different
ways of flowing the air through the bags.
• There are 3 common types of baghouse based
on cleaning method
– Reverse-air
– Shaker
– Pulse-jet
Fabric Filters
Fabric Filters
A shaker baghouse
Filter
compartements
Fabric Filters
Fabric Filters
Filtration Theory
Filtration Theory
Filtration Theory
Figure 6.2 pp 186
Filtration Theory
Filtration Theory
Design Considerations
Cleaning Cycles
• tf: time interval between two cleanings of the same
compartment
• tr: time interval between cleanings of any two
compartment
Variation of pressure drop with time
DPm
DP
tr
tc
Time
Cleaning Cycles
Maximum Filtering Velocities in
Shaker or Reverse Air Baghouses
Dusts
Max. Filtering V
(ft/min)
Activated charcoal, carbon black, detergents, metal
fumes
1.5
Aluminum oxide, carbon, fertilizer, graphite, iron ore, lime, 2
paint pigments, fly ash, dyes
Aluminum, clay, coke, charcoal, cocoa, lead oide, mica
soap, sugar, talc
2.25
Bauxite,ceramics,chorme ore, feldsapr, blour, flint, glass,
gypsum, plastics, cement
2.5
Asbestos, limestone, quartz, silica
2.75
Cork, feeds and grain, marble, oyster shell, salt
3-3.25
Leather, paper, tobacco, wood
3.5
•
Table 6.1
Fabric Selection
Fabric
Max Temp, C
Acid resistance
Base resistance
Dynel
71
Good
Good
Cotton
82
Poor
Good
Wool
93
Good
Poor
Nylon
93
Poor
Good
Polypropylene
93
Excellent
Excellent
Orlon
127
Good
Fair
Dacron
135
Good
Fair
Teflon
204
Excellent
Excellent
Glass
288
Good
Good
Table 6.2
Pulse Jet Filters
• Introduced 45 years ago captured one-half of the
industrial air filtration market
• Air is filtered through the bags from outside to the
inside, a cage inside each bag prevents the bag
from collapsing
• The bags are cleaned by short blast of high
pressure air (90-100 psi)
• Each bag is pulsed every few minutes
• On stream use
Pulse Jet Filters
• There are no compartments and thus no extra
bags which reduces size and cost (for a large
coal-fired power plant, the baghouse is so large
that it is designed with separate compartments)
• Since bags are placed from the top, no need to
provide walkways between rows of bags
(reducing the size)
• Felted fabrics can be used at much higher air to
cloth ratio (higher filtering velocities)
Pulse Jet Filters
• Table 6.5. Maximum Filtering Velocities for
Various Dust or Fumes
Dusts or Fumes
Maximum Filtering Velocity
(ft/min)
Carbon, Graphite, Metallurgical
Fumes, Soap, Detergents;Zinc
oxide
5-6
Cement (Raw), Clay (Green),
Plastics, paitn Pigments, Starch,
Sugar, Wood, Gypsum, Zinc
7-8
Aluminum oxide, cement (finished),
Clay (vitrifies), Lime, Limestone,
Mica,Quartz, soybean, Talc
9-11
Cocoa, Cholocate,Flour,Grains,
Leather Dust, Sawdust,tobacco
12-14
Advantages
Disadvantages
Example
Example
Example
Other Considerations
• Temperature and Humidity : Fabrics have
different maximum allowable teperatures. Low T
can cause condensation of acid and/or blinding
of the fabric with wet dust
• Chemical nature of gas: Different fabrics hav
different resistance to acisd or alkalies
• Fire/explosion: Some fabric are flammable;
Some dust are explosive
• Dust Handling: dust removal rate, conveyor
system, and hopper slope should all be
considered
Wet Scrubbers
Particulate Scrubbers
Reading: Chap. 7
• Types of scrubbers: spray chamber and
venturi scrubber
• Theory and design consideration
• Pressure drop
• Contacting power
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Collecting medium:
 Liquid drops
 Wetted surface
Recirculated water
Spray Chamber
Water to settling basin and recycle pump
Vertical spray chamber (countercurrent flow)
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Cyclone Spray
Chamber &
Impingement Scrubber
Flagan & Seinfeld, Fundamental of Air
Pollution Engineering, 1988
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Venturi Scrubber
High efficiency even for small particles
QL/QG: 0.001 - 0.003
VG: 60 - 120 m/s
Handbook of Air Pollution Control Engineering & Technology, Mycock, McKenna & Theodore, CRC Inc., 1995.
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Theory: Spray
Chamber

Volume of each droplet
d 
6
d d3
Total number of droplets that pass the chamber per second
QL
QL
6QL
Nd 

 3
d  d 3 d d
d
6
VG
QL: volumetric liquid flow rate
Droplet concentration in the chamber
Nd
6QL
nd 
 3
AcVd d d AcVd
Vd
Vtd
Vd  Vtd  VG
Vd: droplet falling velocity relative to a fixed coordinate
Vtd: droplet terminal settling velocity in still air (i.e. relative to the gas flow)
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At a given time dt, the distance a droplet falls is
dz  Vd dt
Volume of air that flows through the cross-section area of a single
droplet during the time dt
Vair, single
 2 
  2  Vtd
  d d Vtd dt   d d 
dz
4 
 4  Vd
Total effective volume of gas swept clean per second by all
droplets in dz
 d d2  Vtd 6QL

Vair,all   d 
dz 3
d d
 4  Vd
Total number of particles swept clean per second by all droplets in dz
 d d2  Vtd 6QL

dN p   d 
dz 3 n
d d
 4  Vd
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p, z
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Total number of particles removed per second over dx

dN p  VG Ac n p , z  dz / 2  n p , z  dz / 2

QL
Particle penetration in a countercurrent vertical
spray chamber

3QLVtd d z 

Pd  exp 
 2QG d d (Vtd  VG ) 
 AdVtd d 

 exp 
QG 

N
z dz / 2
N
N
z
z dz / 2
Cross-sectional area of all the droplets
 6QL
Ad   Ac z   3
 d d AcVd
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  d d2 
3QL z
  
 
  4  2d d Vtd  VG 
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QG
36
 6.12104 QLVtdd z 

Pd  exp 
QG d d (Vtd  VG ) 

If QL in gal/min and QG in cfm, z in ft and dd in mm
Particle penetration in a cross-flow spray chamber
 3  QL   d  
 AdVtd d 







Pd  exp  
z   exp 



QG 

 2  QG  d d  
Q: How do we have higher collection efficiency?
Q: What are the collection mechanisms (we need it for d)?
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Deposition of Particles on a Spherical Collector
Re 
mG
Sc 
G D
d dVtd  G
mG
Particle Reynolds #

dp
dd
St 
Particle Schmidt #
mL

mG
Diameter ratio
Cc  p d p2Vtd
18mG d d
Particle Stokes #
Viscosity ratio
Single droplet collection efficiency
d
(diffusion)
(interception)
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(impaction)
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Impaction only
 St 
d  I  

 St  0.35 
2
(Impaction parameter Kp is
used in textbook; Kp = 2 St)
p = 2 g/cm3
Venturi Scrubber
• Use intertial
impaction of
suspended
particles on water
droplet formed by
gas atomization
Venturi Scrubbers: Calvert Design
Particle penetration through a venturi scrubber

 QLVG  L d d
Pd  exp

 55QG mG

 K po f  0.7 
0.49



 0.7  K po f  1.4 ln

0.7


 0.7  K po f
 1 



K
 po 

Kpo=2St (aerodynamic diameter) using throat velocity
f = 0.5 for hydrophilic materials, 0.25 for hydrophobic materials
Atomization produces a wide distribution of droplet size. However using the
Sauter mean droplet diameter (dd) equation can be solved with satisfactory
results.
0.5
 mL 

 597
0.5 
  L  
k1 = 58600 if VG is in cm/s
= 1920 if VG is in ft/s
k1
dd 
VG
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 
 
 L 
0.45
1.5

QL 
1000 
QG 

 in dyne/cm, L in g/cm3 and
m should be in poise
QL and QG should be of the
same unit
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Pressure Drop
Venturi Scrubber
QL
Dp  k V
QG
2
L G
k  2(1  X 2  X 4  X 2 )
3lt C Dd  G
X
1
16d d  L
lt: venturi throat length
X: dimensionless throat length
Ex: 10” water, 2 mm,  = ?
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Contacting Power Approach
When compared at the same power consumption, all scrubbers give the
same degree of collection of a given dispersed dust, regardless of the
mechanisms involved and regardless of whether the pressure drop is
obtained by high gas flow rate or high water flow rate
  1  exp( Nt )
Nt: Number of transfer unit
(unitless)
Nt  PT
(PT :contacting power in hp / 1000 cfm)
 and : coefficient and exponent of PT
PT should be determined from the friciton loss across the wetted
portion of the scrubber.
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Contacting Power Approach
Venturi scrubber collecting a metallurgical fume
Contacting power, hp/cfm
Example
Q: Tests of a venturi scrubber show the results listed on the right. Estimate
the contacting power required to attain 97% efficiency.
Friction loss (in H2O)
 (%)
12.7
56
38.1
89
  1  exp( Nt )
Nt: Number of transfer unit
(unitless)

Nt  PT
(PT contacting power in hp / 1000 acfm)
(1 inch of water = 0.1575 hp/1000 cfm)
Example
Convert friction loss to contacting power (hp/1000 cfm): 1 in H20 =0.1575
hp/1000cfm
Friction loss (in
H2O)
PT hp/1000cfm
12.7
2
38.1
6
  1  exp( Nt )

Nt  PT
 (%)
Nt
56
0.821
89
2.207
97
3.506
0.821  2   ln    ln 2  ln(0.821)
2.207   6   ln    ln 6  ln(2.207)
Example
Substractin Eq A from Eq B:
0.821  2   ln    ln 2  ln(0.821)
2.207   6   ln    ln 6  ln(2.207)
0.989   (ln 6  ln 2)   ln(6 / 2)
  0.90
2.207
  0.90  0.44
6

Nt  PT
3.506  0.44P
0.90
T
PT  10hp / 1000cfm
A
B
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Problem 7.1
Solution
 St 

 St  0.35 
2
St 
d  I  
Cc  p d p2Vtd
18mG d d
Impaction
parameter Kp is
used in textbook
d a2 
K P  2St  2
Cc  p d p2Vtd
18mG d d

Cc  p d p2Vtd
9mG d d
d p2  p
pw
wd a2Vtd

9mG d d
Determine the density of water and the viscosity of the air at 80 °F from Appendix B

3QLVtd d z 

Pd  exp 
 2QG d d (Vtd  VG ) 
Solution

3QLVtd d z 

Pd  exp 
 2QG d d (Vtd  VG ) 
Solution
 St 
d  I  

 St  0.35 
2