EPA-450/2-80-066
Mr PoMutton TnMng Mfcutt
MO 20
A 4BO/2-CMW
1981
TrtagtoPvfc.NC 27711
Do not remove. This document
should be retained in the EPA
Region 5 Library Collection.
APTI
Course 413
Control of Particulate
Emissions
Student Manual
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united states
Environmental Protection
Agency
Air Pollution Training Institute
MD20
Environmental Research Center
Research Triangle Park, NC 27711
tf*M
October, 1961
Air
APTI
Course 413
Control of Particulate
Emissions
Student Manual
Written by:
David S. Beachler
James A. Jahnke, Ph.D.
Northrop Services. Inc.
P.O. Box 12313
Research Triangle Park, NC 27709
Under Contract No.
68-02-2374
EPA Project Officer
R. E. Townsend
United States Environmental Protection Agency
Office of Air, Noise, and Radiation
Office of Air Quality Planning and Standards
Research Triangle Park, NC 27711
'-:-S. Knviz^iinental Protection Agency
l>cicn 5, Library f5PL-161
^0 ,1. .i,xenrlorn Gt;-eet, Puon 1670
Chicit; '. XT. 6jG04
-------
Notice
This is not an official policy and standards document. The opinions and selections
are those of the authors and not necessarily those of the Environmental Protection
Agency. Every attempt has been made to represent the present state of the an as
well as subject areas still under evaluation. Any mention of products or organiza-
tions does not constitute endorsement by the United States Environmental Protec-
tion Agency.
Availability
This document is issued by the Manpower and Technical Information Branch
Control Programs Development Division, Office of Air Quality Planning and Stan.
dards USEPA. It was developed for use in training courses presented by the LFA
Air Pollution Training Institute and others receiving contractual or grant support
from the Institute. Other organizations are welcome to use the document.
This publication is available, free of charge, to schools or governmental air
pollution control agencies intending to conduct a training course on the subject
covered. Submit a written request to the Air Pollution Training Institute, USEPA,
MD 20, Research Triangle Park, NC 27711.
Others may obtain copies, for a fee, from the National Technical Information
Service (NTIS), 5825 Port Royal Road, Springfield, VA 22161.
Sets of slides and films designed for use in the training course of which this
publication is a part may be borrowed from the Air Pollution Training Institute
upon written request. The slides may be freely copied. Some films may be coped;
others must be purchased from the commercial distributor.
n
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Table of Contents
Page
Chapter 1. Air Pollution Control Overview J'J
Introduction ^ j
Collection Forces 5
Factors Affecting Control Equipment Selection
2-1
Chapter 2. Basic Concepts of Gases
Expression of Gas Temperature 2"g
Expression of Gas Pressure g 4
Molecular Weight g 5
Mole 2-5
The Laws of Ideal Gases
_ . Z-o
Density 2-7
Viscosity g.g
Specific Heat 2_g
Reynolds Number
3-1
Chapter 3. Particle Dynamics
Forces Acting on a Particle
Balance of Forces on a Particle ' "
Determination of the Flow Regime ^
References
Chapter 4. Particle Sizing
Introduction ^
Size 4_2
Various Sizing Devices '
Comparison of Particle Sizing Devices '
Mathematical Treatment of Data '
Example of a Typical Particle Size Data Reduction £«
References
Chapter 5. Gravity Settling Chambers
Introduction ',
Equipment Description
Baffle Chambers .
Design Parameters & 7
Process Variables g
References
6-1
Chapter 6. Cyclones g ^
Introduction g 2
Cyclone Types ' ,.
Characterizing Cyclone Performance "
Summary of Performance Characteristics '
References
iii
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Chapter 7. Electrostatic Precipitaton 7_1
Introduction 7.!
Types of ESPs 7_2
Theory of Precipitation 7.5
Collection Efficiency 7.9
Design Parameters 7-11
Precipitator Equipment 7-19
Electrostatic Precipitator Applications 7-27
References 7-29
Chapter 8. Fabric Filtration g_l
Filtration for Particle Collection 8-1
Baghouses 3.4
Fabric Filter Material 8-16
Bag Cleaning 8-21
Baghouse Design Variables 8-29
Baghouse Design Review 8-33
References 8-37
Chapter 9. Wet Collectors 9_1
General Characteristics—Paniculate Matter Removal 9-1
Theory of Operation 9.3
General Theories 9.4
Collection Mechanisms 9.5
The Johnstone Equation 9-10
The Cut Power Method 9-12
The Contact Power Theory 9-15
Pilot Methods 9-21
Wet Collector Systems 9-22
Gas Phase Contacting Scrubbers 9-22
Liquid Phase Contacting Scrubbers 9-36
Liquid Phase/Gas Phase Contacting Scrubbers 9-41
Mechanically Aided Scrubbers 9-49
Miscellaneous Devices 9-52
References 9.53
i
Appendix A. Common SI Units A-l
Appendix B. Conversion Factors B-l
Appendix C. Constants and Useful Information C-l
Appendix D. Capital and Operating Cost Estimations D-l
Appendix E. Characteristics of Air Pollution Control Equipment E-l
Appendix F. Industry Pollutant Sources and Typical Control Devices F-l
Appendix G. Characteristics of Particles G-l
Appendix References H-l
IV
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Figures
Page
Figure l 3
1-1 Gravity [[[
1-2 Centrifugal force
1-3 Impaction
1-4 Direct interception
1-5 Diffusion [[[ ^ g
1-6 Electrostatic attraction ..........................................
2- 1 Comparison of degree-units between Fahrenheit and Celsius ........... 2-1
2-2 Two examples of absolute pressure determination .................... *•
2-3 Shearing stress in a moving fluid ..................................
o o
3-1 Identical objects in two different fluids ......... . ................... '
3-2 Buoyant and gravitational forces acting on a particle 3-3
3-3 Drag force .......... 1' ' ' ' '.V I" ' Y ".'^^L^L ............ '.'.... S-5
3-4
Drag coefficient versus Reynolds'Number for spheres.
Collisions of air molecules on particles greater than 3 /
3-6 Collisions of air molecules on particles less than 3 /un in diameter 3-7
3-
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Figure Page
5-1 Horizontal flow settling chamber 5-2
5-2 Howard settling chamber (multiple tray) 5-3
5-3 Baffle chamber 5-3
5-4 Settling chamber dimensions 5-4
5-5 Fractional efficiency curve for dusts from a sinter plant 5-6
6-1 Particle collection mechanisms 6-1
6-2 Types of cyclones 6-3
6-3 Common cyclone 6-4
6-4 Inlet interference 6-5
6-5 Types of cyclone inlets 6-6
6-6 Nomenclature for a tangential entry cyclone 6-7
6-7 Standard cyclone designs 6-8
6-8 Cyclone vortices 6-9
6-9 Fines eductor 6-11
6-10 Discharge systems 6-12
6-11 Cyclone outlet devices 6-14
6-12 Typical size efficiency curve 6-16
6-13a Cut size in micrometers for cyclones of conventional type 6-17
6-lSb Viscosity and velocity correction factors for cut size particle
of conventional cyclones 6-18
6-lSc Inlet width/cyclone diameter and effective number of turns correction
factors for cut size particle of conventional cyclones 6-18
6-14 Cyclone efficiency versus particle diameter. Experimental results and
theoretical predictions 6-21
6-15 Cyclone efficiency versus particle size ratio 6-22
6-16 Cyclones in series 6-24
6-17 Battery of four involute cyclones in parallel 6-25
6-18 Battery of vane axial cyclones 6-26
7-1 Typical plate and wire single-stage electrostatic precipitator 7-1
7-2 Typical two-stage precipitator 7-3
7-3 Gas flow through wire and tubular pipe precipitators 7-4
7-4 Gas flow through a wire and plate precipitator 7-4
7-5 Electric field generation (top view) 7-5
7-6 Generation of corona 7-6
7-7 Avalanche multiplication 7-6
7-8 Gas ionization in the inter-electrode region 7-7
7-9 Field lines modified by the particle 7-8
7-10 Effect of temperature and moisture content on apparent resistivity
of precipitated cement dust 7-13
7-11 Fly ash resistivity versus coal sulfur content for several flue gas
temperature bands 7-14
7-12 Gas inlet with diffuser-perforated plates 7-16
7-13 Stage or field sectionalization 7-18
VI
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Page
7-18
7-14 Parallel sectionalization •
7-15 Guide frames and shrouds for discharge wires ''»
7-16 Typical discharge wire shapes
7-17 Rigid frame discharge electrode design ^
7-18 Typical collection plates
7-19 Typical hammer/anvil collection plate rapper '-"
7-20 Typical impulse collection plate rappers 1_
7-21 Typical vibrator rappers used for discharge electrodes /•»
7-22 High voltage system
8-1
8-1 Impaction g 2
8-2 Direct interception
8-3 Diffusion g 4
8-4 Bags and support
8-5 Envelope baghouse g g
8-6 Bags and hopper _
8-7 Positive and negative pressure baghouses •. • • • • '
8-8 Interior filtration (particles collected on the inside of the bag) 8-8
8-9 Exterior filtration (particles collected on the outside of the bag) .8-9
8-10 Dust inlet to the baghouse '^
8-11 Bag attachment g"13
8-12 Hopper g .,
8-13 Trickle valve discharge device
8-14 Rotary airlock discharge device ^
8-15 Screw conveyor g lg
8-16 Pneumatic conveyor '
8-17 Woven fabric filter; twill weave and sateen weave »-J^
8-18 Sieving g lg
8-19 Felted fabric filter g"22
8-20 Shaking g 23
8-21 Reverse air cleaning &
8-22 Pulse jet cleaning g"2g
8-23 Pulse jet air supply "
8-24 Reverse jet cleaning using blow rings '
8-25 Performance curve for a single bag of a fabric filter »-3
8-26 Overall pressure drop of a multicompartment baghouse 8-3
9-1 Zones of a wet scrubber
9-2 Collection efficiency for a mobile bed scrubber as a function ^
of particle size •• \ "
9-3 Inertia! impaction coUection efficiency: target efficiency »-o
9-4 Target efficiency: the area ratios "
9-5 Collection by interception '
9-6 Penetration and the cut diameter • • • • •
9-7 Correlation of scrubber outlet dust loading with theoretical
power consumption
vu
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Figure Page
9-8 Cut diameter [dp]^ as a function of gas pressure drop
and power consumption 9-20
9-9 Sieve plate scrubber 9-23
9-10 Impingement plate scrubber 9-24
9-11 Detail of an impingement plate 9-25
9-12 Detail of bubble caps 9-25
9-13 Detail of orifice action 9-27
9-14 Swirl orifice scrubber 9-28
9-15 Typical venturi scrubber 9-29
9-16 Swirl venturi scrubber 9-31
9-17 Spray venturi scrubber 9-32
9-18 Variable throat venturi scrubber 9-33
9-19 Variable throat venturi scrubber 9-34
9-20 Venturi-rod scrubber 9-35
9-21 Types of spray nozzles 9-37
9-22 Simple spray chamber 9-38
9-23 Mist eliminators 9-39
9-24 Ejector venturi scrubber 9-40
9-25 Irrigated cyclone scrubber 9-42
9-26 Cyclonic spray scrubber 9-43
9-27 Centrifield scrubber 9-44
9-28 Moving bed scrubber ^ 9-45
9-29 Baffle spray scrubber 9-47
9-30 Combination device A 9-48
9-31 Combination device B 9-49
9-32 Centrifugal fan scrubber 9-50
9-33 Vertical spray rotor scrubber 9-51
9-34 Common packing materials; 9-53
9-35 Countercurrent packed tower 9-54
9-36 Cocurrent packed tower 9-55
9-37 Crossflow scrubber 9-56
9-38 Fiber bed scrubber 9-57
D-l Dry type electrostatic preci pita tor purchase prices versus plate area.
Data valid for December 1977 D-2
D-2 1/8" thick carbon steel fabricated scrubber price versus volume.
Data valid for December 1977 D-4
D-S Metal thickness required versus volume and design pressure D-4
D-4 Price adjustment factors versus plate thickness and volume.
Data valid for December 1977 D-5
D-5 Scrubber internal surface area and separator diameter and height
versus waste inlet gas volume D-5
D-6 Internal gas cooler bubble tray cost versus separator diameter.
Data valid for December 1977 D-6
D-7 Intermittent, pressure, mechanical shaker baghouse prices versus net
cloth area. Data valid for December 1977 D-7
vm
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Figure
D-8 Continuous, suction or pressure, pulse jet baghouse prices versus net
cloth area. Data valid for December 1977 D'8
D-9 Continuous, pressure, mechanical shaker baghouse prices versus net
cloth area. Data valid for December 1977 D'8
D-10 Continuous, pressure, reverse air baghouse prices versus net cloth
area. Data valid for December 1977 D'9
D-ll Custom pressure or suction baghouse prices versus net cloth area.
Data valid for December 1977 £'*
D-12 Capacity estimates for cyclones = ****
D-1S Critical particle size estimates for cyclones D'1Z
D-14 Cyclone prices for carbon steel construction versus inlet area. Data
valid for December 1977 D'12
D-15 Cyclone prices for stainless steel construction versus inlet area. Data
valid for December 1977 • °-1S
D-16 Cyclone support prices versus collector inlet area. Data valid for
December 1977 ; • 'D'1S
D-17 Cyclone dust hopper prices for carbon and stainless steel construction
versus collector inlet area. Data valid for December 1977 .. .D-14
D-18 Cyclone scroll outlet prices for carbon and stainless steel construction
versus collector inlet area. Data valid for December 1977 D-14
Tables
Table
3-1 Values of C/ (for air at atmospheric pressure) ........................ 3'7
3-2 K values for flow regime determination ........................... s'10
4-1 Size range capabilities of measuring devices ........................ 4'11
4-2 Comparison of particle sizing devices ............................. 4-11
4-3 Size ranges in arithmetic increments .............................. 4"14
4-4 Size ranges with the same ratio .................................. 4"15
4-5 Typical particle size data ....................................... 4"*9
4-6 Cumulative particle size data .................................... 4"Z1
5-1 Pickup velocities of various materials .............................. 5'8
6-1 Dimensionless design ratios for tangential entry cyclones .............. 6-8
6-2 Changes in performance characteristics ........................... 6-23
IX
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Table Page
7-1 Typical precipitation rate parameters for various applications 7-10
7-2 Typical design parameter ranges for fly ash precipitators 7-28
8-1 Triboelectric series for some production fabrics 8-3
8-2 Typical fabrics used for bags 8-19
8-3 Typical air-to-cloth ranges 8-35
8-4 Typical A/C ratios [(ft3/min)/ft»] for selected industries 8-36
8-5 Typical A/C ratios for fabric filters used for control of paniculate
emissions from industrial boilers 8-37
9-1 Parameters a and /3 for the contact power theory 9-17
9-2 Methods for predicting venturi scrubber pressure requirements 9-21
9-3 Categories of wet collectors and energy input 9-22
9-4 Operating characteristics of plate scrubbers 9-26
9-5 Operating characteristics of orifice scrubbers 9-29
9-6 Operating characteristics of venturi scrubbers 9-36
9-7 Operating characteristics of spray towers 9-39
9-8 Operating characteristics of ejector Venturis 9-41
9-9 Operating characteristics of cyclonic scrubbers 9-45
9-10 Operating characteristics of moving bed scrubbers 9-46
9-11 Baffle spray chamber 9-47
9-12 Operating characteristics of mechanically aided scrubbers 9-52
9-13 Operating characteristics of packed towers 9-57
D-l Approximate guide to estimate gross cloth area. D-7
D-2 Bag prices ($/ft*). Data valid for December 1977 D-10
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Nomenclature
ap acceleration of the particle
A effective collecting plate area of an ESP
A* area of a bag
A. area of cloth filter
parameter characterizing particle size distribution for cut power
method
area cleared of particles
A/C air-to-cloth ratio
A*,, frontal area of a water droplet
A.R. aspect ratio
B width of a settling chamber
Be cyclone inlet width
B^ empirical constant for cut power method
c cyclone dimension factor
c< dust concentration loading
CD drag coefficient
Cf Cunningham correction factor
d' diameter of the area cleared of particles
d^ geometric mean diameter
d,, droplet diameter
dp particle diameter
dp* minimum particle size (diameter) collected in a settling chamber
[dp]c« particle cut diameter
[dp]cr,t particle critical diameter
dp maximum particle diameter
dv/dt change in velocity with respect to time
dv/dy velocity gradient
Dc cyclone body diameter
D, cyclone exit tube diameter
c base of the natural logarithm =2.718
E, strength of electric field in which particles are charged
E, strength of electric field in which particles are collected
F force
FB buoyant force
FD drag force
FG gravitational force
Fj, resultant force
g acceleration due to gravity
a. dimensional constant
h, height particle must fall to be collected in a settling chamber
H settling chamber height
He cyclone inlet height
XI
-------
k dimensionless parameter
^ fabric resistance
kx filter cake resistance
kj Boltzmann's constant
kc dimensionless factor descriptive of cyclone vanes
K factor to determine flow regimes
KC' proportionality factor (cyclones)
L settling chamber length
LC cyclone body length
m mass
m. mass of air
m, true weight of water (specific gravity calculation)
m, mass of a particle
M molecular weight of a gas
n vonex exponent (cyclones)
n, effective number of turns (vortex in a cyclone)
Ne number of parallel chambers (Howard settling (chamber)
N, number of transfer units
p partial pressure of each individual gas compommt
Pt gaSe pressure
p£ liquid inlet pressure (scrubber)
Ap pressure drop
Ape pressure drop across a filter cake
Ap/ pressure drop across a fabric filter
Apr total pressure drop (across a baghouse)
P absolute pressure
P» barometric pressure
PT total pressure of a gas mixture
Pe Peclet number
Pt penetration
,^G power input from gas stream
&L power input from liquid stream
&T total contacting power
q particle charge
Q. volumetric flow rate
Qp gas flow rate
Qt liquid flow rate
r radius of a circular path
R universal gas constant
Re Reynolds Number
Re, Reynolds Number of a particle
xu
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S filter drag
Se length of exit tube in a cyclone
S0 total number of exposures
SCA specific collection area
t time (filtration time)
T absolute temperature
v velocity
v/ filtration velocity
v, inlet gas velocity
vp particle velocity
VK relative velocity of a gas to liquid
v, settling velocity
v, horizontal velocity
vy vertical velocity
V volume
Vp volume of a particle
w migration velocity
w< weight fraction
Zc cone length
a empirical constant for contact power theory
£ empirical constant for contact power theory
77 collection efficiency
rjc collection efficiency of a droplet
77, fractional efficiency
tfror overall collection efficiency
p gas viscosity
ft, liquid viscosity
p° gas viscosity at 0°C and prevailing pressure
v kinematic viscosity
Q density
Q, fluid density (air)
Qt gas density
Q, density of a liquid
Qm density of manometer fluid used to determine v, for cyclones
QP panicle density
a standard deviation
a^ geometric standard deviation
a, liquid surface tension
T shearing stress
^ impaction parameter
xin
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Chapter 1
Air Pollution Control Overview
Introduction
standards are proposed for that industry.
-c,
brfore the proposal date of the emission standard.
'
* c°™d
n comiui. *»~ . less
LAER.
1-1
-------
Best Available Control Technology is control technology applied to new and
modified sources. Prevention of Significant Deterioration (PSD) is the review
process for sources in attainment areas and is required for each pollutant subject to
regulation under the Act. An attainment area is an air quality control region that
meets the National Ambient Air Quality Standards.
New Source Performance Standards (NSPS) are set for a number of source
categories. Best Available Control Technology must reduce emissions at least as
much as the NSPS limit. New Source Performance Standards that have been pro-
mulgated are published first in the Federal Register and then in the U.S. Code of
Federal Regulations. BACT applied to a source must be technologically feasible
and must also reflect considerations of cost and energy usage. The BACT require-
ment is to be applied on a case-by-case basis review by EPA regional engineers.
Lowest Achievable Emission Rate refers to control technology applied to new and
modified sources in a nonattainment area. A nonattainmeni: area is an area or
region where the National Ambient Air Quality Standards are being violated.
LAER is the most stringent emission limitation which is contained in the SIP of
any State or the most stringent emission limitation achieved in practice by that
source category. LAER may be in some cases considered to be technology forcing,
involving the transfer of technology from one source category to another. LAER is
usually more stringent than BACT or RACT but must be at least as stringent as
control specified by the NSPS.
The type of control equipment which can satisfy the requirements of RACT,
BACT or LAER is generally specific to a given source category. For example, a
paniculate emission source may satisfy RACT requirements by applying a wet
scrubber. BACT may require the application of an electrostatic precipitator. In
another industry however, due to the explosive nature of the materials, wet
scrubbers might be the only feasible type of collection equipment that can be used.
BACT, in this case, might specify the use of a wet scrubber at a given pressure
drop.
These control limitations clearly show that paniculate air pollutants must be
controlled from industrial sources. The stringency of control might also include
other methods in addition to the traditional add-on control devices. This calls for
the consideration and application of alternative production procedures, modifica-
tions of processes and control techniques that result in minimum emissions from a
source.
This training manual is intended to be used hi APTI Course 413, Control of Par
ticulate Emissions. This document presents the fundamental concepts of the opera-
tion of paniculate emission control equipment for stationary sources. The authors
of this manual suggest that the reader turn to the many EPA publications from the
Office of Air Quality Planning and Standards and the Industrial Environmental
Research Laboratory for additional information concerning this subject. Many
recent publications include NSPS and LAER control strategies that describe the
latest technological advances in the Geld of paniculate emission control.
1-2
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Collection Forces
All participate emission control devices collect particles by mechanisms involving an
applied force, the simplest being gravity. Large particles moving slow enough in a
«!i stream can be overcome by gravity and be collected (Figure 1-1) Gravity is
responsible for particle collection in the simplest devices such as settling chambers.
-Particle
Figure 1-1. Gravity.
Centrifugal force is another collection mechanism used for particle capture. The
shape or curvature of the collector causes the gas stream to rotate in a spiral
motion Larger particles move toward the outside of the wall by virtue of their
momentum (Figure 1-2). The particles lose kinetic energy there and are separated
from the gas stream. Particles then are overcome by gravitational force and are
collected. Centrifugal and gravitational forces are both responsible for particle col-
lection in a cyclone.
Figure 1-2. Centrifugal force.
1-5
-------
In both fabric filters and wet collectors three separate forces are responsible for
panicle collection: impaction, direct interception, and diffusion. In a fabric filter,
the target object for particle capture is a stationary fiber. In a wet collector, the
target object is a water droplet that is introduced into the gas stream generally
through a nozzle.
Consider the case of an individual fiber in a fabric filter. Impaction occurs when
the particle is so large that it cannot follow the gas streamlines around the sta-
tionary fiber and impacts on the fiber (Figure 1-3). Direct interception is a special
case of the impaction mechanism. The center of a particle may follow the
streamlines formed around the fibers. A collision will occur if the distance between
the particle center and the collection surface is less than the particle radius (Figure
1-4). Particles below 0.1 /un in aerodynamic diameter undergo Brownian motion,
randomly moving or diffusing throughout the gas volume. The mechanism of diffu-
sion is responsible for the collection of particles which are so small that they
become affected by collisions of molecules in the gas stream. The randomly moving
particles then move or diffuse through the gas to impact on the fiber and are col-
lected (Figure 1-5).
Figure 1-3. Impaction.
Figure 1-4. Direct interception.
1-4
-------
Figure 1-5. Diffusion.
The other primary particle collection mechanism involves electrostatic forces.
The particles can be naturally charged, or, as in most cases involving electrostatic
attraction, be charged by subjecting the particle to a strong electnc field. The
charged particles migrate to an oppositely charged collection surface (Figure 1-6).
This is the collection mechanism responsible for particle capture in both elec-
trostatic precipitators and charged droplet scrubbers. In an electrostatic
precipitator, particle collection occurs due to electrostatic forces only. In a charged
droplet scrubber, particle removal occurs by the combined effects of impaction,
direct interception, diffusion and electrostatic attraction. Particles are charged in
these scrubbers in order to enhance both diffusion and direct interception.
Figure 1-6. Electrostatic attraction.
Particle collection can occur from the combined effect of the mechanisms
discussed. In addition, particles can agglomerate or grow in size by cooling,
increasing humidity or from electrostatic effects. Agglomerated particles thus have
a larger aerodynamic diameter and can be collected by impaction, interception or
gravitational forces.
Factors Affecting Control Equipment Selection
Many factors influence the choice of a control device used to reduce industrial par-
ticulate emissions. The composition of the particles in terms of concentration size,
chemical and physical characteristics must be considered. If emitted matenal can
be used in the process, dry collection should be used. If the pollutant has little
1-5
-------
economic value, collection should be accomplished and the pollutant disposed of
safely and economically. The industrial process and potential control devices must
both be carefully reviewed. The conversion of an air pollution problem into a
water pollution problem can create a more difficult disposal problem.
One selection factor to be considered is the concentration, or grain loading of
paniculate pollutants in the process exhaust stream. Pollutant concentration is
typically expressed in terms of pounds per cubic foot (lb/fts), grains per cubic foot
(gr/fts), and grams per cubic meter (g/ms). Both the level and fluctuation of grain
loading are very imponant. Some control devices are not affected by high levels or
great fluctuations in panicle concentration such as in fabric filters. Others, such as
electrostatic precipitators, do not function effectively with large fluctuating concen-
tration levels. Another related problem can occur when the exhaust gas velocity
changes rapidly. Some control devices are designated to operate at specific exhaust
gas velocities; large changes can drastically affect the collection efficiency of the
unit.
Panicle characteristics such as size, shape, and density must also be considered.
Particle size is usually expressed in terms of the aerodynamic: diameter. The
aerodynamic diameter describes how the panicle moves in a gas stream. The larger
particles (> 60 /zm aerodynamic diameter) can be collected in simple devices such
as settling or baffle chambers. Particles greater than 10 fim can be collected in
cyclones or multicyclones. Smaller particles (<5 /mi) must be collected in more
sophisticated devices such as scrubbers, baghouses or electrostatic precipitators.
Particle size thus plays a large role in the collection efficiency of a specific control
device. Methods used to size particles and the effect of size on particle collection
will be discussed in later chapters.
Chemical and physical properties of particulate emission!, greatly affect the
selection of control devices. Electrical properties of the particle can be both a
hinderance and aid to collection. Static electricity can create solid buildup in both
inertial and baghouse collectors. Cakes can form on the bag filters as a result of
the static electric forces and can be difficult to dislodge. In an electrostatic
precipitator, on the other hand, the collection of particulate matter depends on the
ability of the precipitator to charge the particle. Particles passing through an elec-
tric field are charged and consequently migrate to an oppositely charged collection
plate. Although there are many factors that govern the ease of charging the par-
ticles, the primary factor governing adequate particle collection is panicle dust
resistivity (a term that describes the resistance of the collected dust layer to the flow
of electric current).
If the panicles in the gas stream are explosive, electrostatic precipitators cannot
be used. Fabric filters might be used, but only if no static electric effects exist in
the baghouse. The logical choice of control would be a scrubber in which water is
used as the scrubbing liquid. The water has a dampening effect on the explosive
dust.
Hygroscopicity is the tendency of a material to absorb water. It is a physical
characteristic of some particles which causes changes in the crystal structure as
water is added. Some particles such as sodium sulfate can absorb up to 12 moles of
1-6
-------
water per mole of anhydrous salt in high humidity conditions. Other particles from
processes such as alfalfa dehydration and pelleting, and cotton ginning are
hygroscopic to varying degrees. The hygroscopic nature of the particle affec* the
performance of mTchanical collectors by causing dust deposes to build up on the.
External surfaces. This may cause internal plugging or unpredictable dust cake
Sparges into collection hoppers at various times. Hygroscopic particles also affect
^choice of cleaning in a baghouse by forming cakes on the bags that are difficult
to remove.
to remo. .
Particle toxicity will influence the location of the control device and the air
moving system (fan). Highly toxic materials require the use of a negative pressure
^Tsfthat leJwill be contained within the collector. A positive .pressure
system could cause fugitive emissions creating an occupational health and safety
problem. In a negative pressure system the fan is located downstream of the a r
£uuti?n control device'The volume of gas to be handled may increase shght^ by
air leakage into the collector, but little or no contaminant leakage from the collec-
tor should occur. .
The behavior of the carrier gas stream is also important in the design phases of
air pollution control systems. Gas stream temperature affects a Dumber of
variables in the design stages of the control device. The sue and thus Ae cost of
the unit depends on the temperature of the exhaust gas stream being treated. The
vo^uTe of g^s to be cleaned would be larger at high temperatures than that at cor-
responding lower gas temperatures. Reducing the temperature reduces the volume
of exhaust gas to be handled, however this could create some additional problem .
The gas strLn temperature must be maintained above the dew point of the gas to
prevent water and acid from condensing in the collector. Water and -odm*.
could cause corrosion and complete deterioration of the structural material of the
collector. High gas stream temperatures can also cause equipment failure to com-
ponents of a fabric filtration system. At exhaust gas temperatures greater than
SOO'C most fabric materials deteriorate. Gas temperature can also affect condit ons
such as particle resistivity. By changing the temperatures of the ^"««™£ ^
an electrostatic precipitator, one can also change the resistivity of the particles, and
thus the collection efficiency of the unit.
Efficiency/Cost Trade-Offs
In air pollution work, the control equipment should be designed to meet «*«"
limitations at minimum cost with maximum reliability. The basic trade-offs involve
decisions between collection efficiency, pressure drop, installauon cost and
operating costs. Of these, the principle one is the trade off between collecuon eff,
ciency and pressure drop (which can be translated into power requirements) across
the control device. _
Collection efficiency (by weight) can be defined by the following formula:
inlet loading -outlet loading x 1 00 or
collection efficiency = - ^ loading
1-7
-------
Emission limits are usually set by existing air pollution regulations. The control
to be achieved is dependent on the allowed outlet concentration and the quantity
of emissions generated from the process.
Equipment should be operated at the pressure drop specified by the design.
Pressure drop describes the pressure loss between the inlet and outlet sections of the
control device. Collectors with large pressure drops would require larger fans (and
greater power requirements) to either push or pull the exhaust gas through the
system. An increase in pressure drop means that there is a larger pressure loss in
the system. Some control devices such as venturi scrubbers are designed to operate
at high pressure drops (as great as 60 in. H«O; 14.9 kPa). On the other hand, elec-
trostatic precipitators are designed to operate at much lower pressure drops (usually
less than 10 in. H,O; 2.49 kPa) for similar collection efficiencies as venturi scrub-
bers. It may be advantageous however, to choose the scrubber as the control device
in this case, especially if the dust to be collected is explosive.
Other conditions such as space limitations for installation may affect the ultimate
decision for control equipment. Scrubbers generally require less installation space
than either baghouses or electrostatic precipitators. There are, of course, water
disposal problems with wet collectors that could sway the choice toward some other
type of control. Installed cost and operating costs vary for each type of collector.
For a given process, electrostatic precipitators may be more expensive to install
than baghouses. However, baghouses may be more expensive to operate and main-
tain on an annual basis. The cost trade-offs must be examined carefully in making
the final choice in control equipment..
This training manual discusses the operating principles, design specifications,
parameters affecting efficiency, and examples of equipment used in selected
applications. It is intended to assist in evaluating plans for paniculate emission
control systems and in conducting plan reviews.
1-8
-------
Chapter 2
Basic Concepts of Gases
Expression of Gas Temperature
The Fahrenheit and Celsius Scales
The range on the Fahrenheit scale between freezing and boiling is 180 degree-
units; on the Celsius or Centigrade scale, the range is 100 degree-units. Therefore,
1.0 Celsius degree-unit is equivalent to 1.8 Fahrenheit degree-units on a tempera-
ture scale (Figure 2-1). The following relationships convert one scale to the other:
(Eq. 2-1)
(Eq. 2-2)
°F=1.8°C+S2
c = (°F-S2)/1.8
Where: °F = degrees Fahrenheit
°C = degrees Celsius or degrees Centigrade
Fahrenheit
212° ~
Celsius
180 units (
32'
•
•
•
•
•
«
•
^
•
•
«
m
m
m
— Douing ivu — •
•
»
•
»
»
«
4
•
•
. 1.8
— freezing 0° —
t
.1.0
I 100 units
Figure 2-1. Companion of degree-units between Fahrenheit and Celsius.
NOTE: SI units arc listed in Appendix A.
2-1
-------
Absolute Temperature
Experiments with perfect gases have shown that, under constant pressure, for each
change in Fahrenheit degree below 32°F the volume of gas changes 1/491.67.
Similarly, for each Celsius degree, the volume changes 1/278.16. Therefore, if this
change in volume per temperature degree is constant, the volume of gas would,
theoretically, become zero at 491.67 Fahrenheit degrees below 32 °F or at
-459.67°F. On the Celsius or Centigrade scale, this condition occurs at 273.16
Celsius degrees below 0°C, or at a temperature of - 273.16°C.
Absolute temperatures determined by using Fahrenheit units are expressed as
degrees Rankine (°R); those determined by using Celsius units are expressed as
degrees Kelvin (K).* The following relationships convert one scale to the other:
(Eq. 2-3) °R=°F+459.67
(Eq. 2-4) K=°C + 273.16
The symbol T will be used to denote absolute temperature.
Expression of Gas Pressure
Definition of Pressure
A body may be subjected to three kinds of stress: shear, compression, and tension.
Fluids are unable to withstand tensile stress; hence, they are subject to shear and
compression only. Unit compressive stress in a fluid is termed pressure and is
expressed as force per unit area (e.g., lb/in.J or psi, Newtons/m* or Pa). Pressure
is equal in all directions at a point within a volume of fluid, and acts perpen-
dicular to a surface.
Barometric Pressure
Barometric pressure and atmospheric pressure are synonymous. These pressures are
measured with a barometer and are usually expressed as inches, or millimeters, of
mercury (Hg). Standard barometric pressure is the average atmospheric pressure at
sea level, 45° north latitude at 35 °F. It is equivalent to a pressure of 14.696
pounds force per square inch exerted at the base of a column of mercury 29.92
inches high. Weather and altitude are responsible for barometric pressure
variations.
•NOTE: Using the metric system the degree mark (°) is not used with the unit in the Kelvin
system. Throughout this manual degrees Celsius will be °C, degrees Kelvin will be K.
2-2
-------
Gage Pressure
Measurements of pressure by ordinary gages are indications of difference in
pressure above, or below, that of the atmosphere surrounding the gage. Gage
pressure, then, is ordinarily the pressure of the system. If greater than the pressure
prevailing in the atmosphere, the gage pressure is expressed as a positive value; if
smaller, the gage pressure is expressed as negative. The term vacuum designates a
negative gage pressure.
The symbol Pl is used to specify a gage pressure. Also, psig means pounds force
per square inch gage pressure.
Absolute Pressure
Most pressure measurements are taken with atmospheric (barometric) pressure as a
reference. The gage pressure (either positive or negative) is the pressure relative to
atmospheric pressure. The absolute pressure is the algebraic sum of the
atmospheric pressure and the gage pressure and is given as:
(Eq. 2-5) P=P» + P,
•Where: P = absolute pressure
Pk = barometric pressure
Pi = gaSe pressure
The symbol P is used to indicate that the pressure is absolute. Also, psia means
pounds per square inch absolute pressure.
The absolute pressure allows conversion of one pressure system to the other.
Relationship of the pressure systems are shown graphically in Figure 2-2 using two
typical gage pressures, (1) and (2). Gage pressure (1) is above the zero from which
gage pressures are measured, and, hence, is expressed as a positive value; gage
pressure (2) is below the gage pressure zero, and, therefore, is expressed as a
negative value.
2-3
-------
I
\
pf(l)
Gage pressure zero
/
i
i
o-
J
p«»
•""• *\
1
>P(2)
Absolute pressure zero
Figure 2-2. Two examples of absolute pressure determination.
Dalton's Law of Partial Pressure
Dalton's Law is used to determine the partial pressure of a gas compound in a gas
mixture. The pressure exerted by a mixture of gases is equal to the sum of the par-
tial pressures which each gas would exert if it alone occupied the whole volume.
The pressure exerted by one component of a gas mixture is called partial pressure.
The total pressure of the gas mixture is the sum of the partial pressures. Dalton's
Law is given as:
(Eq. 2-6)
Where:
Pr= total pressure of the gas mixture
p = partial pressure of each individual gas component
Molecular Weight
The molecular weight of a compound or an element is simply the sum of all the
atomic weights of all the atoms in the molecule. The atomic weight of an element
is the average isotopic mass and can be generally assumed to be constant. The
chemical identity of an atom is determined by the number of protons contained in
the nucleus. The number of protons contained in the nucleus is also called the
atomic number.
2-4
-------
Examining the element sulfur from the Periodic Table of Elements, one can see
the following:
16 Atomic number
s
52 Atomic weight
The atomic number specifies that the number of protons in the nucleus of the
sulfur atom is 16. Sulfur has an atomic weight of 32 (amu) atomic mass units.
However, a molecule of oxygen, Os, is comprised of two atoms of oxygen. Since
the atomic weight of oxygen is 16, one molecule of oxygen gas would have a
molecular weight of 32 amu.
0 = 0
16 16
Molecular weight = 32 amu
Mole
The mole concept in chemistry is described as the amount of substance that
numerically equals the molecular weight of that compound (usually expressed in
Krams). Thus, one mole or gram-mole of oxygen gas, O,, weighs 32 grams and one
mole of water, H,O, weighs 18 grams. The mole concept is used extensively in
chemistry since it greatly simplifies calculations.
The Laws of Ideal Gases
The Laws of Boyle and Charles
Boyle's Law states that, when the temperature (T) is held constant, the volume (V)
of a given mass of a perfect gas of a given composition varies inversely as the
absolute pressure, i.e.,
V«—at constant T
P
Where: « = proportional to
Charles' Law states that, when the volume is held constant, the absolute
pressure of a given mass of a perfect gas of a given composition varies directly as
the absolute temperature, i.e., P«T at constant volume.
2-5
-------
The Ideal Gas Law
Both Boyle's and Charles' Law are satisfied in the following equation:
(Eq. 2-7) PV= ^£!
M
Where: P=absolute pressure
V=volume of a gas
m — mass of gas
M — molecular weight of a gas
T= absolute temperature
R = universal gas constant
The unit of R depends upon the units of measurement used i.n the equation. Some
useful values are:
1544_CbXftL.
(lb mole)(°R)
3 (in. Hg)(ft')
(lbmole)(°R)
6jmm Hg)(ft')
(lbmole)(°R)
82.06-
(gmolcXK)
Any value of R can be obtained by using the fact, with appropriate conversion fac-
tors, that there are 22.414 liters per g mole or 359 fts per lb mole at 32 °F and
29.92 in. Hg.
Density
Density is defined as mass per unit volume, or:
(Eq. 2-8) C=H
*
2-6
-------
In the case of liquids and solids, the temperature at which density is measured is
given in tables of physical data such as those in Perry's Chemical Engineer Hand-
book. Gas densities refer to the density of that particular gas at a given
temperature and pressure.
A concept related to density is specific gravity, which is defined as the ratio:
(Eq. 2-9) m/V/m./V
Where m is the true weight of the substance being measured, and m. is the true
weight of water, both in the same volume and at a specified temperature.
Density is related to specific gravity by the following relationship:
Density= specific gravity x density of water.
As an example, the specific gravity of a calcium sulfate particle is 2.61. The
density of the particle is:
Density = 2.61 x 62.4 lb/fts
= 162.861b/fts
Viscosity
Origin and Definition of Viscosity
Viscosity is the proportionality constant associated with a fluid resistance to flow-
Viscosity is the result of two phenomena: (a) intermolecular cohesive forces, and (b)
momentum transfer between flowing strata caused by molecular agitation perpen-
dicular to the direction of motion. Between adjacent strata of a moving fluid, a
shearing stress (T) that is directly proportional to the velocity gradient occurs
(Figure 2-S).
Figure 2-S. Shearing streM in a moving fluid.
2-7
-------
This is expressed in the equation:
(Eq. 2-10) T = M —
dy
Where: T = unit shearing stress between adjacent layers of fluid
dv/dy= velocity gradient
H = proportionality constant (viscosity)
The proportionality constant, p, is called the coefficient of viscosity, or merely,
viscosity. It should be noted that the pressure does not appeair hi Equation 2-10
which indicates that the shear (T) and the viscosity (jt) are independent of pressure.
(Viscosity actually increases very slightly with pressure but this variation is negligi-
ble in most engineering problems.)
Kinematic Viscosity
The ratio of the absolute viscosity to the density of a fluid often appears in dimen-
sionless numbers such as the Reynolds Number. The expression for kinematic
viscosity is used to simpify calculations. Kinematic viscosity is defined according to
the following relationship:
(Eq. 2-11) v=JL
Where: v = kinematic viscosity, nWsec
/t= viscosity of the gas, Pa• sec
Q = density of the gas, kg/m1
Liquid Viscosity
In a liquid, transfer of momentum between strata having a relative velocity is small
compared to the cohesive forces between molecules. Hence, shear stress is
predominantly the result of intermolecular cohesion. Because forces of cohesion
decrease rapidly with an increase hi temperature, the shear stress decreases with an
increase hi temperature. Equation 2-10 shows that shear stress is directly pro-
portional to viscosity. Therefore, liquid viscosity decreases when the temperature
increases.
Gas Viscosity
In a gas, the molecules are too far apart for intermolecular cohesion to be effec-
tive. Thus, shear stress is predominantly the result of an exchange of momentum
between flowing strata caused by molecular activity. Because molecular activity
increases as temperature increases, the shear stress increases with a rise hi the
temperature. Therefore, gas viscosity increases as the temperature rises.
2-8
-------
Determination of Viscosity of Gases
The viscosity of a gas for prevailing conditions may be found accurately from the
following formula:
* -/ T V
- 2-12) —0 ~ { 273.2 /
Where • M = viscosity prevailing
Mo_ viscosity at 0°C and prevailing pressure
T= absolute prevailing temperature (K)
n= an empirical exponent (n= 0.768 for air)
The viscosity of air and other gases at various temperatures and at a pressure of 1
atmosphere may be found in engineering tables.
Units of Viscosity
The metric unit used to describe absolute viscosity is the Pascal second (Pa-sec).
EnglSi units are obtained by multiplying the value of Pascal second by 0.672.
English units for viscosity are lbm/ffsec.
Specific Heat
The specific heat of a gas is the amount of heat required to change the
r , ~f /T-.C «•.«*» t«rn"'"-!>tnTp-neoTee. Units OI speci
temperature of a unit-mass ol gas one iem
are, therefore, (Btu/lb»°F) or (joule/kg«K
USHeat may be added while the volume or pressure of the gas remains constant.
Hence Sere may be two values of specific heat: the specific heat at constant
vlCe C), and the specific heat at constant pressure (C,). Because the heat
ene^ added at constant pressure is used in raising the temperature and doing
work against the pressure as expansion takes place, C, is greater than U-
Reynolds Number
Definition
A tvpical inertial force per unit volume of fluid is Cv'/L; a typical viscous force per
^volumeTf fluid is £/L«. The first expression divided by the second pro-
vides the dimensionless ratio known as Reynolds Number:
(Eq. 2-13) Re =
2-9
-------
Where: Q = density of the fluid (mass/volume)
v = velocity of the fluid
L= linear dimension
ft = viscosity of the fluid
Re = Reynolds Number
The larger the Reynolds Number, i:he smaller is the effect of viscous forces; the
smaller the Reynolds Number, the greater the effect of the viscous forces.
The linear dimension, L, is a length characteristic of the flow system. It is equal
to four times the mean hydraulic radius, which is the cross-sectional area divided
by the wetted perimeter. Thus for a circular pipe, L= diameter of the pipe; for a
particle settling in a fluid medium, L = diameter of the panicle; for a rectangular
duct, L = twice the length times the: width divided by the sum; and for an annulus
such as a rotameter system, L = outer diameter minus the inner diameter.
Laminar and Turbulent Flow
In laminar flow, the fluid is constrained to motion in layers (or laminae) by the
action of viscosity. The layers of fluid move in parallel paths that remain distinct
from one another; any agitation is of a molecular nature only. Laminar flow occurs
when the Reynolds Number for circular pipes is less than 2000 and less than 0.1
for particles settling in a fluid medium.
In turbulent flow, the fluid is not restricted to parallel paths but moves forward
in a haphazard manner. Fully turbulent flow occurs when Reynolds Number is
greater than 2500 for circular pipes and greater than 1000 for settling particles.
2-10
-------
Chapter 3
Particle Dynamics
Collection of solid or liquid particles in an air pollution control device is based
upon the movement of a particle in the gas (fluid) stream. In order to understand
the mechanisms of particle capture, it is necessary to examine the basic concepts of
particle behavior in a fluid. To be captured, the particle must be subjected to
external forces large enough to separate the particle from the gas stream. In this
chapter, we will examine the various forces acting on a particle and how they
affect particle collection.
Forces Acting on a Particle
A force acting on a particle can be described by the general equation:
(Eq. 3-1) F = m,ap
Where: F= the force on the particle (kg»m/sec*) or (N)
mp = mass of the particle (g)
a, = accelerajion of the particle (m/secT)
A number of forces act on a particle moving in a fluid. Three major ones are
the gravitational force, Fc, the buoyant force, F,, and the drag force, FD. Others
are magnetic, inertial, electrostatic and thermal forces. All forces must be con-
sidered when evaluating the capture of a particle in an air pollution control device.
This discussion of particle movement in a fluid will examine the gravitational,
buoyant and drag forces. Electrostatic, inertial, and other forces will be discussed
in more detail in later chapters.
Gravitational Force, FQ
All particles are subjected to gravity. The gravitational force, F0, which causes par-
ticles and masses to fall to the earth can be expressed in the form of the general
Equation 3-1. Assuming that there are no other forces acting on the particle Fc is
given as:
(Eq. 3-2) Fc=m,g
Where: m, = mass of the particle (kg)
g= acceleration of the particle due to gravity (980 cm/sec1)
3-1
-------
The mass of the particle is equal to the panicle density ({>,) multiplied by the
particle volume (V,).
The volume of a spherical particle is equal to:
Where: d, = particle diameter
Equation 3-2, gravitational force, can then be written as:
(Eq. 3-3)
Where: QP = density of the particle (g/ons)
dp = particle diameter (/im)
g= acceleration of the particle due to gravity (980 cm/sec*)
Buoyant Force, FB
Another force acting on a particle suspended in an air stream is the buoyant force,
FB. The buoyant force acting on a particle is equal to the weight of the displaced
fluid. The concept of buoyant forces can be seen from the following example. Let's
say we have two identical buckets, one containing water, the other containing air
(Figure 3-1). A block of wood with identical size, shape, and density is placed in
each bucket. The density of air is much less than the density of water. The
buoyant force of the fluid (air) acting on the piece of wood in the bucket filled
with air is not great enough to displace the weight of the object. However, the
buoyant force of the fluid (water) in the second bucket is large enough to displace
the object. The object thus rises and floats on the fluid.
Figure 5-1. Identical objects in two different fluids.
3-2
-------
Particles in a fluid have both a gravitational and buoyant force acting on them
as shown in Figure 3-2.
Figure 5-2. Buoyant and gravitational forces acting on a particle.
The buoyant force can also be written from the general equation (Equation S-1) of
a force.
(Eq. 3-4) FJ, = m.g
Where: m. = mass of air or fluid displaced (g)
g= acceleration of the particle due to gravity (980 cm/sec1)
Fj can also be written as:
(Eq. 3-5) F* = C.V.g = C- ^- g
Where: c« = density of the (air) fluid (g/cms)
Comparing Equations 3-3 and 3-5, one can see that these equations are very
similar except that o, and C. represent the density of the particle and the density
of the fluid, respectively. For air pollution applications, QP is always much greater
than Q, when air is the fluid in which the particle is suspended. Therefore, the
term F« can frequently be ignored.
3-3
-------
Drag Force, FD
Whenever there is particle motion in a gas stream, there wiill be a resisting force
caused by the fluid (gas) molecules resisting the motion of the particle (Figure 3-3).
Particle movement
Figure 5-3. Drag force.
The resistive force caused by the fluid on the particle is called the drag force, FD.
The drag force is given by the expression:
(Eq. 3-6) F,- T
,
8
Where: CD = drag coefficient (unitless)
Vp = particle velocity (m/sec)
dp = particle diameter (/on)
Q. = density of the (air) fluid (g/cms)
The drag force arises when a particle moves through a fluid. The panicle cleaves
or displaces the fluid immediately in front of it, imparting momentum to the fluid.
The drag force produced is equal to the momentum (mv) per unit time imparted
to the fluid by the particle. Since the moving panicle has a velocity, v,, a portion
of the particle's velocity is transferred by momentum to the fluid as fluid velocity,
va. The amount of energy imparted from vp to va is related to a friction factor
which is called the drag coefficient, CD.
The value of CD is related to the velocity of the particle and the flow pattern of the
fluid around the particle. The Reynolds Number, Rep, of the particle is used as an
indication of this flow pattern. The Reynolds Number of the particle is a function
of the fluid density, particle diameter, particle velocity and fluid viscosity (and is
dimensionless). The Reynolds Number (developed by Osborne Reynolds in 1883) is
given as:
(Eq. 3-7) Re, = ^.'L (dimensionless)
M
Where: Q. = density of the (air) fluid (g/cms)
vp = particle velocity (m/sec)
d,, = particle diameter (fj.ro.)
ft= air (fluid) viscosity (g/on»sec)
3-4
-------
A mathematical expression describing CD is very complex, if one could be written
at all. Values of CD can be estimated by using plots of CD versus Reynolds Number
constructed from experimental data. It is essential to determine CD so that one can
solve for FD, the drag force, as in Equation 3-6. From experiment it has been observed
that three particle flow regimes exist. The three regimes are laminar (or Stokes),
transition and turbulent (sometimes called the Newtonian regime). These regimes
are related to the Reynolds Number of the particle (Re,). The relationship of CD
versus Re, is shown in Figure 3-4.
1000 -
Drag 100
coefficient,
CD 10
10-« 10-' 2.0 10 10110» 104 10'
Reynolds Number, Re,
Source: Chemical Engineer's Handbook, 1951.
Figure 3-4. Drag coefficient versus Reynolds Number for spheres.
For low values of the Reynolds Number (Re,<2) the flow is said to be laminar.
Laminar flow is defined as flow in which the fluid moves in layers, one layer
gliding smoothly over an adjacent layer with only a molecular interchange of
momentum between layers. For much higher values of the Reynolds Number
(Re,> 1,000) the flow is turbulent. Turbulent flow has erratic motion of fluid, with
a violent interchange of momentum throughout the fluid. For Reynolds Number
values between 2 and 1,000, the flow pattern is said to be in the transition regime
where the flow can be either laminar or turbulent.
Mathematical expressions relating the values of CD and Re, can be derived from
Figure 3-4. Equations for determining CD in each flow regime are:
(Eq. 3-8)
24
Re,
(dimensionless)
Laminar (Re,<2.0}
(Eq. 3-9)
Cn =
18.5
Re,
0.6
(dimensionless)
Transition (2 < Re, < 500)
3-5
-------
(Eq. 3-10) CD = 0.44 Turbulent (500 < Re, < 2 x 10s)
Cunningham Correction Factor, C/
If the size of the particle is greater than 3 /tm hi diameter, the fluid appears
continuous around the particle. What we mean by the description continuous is
that the particle is not affected by collisions with individual air molecules. Colli-
sions occur frequently on all sides of the particle (Figure 3-5),,
is- Air molecula
Figure 3-5. Collision* of air molecules on panicles greater than 3 pm in diameter.
However, if the particles are smaller than 3.0 /on in diameter, the fluid appears
discontinuous. This occurs for particles in the laminar flow regime. In this case,
the particles are affected by collisions of air molecules. These: collisions will cause
the particle to move in a direction related to the combined forces acting on the
particle. The particle is said to be slipping between the fluid molecules (Figure
3-6). To correct for this, Cunningham deduced that the drag coefficient should be
reduced. Thus the drag coefficient equation includes a term called the
Cunningham slip correction factor, C/. In the laminar flow regime Equation 3-8 is
corrected to include C/. The drag coefficient for the laminar regime thus becomes:
(Eq. 3-11) CD
Where: C/= Cunningham correction factor (dimensionless)
3-6
-------
Air molecules
Figure 5-6. Collisions of air molecules on particles less than 5 pm in diameter.
The Cunningham correction factor can be estimated by:
_ . (6.21 x 1Q-«)(T)
(Eq. 5-12) Q= 1 + -i ^
Where: T= absolute temperature (degrees Kelvin)
dp = particle diameter (/on)
A compilation of values of C, at various temperatures is provided in Table S-1.
Table S-1. Values of C/ (for air at atmospheric pressure).
Particle diameter
(micrometers)
0.1
0.25
0.5
1.0
2.5
5.0
10.0
Value of Cf at temperature of:
70 "F
2.88
1.682
1.325
1.160
1.064
1.0S2
1.016
212°F
3.61
1.952
1.446
1.217
1.087
1.043
1.022
500°F
5.14
2.528
1.711
1.338
1.133
1.067
1.033
Source: Lapple, 1951.
3-7
-------
Calculation of fD
The drag force can be calculated by substituting the proper CD expression into
Equation 3-6. The equations for calculating FD in all three flow regimes are:
F.» ?2Sfe4 Laminar (Stokes)
vr
(Eq. S-14) FD = 2.SlT(d,v,)>V'C.04 Transition
(Eq. S-15) FD - O.OBBT^v^'o. Turbulent
The streamline (laminar) flow of a fluid around a sphere (particle) was
investigated by G. Stokes in 1845 for small values of the Reynolds Number. Stokes
found that the drag force of the fluid around a panicle is a function of the gas
viscosity, particle velocity, and particle diameter and is given by Equation 3-13. This is
known as Stokes law and is valid for Reynolds Numbers less than 2.0. Stokes law is
important in determining the settling velocity of a particle and is used in the design
of settling chambers for paniculate emission control.
Balance of Forces on a Particle
Newton's second law of motion states that the acceleration produced in a given
mass by the action of a given force is proportional to the force and in the direction
of that force. The second law is simply a statement of the equation F = ma. The
sum of the forces can be written as:
&dv
= ma = m —
dt
Where: dv/dt = acceleration or change in velocity with respect to time
3-8
-------
A particle in motion in a fluid will be affected by a number of forces
(Figure 3-7).
Figure 5-7. Vector nun of forces acting on a particle.
The vector sum of the forces is equal to a resultant force, F*.
_ _ dv
fa"_ SI r f* ^™* P o ^™ f n ^™ LU ««^^»
** *G " D dt
As previously stated, the density of the particle is much greater than the density
of air and the term F, can be ignored. As the particle accelerates the velocity wUl
increase The drag force on the particle also increases with increasing velocity .-At
some point there will be a value of velocity where FD will be as large as the other
forces At this point the resultant force will be zero, and the particle will no longer
accelerate. If the particle is not accelerating, then it must be at a constant velocity.
This constant velocity, where all the forces balance out, is called the terminal
settling velocity.
F,, = Fc - Fa - FD = 0 at terminal settling velocity
then:
Derivation of Particle Settling Velocity Equations
The particle settling velocity equation is derived by setting Fc equal to FD.
Substitute the values for Fc and FD from Equations 3-3 and S-lSrespectiyely^
_ gpTdp'g
(Eq. 3-3) Fo= «*-£"
SydpV,/*
(Eq. 3-13) F,- -57-
3-9
-------
Solving for vp (which is now v,):
(Eq. 3-16) v,= -%^-C, (jri/sec) Laminar Regime
18 n
Similar derivations for the settling velocity for the other regimes are:
(Eq.W7) v.~ (U5Sr.d..ue,... • Transition R.gta,
.|t MO« _
(Eq. S-18) v,= iW-Sifii.)' '' (m/sec) Turbulent Regime
Determination of the Flow Regime
To solve for an unknown particle settling velocity, the flow regime of particle
motion must be determined. This is done to select the correct settling velocity
equation. In other words, the Re, and consequent flow regime cannot be deter-
mined because the velocity is unknown. The flow regime can be determined by the
following equation:
(Eq. 3-19) K = dp - (dimensionless)
Values of K correspond to the different flow regime as can be seen from
Table 3-2.
Table 5-2. K value* for flow regime determination.
Laminar regime
Transition regime
Turbulent regime
K<3.3
S.S43.6 .
Once the flow regime has been determined, the correct formula can be used to
calculate the settling velocity of the particle.
3-10
-------
Example Problem
Calculate the settling velocity of a particle moving in a gas stream. Assume the
following information:
o, = 0.899 g/cms
g=980 cm/sec*
d,, = 45 jim
Q= 1.0 (if applicable)
Solution:
1. Calculate the K parameter to determine the proper flow regime. Use Equation S-19.
= 45 x 10'4 cm
'3.07
-82X10-*
Therefore, the flow regime is laminar.
2. The settling velocity is calculated using Equation 3-16.
18
/980-E3EL\/o.899-M (45 X 10-«cm)'
\ sec* A cm* /
isfi.
(1.0)
82 x 10'*
cm»sec/
= 5.38 cm/sec
5-11
-------
References
1. Notes and Personal Communication with Donald A. Deieso, Rutgers Uni-
versity,, 1980.
2. Notes and Personal Communication with Cliff I. Davidson, Carnegie Mellon
University, 1979.
3. Lapple, C. E. 1951. Fluid and Particle Mechanics. Newark, Delaware: Univer-
sity of Delaware.
4. Theodore, L. and Buonicore, A. J. 1976. Industrial Air Pollution Control
Equipment. Cleveland: CRC Press, Inc.
5. Cheremisinoff, P. N. and Young, R. A., eds. 1977. Air Pollution Control and
Design Handbook, Part 1. New York: Marcel Dekker, Inc.
6. Sweeter, V. and Wylie, E. B. 1975. Fluid Mechanics. 6th ed. New York:
McGraw-Hill Book Company.
7. Perry, J. H., ed. 1950. Chemical Engineer's Handbook. 3rd ed. New York:
McGraw-Hill Book Company.
3-12
-------
Chapter 4
Particle Sizing
Introduction
With the high cost of paniculate control equipment and more stringent regula-
tions, particle sizing data has become an increasingly important consideration in
the design of paniculate control equipment. Appropnate design is directly
dependent on good particle size data.
Several methods are used to obtain particle size data from industrial sources.
This section will briefly describe a number of these methods and their operating
principles. The mathematical treatment of collected data will also be presented.
Size
Particle size is usually expressed by some diameter that is measured A particle
diameter, however, is dependent on the measurement method by which it is deter-
mined. Diameters can be based on projected area, surface area, volume, mass, etc.
Particle size is uniquely defined by particle diameter only for the case of
spherical particles. Unfortunately, except for liquid droplets, certain metallurgical
fumes, and combustion emissions, particles are usually not spherical To deal with
nonspherical particles it becomes necessary to define an equivalent diameter term
that depends upon the various geometrical or physical properties of the particles^
Some of the methods used to express the size of a nonspherical particle measured
by microscopy are illustrated in Figure 4-1.
Circle of equal area
Figure 4-1. Diameten of nonipherical particles.
4-1
-------
Ferret's diameter is the mean length between two tangent*; on opposite sides of
the particle perpendicular to the fixed direction of the microscope scan. Martin's
diameter measures the diameter of the particle parallel to the microscope scan that
divides the particle into two equal areas. The diameter of a circle of equal area is
obtained by estimating the projected area of the particle and comparing it with a
sphere that approximates its size.
In terms of paniculate control equipment, interest is centered primarily on the
aerodynamic behavior of the particle. The aerodynamic diameter is defined as the
diameter of a sphere of unit density having the same falling speed in air as the par-
ticle (Figure 4-2). The aerodynamic diameter is a function of the physical size,
shape, and density of the particle. The aerodynamic diameter is useful when
designing certain control devices and is usually measured by a device called an
impactor.
Figure 4-2. Aerodynamic diameter.
Various Sizing Devices
If one were able to design an ideal particle measuring device, the device would
have the following features.
It would be able to:
1. measure the exact size of each particle.
2. report data instantaneously without averaging data over some specified
time interval.
3. determine the complete corapasition of each panicle including shape,
density, chemical nature, etc.
4-2
-------
,, Would be an extremdy difficult
Microscopy m
time consuming and extremely tedious.
Figure 4-3. Microicope.
panides can be coUected
4-S
-------
tides existed in the process stream as agglomerates of smaller particles. In spite of
the limitations of the microscopic method, this method is useful in the determina-
tion of some properties of interest.
Generally speaking, the chemical composition of the particle cannot be obtained
by using an optical microscope. However, a subsequent chemical analysis can be
performed on the sample. The electron microscope, on the other hand, can give a
detailed chemical analysis of the particle. The electron microscope is used in con-
junction with an x-ray diffraction attachment to determine the molecular weight
of the particle. Particles with molecular weights greater than carbon can be deter-
mined by the amount of radiation diffracted by each particle analyzed.
The optical microscope can measure particles from about 0.5 pm to about
100 fun in diameter. Electron microscopes can measure particles with diameters as
small as 0.001 jon. This could be useful for examining extremely minute particles
(see Table 4-1). F
Optical Counters
Optical particle counters have not been widely used for particle sizing because they
cannot be directly applied to the stack exhaust gas stream. The sample must be
extracted, cooled, and diluted before entering the counter. This procedure must be
done with extreme care to avoid introducing serious errors in the sample. The
major advantage of the counter is its capability of observing emission (particle)
fluctuations on an instantaneous level. One can size particles as small as 0.3 /un
with the optical counter (Table 4-1).
Optical particle counters work on the principle of light scattering. Each particle
in a continuously flowing sample stream is passed through a small illuminated
viewing chamber. Light scattered by the particle is sensed by the photodetector
during the time the particle is in the viewing chamber (Figure 4-4). The intensity
of the scattered light is a function of particle size, shape and index of refraction.
Optical counters will give reliable particle size information if only one particle is in
the viewing chamber at a single time. The simultaneous presence of more than one
particle can be interpreted by the photodetector as a larger sized particle. This
error can be avoided by maintaining sample dilution less than 300 particles per
cubic centimeter.
A disadvantage of the optical counter is the dependence of calibration instru-
ment upon the index of refraction and shape of the particle. Errors in counting
can also occur from the presence of high concentrations of very small particles
which are sensitive to the light wavelength used.
4-4
-------
Inlet
Senior chamber
View volume
Photomultiplier
Lamp
Calibrator
Source: EPA, 1979. EPA/4-79-028.
Figure 4-4. Operating principle for an optical particle counter.
Electrical Aerosol Analyzer
The electrical aerosol analyzer (EAA) is an aerosol size distribution measuring
device that was commercially developed at the University of Minnesota. The EAA
uses an electric field (which is set at an intensity dependent upon the size and mass
of the particle) to measure the mobility of a charged aerosol. The analyzer operates
by first placing a unipolar charge on the aerosol being measured, and measuring
the resulting mobility distribution of the charged particles by means of a mobility
analyzer.
4-5
-------
Charged particles enter through a narrow passage (A) and experience a radial
force toward the central cylinder due to the applied field. By moving the sampling
groove (B), axially, or by varying the applied field, the mobility of the charged
particles can be measured. One type of EAA is shown in Figure 4-5 (Hewitt, 1957).
Charged panicle*
Variable direct
voltage
Discharged particles
Source: EPA, June 1977. EPA 600/7-77-059.
Figure 4-5. Coaxial cylinder mobility analyzer.
The EAA has been used for source analysis by pulling a sample from the stack
into the chamber and introducing the gas stream into the analyzer. The instrument
requires that enough particles pass through the chamber so that a charge can be
detected. The concentration range for most efficient operation of the EAA is from
1 to 1000 /ig/m*. Since stack gas concentrations usually exceed 1000 ng/ms, sample
dilution with dean air is required (EPA, 1979, EPA 600/7-79-028). No information
on the chemical composition of the panicles is possible since the particles are not
collected. The major advantage of the EAA is that the instrument can measure
particles from 0.003 to 1.0 /tm in diameter.
4-6
-------
Bahco Microparticle Classifier
The Bahco (Figure 4-6) is a versatile particle classifier used for measuring powders,
dust, and other finely divided solid materials. The Bahco's working range is
approximately 1 to 60 /on (Table 4-1). Developed in the 1950s, the Bahco has lost
some of its initial appeal to more recently developed techniques.
The Bahco uses a combination of elutriation and centrifugation to separate par-
ticles in an air stream. Particles can be collected onto a filter by using an EPA
Method 5 sampling train. The collected particles are subsequently analyzed in the
lab (Figure 4-6).
A weighed sample, usually 5 grams, is introduced into a spiral-shaped air cur-
rent to separate the particle fractions. The larger particles overcome the viscous
forces of die fluid and migrate to the wall of the chamber, while the smaller par-
ticles remain suspended. After the two size fractions are separated, one of them is
^introduced into the device and is fractionated further. A different spin speed is
used to give a slightly different centrifugal force. This is repeated as many tunes as
desired to give an adequate size distribution. The measurements are grouped into
discrete size ranges (i.e., 40-60 Mm, 20-40 fim, etc.).
The Bahco provides information on the aerodynamic size of particles. This data
can be translated into settling velocity information useful in the design of emission
control devices. Several hours are required to complete the fractionation analysis.
Once the particles have been fractionated into the discrete ranges, a chemical
analysis can be done on the collected particles.
Source: Stockman and Fochtman, 1977.
Figure 4-6. Bahco sampler.
4-7
-------
Some of the major drawbacks of the Bahco are that:
• the working range is limited between 1 and 60 /un;
• care must be exercised when measuring certain types of panicles especially
those which are friable, or hygroscopic;
• the sample may not be representative due to panicle agglomeration either in
the stack or during the transfer of the collected sample to the lab;
• the length of time required for analysis is several hours or more; and
• the size grading regimes are not as sharp as with newer devices.
Importers
Inertia! impactors are commonly used to determine the panicle size distribution of
exhaust streams from industrial sources. Inenial impactors measure the
aerodynamic diameter of the particles. The inenial impactor can be directly
attached to an EPA Method 5 sampling train and easily inserted into the stack of
an industrial source.
The mechanism by which an impactor operates is illustrated hi Figure 4-7. The
impactor is constructed using a succession of stages each containing orifice
openings with an impaction slide or collection plate behind the openings. In each
stage, the gas stream passes through the orifice opening and forms a jet which is
directed towards the impaction plate. The larger particles will impact on the plate
if their momentum is large enough to overcome the drag of the air stream as it
moves around the plate. Since each successive orifice opening is smaller than those
on the preceeding stage, the velocity of the air stream, and'therefore that of the
dispersed particles, is increased as the gas stream advances through the impactor.
Consequently, smaller particles eventually acquire enough momentum to break
away from the gas streamlines to impact on a plate. A complete particle size
classification of the gas stream is therefore achieved.
A--.S.-; *«-iy. Path of small
r V particle
Figure 4-7. Schematic diagram, operation of an inenial impactor.
4-8
-------
many as 400 precisely drilled jet orifices, 1(1^n""1 4.g) Adhesive> dec-
decreasing in diameter in each succeeding,staS ^ * fa fa ^ to ^ Col-
trostatic, and van der Waals forces hold Ae Part^°o^e collecting platc by
surfaces. Moreover, the particles are^not^D o ^^ ^ ^ ^ turbulent
Noule
Inlet cone
base
Sierra model 226
Backup
filter
Plate
holder
Jet wage (9 total)
Spacers
Glass fiber
collection
substrate
Nozzle
4 Inlet
Core
Anderson Mark HI
Figure 4-8. Schematics of two commercial cascade impactors.
4-9
-------
Particles are collected on preweighed individual stages, usually filters made of
glass fiber or thin metal foil. Once the sample is complete, the collection filters are
weighed again, yielding particle size distribution data for the various collection
stages. Occasionally there are some dusts that are very difficult to collect, and
require grease on the collection filter for adequate particle capture. Once the par-
ticles have been fractionated into discrete ranges, a chemicail analysis can be per-
formed on the collected particles.
The effective range for measuring the aerodynamic diameter is generally between
0-9 and 20jtmJTable 4-1). Some vendors have claimed size fractionation as small
as O.O^jtm with the use of 20 or morestajres. Impactors are one of the most useful
devices for determining particle sizeTTEsbbecause of the iimpactor's compact
arrangement, mechanical stability, and its ability to draw a sample directly from a
stack. In addition, the impactor measures the aerodynamic diameter of particles,
which describes the movement of the particles in a gas stream. Particle movement
information is extremely useful in designing air pollution control equipment,
especially mechanical collectors which depend on aerodynamic drag forces for par-
ticle collection.
Comparison of Particle Sizing Devices
Five particle sizing instruments have been briefly described in the previous sections.
These are by no means the only ones available; there are others such as the con-
densation nuclei counter and the diffusion battery, cyclones placed in series, laser
backscattering instruments, and multiwavelength transmissometers (EPA 1979
EPA-600/7-79-028).
Comparisons have been made among the various devices mentioned by listing
several advantages/disadvantages of each.* As previously stated, the ideal instru-
ment would measure the exact size of each particle, yield instantaneous response,
and determine the complete composition of the particle. Table 4-1 shows the effec-
tiveness of each device over different particle size ranges. Table 4-2 shows the
major features of each device.
From Table 4-2 one can compare the major features of the various measuring
devices. One can see that the single particle level symbol «^)) describes how the
device measures each individual particle from the sample. The discrete ranges
symbol (-^) is used to describe the devices that classify the particles into discrete
ranges such as 5-10 fan, 10-15 fim, 15-20 fan, etc. The integrated averaging process
—• •^^••^H • • - I » — — — - — —— •— —— WWQ* VB*^. w» *A v ^* MKAAAK r" »•
symbol (| / j) is used to describe the time period (averaged) in which the device
measures size and chemical composition of the particle.
•EPA Guidelines for Paniculate Sampling in Gaseous Effluents from Industrial Processes
EPA-600/7-79-028, IERL, RTF, NC 27711, January 1979.
4-10
-------
Table 4-1. Size range capabilities of measuring devices.
Optical counter J
Electrical aerosol analyzer
Bahco counter
Inertial impactor
J_
J_
J.
0.001
0.01
0.1 1
(microns)
10
100
Table 4-2. Comparison of particle sizing devices.
Device
Ideal
Microscope
Optical counter
EEA
Bahco counter
Impactor
Size
O
o
<§
^
^
^
Time
<0
/
o
<§
CD
/
Composition
O
^
^
Single particle level
Discrete ranges
f \ Integrated averaging process
4-11
-------
Mathematical Treatment of Data
There are various ways to analyze or reduce the data generated from a particle
sizing device. The most common methods of expressing particle size data are
through the use of frequency distribution curves (histograms) or cumulative
distribution curves. The cumulative plot is a cumulative distribution curve. All
forms of graphical presentation employ one axis to represent particle size and the
other axis to represent particle amount. The particle amount can be expressed by
either the mass of particles or the number of particles.
Frequency Distribution Curve
Frequency distribution curves are usually plotted on regular coordinate (linear)
paper. The curve describes the amount of material (particles) falling within each
size range. When gas borne particles produced in industrial operations are
measured, the data has a tendency to show a preferential particle size. A plot of
percent mass versus particle size (d,) on a linear scale gives a curve with a peak at
the preferential size. Such a curve is shown in Figure 4-9.
I I I I I I 1 T
Particle size (d,), /on (linear scale)
Figure 4-9. Particle size distribution with one preferential size.
Figure 4-9 shows a normal probability distribution which is symmetrical about
the preferential size. This curve is rarely encountered for dusts consisting of very
fine particles. This curve may be found for particles such as fumes formed by
vapor phase reaction and condensation or for tar and acid mists.
4-12
-------
Another representation of the normal probability of the distribution can be seen
in Figure 4-10. This curve is skewed or off-center about the preferential sue. This
type of curve usually occurs when very fine industrial dust data are plotted.
Particle size (d,), fun (linear icale)
Figure 4-10. Skewed particle size distribution plotted on a linear scale.
The normal probability of the distribution can also be plotted on log scale. The
representation of the distribution can be seen in Figure 4-11. The percent mass
(concentration) is plotted on a linear scale as the y-axis. The particle size d,, is
plotted on a log scale as the x-axis. If the curve takes on the bell-shape, the
distribution appears normal.
Particle size (d,), i»m dog scale)
Figure 4-11. Typical particle size distribution plotted on a log scale.
4-1S
-------
Figure 4-12 shows a particle size distribution with two peaks. The curve simply
shows that the data has two preferential sizes instead of one, as shown in
Figure 4-9.
!
Particle size (d,), /tin (linear scale)
Figure 4-12. Typical particle size distribution with two preferential sizes.
Construction of the Frequency Distribution Curve
Frequency distribution curves are usually plotted on linear Coordinate paper. The
percentage by weight (or the frequency) is plotted as the ordinate. The average
particle size of each size range (size fraction) is plotted on the abscissa.
Since the curve is plotted using average particle diameters within each range,
there may be several positions of the curve for a given dust. Each position is depen-
dent on the series of size ranges used.
There are a number of methods for selecting size ranges for the construction of
the frequency distribution curve. Two of the more common are:
• select equal arithmetic increments of size as shown in Table 4-3.
Table 4-3. Size ranges in arithmetic increments.
Size range (/on)
0- 2
2- 4
4- 6
6- 8
8-10
>10
Percent in size
10
15
SO
SO
10
5
range
4-14
-------
• choose size ranges bounded by sizes having the same ratio to each other as
shown in Table 4-4.
Table 4-4. Size ranges with the same ratio.
Size range (pm)
0- 5
5-10
10-20
20-40
40-80
>80
Percent in size range
15
10
SO
20
20
10
It is evident from this discussion that a large number of points are necessary to
fix the position of the frequency curve. If this were the only method of representing
the particle size data of a paniculate sample, particle size analysis would be an
extremely laborious and time consuming procedure.
Cumulative Distribution Curves
Particle size data can also be plotted as a cumulative plot. Particle size of each size
range is plotted on the ordinate. The cumulative percent by weight (frequency) is
plotted on the abscissa. The cumulative percent by weight can be given as
cumulative percent less than stated particle size or cumulative percent larger than
stated particle size. The cumulative percent by weight can be plotted on either a
linear percentage or a probability percentage scale. The particle size range
(ordinate) is usually a logarithmic scale.
If the particle size, d^of each size range is plotted versus the cumulative percent
larger than dp (linear scale) one could get a distribution curve as shown in
Figure 4-13.
1 1—i r—
T
J L
JL-J 1 1 1 1—3
100
Mast larger than % (linear scale)
Figure 4-13. Cumulative particle size distribution plotted on a linear scale.
4-15
-------
In Figure 4-13, the distribution approaches the 0% and 100% values
asymptotically. It is evident that the cumulative distribution in this figure is not a
straight line for the entire range of particle size in the sample. The majority of the
size ranges occur toward the 0% size and the 100% size.
More frequently the cumulative distribution is plotted on special coordinate
paper called log probability paper. The panicle size of each size range is plotted on
the logarithmic ordinate. The percent by weight larger than d, is plotted on the
probability scale as the abscissa (Figure 4-14). This allows one to expand the
cumulative distribution axis near 0% and near 100%. By expanding the axis, the
distribution plots out as a straight line if the frequency distribution plot is skewed
as in Figure 4-10. The cumulative distribution plot in Figure: 4-14 is identical to
that in Figure 4-IS, except that the percentage scale is expanded near 0% and
100%. It should be noted that one can just as easily plot percent mass less than d,
on the abscissa.
The Log-Normal Distribution
When measuring dusts from industrial sources, the graph of the particle size
distribution often displays the logarithmic variation of the normal distribution. As
can be seen in Figure 4-10 the normal distribution has a fundamental defect
related to its use in particle sizing analysis. Implicit in the statement that a random
variable is normally distributed is the concept that the values of particle size are at
equal distances from the central tendency or preferential size. Suppose the mean
particle size or central tendency of the distribution were 20 /,an. It would be equally
probable to find either a 15 /tin or 25 fim particle. One might also find a particle
the size of 50 /on in the distribution. Were the distribution normal, it would be
equally likely to find a particle the size of minus 10 fan.
However, if it can be assumed that the logarithm of the particle size is randomly
distributed, then this problem is avoided. The ratios of particle size about a central
tendency are equally probable, and the ratios are bounded on the lower end by
zero.
The usefulness of the log-normal distribution is most evident when the frequency
distribution curve is characteristically skewed, as in Figure 4 10. The data is
plotted as a cumulative plot on log probability paper. If the distribution follows the
log-normal relationship, then the plot will result hi a straight line. The linearity of
the relationship allows one to describe the distribution statistically with a minimum
of individual observations. The distribution is completely specified by two
parameters, the geometric mean, d,m, and the geometric standard deviation, afm.
The geometric mean value of a log-normal distribution can be read directly
from a plot similar to that represented hi Figure 4-15. The geometric mean size is
the 50% size on the plot.
4-16
------- An error occurred while trying to OCR this image.
------- An error occurred while trying to OCR this image.
-------
The geometric standard deviation is a good measure of the dispersion or spread
of a distribution. The geometric standard deviation is the root-mean square devia-
tion about the mean value. Its derivation and application in significance testing
and setting of confidence levels can be found in most statistics textbooks. The
geometric standard deviation is identical for specifying the size distribution of a
log-normal distribution, whether by particle number, surface, mass, or any other
quantity of the form kd,-, where k is a parameter common to all particles and d, is
the diameter. Plots of cumulative distribution on log-probability paper are then
parallel straight lines for number, mass, or surface which leads to a great
simplification and easy graphical technique.
The geometric standard deviation can be read directly from a plot such as shown
in Figure 4-15. For a log-normal distribution (that plots d, maximum versus per-
cent mass larger than d,), the geometric standard deviation is given by:
50% size
84.13% size
or
15.87% size
50% size
I
All one must do is determine the 50% size and the 84.13% size from the plot
and divide to determine the geometric standard deviation.
Example of a Typical Particle Size Data Reduction
Consider the data listed in Table 4-5:
Table 4-5. Typical particle rize data.
Size range
(m)
1- 2
2- 4
4- 6
6-10
10-20
20-40
>40
Concentration
0.8
12.2
25.0
56.0
76.0
27.0
3.0
Concentration
Ad,
0.8/(2. 0-1.0)= 0.8
6.1
12.5
14.0
7.6
1.S5
0.075
Concentration
A log dp
0.8/[log2-logl]= 2.66
40.53
142.0
252.3
253.2
89.7
10.0
Suppose we were asked to determine if the distribution was log-normal, and if
so, determine the geometric mean diameter (d^) and the geometric standard
deviation (a,,). How would one approach this problem?
4-19
-------
First we could plot the mass concentration (concentration/Adp) versus particle
diameter (dp) on a linear scale (Figure 4-16.)
20
I
5 10-
j
20 SO 40 50 60
Particle size d,, pm (linear scale)
70
80
Figure 4-16. Man concentration versus panicle diameter on a linear scale.
The data appears to be skewed and would therefore lead one to believe that the
distribution could be log-normal.
We can now plot mass concentration (concentration/A log dp) on a log scale
(Figure 4-17). The particle size dp (x-axis) is plotted on a log scale. The area under
the smooth curve between two different values of dp represents the total particle
mass concentration between the two values of dp. If the y-axis is not "concentration
divided by A log dp" then the distribution shape will be partly determined by the
size ranges of the sampling device (i.e. impactor stages) rather than the true parti-
cle distribution shape.
4-20
-------
We can now plot mass concentration versus particle diameter (d,) on a log scale
(Figure 4-17).
300
250
200
150
100
50
45 10
Particle rize d,,
20 30 40 50
(logicale)
100
Figure 4-17. Man concentration veriiu particle rize on a log icale.
This plot yields an appropriately shaped bell curve that appears to indicate that
this is a log-normal distribution. However, it is extremely important to plot the
data on log-probability paper to see if a straight line results. If the distribution
plots a straight line on log probability paper, then the distribution is log-normal.
First we must calculate the percent in each size range and then the cumulative
percent larger than dp maximum (Table 4-6).
Table 4-6. Cumulative panicle rize data.
Size range
dan)
0- 2
2- 4
4- 6
6-10
10-20
20-40
>40
Concentration
(/ig/m')
0.8
12.2
25.0
56
76
27
3
200
Percent
weight
in rize range
0.4
6.1
12.5
28
38
13.5
1.5
Cumulative
percent larger
99.6
93.5
81.
53.
15.
1.5
^"^
4-21
------- An error occurred while trying to OCR this image.
-------
The plot (in Figure 4-18) on log probability paper yields a straight line.
Therefore the distribution is log-normal. To determine the geometric mean
diameter one can read the 50% size from Figure 4-18.
,= 10.5 micrometers
The geometric standard deviation is:
50% size
^ 84.13% size
= 10-5
5.5 .
= 1.9
References
1. Stockman, J. D. and Fochtman, E. G. eds. 1977. Particle Size Analysis. Ann
Arbor, Mich: Ann Arbor Science Publishers Inc.
2. McFarland, A. R., 1978. Aerodynamic Particle Sizing. College Station, Texas:
Air Quality Laboratory Publication.
3 Environmental Protection Agency (EPA). 1979. Guidelines for Particulate
Sampling in Gaseous Effluents from Industrial Processes. EPA 600/7-79-028.
4 Environmental Protection Agency (EPA). 1977. Inertial Cascade Impactor
Substrate Media for Flue Gas Sampling. EPA 600/7-77-060.
5. Environmental Protection Agency (EPA). 1976. Particulate Sizing Techniques
for Control Device Evaluation: Cascade Impactor Calibrations. EPA
600/2-76-280.
6. Environmental Protection Agency (EPA). 1979. Proceedings: Advances in Par-
ticle Sampling and Measurement (Asheville, NC, May 1978). EPA
600/7-79-065.
7. Environmental Protection Agency (EPA). 1977. Procedures Manual for Elec-
trostatic Precipitator Evaluation. EPA 600/7-77-059.
8. Hewitt, G. W. 1957. The Charging of Small Particles for Electrostatic
Precipitation. AIEE Winter General Meeting. Paper no. 73-283. New York.
9. Notes and Personal Communication with Cliff I. Davidson, Carnegie Mellon
University, 1979, 1981, 1982.
4-23
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Chapter 5
Gravity Settling Chambers
Introduction
Long used by industry for removing solids or liquid paniculate matter from
gaseous streams, gravity settlers or gravity settling chambers have the advantages of
simple construction, low initial cost and maintenance, low pressure losses, and
simple disposal of collected materials.
The gravity settler was one of the first devices used to control paniculate emis-
sions. It is an expansion chamber in which particle velocity is reduced, thus
allowing the particle to settle out under the action of gravity. One primary feature
of this device is that the external force causing separation of particles from the gas
stream is provided free by nature. This chamber's use in industry, however, is
generally limited to the removal of larger sized particles, 40-60 /tm in diameter.
Today's demands for cleaner air and stricter emission standards have relegated
the settling chamber to use in research for testing or as a precleaner for other par-
ticulate control devices (cyclones, fabric filters, electrostatic precipitators, and
scrubbers).
Settling chambers have been used in a number of research projects to study the
flow of particles in a gas .stream. The data generated from these studies is useful in
the design of other paniculate emission control devices.
Equipment Description
There are basically two types of gravity settlers: the simple expansion chamber and
the multiple-tray settling chamber. A typical horizontal flow (simple expansion)
gravity settling chamber is presented in Figure 5-1. The unit is constructed in the
form of a long horizontal box with inlet, outlet and dust collection hoppers. These
units depend on gravity for collection of the particles. The particle-laden gas
stream enters the unit at the gas inlet. The gas stream then enters the expansion
section of the duct. Expansion of the gas stream causes the gas velocity to be
reduced. All particles in the gas stream are subject to the force of gravity.
However, at reduced gas velocities (in the range of 0.305 to 3.05 m/sec) the larger
particles can be overcome by gravity and fall into the dust hoppers. Theoretically,
a settling chamber of infinite length could collect even the very small particles
(<10 jun).
5-1
-------
The collection hoppers located at the bottom of the settler are usually designed
with positive seal valves and must be emptied as dust buildup occurs. Dust buildup
will vary depending on the concentration levels of paniculate matter in the gas
streams, especially for heavy concentrations of particles greater than 60 /un in
diameter.
Particle laden ,
gas inlet
Gas outlet
~ Dust collection hoppers
Figure 5-1. Horizontal flow settling chamber.
The multiple-tray settling chamber, also called the Howard settling chamber, is
shown in Figure 5-2. Several horizontal collection plates are introduced to shorten
the settling path of the particle and to improve the collection efficiency of small
particles (as small as 15 /un in diameter). Each shelf or tray in the unit can collect
dust that settles out by gravitational force. Since the vertical distance that a par-
ticle must fall to be captured is less than the distance in a horizontal settling unit,
the overall collection efficiency of the Howard settling chamber can be greater than
the horizontal chamber. The gas must be uniformly distributed as it passes through
each tray throughout the chamber. Uniform distribution is usually achieved by the
use of gradual transitions, guide vanes, distributor screens, and perforated plates.
The particles will settle on the individual trays which must be cleaned periodically.
The vertical distance between trays may be as little as 1 inch, making cleaning
much more difficult than with the horizontal settling chamber. Other dis-
advantages include the tendency of trays to warp during high temperature opera-
tion and the inability of the unit to handle dust concentrations exceeding approx-
imately 1 grain/ft* (2.29 g/ms). For these reasons, the Howard settling chamber is
not used very frequently for paniculate emission control.
5-2
-------
Figure 5-2. Howard Kttling chamber (multiple tray).
Baffle Chambers
A variation of the gravity settling chamber is a baffle chamber, sometimes refer ed
to as an inertial separator. These units have baffles within the chamber to enhance
particle separation and collection. This arises by changing the direction of the gas
velocity and imparting a downward motion to the particle. This induced motion is
superimposed on the motion due to gravity. Thus, particle collection is accom-
plished by gravity and an inertial or momentum effect. Particles as small as 20 to
40 jim can be collected. An example of this device is shown in Figure 5-S. These
units are more compact and require less space than gravity settling chambers. The
pressure drops are slightly higher, ranging from 0.1 to 1.0 in. of H,O (0.25 to 2.5
cm H,O).
Figure 5-3. Baffle chamber.
5-S
-------
Design Parameters
Understanding the principles governing particle collection in a gravity settler begins
by examining the behavior of a single spherical particle in the chamber (Figure
5-4). The bulk flow air velocity profile in this case is assumed, to be uniform
throughout the chamber. The particle flows with the same velocity as the gas
stream, and the horizontal velocity is given as v,. The particle also has a vertical
velocity, vy. The term v, is also called the terminal settling velocity (v,) and was
discussed hi Chapter 3 of this manual. The length, width, and height of the
chamber are L, B, and H respectively.
Suppose the particle enters the chamber at a height hp. The particle must fall
this distance (hp) before it travels the length of the chamber if the particle is to be
collected. In other words, the particle will settle if the time required for the par-
ticle to settle is less than the time that the particle resides in the chamber.
Figure 5-4. Settling chamber dimensions.
The theoretical collection efficiency of the settling chamber is given by the
expression.
(Eq. 5-1) i? = -^_
Where: 17 = fractional efficiency of particle size dp (one size)
v, = vertical settling velocity
v, = horizontal gas velocity
L = chamber length
H = chamber height (greatest distance a particle must fall to be collected)
5-4
-------
The settling velocity can be calculated from Stokes law as previously discussed in
Chapter 3. As a rule of thumb, Stokes law applies when the particle size dp is less
than 100 ^im in size. The settling velocity is:
(V* *-<>\ v
(Eq. 5-2) v,
Where: v, = settling velocity in Stokes law range, m/sec (ft/sec)
g= acceleration due to gravity, 9.8 in/sec* (32. 1 ft/sec*)
dp = diameter of the particle, fan
gp = particle density kg/m*. Qb/ft*)
C. = gas density (usually air), kg/ms (lb/ftj)
viscosity, Pa«sec (Ib/ft sec)
Note:Pa = N/m*
N = Kg«m/sec*
Thus, we can rearrange Equation 5-2 to determine the minimum particle size
that can be collected in the unit with 100% efficiency. The minimum particle size
dp* is given as: ___ __
(Eq. 5-3 given in /on) dp* = ( ,V'18M )
\g(Qp~Q')/
The density of the particle QP is usually much greater than the density of the gas
C.. Therefore, the quantity (QP- c.) reduces to QP. The velocity can be written as
v= Q/BL where Q, is the volumetric flow rate and B and L are the width and
length respectively. Equation 5-3 is reduced to: __
From Equation 5-1 and 5-2, one can rewrite the efficiency equation to be:
Where: Ne = number of parallel chambers: 1 for simple settling chamber
and N trays + 1 for a Howard settling chamber
In Equation 5-5, Qf (particle density) was assumed to be much greater than Q,
(gas density), hence QP- Q.— Qf. The term in the brackets in Equation 5-5 is often
multiplied by a dimensionless empirical factor to correlate theoretical efficiencies
with experimental data. If no information is available, it is suggested that 0.5 be
used*. Thus Equation 5-5 can be written:
Theodore and Buonicore, 1976.
5-5
-------
Equations 5-1 and 5-5 give the theoretical collection efficiencies of a settling
chamber for a single sized particle. Since the gas stream entering a unit consists of
a distribution of particles of various sizes, a fractional efficiency curve must be used
to determine the overall collection efficiency. This is simply a curve describing the
collection efficiency for particles of various sizes (Figure 5-5).
100
90
80
70
M
50
*
30
20
10
0
Simple fettling chamber:
H« (10 ft) (3.05m)
B« (10 ft) (3.05m)
L« (20 ft) (6.10m)
Q» (33,330 cfmftVmin)
(943.8 m'/min)
20
70 80
30 40 50 60
Particle diameter, p
Source: Jennings, R. F., J. Iron Steel Inst. London, 164, 305, 1950.
Figure 5-5. Fractional efficiency curve for dusts from a sinter plant.
One can see from Figure 5-5 that the fractional efficiency for a 40 micrometer
iron oxide particle is 22%. The fractional efficiency of an 80 micrometer iron
oxide particle is 88%. Figure 5-5 shows that the larger particles have a higher frac-
tional efficiency than the smaller particles. The overall efficiency can be calculated
using:
(Eq. 5-7)
Where:
I\TOT — overall collection efficiency
rfi = fractional efficiency of specific size particle
w, = weight fraction of specific size particle
5-6
-------
Equations 5-1 and 5-2 were developed with the assumption that the gas flow
through the settling chamber is laminar. This assumption is usually incorrect. The
gas flow is usually turbulent. The equation for determining efficiency when the
flow is turbulent is:
-I—
(Eq. 5-8) i;=l-e LHvJ
One must be careful when using Equations 5-1 through 5-6. For example. Equa-
tion 5-4 is used to find the minimum particle size collected with 100% efficiency.
This equation assumes that Stokes law describes particle settling. However, Stokes
law does not work for particles greater than 100 micrometers, the particle size more
suitable for collection in a settling chamber. Equation 5-4 can, in some cases, give
results that are gravely in error.
Process Variables
The process design variables for a settling chamber consist of length (L), width (B),
and height (H). These parameters are usually chosen by the chamber manufacturer
to remove all particles above a specified size. The chamber design must provide
conditions for sufficient particle residence time to capture the desired particle size
ranee This can be accomplished by keeping the velocity of the exhaust gas
through the chamber as low as possible. If the velocity is too high dust reentram-
ment will occur. However, the design velocity must not be so low as to cause the
design of the chamber volume to be exorbitant. Consequently the units are
designed for gas velocities in the range from 1 to 10 ft/sec (0.305 to S.05 m/sec).
Errors in estimating settling velocities from equations such as Equation 5-2 can occur
due to agglomeration and electrostatic effects. Therefore, in actual practice, the
terminal settling velocities used for design purposes are based upon experience and
tests under normal process conditions.
In settling chamber designs, the velocity at which the gas moves through the
chamber is usually called the throughput velocity. The velocity at which settled
materials (particles) become reentrained is called the pickup velocity. In order to
avoid reentrainment of collected dust, the throughput velocity must not exceed the
pickup velocity. Experimental data or equipment supplier data such as that
presented in Table 5-1 should be used to estimate the pickup velocity. As can be
seen from this table, the pickup velocity can exceed 10 ft/sec H no data for deter.
mining the pickup velocity is available, the pickup velocity should be assumed to be
10 ft/sec. In this case, the velocity of gas through the settling chamber (throughput
velocity) must be less than 10 ft/sec.
5-7
-------
Table 5-1. Pickup velocities of various materials.
Material
Aluminum chips
Asbestos
Nonferrous foundry dust
Lead oxide
Limestone
Starch
Steel shot
Wood chips
Wood sawdust
Density
(g/cm')
2.72
2.20
3.02
8.26
2.78
1.27
6.85
1.18
Median size
dan)
335
261
117
14.7
71
64
96
1370
1400
Pickup
velocity
(ft/sec)
14.2
17.0
18.8
25.0
21.0
5.8
15.2
13.0
22.3
Source: Theodore and Buonicore, 1976.
In terms of overall design considerations for gravity settlers, advantages include:
• low cost of construction and operation,
• few maintenance problems,
• relatively low operating pressure drops in the range of approximately 0.2-0.5
in. (0.51-1.27 cm) of water,
• temperature and pressure limitations imposed only by the materials of con-
struction used, and
• dry disposal of solid particulates.
The disadvantages include:
• large space requirements, and
• relatively low overall collection efficiencies (typically ranging from 20 to 60%).
In general, most gravity settlers in use today are precleaners removing the
relatively large panicles (greater than 60 /on) before the gas stream enters a more
efficient paniculate control device such as a cyclone, baghouse, electrostatic
precipitator, or scrubber.
References
1. Bethea, R. M. 1978. Air Pollution Control Technology: An Engineering
Analysis Point of View. New York: Van Nostrand Reinhold Company.
2. Theodore, L. and Buonicore, A. J. 1976. Industrial Air Pollution Control
Equipment for Particulates. Cleveland: CRC Press.
5. Cheremisinoff, P. N. and Young, R. A., eds. 1977. Air Pollution Control and
Design Handbook Part 1. New York: Marcel Dekker, Inc.
5-8
-------
Chapter 6
Cyclones
Introduction
Cyclones provide a relatively low-cost method of removing paniculate matter from
exhaust gas streams. Cyclones are somewhat more complicated in design than
simple gravity settling systems and their removal efficiency is accordingly much bet-
ter than that of settling chambers. However, cyclones are not as efficient as
baghouses and electrostatic precipitators, but are often installed as precleaners
before these more effective devices.
Cyclones come in many sizes and shapes and have no moving parts. From the
small one- and two-centimeter diameter source sampling cyclones used for particle
size analysis to the large five-meter diameter cyclone separators used after wet
scrubbers, the basic separation principle remains the same. Particles enter the
device with the flowing gas (Figure 6-1); the gas stream is forced to turn, but the
larger particles have too much momentum and can't turn with the gas. These
larger particles impact and fall down the cyclone wall, then are collected in a
hopper. The gas stream actually turns a number of times in a helical pattern,
much like the funnel of a tornado. The repeated turnings provide many oppor-
tunities for particles to hreak through the streamlines, thus hitting the cyclone wall.
Inlet
Figure 6-1. Particle collection mechanisms.
6-1
-------
The range of particle sizes collected in a cyclone is dependent upon the overall
diameter and relative dimensions of the device. Various refinements such as the use
of skimmers, turning vanes, and water sprays can in some caises improve efficiency.
Stacking cyclones in series or in parallel can provide further alternatives for
improving overall collection efficiency.
Cyclone Types
Three types of cyclones are shown in Figure 6-2. The first diagram, Figure 6-2a,
shows a typical tangential entry cyclone arrangement. These cyclones have a
distinctive and easily recognized form and can be found in almost any industrial
area of a town or a city—at lumber companies, feed mills, cement plants, power
plants, smelters, and at many other process industrial sites. Since top inlet type
cyclones are so widely used, most of this chapter will be devoted to their opera-
tional characteristics.
In axial entry cyclones, Figure 6-2b, the gas inlet is parallel to the axis of the
cyclone body. Here, the exhaust process gases enter from the top and are directed
into a vortex pattern by the vanes attached to the central tube. Axial entry
cyclones are commonly used in multicyclone configurations.
The large cyclonic type separator shown in Figure 6-2c is often used after wet
scrubbers to collect paniculate matter entrained in water droplets. The gas enters
tangentially at the bottom of the drum, forming a vortex. The large water droplets
are forced against the walls and are removed from the gas stream.
There are other variations in the design of cyclones. One classification scheme,
Caplan (1977), characterizes the various types in terms of where the gas enters and
exits the cyclone body (tangentially, axially, or peripherally).
6-2
------- An error occurred while trying to OCR this image.
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The common cyclone shown in Figure 6-S has four major design areas: inlet,
cyclone body, dust discharge system, and outlet. Each part of the cyclone is impor-
tant to its overall effectiveness.
Inlet
•Outlet
Cylinder \ Cyclone
body
Cone
Dust
discharge
Figure 6-3. Common cyclone.
The inlet directs the gas into the cyclone body and is important in forming the
vortex. In the cylindrical section the particulate matter is forced to the wall. The
gas spirals down to the cone section where the body is tapered to give the gas
enough rotational velocity to keep the dust against the wall. The particulate matter
is collected in the hopper or vortex arrester, and is continuously or periodically
removed. The gas vortex changes from the downward to the upward direction .at
the bottom of the cone.
A tube extends from the top of the cyclone into the cylinder of the body. This
tube extension is sometimes called a vortex finder. The ascending vortex enters the
tube extension and then exits. A helical exhaust duct or a dium may be placed on
the exit tube to straighten the gas flow before it flows through further ductwork.
6-4
-------
Inlet
Gas coming from a duct into a cyclone is transformed from linear flow mto ,i cir-
cular vonex pattern. The shape of the inlet assists in this transformation however,
several problems can arise. First, gas is already present in the cyclone and the
incoming gas from the duct has to be squeezed between the cyclone body and the
c^t tube Consequently, the gas is accelerated and added to the already rotatmg
eas in the cyclone. In other words, the pressure drop increases and it
fates more^power to move the gas through the system with increasing pressv^e
drop If the inlet is poorly designed, turbulence can occur and the energy of the
gas stream leading into the inlet is not immediately incorporated into the vortex
(Figure 6-4).
Area of turbulence
Inlet
Inner vortex
Outer vortex
Figure 6-4. Inlet interference.
6-5
-------
Several methods have been developed in order to merge the incoming gas into
that already in the cyclone. These are shown in Figure 6-5.
a. Tangential entry
b. Tangential entry
with deflector vane
c. Helical entry
d. Involute entry
Figure 6-5. Types of cyclone inlets.
The straight tangential inlet shown in Figure 6-5a can cause some turbulence
and loss of particles directly through the outlet tube. Inlet deflector vanes added to
a tangential entry as shown in Figure 6-5b can narrow and guide the gas stream to
move tangentially against the wall. Inlet vanes will also reduce the pressure drop. It
has been found, however, that vanes can suppress the development of the vortex
and reduce efficiency.
Helical inlets have been designed to deflect the incoming gas both tangentially
and downward in order to provide at smooth transition to the vortex (Figure 6-5c).
Present experimental data is inadequate to determine whether this general tech-
nique can gain anything in terms of increased efficiency or even in reducing the
pressure drop (Stern, 1955).
6-6
-------
The involute or wraparound design shown in Figure 6-5d allows the incoming
gas to enter the top section of the cyclone with a minimum amount of turbulence.
It has been found that the pressure drop for the involute design is less than for the
tangential entry and that the efficiency is much higher (Bethea, 1979). The par-
ticles are thrown to the wall more effectively and fewer fine particles are lost
through the exit tube due to turbulence.
Cyclone Body and Cone
The removal efficiency of a cyclone for a given size particle is very dependent upon
the cyclone dimensions. The pressure drop at a given volumetnc flow rate is most
affected by the diameter. The overall length determines the number of turns of the
vortex The greater the number of turns, the greater the efficiency.
The length and width of the inlet are also important, since the smaller the inlet,
the greater the inlet velocity becomes. A greater inlet velocity gives greater effi-
ciency but also increases the pressure drop. Consider the dimensions shown in
Figure 6-6.
D. • body diameter
I« « body length
Z. • cone length
DC « exit tube diameter
Se« length of exit tube in cyclone
H, • inlet height
Bc * inlet width
Figure 6-6. Nomenclature for a tangential entry cyclone.
6-7
-------
Many different types of cyclones have been designed by merely varying the
dimensions highlighted in Figure 6-6. Table 6-1 gives dimensional characteristics of
a number of designs reported in the literature. Dimensions are given relative to the
body diameter De.
Table 6-1. DimensionleM design ratios for tangential
Symbol
D.
H,
B«
s.
D.
L,
z.
Nomenclature
Body diameter
Inlet height
Inlet width
Outlet length
Outlet diameter
Cylinder length
Cone length
High efficiency
Stairmand
1.0
0.5
0.2
0.5
0.5
1.5
2.5
Medium efficiency
1.0
0.75
O.S75
0.875
0.75
1.5
2.5
Conventional
Lapple
1.0
0.5
0.25
0.625
0.5
2.0
2.0
High efficiency cyclones generally have smaller inlet and exit areas with a smaller
body diameter and possibly longer overall length. A conventional cyclone will be
from 4 to 12 feet (1.2 to 3.6 m) in diameter, with a pressure drop of from 2 to
5 inches (5 to IS cm) of water. A high efficiency cyclone will be less than 3 feet
(0.9 m) hi diameter with a pressure drop of from 2 to 6 niches (5 to 15 cm) of
water. Two of these designs are shown in Figure 6-7.
0.5 Dc
0.2 Dc
1.5 De
0.375 Dc
2.5 D,
High efficiency
Source: Strauss, 1975.
1.5 Dc
2.5 D.
Medium efficiency
Figure 6-7. Standard cyclone designs.
6-8
-------
The motion of the gas in the cyclone is not as simple as it first might appear.
There are two vortices, one descending, the other ascending. The descending
vortex is often called the main vortex. Figure 6-8 shows a descending vortex which
has a right-handed orientation.
Right-handed
down
Left-handed
up
Figure 6-8. Cyclone vortices.
If the thumb of the right hand is pointed downward, the curl of the fingers will
show the radial direction of the gas stream as if it were viewed from the top. For a
properly designed cyclone, the vortex will change direction and ascend at the
bottom of the cone.
The ascending vortex is smaller in radius, with faster tangential velocities than
the descending vortex. It has a left-handed orientation. If the thumb of the left
hand is pointed upwards, the curl of the fingers will again show the radial direc-
tion of the vortex. Note that the radial directions are the same in both cases.
6-9
-------
The cone primarily serves as a mechanism to remove paniculate matter down
the walls to the hopper, and to provide greater tangential velocities near the
bottom for removal of smaller particles. The vortex formed in a cyclone is,
however, eccentric. Just as a tornado moves at an angle to the ground, the
cyclone's main vortex can deviate from the vertical axis. For this reason, it has
been found that the bottom of the cone should have a diameter of at least Vi of
DC, the cylinder diameter. The outer vortex otherwise could touch the cone wall
and entrain already separated paniculate matter into the ascending vonex.
Even if optimum dimensions are selected, problems can occur within the cyclone
which can reduce the efficiency. Rough walls will slow down the gas, decreasing
the velocity of the vonex, thus causing smaller particles to be lost. Panicles can
bounce off the walls into the inner vonex and also be lost. Secondary flow patterns
or eddies can form in the annular region at the inlet. The rccirculation and tur-
bulence here can prevent small panicles from entering the vonex and instead be
drawn into the outlet tube. The lower pressure in the inner vortex caused by its
faster velocity can cause particles to drift hi from the main vortex (a Bernoulli
effect). These problems limit the efficiency of a cyclone.
These problems can, however, be minimized in some cases;. First, care should be
taken to have the cyclone constructed with smooth interior walls. A fines eductor or
skimmer can also be added which will bleed off a portion of the gas (and fine par-
ticles) from the upper part of the cyclone (Figure 6-9). Since a pressure drop exists
from the upper part of the cyclone cylinder to the bottom part, the fines can be
reinjected into the main vortex at the cone junction merely-by taking advantage of
the small vacuum which exists. The fines are bombarded by other particles and
forced to the wall, thus increasing the efficiency. If particle bounce is a great
problem, water sprays can be added to wet the walls and entrain the particles.
6-10
-------
Fine particles
entry to eductor
Fines eductor
Fines re-entry
Figure 6-9. Fines eductor.
Dust Discharge System (Hopper)
The vortex will extend into the discharge bin if the bin is immediately below the
cone and nothing is added at the bottom of the cone to arrest the vortex. Since the
static pressure in the vortex core is slightly negative, dust can be reemrained from
the hopper into the inner vortex. Also, if leaks exist in the bin, dust can be sucked
back up into the cyclone. •
6-11
-------
Several methods are used to avoid reentrainment. Straightening vanes added to
the inside of the pipe leading from the cone to the hopper can arrest the vortex, as
can axial discs added above the discharge tube (Stem, 1955). Reentrainment can
be minimized by making the hopper large in volume and deep enough so that the
collected dust level will lie below the point where the vortex ends. The addition of
a mechanical valve which can periodically or continually remove the dust from the
cyclone, can effectively reduce inflow from the hopper. A number of designs are
shown in Figure 6-10.
Slide gate
a. Simple manual slide gate
c. Discharge screw feeder
7
b. Rocary valve
Counterweight
Flap valve
d. Automatic flap valve
Figure 6-10. Discharge systems.
A valve between the cyclone and bin can be a simple manual device as shown in
Figure 6-10a, or can provide a continuous discharge as with i:he rotary valve and
screw feeder shown in Figures 6-10b and c. Automatic flap valves shown in Figure
6-10d can periodically swing to discharge accumulated dust in a double-valved
arrangement.
6-12
-------
Cyclone efficiency can also be improved if a portion of the flue gas is drawn
through the hopper. An additional vane or lower pressure duct can provide this
flow. However, it may then become necessary to recirculate or otherwise treat this
purge exhaust to remove uncollected paniculate matter.
Cyclone Gas Outlet
The exit tube is an important consideration in the design of any cyclone. Its length
must extend beyond the inlet so that the eddies created in the annulus between the
tube and the walls do not mix particles up and into the exit tube.
Some of the energy due to the radial motion of the ascending gases can be
recovered by the application of scroll devices or by placing outlet drums on top of
the exit tube (Figure 6-11). These are essentially flow straighteners and can effec-
tively reduce the pressure drop across the cyclone without reducing collection effi-
ciency. This is a unique situation since most other modifications which reduce the
pressure drop in a cyclone also reduce the efficiency. For example, straightening
vanes added to the inside or bottom of the outlet tube can suppress the vortex and
therefore reduce efficiency (Stern, 1955).
It should be apparent from the above discussion, that many things will affect the
efficiency and pressure drop of a cyclone. Although the manufacturers of these
control devices will start with an optimized set of dimensions such as those given in
Table 6-1 " . .there is no single cyclone design which will perform best for all dust
collection problems" (Leith, 1973). Basic process characteristics such as dust caking
properties, particle size distributions, and gas volumetric flow rates and
temperatures must be considered. Most of the theoretical cyclone design equations
have been developed for simple cyclones not having modifications such as skimmers
and outlet drums. The basic design equations can, however, in some cases be used
to compare the performance of similar cyclones.
6-13
------- An error occurred while trying to OCR this image.
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Characterizing Cyclone Performance
Objects moving in circular paths tend to move away from the center of their
motion. The object moves outward as if a force were pushing it out This force *
CI as centrifugal force. The whirling motion of the gas in a cyclone,:au*es par-
ticulate matter in the gas to feel this force and move out to the walls. An expres-
sion for this phenomenon is as follows:
P. frW
(Eq. 6-1) F F~~
Where: C, = particle density, lb/ft3 (kg/m8)
dp = particle diameter, in. (tan)
Vp = particle tangential velocity, ft/sec (m/sec)
r = radius of the circular path, ft (m)
F is the force that the particulate matter views as acting on it. This expression
explains several of the cyclone characteristics discussed in the previous section For
example o,d > » merely proportional to the mass of the particle. The larger the
ma^ the Later the force. The tendency to move toward the walls is consequently .
"Teased Sd larger particles are more easily collected. The reason all of the par-
tides don't move to the wall is because of the drag resistance of the air. The buf-
feting molecules in the gas resist the outward motion and act like an opposing
force Particles move to the wall when the centrifugal force is greater than the
opposing drag force.
Note also from Equation 6-1, that as r (the radius of the circular path)
decreases, the force again increases. This is why smaller cyclones are more efficient
for the collection of smaller sized particles than are large cyclones.
These types of considerations in conjunction with considerations of cyclone
geometry, and vortex formation have led to the development of numerous perfor-
£ance equations. These equations attempt to characterize the behavior of cyclones.
Some work well and some do not (Leith, 1973). None adequately describe perfor-
mance under all operating conditions, such as at high pressure and high
temperatures (Parker, 1981). .
There are three important parameters which can be used to characterize cyclone
performance. These are:
[dp]c«t= cut diameter
Ap = pressure drop
ij = overall collection efficiency
Equations for each of these parameters will be given in this section. The equations
should be used with caution, however, since there are strict limitations on their
applicability.
6-15
-------
Cut Diameter
The cut diameter is defined as the size (diameter) of particles collected with 50%
efficiency. It is a convenient way of defining efficiency for a control device since it
gives an idea of the effectiveness for a particle size range.
A frequently used expression for cut diameter is that given by Lapple (1965):
(Eq. 6-2)
Where:
2rn,v,(o,-ot)
H= viscosity, Ibs/sec ft (Pa»s)
n, = effective number of turns (5 to 10 for common cyclone)
v, = inlet gas velocity, ft/sec (m/sec)
Qf = particle density, Ibs/ft8 (kg/m8)
Qg = gas density, Ibs/ft8 (kg/m8)
Bc = inlet width, ft (m)
The cut diameter, [d,]^, is a characteristic of the control device and should not ^
be confused with the geometric mean particle diameter, d,^,, of the size
distribution.
Figure 6-12 shows a size efficiency curve and points out the cut diameter and the
critical diameter, [dp]errt, the panicle size collected at 100% efficiency. Values of
[dpjcrrt are difficult to obtain from such curves so the cut size is often determined
instead.
~ 100
z
•§ 50
[d,L
Particle diameter, d, (/un)
Figure 6-12. Typical size efficiency curve.
A number of formulas exist for the calculation of the cut idiameter and critical
diameter. Stern (1955) and Strauss (1975) review a number of the expressions. A
value of n,, the number of turns, must be known in order to solve Equation 6-2 for
[dp]«,. Given the volumetric flow rate inlet velocity, and dimensions of the cyclone,
n, can be easily calculated (Strauss, 1975).
6-16
-------
Using Figure 6-lSa, Equation 6-2 can be solved graphically for a conventional
cyclone having the relative dimensions given in Table 6-1 and if n, is assumed to
equal 5. (Lapple, 1951. See also USEPA AP 40, Air Pollution Engineering
Manual).
Data valid for:
ft>0.02cp
Y,» 50 ft/sec
n,»5
B.-D./4
UK correction factors
for other conditions
3 45 ~o 20 SO 40 50 1C
Cyclone diameter, De (inches)
Source: EPA, 1973.
Figure 6-13a. Cut size in micrometers for cyclones of conventional type.
Figure 6-lSa assumes certain values for inlet velocity, viscosity, number of turns
and relative dimensions. Figures 6-lSb and c could be used to correct the [d,]^ of
Figure 6-lSa if these values are different from those assumed.
6-17
-------
10
Cyclone inlet velocity, v, (ft/sec)
20 SO 40 50 60 100
4.0
3.0
>
s
* 2.0
£
b
1
* i.o
6 0.9
J 0.8
0.7
0.6
0.5
0.
^
"V,
-*•
s>
s^
4
J,
^
*
T
>^^
^~^
-------
The expression for the cut diameter (Equation 6-2) has been found to agree
within Vi micrometer for some experimental data (Lucas, 1974). However, other
experimental work (Parker, 1981) has shown limitations to its application. A high
efficiency cyclone will have a cut diameter of typically 5 to 10 micrometers. Equa-
tion 6-2 is typical of most of those devised for determining the cut or critical
diameter Note that an increase in the number of turns, inlet velocity, or the par-
ticle density will decrease the cut size as one would expect. A decrease in viscosity
will decrease the drag force opposing the centrifugal force and therefore also
reduce the cut size (i.e. smaller size particles will be collected).
Pressure Drop
The pressure drop across a cyclone is an important parameter to the purchaser of
such equipment. Increased pressure drop means greater costs for power to move
exhaust gas through the control device. With cyclones, an increase in pressure drop
usually means that there will be an improvement in collection efficiency (one
exception to this is the use of pressure recovery devices attached to the exit tube;
these reduce the pressure drop but do not adversely affect collection efficiency). For
these reasons, there have been many attempts to predict pressure drops from design
variables. The idea is that having such an equation, one could work back and
optimize the design of new cyclones.
One of the simplest pressure drop equations which correlates reasonably well
with the experiment (Leith, 1973) is that developed by Shepherd and Lapple (1939).
= v-4) <*P /T \u/Z,\W
Where: Q,= volumetric flow rate
(In this case k« is a dimensionless factor descriptive of cyclone inlet vanes. It is
equal to 0.5 for cyclones without vanes, 1.0 for vanes that do not expand the
entering gas or touch the outlet wall, and 2.0 for vanes that expand and touch the
outlet wall.) This equation, when compared to experimental data wasjound to
have a correlation coefficient of only 0.53 (Leith, 1977).
6-19
-------
It should be noticed from both of these equations that the pressure drop is a
function of the square of the inlet velocity. Most of the empirical pressure drop
equations have the form:
(Eq. 6-5) Ap = Kc'o^v,* (Consistent units)
Where: K«'= a proportionality factor
If Ap is measured in inches of water, K«' can vary from 0.013 to 0.024. Velocities
for cyclones range from 20 to 70 ft/sec (6 to 21 m/sec), although common
velocities range from 50 to 60 ft/sec (15 to 18 m/sec). At velocities greater than 80
ft/see (24 m/sec), turbulence increases in the cyclone and efficiency will actually
decrease. Also, at high loads of paniculate matter and high velocities, scouring of
the cyclones by the particles will rapidly increase. To minimize erosion in such
cases, a cyclone would be designed for lower inlet velocities.
Pressure drops for single cyclones vary depending upon both size and design.
Common ranges are:
low efficiency cyclones 2-4* H,O (5-10 cm H,O)
medium efficiency cyclones 4-6* H,O (10-15 cm H,O)
high efficiency cyclones 8-10* H,O (20-25 cm H,O)
Collection Efficiency
A number of formulations have been developed for determining the fractional
cyclone efficiency, 17,, for a given size particle. Fractional efficiency is defined as
the fraction of panicles of a given size collected in the cyclone, compared to those
of that size going into the cyclone. An excellent discussion and comparison of these
theories is given by Leith (1973). Figure 6-14 reproduced from Leith (1973) gives
an estimate of the applicability of several theories for the calculated efficiency of a
simple cyclone.
No efficiency theory or calculation method provides a description for all
cyclones. The modification of inlets and outlets, addition of fines eductors, etc.
introduce variables which are difficult to treat theoretically. Although theoretical
efficiencies can give estimations of cyclone performance, it should be kept in mind
that designers of equipment commonly rely on comparative evaluations between
similar designs and on experience.
This section will describe two methods of calculating cyclone efficiency. The
Leith and Licht theory for calculating fractional efficiency will be discussed first. A
convenient graphical method for estimating efficiency, developed by Lapple (1951),
will also be discussed.
6-20
-------
Peterson and Whitby
experimental data (1965)
Earth theory (1956)
Lapple theory (1951)
Sproull theory (1970)
Leith and Licht theory (1972
5 10
Particle diameter, d, (/on)
Source: Leith and Mehta, 1973.
Figure 6-14. Cyclone efficiency versus particle diameter.
Experimental results and theoretical predictions.
Leith and Licht Theory
The fractional efficiency equation of Leith and Licht has a form similar to many
of those developed for paniculate control devices. Fractional efficiency is expressed
as:
(Eq.6-6) m-1-0- -—
Where: T;, = fraction efficiency
c = cyclone dimension factor
\l/ = impaction parameter
n = vortex exponent
This exponential form is quite common since it can give the general form of effi-
ciency plots such as shown in Figure 6-14. In this expression, c is a factor which is
a function only of the cyclone's dimensions. The symbol t, expresses characteristics
of the particles and gas as:
!Xi(n+l)
(Eq. 6-7) iH
and is known as inertia or impaction parameter. Note that o, times v, essentially
expresses the particle's initial momentum. The value of n is dependent upon the
cyclone diameter and temperature of the gas stream.
6-21
-------
The calculations involved in this method are straightforward, although tedious.
The original reference (Leith, 1973) should be consulted if it is to be applied. The
method has also been extended to provide a graphical method of optimizing cer-
tain parameters of cyclone design (Koch, 1977).
Lapple Method
An older method of calculating cyclone fractional efficiency and overall efficiency
was developed by Lapple (1951). (See also USEPA AP 40, Air Pollution Engineer-
ing Manual, pp. 94-99). Lapple first computed the ratios d^/fdp],,,,, the particle
diameter versus the cut diameter as determined from Equation 6-2 or Figure 6-12.
He found that cyclone efficiency correlates in a general way with this ratio. For a
common cyclone, efficiency will increase as the ratio increases as shown in Figure
6-15.
iUU
^>°"
"""iio W
»K
*** ajf
«
10
/
/
/
>
x
/
/
/
y
s*
^***
^•g
9M
••
••
•
^
•
0.4 0.5 1 234
Particle size ratio, dp/[dPL.<
Source: Lapple, 1951.
Figure 6-15. Cyclone efficiency versus particle size ratio.
10
As a universal curve for common cyclones, this correlation has been found to
agree within 5% at a dp/fd,]^ near 1.0 (AP 40). To calculate fractional efficien-
cies, the following procedure is followed:
1. Calculate [dp] '„,, for the cyclone being investigated using Equation 6-2 or
Figure 6-12.
2. Multiply [dp] ^ times several values of the ratio dp/[df]e* given in
Figure 6-15.
S. Replot the efficiency given at each dp/fdp],^ versus the values obtained in step
6-22
-------
The replot will give the fractional efficiency curve for the cyclone being
evaluated Since this procedure only gives an estimate (see Figure £" j«
deviations), in practice, a range of [dj L, values are used ^tea^h * rive
Maximum and minimum curves determined from these values will then give a
ranee of efficiency for estimation purposes.
The overall efficiency can also be determined if the inlet particle size distribution
is known. This can be easily done by constructing a table with the following
headings: _ . _
ij< for each d, from
experiment or
Lapple's method
The fractional efficiency <„) for each (d,/[d,W x d,] ^ is found using
previous method. Each fractional efficiency is multiplied times the weight percent
Fn each range. The sum of these products in the last column will give the overall
efficiency.
Summary of Performance Characteristics
Efficiency, pressure drop, and costs are intimately related in cyclones as they are
^L mo* other paniculate control equipment. Many factors affect efficiency and
pressure drop. A number of factors can be taken into account in the theoretical
formulations, others cannot. A thorough evaluation of cyclone design will depend
on previous experience or empirical information derived from experiments on
similar cyclones. _
A summary of changes in performance characteristics produced by changes in
cyclone design and exhaust gas properties is given in Table 6-2^
Table 6-2. Changes in performance characteristics
_————-^——^~
Cyclone and process
design changes
Increase cyclone size (D«)
Lengthen cylinder (L.)
Lengthen cone (Z.)
Increase exit tube
diameter (D.)
Increase inlet area—
maintaining velocity
Increase velocity
Increase temperature
(maintaining velocity)
Increased dust concentration
Increasing particle size
and/or density
Source: Bhatia and Cheremisinoff, 1977.
6-25
CSS
D.)
)
y
ty)
itration
X
Pressure
drop
Decreases
Decreases slightly
Decreases slightly
Decreases
Increases
Increases
Decreases
Decrease for
large increases
No change
Efficiency
Decreases
Decreases
Increases
Decreases
Decreases
Increases
Decreases
Increases
Increases
.1.
Cost
Increases
Increases
Increases
Increases
Decreases
Operating costs
higher
No change
No change
No change
-------
It should be remembered that the addition of fines eductors (skimmers) can also
increase efficiency with little effect on pressure drop. Adding water sprays to wet
the walls can also improve performance.
Cyclone Arrangements
It should be apparent from the above discussion that small cyclones are more effi-
cient than large cyclones. Small cyclones, however, have a higher pressure drop
and are limited with respect to volumetric flow rates. Smaller cyclones can be
arranged either in series or in parallel to substantially increase efficiency at lower
pressure drops. These gains are somewhat offset however, by increased
maintenance problems. Multicyclone arrangements tend to plug more easily. When
common hoppers are used in such arrangements, different flows through cyclones
can lead to reentrainment problems.
Series Arrangements
A typical series arrangement is shown in Figure 6-16.
Clean gas
Dirty gaa
Dust
Dust
Figure 6-16. Cyclones in series.
6-24
-------
Larger particles can be collected in the first cyclone and a smaller, more effi-
cient cyclone can collect smaller particles. Such an arrangement can reduce dust
loading in the second cyclone and avoid problems of abrasion and plugging. Also,
if the first cyclone should plug there still will be some collection occurring in the
second cyclone. The additional pressure drop produced by the second cyclone adds
to the overall pressure drop of the system. The higher pressure drop can be a
disadvantage in such a series system design.
Parallel Arrangements
Many types of parallel arrangements have been designed for cyclones. An example
of a parallel arrangement using tangential entry cyclones is shown in Figure 6-17.
dean gas
Junction duct
Grouped inlets
Top outlet box
Vortex finder
Figure 6-17. Battery of four involute cyclones in parallel.
6-25
-------
With batteries of cyclones using a common inlet plenum, higher volumes of gas
can be treated at reasonable pressure drops. In configurations where a common
hopper is used, each cyclone should have the same pressure drop or the gas will
preferentially channel through one cyclone or several cyclones.
Another type of parallel arrangement uses the axial entry cyclone shown in
Figure 6-16. Arrangements of high efficiency, small diameter axial cyclones can
provide increases in collection efficiency with reductions in pressure drop, space,
and cost. Such a multiclone arrangement is shown in Figure 6-18. Pressure drops
commonly range from 4 to 6 inches (10 to 15 cm) of water.
dean gas
Dirty gas
Figure 6-18. Battery of vane axial cyclones.
The axial entry minimizes the eddy formation that is common in tangential entry
cyclones. Here, the inlet guide vanes create the vortex.
Care must be taken in designing the inlet plenum for the multiclone since the
inlet exhaust gas should have an even distribution to each individual cyclone.
Sticky materials should not be collected using multiclones since the vanes and
smaller outlet tubes are quite prone to plugging. A good discussion of operation
and maintenance problems occurring with multiclones is given by Schneider (1975).
6-26
-------
References
1. Bethea, R. M. 1978. Air Pollution Control Technology. New York: Van
Nostrand Reinhold, pp. 117-144.
2 Bhatia M. V. and Cheremisinoff, P. N. 1977. Cyclones. In Air Pollution Con-
' trol and Design Handbook. P. N. Cheremisinoff and R. A. Young, eds. pp.
281-516, New York: Marcel Dekker, Inc.
3. Caplan, K. 1964. All About Cyclone Collectors. Air Eng. Sept.: 28-38.
4 Caplan, K. 1977. Source Control by Centrifugal Force and Gravity. In Air
Pollution Vol. IV Engineering Control of Air Pollution, A. C. Stern, ed. pp.
97-148, New York: Academic Press.
5. Danielson, J. A., ed., 197S. Air Pollution Engineering Manual, Research
Triangle Park, NC: US Environmental Protection Agency, pp. 91-99.
6. Doerschlag, C. and Miczek, G. 1977. How to Choose a Cyclone Dust Collector.
Chem. Eng. Feb.: 64-72.
7. Hesketh, H. E. 1979. Air Pollution Control. Ann Arbor: Ann Arbor Science
Publishers, pp. 184-193.
8. Koch, W. H. and Licht, W. 1977. New Design Approach Boosts Cyclone Effi-
ciency, Chem. Eng. Nov.: 79-88.
9. Leith, D. and Mehta, D. 1973. Cyclone Performance and Design. Atmos.
Environ. 7:527-549.
10. McCarty, R. E. 1962. How to Evaluate and Specify Mechanical Dust Collec-
tors. Air Eng. Feb.: 22-49.
11 Parker, R.; Jain, R.;-Calvert, S.; Drehmel, D.; and Abbott, J. 1981. Panicle
Collection in Cyclones at High Temperature and High Pressure. Environ. Sci.
Technol. 15:451-458.
12. Schneider, A. G. 1975. Mechanical Collectors. In Handbook for the Operation
' and Maintenance of Air Pollution Control Equipment, F. L. Cross, Jr. and
H E Hesketh, eds. pp. 41-68, Westport: Technomic Publishing.
IS. Stern, A. C., Caplan, K. J, and Bush, P. D. 1955. Cyclone Dust Collectors.
Amer. Petrol. Inst., New York.
14. Strauss, W. 1975. Industrial Gas Cleaning. International Series in Chemical
Engineering, Vol. 8. Oxford: Pergamon Press, pp. 216-276.
15. Theodore, L. and Buonicore, A. J. 1976. Industrial Air Pollution Control for
Particulates. Cleveland: CRC Press, pp. 91-137.
16. Lapple, C. E., 1951. Processes Use Many Collection Types. Chem. Eng.
58:145-51 (May).
17. Shepherd, C. B. and Lapple, C. E. 1939. Flow Pattern and Pressure Drop in
Cyclone Dust Collectors. Ind. Eng. Chem. 31:972-984.
6-27
-------
Chapter 7
Electrostatic Precipitators
Introduction
The fundamental principles underlying the application of electrostatic forces to
precipitate particles suspended in a gas were known in the 18th century, however,
the successful development of a device that employed electrical gas cleaning
methods did not take place until professor Fredrick Cottrell designed and built the
first industrial ESP at the Detroit-Edison Trenton Channel Steam generator in
1923. Today there are thousands of ESPs hi operation for the control of fly ash
emissions from steam generators. The ESP is also an effective device for controlling
emissions from cement kilns, pulp and paper plants, acid plants, sintering opera-
tions, and other industrial sources. The method is extensively used where dust emis-
sions are less than 10-20 fan in size with a predominant portion in the submicron
range.
The electrostatic precipitator is comprised of four essential components, each of
which will be discussed in detail in this chapter (see Figure 7-1). The major com-
ponents are:
• discharge electrodes
• collection electrodes _
• rappers
• hoppers
Discharge
electrodes
Collection
electrodes
Hoppers
Figure 7-1. Typical plate and wire single-*Uge electrostatic precipitator.
7-1
-------
The discharge electrode is normally a wire where a corona discharge occurs. This
electrode is used to ionize the gas (which charges the particles) and create an elec-
tric field. The collection electrode consists of either a tube or flat plate which is op-
positely charged (relative to the discharge electrode) and is the surface where the
charged particles are collected. The rapper is a device used to impart a vibration
or shock to dislodge the deposited dust on the electrodes. Rappers are used to
remove dust accumulated on both the collection electrodes and discharge elec-
trodes. Hoppers are located at the bottom of the precipitator and are used to col-
lect and store the dust removed by the rapping process.
Types of ESPs
There are basically two types of electrostatic precipitators: high voltage single-stage
and low voltage two-stage. The high voltage single-stage precipitator is the more
popular type and has been used successfully to collect both solid and liquid par-
ticulate matter in industrial facilities such as smelters, steel furnaces, cement kilns,
municipal incinerators, and utility boilers. Low voltage two-stage precipitators are
limited almost exclusively to the collection of liquid aerosols discharged from
sources such as meat smokehouses, pipe coating machines, asphalt paper
saturators, and high speed grinding machines.
Low Voltage, Two-Stage ESP
Low voltage two-stage precipitators were originally designed for air purification in
conjunction with air conditioning systems (they are also refeired to as electronic air
filters). Two-stage ESPs have been used primarily for the control of finely divided
liquid particles. Controlling solid or sticky materials is usually difficult, and the
collector becomes ineffective for dust loadings greater than 0.4 grains per standard
cubic foot (7.S5 x 10"* g/ms). Therefore, two-stage precipitators have limited use
for paniculate emission control.
The low voltage two-stage precipitator differs from the high voltage single-stage
precipitator in terms of both design and amount of applied voltage. The two-stage
ESP has separate particle charging and collection stages (Figure 7-2). The ionizing
stage consists of a series of small (0.007 inch diameter) positively charged wires
equally spaced 1 to 2 inches from parallel grounded tubes or rods. A corona
discharge between each wire and a corresponding tube charges the panicles
suspended in the air flow through the ionizer. The direct-current potential applied
to the wires is approximately 12 to IS kV.
The second stage consists of parallel metal plates less than 1 in. (2.5 cm) apart.
The liquid particles receive a positive charge in the ionizer stage and are collected
at the negative plates in the second stage. Collected liquids drain by gravity to a
pan located below the plates.
7-2
-------
Charging
particles
Uncharged
particles
Dirty air
Source: EPA, 1973.
Collector cell
(to collect particles)
Precipitated
particles
Ionizer*
(to charge particles)
Positively
charged
particles
Figure 7-2. Typical two-stage precipitator.
High Voltage, Single-Stage
The two major types of high voltage single-stage ESP configurations are tubular
and plate. Particles are both charged and collected in a single stage.
Tubular precipitators consist of cylindrical collection electrodes with discharge
electrodes located in the center of the cylinders. Dirty gas flows into the cylinder
where precipitation occurs. The negatively charged particles migrate to and are
collected on grounded collecting tubes. The collected dust or liquid is removed by
washing the tubes with water sprays located directly above the tubes (Figure 7-3).
These precipitators are generally referred to as water-walled ESPs. Tubular
precipitators are generally used for collecting mists or fogs. Tube diameters typi-
cally vary from 0.5 to 1 ft (0.15 to O.SI m), with length usually ranging from 6 to
15 ft (1.85 to 4.6m).
7-S
-------
Gas flow in
Gai flow out
Watei-walled
tiibet
Figure 7-3. Gas flow through wire and tubular pipe precipitators.
m
Plate electrostatic precipitators are used more often than tubular ESPs in
industrial applications. High voltage is used to subject the particles in the gas
stream to an intense electric field. Dirty gas flows into a chamber consisting of a
series of discharge electrodes (wires) spaced along the center line of adjacent plates
(Figure 7-4).
Discharged
electrode
Collection
plate
Figure 7-4. Gas flow through a wire and plate precipitator.
7-4
-------
Charged particles migrate to and are collected at oppositely charged collection
plates. Collected particles are usually removed by rapping (dry precipitator) or by a
liquid film (wet precipitator). Particles fall by force of gravity into hoppers where
they are stored prior to removal and final disposal. The remainder of the chapter
will be devoted to the high voltage single-stage plate ESP.
Theory of Precipitation
Charging the Particles
Electrostatic precipitation occurs in the space between the discharge electrode and
the collection surface. A high voltage, pulsating direct current is applied to an
electrode system consisting of a small diameter discharge electrode, which is usually
negatively charged, and a collecting plate electrode, which is grounded. This pro-
duces a unidirectional, nonuniform electric field whose magnitude is highest near
the discharge electrode (Figure 7-5).
Collection electrode
n n
\ \
\ N
'
\ I / /
x Electric field
Discharge
/
I/
/ f 1
t 1
/ I
! n!
\ x \ "
i \
» ^ \
In \ \i
LJ LJ
Collection electrode
Figure 7-5. Electric field generation (top view).
7-5
-------
Corona Generation
The applied voltage is increased until it produces a corona discharge (corona),
which can be seen as a luminous blue glow around the discharge electrode. The
corona is a discharge phenomenon in which gaseous molecules are ionized by elec-
tron collisons in the region of a high electric field. The intense electric field close
to the discharge electrode accelerates the free electrons that are present hi the gas.
These electrons acquire sufficient velocity to ionize gas molecules upon collison;
producing a positive ion and an additional free electron (Figure 7-6).
Blue glow
f
Positive ion
Electron!
Figure 7-6. Generation of corona.
The additional free electrons create more positive ions and free electrons as they
collide with additional gas molecules. This process is called avalanche multiplica-
tion, and occurs in the corona glow region (Figure 7-7). Avahmche multiplication
will continue until the local electric field strength decreases to the point where
there is insufficient energy to perpetuate ionization.
Figure 7-7. Avalanche multiplication.
7-6
-------
The sluggish positive ions migrate back to the negative discharge electrode and
form new free electrons upon impaction with the discharge wire or gas space
around the wire. The electrons produced during the avalanche multiplication pro-
cess follow the electric field toward the grounded collection electrode.
The electrons leave the corona region and enter the inter-electrode region. The
magnitude of the electric field is diminished and the free electrons' velocity
decreases. When electrons impact on gas molecules in the inter-electrode region,
they are captured, and negative gas ions are created (Figure 7-8). These negative
ions serve as the principal mechanism for charging of the dust.
Figure 7-8. Gas ionization in the inter-electrode region.
Negative gas ions migrate toward the grounded collection electrode. A space
charge which is a stable concentration of negative gas ions forms in the inter-
electrode region. Increases in the applied voltage will increase the field strength
and ion formation until sparkover occurs. Sparkover refers to internal sparking
between the discharge and collection electrodes. It is a sudden rush of localized
electric current through the gas layer between the two electrodes. Sparking causes
an immediate short-term collapse of the electric field. In general, it is very
desirable to operate at voltages high enough to cause some sparking but not at a
frequency such that the electric field constantly collapses. The average sparkover
rate for optimum precipitator operation is between 50 and 100 sparks per minute.
At this spark rate the gains in efficiency associated with increased voltage compen-
sates for decreased gas ionization due to collapse of the electric field. For optimum
efficiency the electric field strength should be as high as possible. This is
accomplished by applying a high voltage to the discharge electrode and the
consequent high corona current flow from the discharge electrode to the collection
electrode.
7-7
-------
Field and Diffusion Charging
The negative gas ion movement has two main charging effects on dust particles in
the inter-electrode region. These effects are called field charging and diffusion
charging. Each type of charging is used to some extent in panicle charging, but
one dominates depending on the particle size. Field charging dominates for par-
ticles with a diameter > 1.0 /un, while diffusion charging dominates for particles
with a diameter between 0.1 and 0.3 /on. A combination of the two mechanisms
occurs for particles in the range between 0.3 and 1.0 /on in diameter. It is also
possible to charge particles by electron charging. In this case free electrons that do
not combine with gas ions are moving at an extremely fast rate. These electrons hit
the particle and impart a charge. However, this effect is responsible for very little
particle charging.
In field charging, as particles enter the electric field they cause a local disloca-
tion of the field (Figure 7-9). Negative gas ions traveling along the electric field
lines collide with the suspended particles and impart a charge to them. The ions
continue to bombard the particle until the charge on the particle is sufficient to
divert the electric lines away from the charged particle. This prevents new ions
from colliding with the dust particle. When a particle no long;er receives an ion
charge it is said to be saturated. Saturated charged particles then migrate to the
collection electrode and are collected.
Figure 7-9. Field 'lines modified by the particle.
Diffusion charging is associated with the random Brownian motion of the
negative gas ions. The random motion is related to the thermal velocity of the gas
ions: the higher the temperature, the more movement. Negative ions collide with
the particles because of the random thermal motion of the ions and impart a
charge on the particle. The charged particles migrate to the collection electrode.
This mechanism is important for charging particles in the submicron range. In the
intermediate size range from 0.3 to 1.0 jon in diameter, both field and diffusion
charging are important.
7-8
-------
Discharging the Particle at the Collection Electrode
Resistivity is related to the ability of a particle to take on a charge. In most
industrial applications, the resistivity of the particle is such that the charge on the
particle is only partially discharged upon contact with the grounded collection elec-
trode. A portion of the charge is returned and contributes to the intermolecular
cohesive and adhesive forces which hold the particles to the collection surfaces. The
dust layer builds up on the collection plate to a thickness between 0.03 and 0.5 in.
(0.08-1.27 cm). If the dust layer becomes too thick, it is possible for the accumu-
lated layer to act as an insulator, reducing the flow of the electric field lines.
Rapping Particles into the Hopper
To maintain the continuous process of precipitation, it is necessary to periodically
remove collected dust particles from the discharge and collecting electrodes. In
wet-walled precipitators, the electrodes are cleaned by washing with water sprays.
In most other precipitators deposited dry particles are dislodged by mechanical
impulses or vibrations to the electrodes, called rapping. The electrodes are rapped
with sufficient intensity to cause particles collected on them to fall into a hopper.
Rapping is done when the accumulated dust layer is approximately 0.OS to 0.5
inches thick. This allows the dust layer to fall off the collecting plates as large
aggregate sheets to help eliminate particle reentrainment. Most precipitators will
use adjustable rappers that can change the rapping intensity and frequency
according to dust grain-loading conditions. Rapping is done while the ESP is
on-line.
Dislodged dust falls from the collecting plates into the hopper. The hopper is a
simple collection bin. Hoppers should be cleaned frequently to prevent dust
buildup. Most hoppers are emptied by some type of screw or pneumatic conveyer.
Collection Efficiency
Migration Velocity
Once the particle is charged, it migrates toward the grounded collection electrode.
An indicator of particle movement toward the collection electrode is denoted by
the symbol w and is called the particle migration velocity or drift velocity. The
migration velocity parameter represents the collectability of the particle within the
confines of a specific collector. The migration velocity can be expressed in terms of:
(Eq. 7-1) W
~~^——
Where: d, = diameter of the particle, micrometers
E, = strength of field in which particles are charged, volts per meter
= (represented by peak voltage)
Ef = strength of field in which particles are collected, volts per meter
(normally the field close to the collecting plates)
H = viscosity of gas, Pascal'second
7-9
-------
Migration velocity is quite sensitive to the voltage since the electric field appears
twice in Equation 7-1. Therefore, the precipitator must be designed using the max-
imum electric field for maximum collection efficiency. The migration velocity is
also dependent on particle size; larger particles are collected more easily than
smaller ones.
Particle migration velocity can be determined by the following equation:
(Eq. 7-2) w =
Where:
q= particle charge (charges)
Ep = strength of field in which particles are collected (V/m)
jt= gas viscosity (Pa»sec)
r= radius of the particle (ion)
The particle migration velocity can be calculated using either Equation 7-1 or
7-2. However, most ESPs are designed using a particle migration velocity based on
field experience rather than theory. Typical particle migration velocity rates such
as those listed in Table 7-1 have been published by various ESP vendors. These
values can be used to estimate the collection efficiency of the ESP.
Table 7-1. Typical precipitation rate parameters for varioui applicationi
Application
Utility fly ash
Pulverized coal fly ash
Pulp and paper mills
Sulfuric acid mist
Cement (wet process)
Cement (dry process)
Gypsum
Smelter
Open-hearth furnace
Blast furnace
Hot phosphorous
Flash roaster
Multiple hearth roaster
Catalyst dust
Cupola
Precipitation rate
0.13-0.67
0.33-0.44
0.21-0.31
0.19-0.25
0.33-0.37
0.19-0.23
0.52-0.64
0.06
0.16-0.19
0.20-0.46
0.09
0.25
0.26
0.25
0.10-0.12
(cm/ sec)
4.0-20.4
10.1-13.4
6.4- 9.5
5.8- 7.62
10.1-11.3
6.4- 7.0
15.8-19.5
.8
4.9- 5.8
6.1-14.0
2.7
.6
7.9
7c
.O
3.0- 3.7
Sources: Theodore and Buonicore, 1976; EPA, 1979.
Deutsch-Anderson Equation
Probably the best way to gain insight into the process of electrostatic precipitation
is to study the relationship known as the Deutsch-Anderson Equation. This equa-
tion is used to determine the collection efficiency of the precipitator under ideal
conditions. The simplest form of the equation is:
(Eq. 7-3)
7J=l-«
7-10
-------
Where: »? = collection efficiency of the precipitator
A = the effective collecting plate area of the precipitator, ft* (m1)
Q= gas flow rate through the precipitator, acfs (acms)
e = base of natural logarithm = 2.718
w = migration velocity, ft/sec (cm/sec)
Sources: Deutsch, 1922; Anderson, 1924.
This equation has been used extensively for many years for theoretical collection
efficiency calculations. Unfortunately, while the equation is scientifically valid,
there are a number of operating parameters that can cause the results to be in
error by a factor of two or more. The Deutsch-Anderson Equation neglects three
significant process variables. First, it completely ignores the fact that dust reen-
trainment may occur during the rapping process. Second, it assumes that the par-
ticle size and, consequently, the migration velocity is uniform for all panicles in the
gas stream. Third, it assumes that the gas flow rate is uniform everywhere across
the precipitator and that particle sneakage through the hopper section does not
occur. Therefore, this equation should be used only for making preliminary
estimates of precipitation collection efficiency.
Design Parameters
Many parameters must be taken into consideration in the design and specification
of electrostatic precipitators. The focus of the remainder of this chapter will be on
typical design parameters such as those associated with resistivity, specific collection
area, aspect ratio, gas flow distribution, electrical sectionalization, and precipitator
equipment.
Resistivity
Particle resistivity is a condition of the particle in the gas stream that can alter the
actual collection efficiency of an ESP design. Resistivity is a term that describes the
resistance of the collected dust layer to the flow of electrical current. By definition,
the resistivity is the electrical resistance of a dust sample 1.0 cm* in cross-sectional
area, 1.0 cm thick, and recorded in units ohm»cm. It can also be described as the
resistance to charge transfer by the dust. Dust resistivity values can be classified
roughly into three groups:
• between 10* and 10r ohm • cm—low resistivity
^ between 107 and 1010 ohm*cm—normal resistivity
"* above 10" ohm»an—high Resistivity'"
Low Resistivity
Particles that have low resistivity are difficult to collect since they are easily
charged and lose their charge upon arrival at the collection electrode. This hap-
pens very fast and the particles can take on the charge of the collection electrode.
Particles thus bounce off the plates and become reentrained hi the gas stream.
Examples of low resistivity dusts are unburned carbon in fly ash and carbon black.
7-11
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If the conductive particles are coarse, they can be removed upstream of the
precipitator with another device such as a cyclone. Baffles are often installed on
the collection plates to help eliminate this precipitation-repulsion phenomenon.
The use of liquid ammonia (NHS) as a conditioning agent has found wide use in
recent years. It is theorized that ammonia reacts with H»SO4 to form an
ammonium sulfate substance that increases the resistivity of the dust. Ammonia
vapor at rates of 15 to 40 ppm by volume is injected into the duct leading to the
precipitator. The injection of NH, ha? improved the resistivity of fly ash from coal
fired boilers with low flue gas temperatures (Katz, 1979).
Normal Resistivity
Particles that have normal resistivity do not rapidly lose their charge upon arrival
at the collection electrode. These particles slowly leak their charge to ground and
are retained on the collection plates by intermolecular adhesive and cohesive forces.
This allows a paniculate layer to be built up, which is then dislodged into the
hoppers. At this range of dust resistivity (between 107 and 101" ohm*on) flyash can
be collected most efficiently.
High Resistivity
Particles that exhibit high resistivity are difficult to charge. Once they are finally
charged they do not readily give up the acquired negative charge upon arrival at
the collection electrode. As the dust layer builds up on the collection electrode, the
layer and the electrode form a high potential electric field. The surface of the dust
layer is negatively charged, the interior is neutral, and the collection electrode is
grounded. This causes a condition known as back corona. Under the influence of
the corona discharge, the dust layer breaks down electrically, producing small holes
or craters (in the layer) from which back corona discharges occur. Positive ions are
generated within the dust layer and are accelerated toward the negative (discharge)
electrode. The result of this event would be to counteract the ion generation of the
charging electrode with a corresponding reduction in collection efficiency. Disrup-
tions of the normal corona process greatly reduce precipitator collection efficiency,
which, in severe cases, may fall below 50%. (White, 1974).
Reducing High Resistivity
High resistivity can generally be reduced by adjusting the temperature and
moisture content of the gas stream. Particle resistivity decreases for both high and
low temperatures (see Figure 7-10).
The moisture content of the gas stream also affects particle: resistivity. Increasing
the moisture content of the gas stream lowers the resistivity. This can be
accomplished by spraying water or injecting steam into the duct work proceeding
the ESP. In both temperature adjustment and moisture conditioning one must
maintain gas conditions above the dew point to prevent corrosion problems.
7-12
-------
10'
10'
10
10'
10'
10'
1%
0 100 300 500
Temperature, °F
700
Source: Schmidt, 1949.
Figure 7-10. Effect of temperature and moisture content on apparent resistivity
of precipitated cement dust.
7-13
-------
The presence of SOS in the gas stream has been shown to favor the electrostatic
precipitation process when problems with high resistivity occur. Most of the sulfur
content in the coal burned for combusion sources converts to SOj. However,
approximately 1 percent of the sulfur converts to SO3. The amount of SO» in the
flue gas normally increases with increasing sulfur content of the coal. The resistivity
of the particles decreases as the sulfur content of the coal increases (Figure 7-11).
The use of low sulfur western coal for boiler operations has; caused fly ash
resistivity problems for ESP operations. For coal fly ash dusts, the resistivity can be
lowered below the critical level by the injection of as little as 10-20 ppm SO» into
the gas stream. The SOj is injected into the duct work preceeding the precipitator.
Other conditioning agents such as sulfuric acid, ammonia, sodium chloride, and
soda ash have also been used to reduce particle resistivity (White, 1974).
Two other methods to reduce particle resistivity include: increasing the collection
surface area and by inletting exhaust gas at higher temperatures. Increasing the
collection area of the precipitator will increase the overall cost of the ESP. This
may not be the most desirable method to reduce resistivity problems. Hot
precipitators, which are usually located before the combustion air preheater section
of the boiler, are also used to combat resistivity problems. The use of hot
precipitators is discussed in more detail later in this chapter.
10"-
I
I
0.5 1.0 1.5 2.0 2.5 S.O 3.5
Source: White, 1977.
Figure 7-11. Fly ash resistivity versus coal sulfur content for several flue gas temperature bands.
7-14
-------
Specific Collection Area
The specific collection area (SCA) is defined as the ratio of collection surface area
to the gas flow rate into the collector. The importance of this term is that it
represents the A/Q, relationship in the Deutsch-Anderson equation.
Total collecting surface (ft*)
gas flow rate (1000 acfm)
or in metric units: SCA= ———•
1000 m'/hr
Increases in the SCA of a precipitator design will in most cases increase the col-
lection efficiency of the precipitator. Most conservative designs call for an SCA of
350 to 400 ft1 per 1000 acfm (20-25 m* per 1000 m'/hr) to achieve 99.5* percent
particle removal. The general range of SCA is between 200-800 ft* per 1000 acfm
(11-45 m* per 1000 mVhr) depending on precipitator design conditions and desired
collection efficiency.
Aspect Ratio
The aspect ratio is the ratio of the total length to height of collector surface. The
aspect ratio can be calculated by:
_ Effective length
Effective height
Having a precipitator" chamber many times larger in length than in height would
be ideal. However, space limitations and cost could be prohibitive. The aspect ratio
for ESPs can range from 0.5 to 2.0. For 99.5* percent collection efficiency, the
precipitator design should have an aspect ratio of greater than 1.0.
7-15
-------
Gas Flow Distribution
Gas flow through the ESP chamber should be slow and evenly distributed
throughout the unit. The gas velocities in the duct ahead of the ESP are generally
between 20 and 80 ft/sec (6 and 24 in/sec). The gas velocity into the ESP must be
reduced for adequate panicle collection. This is achieved by using an expansion
inlet plenum (see Figure 7-12).
Gas
distribution
•ection
Figure 7-12. Gas inlet with diffuser-perforated plates.
The inlet plenum contains diffuser-perforated plate openings to evenly distribute
the gas flow throughout the precipitator. Typical gas velocities in the ESP chamber
range from 2 to 8 ft/sec (0.6 to 2.4 rn/sec). With aspect ratios of 1.5, the optimum
gas velocity is generally between 5 and 6 ft/sec (1.5 and 1.8 in/sec).
Electrical Sectionalization
Stage or Field Sectionalization
Precipitator performance is dependent on the number of individual sections or
fields installed. The maximum voltage at which a given field can be maintained
depends on the properties of the gas and dust being collected. These parameters
may vary from one point to another in the unit. To keep each section of the
precipitator working at high efficiency, a high degree of Sectionalization is recom-
mended. Multiple fields or stages are used to provide electrical Sectionalization.
Each field has separate power supplies and controls to adjust for varying gas con-
ditions in the unit.
7-16
-------
Modern precipitators have voltage control devices that automatically limit
precipitator power input. A well designed automatic control system keeps the
voltage level at approximately the value needed for optimum particle charging by
the discharge electrodes.
The voltage control devices operate in the following manner: increases in voltage
will cause a greater spark rate between the discharge and collection electrodes.
Occurrence of a spark counteracts high ESP performance since it causes an
immediate, short term collapse of the precipitator field. Consequently, less useful
power is applied to capture particles. There is, however, an optimal sparking rate
where the gains in particle charging are just offset by corona current losses from
sparkover.
Measurements on commercial precipitators have determined that the optimal
sparking rate is between 50 and 150 sparks per minute per electrical section. The
objective in power control is to maintain corona power input at this optimal
sparking rate. This can be accomplished by momentarily reducing precipitator
power whenever excessive sparking occurs.
The need for separate fields arises mainly because power input requirements dif-
fer at various locations in a precipitator. The paniculate matter concentration is
generally high at the inlet sections of the precipitator. High dust concentrations
tend to suppress corona current. Therefore, a great deal of power is needed to
generate corona discharge for optimal particle charging.
In the downstream fields of a precipitator, the dust loading is usually lighter.
Consequently, corona current flows freer in downstream fields. Particle charging
will more likely be limited by excessive sparking in downstream fields than in the
inlet fields. The power t« the outlet sections must still be high in order to collect
small particles, particularly if they exhibit high resistivity.
If the precipitator had only one power set, the excessive sparking would limit the
power input to the entire precipitator. This would result in a reduction of overall
collection efficiency.
7-17
-------
The precipitator is divided into a series of independently energized 6us sections
or fields (see Figure 7-13). Each bus section has individual transformer-rectifier
sets, voltage stabilization controls, and high-voltage conductors that energize the
discharge electrodes within the section. This allows greater flexibility for individual
field energizing for varying conditions in the precipitator. Most ESP vendors recom-
mend that there be at least four or more fields in the precipitator. It might be
necessary to design the unit with seven or more fields for 99.9* percent collection
efficiency.
SOkVA
Field
1
65kVA
Field
2
85kVA
Field
S
100 kVA
Field
4
Figure 7-13. Stage or field sectionalizatiori.
Parallel Sectionalizadon
Parallel sectionalization provides a means of coping with different power input
needs due to uneven dust and gas distribution (Figure 7-14). Uneven gas distribu-
tions generally occur across the inlet face of the precipitator. Gains in collection
efficiency from parallel sectionalization are likely to be small compared to field or
stage sectionalization.
Figure 7-14. Parallel sectionalization.
7-18
-------
Precipitator Equipment
Discharge Electrodes
The discharge electrodes (wires) in most U.S. precipitator designs are thin round
wires varying from 0.05 to 0.15 inches (0.13 to 0.58 cm) in diameter. Most com-
mon designs use wires approximately 0.1 inch (0.25 on) in diameter. The
discharge electrodes consist of vertically hung wires supported at the top and held
taut and plumb by a weight at the bottom. The wires are usually made from high
carbon steel, but have also been constructed of stainless steel, copper, titanium
alloy, and aluminum. The weights are made of cast iron and are generally 25 Ibs
(11.4 kg) or more.
Discharge wires are usually supported to help eliminate breakage due to
mechanical fatigue. The wires move under the influence of aerodynamic and elec-
trical forces and are subject to mechanical stress. The weights at the bottom of the
wire are attached to guide frames to help maintain wire alignments. In addition,
this will prevent the weights from falling into the hopper in the event that the wire
breaks (Figure 7-15). The bottom and top of each wire is usually constructed with
a shroud of steel tubing. The shroud helps minimize sparking and consequent
metal errosion by sparks at these points on the wire.
Lower guide frame
Guide loop
Top shroud
Bottom ihroud
Weight
Figure 7-15. Guide frames and ihrouds for discharge wires.
7-19
-------
The size and shape of the electrodes are governed by the mechanical
requirements of the system. Most U.S. designs have traditionally used thin, round
wires for corona generation. Some designers have also used twisted wire, square
wire, barbed wire or other configurations. Some of these are illustrated in
Figure 7-16.
r
Figure 7-16. Typical diichargt wire shapes.
European precipitator manufacturers favor the use of rigid support frames for
discharge electrodes. The frames may consist of coiled spring wires, serrated strips,
or needle points mounted on a supporting strip. An example is shown in Figure
7-17. The purpose of the rigid frame is to eliminate the possible swinging of the
discharge wires. These designs have been used as successfully as the U.S. wire
designs. One major disadvantage is the inability to remove a broken wire without
removing the whole frame.
Collection Electrodes
Most precipitators use the plate collection electrodes because of the large gas
volumes treated and high collection efficiencies needed. The plates are generally
made of carbon steel, stainless steel, or some type of alloy, depending upon the gas
stream conditions. The plates range from 0.02 to 0.08 inches (0.05-0.2 cm) in
thickness. Plates are spaced from 4 inches (10 cm) to 12 inches (SO cm) apart. Nor-
mal spacing for high efficiency units is 8 to 9 inches (20-23 on). Wider spaced
ducts are preferred with large collection plates. Plates are usually between 20 and
50 ft (6 to 15 m) high, with higher efficiency precipitators having plates around
30 ft (9 m) high. Lower collection plate height reduces dust reentrainment since
the particle has a shorter distance to fall to the hopper.
Collection plates consist of solid-sheet plates with structural stiffeners. In some
designs the stiffeners have contours designed to improve gas Oow and to lower tur-
bulence in the collecting space near the plate surfaces (Figure 7-18). Baffles are
commonly used to minimize particle reentrainment losses. The structural rigidity of
the plates should be sufficient to maintain consistent electrode spacing. Distorted or
misaligned electrodes contribute to reduced operating voltages and loss of collector
efficiency.
7-20
------- An error occurred while trying to OCR this image.
-------
Shell
The shell structure encloses the electrodes and supports the precipitator com-
ponents in a rigid frame. This is done to maintain proper electrode alignment and
configuration. Supporting structure and foundations for the entire precipitator and
connecting flues must be properly designed. The support structure is especially
critical for hot-side precipitators because of the large temperature expansions of
precipitator components. Excessive temperature stresses can literally tear shell and
hopper joints and welds apart.
Collecting plates and discharge electrodes are normally supported from the top
so that the elements hang vertically under the force of gravity. This allows the
elements to expand or contract with temperature changes without binding or
distorting.
Shells, hoppers, and connecting flues should be covered with insulation to con-
serve heat, and to prevent corrosion due to condensation of moisture and acid on
internal precipitator components. Insulation will also help minimize temperature
differential stresses, especially on hot-side precipitators. Ash hoppers should be
insulated and heated. Cold fly ash has a tendency to cake, making removal
extremely difficult.
The precipitator design should also provide for easy access to strategic points of
the collector. This permits the internal inspection of electrode alignment; facilitates
maintenance; and allows for the cleaning of electrodes, hoppers, and connecting
flues during outages.
Rappers
Removal of the accumulated dust deposit on collection and discharge electrodes is
accomplished by rapping. Dust deposits are dislodged by mechanical impulses or
vibrations imparted to the electrodes. A rapping system is designed so that rapping
intensity and frequency can be adjusted for varying operational conditions. The
system must also be capable of maintaining uniform rapping over long periods of
time without attention.
Rapping of collection plates can be accomplished by a number of methods. One
design uses mechanical rappers consisting of hammers mounted on a rotating shaft.
As the shaft rotates, hammers drop by gravity and strike anvils attached to the col-
lecting plates. The rappers can be mounted on the top of the: collection plates or
on the side as shown in Figure 7-19.
Rapping intensity is governed by the weight of the hammeirs and length of the
hammer mounting arm. The frequency of rapping can be changed by changing
the speed of the rotating shafts. Thus, rapping intensity and frequency adjustments
can be made to deal with varying dust concentration to the precipitator.
7-22
-------
Figure 7-19. Typical hammer/anvil collection plate rapper.
7-23
-------
Magnetic impulse rappers are frequently employed on many U.S. designs to
remove accumulated dust layers from collection plates. A magnetic impulse rapper
consists of a steel plunger that is raised by a current pulse in a coil. The raised
plunger then drops back, due to gravity, striking a rod connected to a number of
plates within the precipitator (Figure 7-20). Rapper frequency and intensity are
easily regulated by an electrical control system. The frequency may be one rap
every few minutes to one rap an hour with an intensity of 10-24 g's (Katz, 1979).
Magnetic impulse rappers usually operate more frequently but with less intensity
than the rotating hammer/anvil type rappers.
Rapper rod
Resilient
Humming
Support
channel
Figure 7-20. Typical impulse collection plate rapi»ers.
The discharge or corona electrodes must also be rapped to prevent buildup of
excessive dust deposits which interfere with corona generation. This is usually
accomplished by the use of air or electric vibrators that gently vibrate the discharge
wires. Vibrators are usually mounted externally on the roofs of the precipitators
and are connected by rods to the high tension frames that support the corona elec-
trodes (Figure 7-21). An insulator, located above the rod, electrically isolates the
rapper while mechanically transmitting the rapping force.
7-24
-------
Rapper
Rapper
insulator
Lower guide
frame
Figure 7-21. Typical ribntor rappers used for discharge electrodes.
7-25
-------
High Voltage Equipment
Transformer-Rectifier Sets
The high voltage equipment is the heart of the electrostatic precipitator. The high
voltage equipment controls the strength of the electric field generated between the
discharge and collection electrodes. This is accomplished by using transformer-
rectifier sets (T-R sets). The transformer-rectifier sets step up normal service
voltages from 400-480 volts to approximately 50,000 volts and convert alternating
to direct current. For fly ash applications, rectifier output ratings are typically 50
kilovolts and 500-2000 milliamperes DC rating.
Development of high voltage equipment to energize precipicators has been an
evolutionary process. The earliest electrostatic precipitators were energized by
mechanical rectifier sets but were consequently replaced by more efficient vacuum
tube rectifiers. Modern precipitators use solid-state silicon type rectifiers and oil or
askerel filled high voltage transformers (Figure 7-22). The T-R ratings usually
range from 15-130 kVA, with more recent design between 60-95 kVA units. The
T-R sets used in conjunction with spark rate, voltage, and current feedback signals
automatically control electrical energization of the ESP.
Support insulator
housing
High voltage
bus duct
Transformer-
rectifier
Discharge electrode
support frame
Discharge electrode
Figure 7-122. High voltage system.
7-26
-------
Control Meters
Control meters are important parts of the control circuitry which monitors the
variations in the electrical power output. Commonly used meters are voltage and
current meters and sparkmeters. Primary voltage and current readings across the
transformer reflect the precipitator power input. Spark meters measure the elec-
trical breakdowns in the precipitator in sparks per minute. From the previous
discussion, it is desirable to generate approximately 100 sparks per minute. These
meters are generally located on the control panel of the precipitator.
Electrostatic Precipitator Applications
Electrostatic precipitators have been used for paniculate emission reduction for
many industrial applications. ESPs have been designed to collect particles hi the
submicron range with 99* percent control efficiency. They are economical to
operate since they have relatively low internal power requirements and low pressure
drop losses across the collector. The ability of ESPs to handle large exhaust gas
volumes at very high temperatures (175-700°C) make them very attractive for many
industries. This is particularly desirable for both cement kiln emission reduction
and control of emissions from basic oxygen furnaces in the steel industry where flue
gas temperatures enter the precipitator at approximately S50°C. ESPs are com-
monly used for paniculate emission reduction in the pulp and paper industry for
black liquor operations, the steel industry for blast furnace and sintering opera-
tions, and for fly ash control from industrial and utility boilers.
Hot-Side Precipitators
Hot-side precipitators have been used in high temperature situations collecting
cement kiln dust and utility boiler fly ash. The hot-side precipitator is located
ahead of the air preheater in a boiler, as compared to the cold side precipitator
which is located after the air preheater. The flue gas temperature for hot-side
precipitators is in the range of 520-420°C. Popularity of hot-side precipitators has
increased with the use of low sulfur coals that produce high resistivity fly ash that is
difficult to collect in the precipitator. The use of hot-side precipitators also helps
reduce corrosion and hopper plugging problems for easier emptying and dust
disposal.
Hot-side precipitators have, however, some disadvantages. Since the temperature
of the gas flow is higher, the volume of gas to be treated hi the ESP is larger. Con-
sequently the overall size of the precipitator will be larger. High gas temperature
also has an effect on the corona, lowering the sparkover voltage and increasing the
corona current. This could decrease the collection efficiency of the unit. Other
major disadvantages include structural and mechanical problems that occur hi the
precipitator shell and support structure. Structural distortions stem mainly from
differential thermal expansion between the shell and the support structure. This
can be overcome with careful engineering and material design of precipitator
components.
7-27
-------
Review of ESP Design Plans
The first step in reviewing design plans for air pollution permits is to read the ven-
dor literature and specifications of the precipitator design. The design specifica-
tions should include at least:
• exhaust gas flow rate and temperature,
• inlet dust concentration,
• specific collection area (SCA),
• gas velocity in the precipitator,
• distance between the plates
• aspect ratio,
• number and size of T-R sets,
• number of fields,
• design migration velocity,
• corona power/1000 m'/min,
• corona current/sq ft plate area,
• design collection efficiency, and
• outlet dust concentration.
The next review step is to determine if the design specifications are within the
range that is typically used in practice by industry. The range of basic design
parameters for fly ash precipitators are given in Table 7-2.
Table 7-2. Typical design parameter ranges for fly ash precipitators.
Parameter
Range (English units)
Range (metric units)
Distance between plates
(duct width)
Gas velocity in ESP
SCA
Aspect ratio (L/H)
Design migration velocity
Number of fields
Corona power/1000 cfm
Corona current/ft1
plate area
Plate area per electrical
(T-R) set
8-12 in. (89 in. optimum)
4-8 ft/sec (5-6 ft/sec optimum)
200-800 ft'/lOOO cfm
(300-400 ft1/1000 cfm optimum)
1-1.5
(keep plate height less than
SO ft for high efficiency)
0.1-0.5 ft/sec
4-8
100-500 watts/1000 cfm
10-80 microamp/fts
5000-80,000 ftVT-R set
(10,000-30,000 ftVT-R set
optimum)
20-30 cm
1.2-2.4 m/sec
11-45 mVlOOO m'/hr
(16.5-22.0 mVlOOO m'/hr)
1.1-5
(keep plate height less than
9 m for high efficiency)
3.05-15.2 cm/sec
4-8
59-295 watts/1000 m'/hr
107-860 microamp/m*
4(55-7,430 mVT-R set
(930-2.790 mVT-R set optimum)
Source: White, 1977.
Finally, the outlet concentration from the ESP must meet the grain loading
requirements of air pollution regulations. The design reviewer can determine if the
calculated outlet values, using the Deutsch-Anderson equation, are within the
regulation limits. In addition, requiring the source to perform a source test to
verify the designed collection efficiency of the ESP would be extremely useful.
7-28
-------
References
1. Deutsch, W. 1922. Ann. Phys. (Leipzig) 68:335.
2. Anderson, E., 1924. Report, Western Precipitator Co., Los Angeles, CA, 1919.
Trans, Amer. Inst. Chem. Eng. 16:69.
3. White, H. J. 1963. Industrial Electrostatic Precipitation. Reading,
Massachusetts: Addison-Wesley.
4. White, H. J. 1977. Electrostatic Precipitation of Fly Ash. APCA Reprint
Series, J of Air Poll. Control Assoc.
5. Katz, J. 1979. The Art of Electrostatic Precipitators. Munhall, Pennsylvania:
Precipitator Technology, Inc.
6. White, H. J. 1974. Resistivity Problems hi Electrostatic Precipiation. J of Air
Poll. Control Assoc. 24:315-338.
7. Schmidt, W. A. 1949. Ind. and Eng. Chem. 41:2428.
8. Rose, H. E. and Wood, A. J. 1956. An Introduction to Electrostatic Precipita-
tion in Theory and Practice. London: Constable and Company LTD.
9. Bethea, R. M. 1978. Air Pollution Control Technology—an Engineering
Analysis Point of View. New York: Van Nostrand Reinhold Company.
10. Theodore, L. and Buonicore, A. J. 1976. Industrial Air Pollution Control
Equipment for Particulates. Cleveland: CRC Press.
11. Cheremisinoff, P. N. and Young, R. A., eds. 1977. Air Pollution Control and
Design Handbook Part 1. New York: Marcel Dekker, Inc.
12. Hesketh, H. E. 1979. Air Pollution Control. Ann Arbor: Ann Arbor Science
Publishers Inc.
13. Stern, A. C. ed. 1977. Air Pollution Third Edition Volume IV Engineering
Control of Air Pollution. New York: Academic Press.
14. Cross, F. L. and Hesketh, H. E. eds. 1975. Handbook for the Operation and
Maintenance of Air Pollution Control Equipment. Westport, Conn.:
Technomic Publishing Co., Inc.
15. Environmental Protection Agency (EPA). 1979. Particulate Control by Fabric
Filtration on Coal-Fired Industrial Boilers. EPA 625/2-79-021.
16. Environmental Protection Agency (EPA). 1976. Capital and Operating Costs of
Selected Air Pollution Control Systems. EPA 450/3-76-014.
17. Sittig, M. 1977. Particulates and Fine Dust Removal Processes and Equipment.
New Jersey: Noyes Data Corporation.
18. Environmental Protection Agency (EPA). 1973. Air Pollution Engineering
Manual. 2nd ed. AP-40.
7-29
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Chapter 8
Fabric Filtration
Filtration for Particle Collection
Fabric filtration is one of the most common techniques used to collect paniculate
matter. There are two basic types of filters: one is disposable; the other is not.
Disposable filters are similar to those used in a home heating or air conditioning
system. Disposable filters can be constructed as mats or as deep beds (12 inches or
more). Mat filters are usually made using fiberglass bats with a thin metal plate on
the outside of the filter used for structural reinforcement. Depth filters are
generally constructed using fiberglass fibers, glass fiber paper or some other inert
material such as fine grade steel to form a deep mesh. The filters are very efficient
(99.97%) for the collection of O.S /tm and larger particles but must be replaced
when they become loaded with paniculate matter (when the pressure drop across
the filter becomes excessive). Depth filters are widely useful for the collection of
toxic dust materials.
Nondisposable fabric filters consist of some type of fabric material (nylon,
wool, or others). This type is commonly used to clean dirty exhaust gas streams
from industrial processes. The particles are retained on the fabric material, while
the cleaned gas passes through the material.
The collected panicles are then removed from the filter by a cleaning
mechanism; by shaking or using blasts of air. The removed particles are stored in a
collection hopper until they are disposed of or are reused in the process.
Collection Mechanisms
Particles are collected on a filter by a combination of the mechanisms described
earlier in this manual. The most important here are impaction, direct interception
and diffusion. In collection by impaction, the particles in the gas stream have too
much inertia to follow the gas streamlines around the fiber and are impacted on
the fiber surface (Figure 8-1).
Particle
Gas streamlines
Figure 8-1. Impaction.
8-1
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In the case of direct interception the particles have less inertia and barely follow
the gas streamlines around the fiber. If the distance between the center of the par-
ticle and the outside of the fiber is less than the particle radius, the particle will
graze or hit the fiber and be "intercepted" (Figure 8-2).
Impaction and direct interception mechanisms account for 99 percent collection
of particles greater than 1 jim aerodynamic diameter in fabric filter systems.
The third collection mechanism is that of diffusion. In diffusion, small particles
are affected by collisions on a molecular level. Particles less than 0.1 fan
aerodynamic diameter have individual or random motion. The particles do not
necessarily follow the gas streamlines, but move randomly throughout the fluid.
This is known as Brownian motion. The particles may have a different velocity
than the fluid and at some point could come in contact with the fiber and be col-
lected (Figure 8-3).
Panicle ^ ••••••
Gas streamlines
Figure 8-2. Direct interception.
Particle
Gas streamlines
Figure 8-3. Diffusion.
8-2
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Other collection mechanisms such as gravitational settling, agglomeration, and
electrostatic attraction may contribute slightly to particle collection. Particles can
agglomerate or grow in size and then be more easily collected by the fibers. Some
particles have a small electrostatic charge and can be attracted to a material of
opposite charge. Electrostatic charges could, on the other hand, have a bad affect
if the charges of the particles and fiber are the same. Electrostatic charges can be
particularly useful for the capture of particles in the submicron range. The use of a
selected fiber material or a specially coated material may enhance particle capture
(Frederick, 1974). Different materials will develop electrostatic charges of varying
degree and sign. A series of these triboelectric effects or electrostatic charges for
various fabric materials was developed by Frederick and is shown in Table 8-1.
Table 8-1. Triboelectric Kiie* for tome production fabrics.
Positive
+ 25
+ 20
+ 15
+ 10
+ 5
- 5
-10
-15
-20
Negative
Wool felt
Glass, filament, heat cleaned and silicone treated
Glass, spun, heat cleaned and silicone treated
Wool, woven felt. T-2
Nylon 66, spun
Nylon 66, spun, heat set
Nylon 6, spun
Cotton sateen
Orion 81, filament
Orion 42, needled fabrics
Arnel, filament
Dacron, filament
Dacron, filament, silicone treated
Dacron, filament, M-S1
Dacron, combination filament and spun
Creslan, spun; Azoton, spun
Verel. regular, spun; Orion 81, spun (55200)
Dynel, spun
Orion 81, spun
Orion 42, spun
Dacron, needled
Dacron, spun; Orion 81, spun (79475)
Dacron, spun and heat set
Polypropylene 01, filament
Orion 39B, spun
Fibravyl, spun
Darvan, needled
Kodel
Polyethylene B, filament and spun
8-3
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Baghouses
General Description
Nondisposable fabric filter systems are developed for industrial application as
baghouse systems. A baghouse consists of the following components:
• filter medium and suppon
• filter cleaning device
• collection hopper
• shell
• fan
The particle collection surface is composed of the filtering material and usually
some type of suppon structure. Most U.S. baghouse designs employ long cylindrical
tubes that contain felted fabric or woven doth as the filtering medium. The cloth
can be supported at the top and bottom of the bag by metal rings or by a cage
that completely supports the entire bag (Figure 8-4). Some European baghouse
designs employ an envelope filter arrangement as shown in Figure 8-5. In this case
the baghouse consists of compartments that contain envelopes of fabric mounted on
frames and attached to the walls of the collector (Figure 8-5).
Metal cap
Rings or clasps
Internal cage
suppon
Figure 8-4. Bags and support.
8-4
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Baghouses are usually constructed using many cylindrical bags that hang ver-
tically in the baghouse. The number of bags can vary from a few hundred to a
thousand or more depending on the size of the baghouse. When dust layers have
built up to a sufficient thickness, the bag is cleaned, causing the dust panicles to
fall into a collection hopper (Figure 8-6). Bag cleaning can be done by a number
of methods. Particles are stored in the hopper and are usually removed by a
pneumatic or screw conveyer. The baghouse is enclosed by sheet metal to contain
the collected dust and to protect the bags from atmospheric environmental
conditions.
Bags
Hopper
Figure 8-6. Bags and hopper.
Dirty gas is either pushed or pulled through the baghouse by a fan. When the
dust laden gas is pushed through the baghouse, the collector is called a positive
pressure baghouse. When the fan is on the downstream side of the baghouse, the
dirty gas is pulled through the baghouse and the collector is called a negative
pressure baghouse (Figure 8-7). The structure of the negative pressure baghouse
must be reinforced because of the suction on the baghouse shell. Vendors can con-
struct positive pressure baghouses with weaker support structure since the positive
pressure will counterbalance the atmospheric pressure on the baghouse shell.
Limitations, however, do exist since the fan is located on the dirty side of the
system. Premature deterioration of fan blades and bearings can occur in this
configuration.
8-6
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Dirty gas.
Fan
Positive pressure baghouse
Dirty gas«
Fan
Negative pressure baghouse
Figure 8-7. Positive and negative pressure baghouses.
8-7
-------
Filtration Designs
There are two filtration designs used in baghouses: interior filtration and exterior
filtration. In baghouses using interior filtration, panicles are collected on the
inside of the bag. The dust laden gas enters either through the top or bottom of
the collector and is directed inside the bag by diffuser vanes and a cell plate. The
cell plate is a thin metal sheet surrounding the bag openings. The cell plate
separates the clean gas section from the baghouse inlet. The particles are filtered
by the bag and clean air exits through the outside of the bag (Figure 8-8).
Cell plate
Dirty gas
Figure 8-8. Interior filtration (particles collected on the inside of the bag).
In exterior filtration systems, dust is collected on the outside of the bags. The
filtering process goes from the outside of the bag to the inside with clean gas
exiting through the inside of the bag (Figure 8-9). Consequently, some type of bag
support is necessary, such as an internal bag cage or rings sewn, into the bag fabric.
8-8
------- An error occurred while trying to OCR this image.
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The dust-laden gas inlet position for both filtration systems often depends on the
baghouse model and manufacturer. If the gas enters the top of che unit, a
downwash of gas occurs which tends to clean the bags somewhat while the bags are
filtering. This usually allows slightly higher gas volumes to be filtered through the
baghouse before cleaning is required. If the gas enters the bottom of the unit, the
inlet is positioned at the very top pan of the dust hopper (Figure 8-10). Bottom or
hopper inlets are easier to design and manufacture structurally than are the top
inlets. However, when using hopper inlets, vendors must carefully design gas flows
to avoid dust reentrainment from the hopper.
Clean gas
Dirty gas -*• ff -*•
Figure 8-10. Dust inlet to the baghouse.
8-10
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Bags
Tubular bags vary in both length and diameter depending on baghouse design and
manufacturer. The length ranges from 10 to 40 feet and the diameter is usually
between 6 and 18 inches. Bags are usually hung vertically in the baghouse and are
attached at the top and bottom by some type of ring, cap, damp or clasp
(Figure 8-11).
Caps
damp
Cell plate
Figure 8-11. Bag attachment.
8-11
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A cell plate (shown in Figure 8-11) is also used in some baghouse systems to
secure bags in place. Exterior filtration baghouses usually have the bags supported
by a cage and the bags are attached to the cage by a clasp.
Housing
Baghouses are constructed as single units or compartmental units. The single unit
is generally used on small processes that: are not in continuous operation such as
grinding and paint spraying processes. Compartmental units consist of more than
one baghouse compartment and are used in continuous operating processes with
large exhaust volumes such as electric melt steel furnaces and industrial boilers. In
both cases, the bags are housed in a shell made of a rigid metal material.
Occasionally it is necessary to include insulation with the shell when treating high
temperature flue gas. This is done to prevent moisture or acid mist from con-
densing in the unit, causing corrosion and rapid deterioration of. the baghouse.
Hoppers
Hoppers are used to store the collected dust before it is disposed in a landfill or
reused in the process. They are designed usually with a 60° slope to allow dust to
flow freely from the top of the hopper no the bottom discharge opening. Some
manufacturers add devices to the hopper to promote easy and quick discharge.
These devices include strike plates, poke holes, vibrators, and rappers. Strike plates
are simply pieces of flat steel which are bolted or welded to the center of the hop-
per wall. If dust becomes stuck in the hopper, rapping the strike plate several times
with a mallet will free this material. Hopper designs also usually include access
doors or ports. Access ports provide for easier cleaning, inspection and
maintenance of the hopper (Figure 8-12).
Some type of discharge device is necessary for emptying the hopper. Discharge
devices can be manual or automatic. The simplest manual discharge device is the
slide gate, a plate held in place by a frame and sealed with gaskets. When the
hopper needs to be emptied, the plate is removed and the material discharges.
Other manual discharge devices include hinged doors or drawers. The collector
must be shut down before opening any manual discharge device. Thus, manual
discharge devices are used on baghouses that operate on a periodic basis.
Automatic continuous discharge devices are installed on baghouses that are used in
continuous operation. Some devices include trickle valves, rotary air lock valves,
screw conveyors or pneumatic conveyers. Trickle valves are shown in Figure 8-13.
As dust collects in the hopper, the weight of the dust pushes down on the
counterweight of the top flap and dust discharges downward (Figure 8-1S). The top
flap then closes, the bottom flap opens, and the material falls out. This type of
valve is available in gravity-operated and motorized versions.
8-12
-------
Access port
Strike
Figure 8-12. Hopper.
Figure 8-15. Trickle valve discharge device.
8-13
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Rotary airlock valves are used on medium or large sized baghouses. The valve is
designed with a paddle wheel which is shaft-mounted and driven by a motor
(Figure 8-1S). The rotary valve is similar to a revolving door: the paddles or blades
form an airtight seal with the housing; the motor slowly moves the blades to allow
the dust to discharge from the hopper.
Figure 8-14. Rotary airlock discharge device.
Other automatic dust discharge devices include screw and pneumatic conveyers.
Screw conveyers employ a revolving screw feeder located at the bottom of the
hopper to remove the dust from the bin (Figure 8-15).
8-14
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Screw
Figure 8-.15. Screw conveyor.
8-15
-------
Pneumatic conveyers use compressed air to blow (remove) dust from the hopper
(Figure 8-16).
Blower or_
compressed air
Figure 8-16. Pneumatic conveyor.
Fabric Filter Material
Construction Types
Woven and felted materials are used to make bag filters. Woven filters are made of
yarn with a definite repeated pattern. Felted filters are composed of randomly-
placed fibers compressed into a mat and attached to some loosely woven backing
material. Woven filters are used with low energy cleaning methods such as shaking
and reverse air. Felted fabrics are usually used with higher energy cleaning systems
such as pulse jet cleaning.
Woven Filters
Woven filters have open spaces around the fibers. The type of weave used will
depend on the design and the actual intended application of the woven filter. The
simplest woven weave is the plain weave. The yarn is woven over and under to
form a checkerboard pattern. This weave is not frequently used. Other weaves
include the twill and sateen (satin). In the twill weave, yarn is woven over two and
under one but hi one direction only (Figure 8-17). This weave is tighter and more
durable than the simple weave. Sateen weave goes one over and four under hi
both directions. Sateen weaves are very tight and allow .the use of very fine yarns.
Different weaving patterns increase or decrease the open spaces between the fibers.
8-16
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This will affect both fabric strength and permeability. Fabric permeability affects
the amount of air passing through the filter at a specified pressure drop. A tight
weave, for instance, has low permeability and is better for the capture of small par-
ticles at the cost of increased pressure drop.
C
c
3
3
TjUu
Twill weave
Sateen weave
Figure 8-17. Woven fabric filter; twill weave and sateen weave.
«
The true filtering surface for the woven filter is not the bag itself, but the dust
layer or filter cake. The bag simply provides the surface for capture of larger par-
ticles. Particles are collected by impaction or interception and the open areas in
the weave are closed. This process is referred to as sieving (Figure 8-18). Some par-
ticles escape through the filter until the cake is formed. Once the cake builds up,
effective filtering will occur until the bag becomes plugged and cleaning is
required. At this point the pressure drop will be exceedingly high and filtering will
no longer be cost effective. The effective filtering time will vary from a time of
approximately 15-20 minutes to as long as a number of hours, depending on the
concentration of paniculate matter in the gas stream.
8-17
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*«•*:* *
Dust
'cake
Woven filter
Figure 8-18. Sieving.
Felted Filters
Felted filters are made by needle punching fibers onto a woven backing called a
scrim. The fibers are randomly placed as opposed to the definite repeated pattern
of the woven filter. The felts are attached to the scrim by chemical, heat, resin and
stitch-bonding methods.
To collect fine particles, the felted filters depend to a lesser degree on the initial
dust deposits than do woven filters. The felted filters are generally 2 to S times thicker
than woven filters. Each individual randomly oriented fiber acts as a target for par-
ticle capture by impaction and interception. Small particles can be collected on the
outer surface of the filter (Figure 8-19).
Felt
Woven
backing
Figure 8-19. Felted fabric filter.
8-18
-------
Felted filters are usually used in pulse jet baghouses. A pulse jet baghouse
generally filters more air per cloth area (higher air-to-cloth ratio) than a shaker or
reverse air unit. Felted bags should not be used in high humidity situations,
especially if the particles are hydroscopic. Clogging and blinding could result in
such situations.
Fibers
The fibers used for fabric filters vary depending on the industrial application to be
controlled. Some filters are made from natural fibers such as cotton or wool. These
fibers are relatively inexpensive but have temperature limitations (< 100°C) and
only average abrasion resistance. Synthetic fibers such as nylon, Orion®, and
polyester have slightly higher temperature limitations and chemical resistance. Syn-
thetic fibers are more expensive than natural fibers. Nomex® is a registered
trademark of fibers made by DuPont. DuPont makes the fibers, not filter fabrics or
bags. Nomex® is widely used due to its relatively high temperature resistance and its
resistance to abrasion. Other fibers such as Teflon® and Fiberglas* can be used
in very high temperature situations (230 to 260 °C). Both materials have good
resistance to acid attack, but are generally more expensive than other fibers.
Table 8-2 lists a number of typical fibers used for fabric filters. The properties of
the listed fibers include temperature limits, acid and alkali resistance, abrasion
resistance, and relative bag costs.
Table 8-2. Typical fabrics used for bags.
Fabric
Cotton
Polypropylene
Wool
Nylon
Orion*
Acrylic
Dacron*
Nomex*
Teflon*
Fiberglas*
Maximum temperature
(•O
Continuous
82
88
93-102
93-107
116
127
135
204
204-232
260
Surges
107
93
121
121
127
137
163
218
260
288
Acid
resistance
poor
good to
excellent
very good
poor to fair
good to
excellent
good
good
poor to good
excellent
•except poor
to fluorine
fair to good
Alkali
resistance
very good
very good
poor
good to
excellent
fair to good
fair
good
good to
excellent
excellent
•except poor
to trifluoride
chlorine and
molten alka-
line metals
fair to good
Flex
abrasion
resistance
very good
excellent
fair to good
excellent
good
good
very good
excellent
fan-
fair
Relative
cost
2.0
1.5
3.0
2.5
2.75
3.0
2.8
8.0
25.0
6.0
Sources: Bethea, 1978; EPA, 1979; Theodore and Buonicore, 1976.
8-19
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Fabric Treatment
Fabrics are usually pretreated to improve their mechanical and dimensional sta-
bility. They can be treated with silicone to give them better cake release properties.
Natural fabrics (wool and cotton) are usually preshrunk to eliminate bag shrinkage
during operation. Both synthetic and natural fabrics usually undergo processes such
as heat setting, flame retardation, and napping; these processes increase fabric life,
improve dimensional stability and permeability, and ease of bag cleaning.
Bag Failure Mechanisms
There are three failure mechanisms that shorten the operating life of a bag. They
are related to abrasion, thermal durability and chemical attack. The chief design
variable is the upper temperature limit of the fabric. The process exhaust
temperature will determine which fabric material should be used for dust collec-
tion. Exhaust gas cooling may be feasible, but one must be careful to keep the
exhaust gas hot enough to prevent moisture or acid from condensing on the bags.
Another problem frequently encountered in baghouse operation is that of abra-
sion. Bag abrasion can result from bags rubbing against each other or from the
type of bag cleaning employed in the baghouse. For instance, in a shaker
baghouse, vigorous shaking may cause premature bag deterioration, particularly at
the points where the bags are attached. In pulse jet units, the continual, slight
motion of the bags against the supporting cages can also seriously affect bag life.
As a result, a 25 percent per year bag replacement rate is usually encountered.
This is the single biggest maintenance problem associated with baghouses.
Bag failure can also occur by chemical attack to the fabric. Changes in dust
composition and exhaust gas temperatures from industrial processes can greatly
affect the bag material. If the exhaust gas stream is lowered to its dew point or a
new chemical species is created, the design of the baghouse (fabric choice) may be
completely inadequate. Proper fabric selection and good process operating prac-
tices can help eliminate bag deterioration caused by chemical attack.
Gas Conditioning
Occasionally it is necessary to cool the process gas stream before the gas goes to the
baghouse. Since there is an upper temperature limit on the fabrics used for bags,
gas cooling is sometimes necessary to preserve bag life. This can be accomplished
by a number of cooling methods.
Dilution of the exhaust gas stream by air is the easiest and cheapest method,
especially at very high temperatures. However, air dilution requires the use of a
larger baghouse to handle the increased volume of air. Other problems can arise
due to the difficulty of controlling the intake of ambient moisture and other con-
taminants from the dilution air intake.
Radiation cooling can also be used to lower the process exhaust gas temperature.
Radiation cooling involves the use of long uninsulated ducts that allow the gas
stream to cool as heat radiates from the duct walls. Ducts can be designed in "U"
8-20
-------
shapes to allow more duct surface area to be exposed for radiation cooling. Radia-
tion cooling would not normally be very effective to cool gas temperatures below
500 °C. This would require substantial surface area, lengthy duct runs, and
increased fan horsepower. Precise temperature control is difficult to maintain and
there is a possibility of the ducts becoming plugged due to panicle sedimentation.
Evaporative cooling is also used to reduce exhaust gas stream temperature.
Evaporative cooling is accomplished by injecting fine water droplets into the gas
stream. The water droplets absorb heat from the gas stream as they evaporate.
Spray nozzles are located in a quench chamber or somewhere in the duct
preceding the baghouse. Evaporative cooling gives a great amount of controlled
cooling at a relatively low installation cost. Temperature control can be flexible
and accurate. However, this cooling method increases the exhaust volume to the
baghouse. The biggest problem with evaporative cooling is keeping the gas
temperature above the dewpoint of the gas (SO,, NO,, HC1, etc.). Otherwise, gases
may condense on the bags causing rapid bag deterioration. In addition, all
moisture injected into the gas must be evaporated to prevent corrosion of metal
parts and blinding or plugging of caked dust on the bags.
Bag Gleaning
Cleaning Sequences
Three basic sequences are used for bag cleaning: intermittent cleaning, which is
done on a demand basis; periodic cleaning, which is performed on a timed or
scheduled basis; and continuous filter cleaning.
In intermittently cleaned baghouses, an entire compartment (or baghouse) is
bypassed and the bags are cleaned either row by row or simultaneously. Inter-
mittent baghouses are used for batch processes that can be shut down for bag
cleaning.
Periodically cleaned baghouses consist of a number of compartments or sections.
One compartment at a time is removed from service and cleaned on a regular rota-
tion basis. The dirty gas stream is diverted from the compartment being cleaned to
the other compartments in the baghouse, so it is not necessary to shut down the
process.
Continuously cleaned baghouses, are fully automatic and can constantly remain
on-line for filtering. The filtering process is momentarily interrupted by a blast of
compressed air that deans the bag, called pulse jet cleaning. In continuous
cleaning, there is always a row of bags which are being cleaned somewhere in the
baghouse. The advantage of continuous cleaning is that it is not necessary to take
the baghouse out of service. Large continuous cleaning baghouses are built with
compartments to help prevent total baghouse shutdown for bag maintenance and
failures to the compressed air cleaning system or hopper conveyers. This allows the
baghouse operator to take one compartment off-line to perform necessary
maintenance.
8-21
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Types of Bag Cleaning
A number of cleaning mechanisms are used to remove caked particles from bags.
The three most common are shaking, reverse air, and pulse jet. Another
mechanism called blow ring or reverse jet is normally not used in modern bag
cleaning systems.
Shaking can be done manually, but is usually performed mechanically in
industrial-scale baghouses. It is a low energy process that gently shakes the bags to
remove deposited particles. The shaking motion and speed depends upon the ven-
dors' design and the composition of dust deposited on the bag. The shaking motion
can be either in a horizontal or vertical direction. The tops of die bags in shaker
baghouses are sealed or closed and supported by some type of book or clasp. Bags
are open at the bottom and attached to a cell plate. The cell plate is shaken as a
unit causing the bag to ripple, releasing the dust (Figure 8-20).
Sonic vibration
Horizontal
Vertical
Figure B-20. Shaking.
Shaking should not be used when collecting sticky dusts. The forces needed for
removing sticky dust can cause the bag to be torn or ripped.
Bag wear on the whole is generally not a problem at the bottom of the bag
which is attached to the cell plate; the greatest wear is at the top of the bag where
the support loop attaches to the bag. Proper shaking frequenqr is therefore impor-
tant to prevent premature bag failure.
8-22
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In a few systems, shaking is accomplished by sonic vibration (Figure 8-20). A
sound generator is used to produce a low frequency sound that causes the bags to
vibrate. The noise level produced by the generator is barely discernable outside the
baghouse. This type of cleaning, however, is not used on many newer baghouse
systems.
Reverse air, the simplest cleaning mechanism, is accomplished by stopping the
flow of dirty gas into the compartment and backwasbing the compartment with a
low pressure flow of air. Dust is removed by merely allowing the bags to collapse,
thus causing the dust cake to break and fall into the hopper. The cleaning action is
very gentle, allowing the use of less abrasion resistant fabrics such as Fiberglas*
(Figure 8-21).
Figure 8-21. Reverse air cleaning.
Reverse air cleaning baghouses are usually compartmentalized to permit a sec-
tion to be off-line for cleaning. Dust can be collected on either the inside or outside
of the bag. If collected on the outside, some type of support is needed to prevent
bag collapse during the filtering process. Bags can be supported by small steel rings
sewn to the inside of the bag.
8-23
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Reverse air cleaning baghouses generally have very low air-to-cloth (A/C) ratios.
Air-to-cloth ratios describe how much dirty gas passes through a given surface area
of filter in a given time. A high air-to-cloth ratio means a large volume of air
passes through the fabric area. A low air-to-cloth ratio means a small volume of air
passes through the fabric. The A/C ratios are usually expressed in units of
(ftVmin)/ft* of cloth [(cmVsecJ/cm* of cloth]. The A/C ratio can be used inter-
changeably with a term called filtration velocity. The units for filtration velocity
are ft/min (cm/sec). When using the A/C ratios for comparison purposes one
should use the units (fts/min)/fts or (cmB/sec)/cmf. Likewise, when using filtration
velocities one should use the units ft/min or cm/sec.
For reverse air baghouses, the filtering velocity (filtration velocity) range is
usually between 1 and S ft/min (0.51 and 1.52 cm/sec). For shaker baghouses, the
filtering velocity ranges between 2 and 6 ft/min (1.02 and S.05 cm/sec). More
cloth is generally needed for a given flow rate in a reverse air baghouse than in a
shaker baghouse. Hence, reverse air baghouses tend to be larger in size.
Occasionally, baghouse cleaning is accomplished by two methods in combina-
tion. Many baghouses have been designed with both reverse air and gentle shaking
to remove the dust cake from the bag.
The third bag cleaning mechanism most commonly used is the pulse jet or
pressure jet cleaning. Baghouses using pulse jet cleaning make up approximately 40
to 50 percent of the new baghouse installations in the U.S. today. The pulse jet
cleaning mechanism uses a high pressure jet of air to remove the dust from the
bag. Bags hi the baghouse compartment are supported internally by rings or cages.
Bags are held firmly in place at the top by clasps and have an enclosed bottom.
Dust-laden gas is filtered through the bag, depositing dust on the outside surface of
the bag.
The dust cake is removed from the bag by a blast of compressed air injected into
the top of the bag tube. The blast of high pressure ah- stops the normal flow of air
through the filter. The air blast develops into a standing wave that causes the bag
to expand as the bubble travels down the bag tube. As the bag flexes, the cake
fractures and deposited particles are discharged from the bag (Figure 8-22). The
air bubble travels down and back up the tube in approximately 0.5 seconds.
8-24
------- An error occurred while trying to OCR this image.
-------
The blast of compressed air must be strong enough to travel the length of the
bag and shatter or crack the dust cake. Pulse jet units use air supplies from a com-
mon header above each bag (Figure 8-23). In most baghouse designs, a venturi
sealed at the top of each bag is used to create a large enough pulse to travel down
and up the bag. This occurs in approximately 0.3 to 0.5 sec. The pressures
involved are commonly between 60 and 100 psig (414 kPa and 689 kPa).
Figure 8-23. Pulie jet air supply.
8-26
-------
Most pulse jet baghouses use bag tubes that are 4 to 6 in. (10.2 to 15.2 cm) in
diameter. The length of the bag is usually around 10 to 12 ft (S.05 to S.66 m), but
can be as long as 25 ft (7.6 m). The shaker and reverse air baghouses use larger
bags than the pulse jet units. The bags in these units are 6 to 18 in. (15.2 to 45.7
cm) hi diameter and up to 40 ft (12.2 m) hi length.
Pulse jet baghouses are designed with filtering velocities between 5 to 15 ft/mm
(2.5 to 7.5 cm/sec). Therefore, these units usually use felted fabrics as bag
material. Felted material holds up very well under the high filtering rate and
vigorous pulse jet cleaning. Pulse jet cleaning methods have the advantage of
having no moving parts within the compartment. In addition, pulse jet units can
clean bags on a continuous basis without isolating a compartment from service.
The duration of the cleaning time is short (< 1.0 sec) when compared to the time
length between cleaning intervals (approximately 20 minutes to several hours). The
major disadvantage of high pressure cleaning methods is that the bags are sub-
jected to more mechanical stress. Fabrics with higher dimensional stability and
high tensile strength are required for these units.
Another bag cleaning mechanism is reverse jet cleaning using a blow ring. Some
older baghouse designs employed this method, but it has lost popularity due to the
great number of moving parts inside the baghouse. Blow ring cleaning involves
reversing the air flow on each bag. This cleaning method does not depend on the
collapse of each bag to crack the cake as in the reverse air baghouse. A traveling
blow ring carriage moves up and down the bag compartment (Figure 8-24). Each
ring has a number of slots where high velocity air jets penetrate the bag tube and
dislodge the accumulated dust layer. The expense and complication of the blow
ring mechanism (motors, drives, and switches for both ring and fan) limits the
applicability of this equipment for air pollution control.
8-27
-------
Dirty
Blow ring
carriage
Blow ring
Figure 8-24. Rrvene jet cleaning wing blow rings.
8-28
-------
Baghouse Design Variables
Baghouses are designed by considering a number of variables: pressure drop, filter
drag, air-to-cloth ratio, collection efficiency and gas conditioning. Although not
always possible or practical, it is a good idea to use a pilot scale baghouse during
the initial stages of the baghouse design. However, previous vendor experience with
the same or similar process to be controlled will generally be adequate for design
purposes. Careful design will reduce the number of baghouse operating problems
and possible air pollution violations.
Pressure Drop
Pressure drop, a very important baghouse design variable, describes the resistance
to air flow across the baghouse. Pressure drop is usually expressed in mm of
mercury or inches of water. It can be related to the size of the fan that would be
necessary to either push or pull the exhaust gas through the baghouse. A baghouse
with a high pressure drop would need a larger fan and more energy to move the
exhaust gas through the baghouse.
Many different relationships have been used to estimate the pressure drop across
a fabric filter. In a baghouse the total pressure drop is a function of the pressure
drop across both the filter and the deposited dust cake. There are also some minor
pressure losses due to friction occurring as the gas stream moves through the
baghouse.
The simplest equation used to predict pressure drop across a filter is derived
from Darcy's law governing the flow of fluids through porous materials and given
as: _
(Eq. 8-1) Ap/=k,v/
Where: Ap/= pressure drop across the clean fabric, in. H,O (cm HtO)
k, = fabric resistance, in. H,O/ft-min (cm H,O/cm-sec)
v/= filtration velocity, ft/min (cm/sec)
The term ki is the fabric resistance and is a function of exhaust gas viscosity and
filter characteristics such as thickness and porosity. Porosity describes the amount
of void volume in the filter.
The pressure drop across the deposited dust cake can be estimated by using
Equation 8-2 (Snyder and Pring, 1955). This formula is also derived from Darcy's
law and the simplified form is given as:
(Eq. 8-2) Ape = k,c
-------
The term kz is the dust-fabric filter resistance coefficient and is determined
experimentally. The coefficient is dependent on gas viscosity, panicle density and
dust porosity. The dust porosity is the amount of void volume in the dust cake.
The porosity is related to the permeability. Permeability is defined in ASTM stan-
dard D7S7-69 as the volume of air which can be passed through one square foot of
filter medium with a pressure drop of no more than 0.5 inches of water.
The total pressure drop equals the pressure drop across the filter plus the
pressure drop across the cake and is given as:
(Eq. 8-3) Apr = Ap, + Ape
ApT=kiV/-»-kfc,v/t
Equation 8-3 should be used as only an estimate of pressure drop across shaker
and reverse air cleaning baghouses. In the industrial filtration process, there are
complicated particle-fabric interactions occurring just after the filtration cycle
begins. In addition, the filter resistance factor kt can take on two values; one value
for the clean filter and another after the filter has been cleaned. When the dust
cake builds up to a significant thickness the pressure drop will become exceedingly
high (> 12 in. H,O or 30.5 cm H,O). At this time the filter must be cleaned. Some
dust will remain on the cloth even after cleaning; therefore, the filter resistance
level will be higher than during original conditions.
Filter Drag
Filter drag is the filter resistance across the fabric-dust layer. It is a function of the
quantity of dust accumulated on the filter and given as:
(Eq. 8-4) S = -^2_
v/
Where: S = filter drag, in. H,O/(ft/sec) [kPa/(cm/sec)]
Ap = pressure drop across the filter and dust cake, in. HtO (cm HtO)
v,= filtration velocity, ft/sec (cm/sec)
It essentially gives the pressure drop occurring per unit velocity.
As previously mentioned, the true filtering surface is not the bag itself, but the
dust layer. Dust bridges the pores or openings in the weave, increasing the drag
rapidly. A filter performance curve of a single bag of a fabric is shown in Figure
8-25. The drag is plotted versus the dust mass deposited on the filter.
The point cr on the graph is the residual drag of the dean filter medium. The
filter drag increases exponentially up to a constant rate of increase. This is the
period of cake repair and initial cake buildup. Effective filtration takes place while
the filter drag increases at a constant rate. When the total pressure drop reaches a
value set by the system design, bag cleaning is initiated. At this point, the pressure
drop decreases (almost vertically on the performance curve) to the initial point.
Cake repair begins when the cleaning cycle stops and the cycle repeats.
8-30
-------
be
I
§
£
Cake
repair
Initiation of
cleaning cycle
• Resistance of dean fabric, c.
Mass of dust deposited
Figure 8-25. Performance curve for a single bag of a fabric filter.
In multicompartment baghouses where the various compartments are cleaned
one at a time, the performance curve takes on a different shape. In this case the
change in the curve is less pronounced than in Figure 8-25. The performance curve
has a slight saw tooth shape for the net pressure drop across the entire baghouse
(Figure 8-26). Each of the minima points on the curve represent the cleaning of an
entire compartment. The average pressure drop would be represented by the
dotted line. For optimum filtration rate and collection efficiency, the baghouse
should be designed to operate at a pressure drop that approaches a constant value.
This involves careful selection of fabrics and cleaning mechanisms for the
baghouse. The weave, and any pretreatment of the fabric can affect the cake
repair time. Poor cleaning will increase the filter drag; therefore, it is essential to
thoroughly clean the bags to reduce the filter drag effect. If cake repair time can
be minimized, the pressure drop will be lower. Consequently, the effective filtration
rate will be longer for optimum filtering use.
Time
Figure 8-26. Overall pressure drop of a multicompartment baghouse.
8-31
-------
Filtration Velocity: Air-to-Cloth Ratio
As previously mentioned, the terms filtration velocity and air-to-cloth ratio can be
used interchangeably. The formula used to^xp^wTfiltratiorrvelocityls: ~"
(Eq. 8-5) Y,= £
Where: v/= filtration velocity, ft/min (cm/sec)
Qj= volumetric air flow rate, ftVmin (cm'/sec)
AC = area of cloth filter, ft* (on*)
Air-to-cloth ratio is defined as the ratio of gas filtered in cubic feet per minute
to the area of filtering media in square feet. Typical units used to express the A/C
ratio are:
(fts/min)/ft* or (cms/sec)/cms
These A/C ratio units essentially reduce to velocity units.
The A/C ratio (filtration velocity) varies for various baghousc designs. Shaker
and reverse air baghouses generally have small A/C ratios. (Shaker units <3:1
(cms/sec)/cm* and reverse air units < 1.5:1 (cmVsecJ/cm1). On the other hand,
pulse jet units usually operate at A/C ratios between 2.5 and 10:1 (cms/sec)/cm*.
For a given flow rate, pulse jet units can be smaller in size (fewer bags) than the
shaker and reverse air baghouse.
The A/C ratio (filtering velocity) is a very important factor vised in the design
and operation of a baghouse. Improper ratios can cause the baghouse to be in
violation of air pollution, regulations. Operating at an A/C ratio that is too high
may lead to a number of problems. Very high ratios can cause compaction of dust
on the bag resulting in excessive pressure drops. In addition, breakdown of the dust
cake could also occur which in turn results in reduced collection efficiency. The
major problem of a baghouse using a very low A/C ratio, is that the baghouse will
be larger in size.
Collection Efficiency
Extremely small particles can be efficiently collected in a baghouse. Baghouse units
designed with collection efficiencies of 99.99 percent are common. Exhaust air
from baghouses can even be recirculated back into the plant for heating purposes,
as long as the particles collected are not toxic.
Baghouses are not normally designed with the use of fractional efficiency curves
as are some of the other paniculate emission control devices. Vendors design and
size the units strictly on experience. The baghouse units are designed to meet par-
ticulate emission outlet loading and opacity regulations. There is no one formula
that can determine the collection efficiency of a specific baghouse. Some
theoretical formulas for determining collection efficiency have been suggested, but
these formulas contain numerous (3 to 4) experimentally determined coefficients hi
the equations. Therefore, these efficiency equations give at best only an estimate of
baghouse performance.
8-32
-------
Baghouse Design Review
The design of an industrial baghouse involves consideration of many factors
including space restriction, cleaning method, fabric construction type, fiber, air-to-
cloth ratio and many construction details such as inlet location, hopper design and
dust discharge devices. Air pollution control agency personnel who review baghouse
design plans should consider these factors during the review process.
A given process might often dictate one type of baghouse for paniculate emission
control. The manufacturers previous experience with a particular industry is
sometimes the key factor. For example, a pulse jet baghouse with its higher filter
rates would take up less space and would be easier to maintain than a shaker or
reverse air baghouse. But if the baghouse was to be used in a high temperature
application (260°C), a reverse air cleaning baghouse with woven Fiberglas® bags
might be chosen. This would prevent the need of exhaust gas cooling for the use of
Nomex® felt bags (on the pulse jet unit) which are more expensive than Fiberglas®
bags. All design factors must be weighed carefully in choosing the most appropriate
.baghouse design.
Review of Design Criteria
The principal design criterion is the gas flow rate to the baghouse, measured in
cubic meters (cubic feet) per minute. The gas volume to be treated is set by the
process exhaust, but the filtration velocity or air-to-cloth ratio is determined by the
baghouse vendor's design. The air-to-cloth ratio depends on a number of variables.
A thorough review of baghouse design plans should consider the following factors.
1. Type, shape, and'density of dust; average and maximum concentrations;
chemical properties such as abrasiveness, explosiveness, electrostatic charge
and agglomerating tendencies.
2. Gas flow rate: average and maximum flow rate, temperature, moisture
content, chemical properties such as dew point, corrosiveness and
combustibility.
5. Fabric construction: woven or felt filters, filter thickness, fiber size, fiber
density, filter treatments such as napping, resin and heat setting, and special
coatings.
4. Fiber type: natural, snythetic, Nomex®, Teflon®, etc.
5. Cleaning methods: low energy which are shaker and reverse air cleaning;
high energy which is pulse jet cleaning.
6. Cleaning time: ratio of filtering time to cleaning time is the measure of the
percent of time the filters are performing; this should be at least 10:1 or
greater.
7. Cleaning and filtering stress: amount of flexing and creasing to the fabric;
reverse air is the gentlest, shaking and pulse jet have the most vigorous stress
on the fabric.
8. Bag spacing: bags must be properly spaced to eliminate rubbing against
each other; bags must be accessible for inspection and maintenance service.
8-SS
-------
9. Compartment design: allowance for proper cleaning of bags; design should
include an extra compartment to allow for reserve capacity and off-line
cleaning, and inspection and maintenance of broken bags.
10. Space and cost requirements: baghouses require a good deal of installation
space; initial costs, and operating and maintenance costs can be high.
11. Emission requirements: efficiency in terms of opacity and grain-loading
regulations.
12. Proper air-to-cloth ratio (A/C); reverse air lowest, shakers; next, pulse jet
baghouses allow the highest A/C ratio.
Simple Cloth Size Check
Baghouse sizing is done by the manufacturer. A simple check or estimate of the
amount of baghouse cloth needed for a given process flow rate can be computed by
using Equation 8-5.
or
-I
For example, if the process gas exhaust rate is given as 4.72 x 106 onVsec
(10,000 ftVmin) and the filtration velocity is 4 cm/sec (A/C is 4:1 (cms/sec)/cm,),
the cloth area would be:
4.72xlO«cmVsec
AC = -^~^~ ~—~ ~~ ^^~^^~
4 cm/sec_
= 1,179875 cm* (cloth required)
= 117.98 m* (cloth required)
To determine the number of bags required in the baghouse, one would simply use
the formula:
Where: A* = area of bag, m (ft)
d=bag diameter, m (ft)
h= bag height, m (ft)
If the bag diameter is 0.203 m (8 in.) and bag height is 3.66 m (12 ft), the area
of each bag is:
A» = 3.14x0.20SmXS.66m
= 2. 33m*
8-34
-------
The calculated number of bags in the baghouse is:
117.98m1
Number of bags =
2.33
51 bags
Typical Air-to-Cloth Ratios
During a permit review for baghouse installations the reviewer should check the
A/C ratio. Typical A/C ratios for shakers, reverse air and pulse jet baghouses are
listed hi Table 8-3.
Table 8-3. Typical air-to-cloth ranges.
Baghouse cleaning
method
Shaking
Reverse air
Pulse jet
Air-to-cloth range
1-3 (cm'/sec)/cm* 2-6 (ftVmin)/ft'
0.5-1.5 (cm»/sec)/cm' 1-3 (ft'/min)/ftl
2.5-7.5 (anVsecj/cm1 5-15 (ftVmin)/ft*
Note: Air-to-cloth ratios are occasionally given as 2.0:1 instead of 2.0 (cm*/sec)/cml
Baghouses should be operated within a reasonable design A/C ratio range. For
example, assume a permit was submitted indicating the use of a reverse air
cleaning baghouse using woven Fiberglas® bags for reducing paniculate emissions
from a small foundry furnace. If the information supplied indicated that the
baghouse would operate with an A/C ratio of 6 (cmVsec)/cm* of fabric material,
one should question this information. Reverse air units should be operated with a
much lower A/C ratio. The fabric would probably not be able to withstand the
stress from such high filtering rates and could cause premature bag deterioration.
Too high an A/C ratio results in excessive pressure drops, reduced collection
efficiency, blinding, and rapid wear. In this case a better design might include
reducing the A/C ratio within the acceptable range, thus adding more bags.
Another alternative would be to use a pulse jet baghouse with the original design
A/C ratio of 6 (cms/sec)/cm* and use felted bags made of Nomex® fibers. Either
alternative would be more acceptable to the original permit submission.
Typical air-to-cloth ratios for baghouses used in industrial processes are listed in
Tables 8-4 and 8-5. These values should be used as a rule of thumb or guide only.
Actual design values may need to be reduced if the dust loading is high or the par-
ticle size is small. When compartmental baghouses are used, the design A/C ratio
must be based upon having enough filter cloth available for filtering while one or
two compartments are offstream for cleaning.
8-35
-------
Table 8-4. Typical A/C ratios [(ftVmiiO/ft*] for Klected industries*.
Industry
Basic oxygen furnaces
Brick manufacturing
Castable refractories
Clay refractories
Coal fired boilers
Conical incinerators
Cotton ginning
Detergent manufacturing
Electric arc furnaces
Feed mills
Ferroalloy plants
Glass manufacturing
Grey iron foundries
Iron and steel (sintering)
Kraft recovery furnaces
Lime kilns
Municipal incinerators
Petroleum catalytic cracking
Phosphate fertilizer
Phosphate rock crushing
Polyvinyl chloride production
Portland cement
Pulp and paper (fluidized bed
reactor)
Secondary aluminum smelters
Secondary copper smelters
Sewage sludge incinerators
Surface coatings— spray booth
Fabric filter
air-to-cloth ratio
Reverse
air
1.5-2.0
1.5-2.0
1.5-2.0
1.5-2.0
—
—
—
1.2-1.5
1.5-2.0
—
2.0
1.5
1.5-2.0
1.5-2.0
—
1.5-2.0
—
—
1.8-2.0
—
—
1.2-1.5
—
—
—
—
—
Pulse
jet
6-8
9-10
8-10
8-10
—
—
—
5-6
6-8
10-15
9
—
7-8
7-8
—
8-9
—
—
8-9
5-10
7
7-10
—
6-8
6-8
—
—
Mechanical
iihaker
2.5-S.O
2.5-3.2
2.5-S.O
2.5-S.2
—
—
—
2.0-2.5
2.5-S.O
S.5-5.0
2.0
—
2.5-3.0
2.5-3.0
—
2.5-3.0
—
—
3.0-3.5
3.0-3.5
—
2.0-3.0
—
2.0
—
—
~*
•High efficiency: a sufficiently low grain loading to expect a clear stack.
Source: EPA, 1976, EPA 450/3-76-014.
8-S6
-------
Table 8-5. Typical A/C ratios for fabric filters wed for
of paniculate emissions from industrial boilers.
control
Size of boiler
(10* Ib steam
per hour)
260 (9 boilers)
170 (5 boilers)
140 (2 boilers)
250
200 (3 boilers)
400 (2 boilers)
75
50
270 (2 boilers)
450 (4 boilers)
380
645
1,440 (3 boilers)
Temperature
<°F)
400°
500°
360°
338°
300°
Stoker, 285° to
300°; pul-
verized coal,
350°
150°
350°
330°
330°
NA
NA
360°
Air-to-cloth
ratio
[
-------
7. Environmental Protection Agency (EPA). 1979. Paniculate Control by Fabric
Filtration on Coal-Fired Industrial Boilers. EPA 625/2-79-021.
8. Environmental Protection Agency (EPA). 1976. Capital and Operating Costs of
Selected Air Pollution Control Systems. EPA 450/S-76-014.
9. Sittig, M. 1977. Particulates and Fine Dust Removal Processes and Equipment.
New Jersey: Noyes Data Corporation.
10. Kraus, M. N. 1979. Baghouses: separating and collecting industrial dusts.
Chem. Eng. 86:94-106.
11. Proceedings: Symposium on the use of fabric filters for the control of sub-
micron participates, April 8-10, 1974, Boston, MA./. Air Pol. Control Assoc.
24:1139-1197, 1974.
12. Frederick, E. R. 1974. Some Effects of Electrostatic Charges in Fabric Filtra-
tion./ Air Pol. Control Assoc. 24:1164-1168.
IS. Environmental Protection Agency (EPA). 1973. Air Pollution Engineering
Manual. 2nd ed. AP-40.
14. Proceedings: The User and Fabric Filtration Equipment III, October 1-3,
1978, Niagara Falls, NY. Air Pollution Control Association Specialty
Conference.
8-38
-------
Chapter 9
Wet Collectors
Wet collectors provide many options for the control of source emissions. These air
pollution control devices can remove both paniculate matter and gases from
effluent gas streams. They can operate at low removal efficiencies or at high
removal efficiencies. They also offer more versatility in design than other air pollu-
tion control devices. This versatility, however, does not come without problems.
Higher efficiency requires higher operating costs; by-products are difficult to
recover, and an air pollution problem can be transformed into a water pollution
problem. In terms of cost, wet collectors are generally more expensive than simple
settling chambers and cyclones but less expensive than high efficiency electrostatic
precipitators and baghouses. They have often been specified as satisfying the
requirements of Reasonably Available Control Technology (RACT) because of the
number of design and cost options they can provide for existing stationary sources.
This chapter will deal with the general characteristics of wet collectors, the design
theories behind them, and their equipment descriptions.
General Characteristics—Paniculate Matter Removal
As the name implies, wet collectors or wet scrubbers, are devices which use a liquid
for removing particles or polluted gases from an exhaust gas stream. Water sprays
can be injected into the gas stream; the gas can be forced to pass through sheets or
films of liquid; or the gas can move through beds of plastic spheres covered with
liquid. Each of these techniques can effectively remove paniculate matter from
process exhaust gases. They can also effectively remove gases such as HC1 or SO*,
but removal conditions must be right. In many cases, the best conditions for
removing paniculate matter are the poorest for removing pollutant gases. In this
chapter, emphasis will be placed on the design and application of wet scrubbers for
the removal of paniculate matter. Optimum operating conditions for paniculate
matter removal will be discussed. Effective techniques for removing gaseous com-
ponents will be contrasted to paniculate emission control methods. Gaseous emis-
sion control methods are discussed in greater detail in the manual for APTI Course
415—"Control of Gaseous Emissions".
Wet collectors exhibit some relative advantages compared to other paniculate
matter control equipment. These advantages follow:
Small space requirements: Systems can be designed for small locations, roof
mounting, etc.
9-1
-------
No secondary dust sources: Once the pollutant is collected, it cannot escape
from hoppers or in transport. The collected slurries resulting from wet scrubbers
can possibly be more easily handled than dry dust.
Collects gases and paniculate matter: This can be an advantage for small
industrial process industries unable to afford separate control systems; particularly
useful for incineration processes.
Handles high temperature, high humidity gas streams: Temperature limits
and condensation problems in baghouses and ESPs can be avoided since wet scrub-
bers cool incoming gases and wash away accumulated paniculate matter.
Ability to humidify a gas stream: The scrubber can reduce the temperature
and volume of an unsaturated gas stream at high temperature by the process of
evaporation. Smaller ducting and fan sizes can then be used downstream of the
collector.
Fire and explosion hazards are at a minimum; Fire and explosion hazards con-
nected with various dry dusts can be eliminated by using water as the control
medium.
Although wet collectors have many advantages, they may not be suitable for all
applications. Some relative disadvantages follow:
Corrosion problems: Water and absorbed gaseous pollutants can form highly
corrosive acid solutions; therefore, choosing proper construction materials for the
control system is important.
Meteorological problems: Highly humidified exhaust gases czin produce a wet,
visible steam plume, especially during cold weather; fog and precipitation from the
plume may cause local meteorological problems.
Pressure drop and power requirement: High collection efficiencies for par-
ticulate matter are attainable only at high pressure drops; the increased fan power
required to move the exhaust gases through the scrubber may result in significant
operating costs.
Water pollution: Adequate precautions must be taken before scrubber waste
liquid is disposed; settling ponds and sludge darifiers are often included in the
design of wet collector systems to meet waste water regulations.
Difficulty of by-product recovery: Recovery of dust for re-use is difficult when
wet collectors are used; costs associated with dewatering and drying of the scrubber
sludge may make other control methods more practical.
A variety of wet collectors are commercially available. The evaluation and selec-
tion of a scrubber system is somewhat simplified by the observation that collection
efficiency is a function of the amount of energy required to operate the scrubber
(Lapple, 1955). This means simply, that independent of design, the more power
put in the system the greater the collection efficiency. Therefore, for systems of
nearly equivalent power inputs the collection efficiencies should be nearly
equivalent. The selection or evaluation between two systems could then concentrate
on ease of operation, potential maintenance problems, and comparative costs.
9-2
-------
Theory of Operation
Wet collectors are designed to incorporate small dust panicles into larger water
droplets. Droplets ranging from 50 to 500 /tm in diameter are produced and
brought into contact with the paniculate matter. These large droplets containing
the captured particles are then collected by simple mechanisms such as gravity,
impaction on baffles, or by cyclonic action. Many wet collectors can be represented
as having two zones: a contact zone and a separation zone (Figure 9-1).
Contact
Separation
Di«y ,VJ
dean
gas
Figure 9-1. Zones of a wet scrubber.
Droplets can be produced by a spray nozzle, by the aspirating effect of the gas
stream shearing a liquid film, or by the motion of a mechanically driven rotor. In
the contact zone, the paniculate matter approaches the water droplets. Contact is
made by three primary mechanisms: inertial impaction, direct interception, and
diffusion (see Chapter 1). Inertial impaction, the breaking through streamlines by
panicles, is the predominant collection mechanism for scrubbers having gas stream
velocities greater than 1 ft/sec (0.3 m/sec) (Perry, 1973). At these and higher
velocities, particles having diameters greater than 1.0 /tm are collected by breaking
through the streamlines encircling the water droplets.
Direct interception (the panicle flows with the streamlines but "bumps" into the
droplet because it gets too close) becomes important when the particle size (d,)
approaches that of the droplet (d.) or when i/d,—1. In scrubbers, droplet sizes
are usually greater than 50 /on and dust particle sizes are usually 5 jim or less.
Therefore, direct interception is a secondary effect compared to menial impaction.
9-3
-------
Diffusion effects become important for very small particle sizes (less than 0.5
/an). Here, the random motion of the small panicles results in eventual contact
with water droplets. The efficiency (77) of a scrubber can increase for smaller par-
ticle sizes because of the predominance of this effect (Figure 9-2).
100
50
I
I
I
0.5 1 2 3
Particle diameter, d,
Source: Calvert, 1977.
Figure 9-2. Collection efficiency for a mobile bed scrubber
as a function of particle size.
Some scrubbing systems can increase collection efficiency through a combination
of other effects. The condensation of steam on dust particles acting as nuck1., or
the sweeping of particles into condensing water droplets provide mechanisms for
particle collection. Charged droplet scrubbers impart an electrical charge to water
droplets produced in the system. These are then collected much like in an elec-
trostatic precipitator. These systems are effective in collecting particles in the range
between 0.3 and 1 /on, the range where the impaction and diffusion mechanisms
have the least effectiveness (Lear, 1975).
General Theories
A number of theories have been developed from basic particle movement principles
to explain the action of wet scrubbing systems. Many of these start from firm scien-
tific concepts, but give only qualitative results when predicting; collection efficien-
cies or pressure drops. The interaction of paniculate matter having a given particle
size distribution with water droplets having another size distribution is not easy to
express in quantitative terms. As a result of this complexity, experimentally deter-
mined parameters are usually needed in order to approach reality.
9-4
-------
This section covers five topics:
• Collection Mechanisms
• The Johnstone Equation
• The Cut Power Method
• The Contact Power Theory
• Pilot Methods
An overview of a number of general methods that are used to evaluate wet col-
lector systems will be presented. Each method described has limitations. One of the
principal problems with most theoretical approaches is the lack of input data for
the equations. Particle size information or specific equation parameters may not be
available for a given process, particularly for a new process. For this reason, it is
often necessary to obtain pilot plant information that can be scaled up to design
the actual control system. Wet collector equipment manufacturers and various con-
sultants also have developed methods and assembled data which can be used to
design or evaluate these systems. However, much of this information is proprietary.
Collection Mechanisms
As stated earlier, wet collectors operate through three primary mechanisms: inertial
impaction, direct interception, and diffusion. Each of these effects can be
characterized by a mathematical expression called the separation number. The
separation numbers are dimensionless groups of parameters— the higher the value
of the separation number, the more effective the mechanism (Perry, 1973). From
these basic expressions, theories have been derived to describe scrubber perfor-
mance. Each of these mechanisms will be discussed in more detail here.
Inertial Impaction
Because of their mass, particles do not always follow the streamlines which diverge
around an obstacle such as a droplet. An impacting particle is one which has
broken through the streamlines and hits the droplet.
The separation number for impaction is obtained by balancing the force of the
moving particle against the resistance of the gas stream to its motion. The separa-
tion number for impaction is known as the impaction parameter, $ and is
expressed by:
• C/o,d,fv
="-
Where: C/= Cunningham correction factor
Q, = particle density (lb/fts)
d, = particle diameter (ft)
v = gas velocity (ft/ sec)
/t = gas viscosity (lb/ft»sec)
d, = droplet diameter (ft)
9-5
-------
Note the similarity of this expression to the impaction parameter given for
cyclones (Equation 6-7).
The impaction parameter can be used with a number of other concepts to
develop a theoretical equation for the impaction collection efficiency, i7/mj,«,to,.
One of these concepts, target efficiency, is defined as the ratio of the actual
number of particles hitting the droplet to the total number of piurticles which could
hit it (Figure 9-5).
• w
Total particles which • • * •. *9 •
could hit the drop * • * , •
* A «B • »
Frontal area
Particles which
actually hit
the drop
Figure 9-3. Inerrial impaction collection efficiency: target efficiency.
9-6
-------
Given a uniform distribution of particles, target efficiency can be thought of as a
ratio of areas, i.e. the ratio of an area cleared of particles, to the frontal area of
the drop (Figure 9-4).
Figure 9-4. Target efficiency: the area ratio*.
The target efficiency is thus the impaction collection efficiency, tympana*, and is
expressed as:
(Eq. 9-2)
For large particles with high momentum, the area cleared (d') will be relatively
large since the particles will be more likely to break the streamlines and be col-
lected. Complete collection would correspond to (d'/d0)*= 1. However, as d,
decreases, the particles tend to follow streamlines, so a point will be reached where
(d'/d,,) = 0 and no particles are collected.
The path of a particle around a single droplet can be calculated by numerical
techniques. Given parameters such as particle density, gas viscosity, and collector
size, the conditions where the particle will be collected or follow the streamlines
around the droplet can be predicted (Theodore, 1976). In other words, d' can be
found numerically and a value for the single collector efficiency, r}imfmaltH,, can be
calculated.
Many other theoretical expressions for rjlmfmelum do not rely on numerical
methods. The impaction parameter, \£, is common to many of them. In general, for
a wet collector system having impaction as the predominant mechanism, the collec-
tion efficiency is expressed as a function of ^, or:
(Eq. 9-3)
9-7
-------
A number of these expressions are given for different flow regimes by Rimberg and
Perry (1977). Another expression, developed by Johnstone, will be discussed later in
this chapter.
Interception
The separation number characterizing ineitial impaction (the impaction
parameter) accounts for the effect that the mass of the particle has in penetrating
streamlines. It does not account for the finite size of the particle as it nears the
droplet.
Particle
Figure 9-5. Collection by interception.
The center of the particle may follow streamlines around a droplet, but if the
particle approaches the droplet by a distance less than dp/2 (as measured from the
particle's center), it will hit and be collected.
The separation number which characterizes this effect is the iratio of the particle
diameter to the droplet diameter and expressed as:
(Eq. 9-4)
do
The collection efficiency associated with this effect is a function of d, and d,, or:
(Eq. 9-5)
Again, a number of expressions have been developed for •t\in<.rc*p
-------
The Brownian diffusion process leading to particle capture is most often
described by a parameter called the Peclet number, Pe:
Qk*T
Where: p = gas viscosity
v= gas stream velocity
d,, = particle diameter
d. = water droplet diameter
kf = Boltzmann's constant
T = temperature of gas stream
Q= Cunningham correction factor
Expressions for collection efficiency by the diffusion process are generally in the
form:
(Eq. 9-7)
As the Peclet number decreases, collection efficiency by diffusion increases. In
terms of Equation 9-6, as the temperature increases, Pe decreases. An increase in
temperature means that gas molecules will move around faster than at lower
temperatures. This will lead to increased bombardment of the small particles,
increased random motion, and increased collection efficiency by this mechanism.
Combined Efficiency
When attempting to collect 100 particles using a collection mechanism having 80%
efficiency, 80 particles would be collected and 20 would escape. If another
mechanism with 60% efficiency was used to collect the remaining 20 particles, 8
would escape this tune. Ninety two particles would therefore be collected, giving a
total efficiency of 92/100 = 0.92 or 92%. This can be expressed as:
[100- 100 (0.80)] - [100- 100 (0.80)]0.60(
[100 - 100(0.80)](1 - 0.60)}
Collected particles = 100 -
= 100--
= 100-
= 100-8
= 92
Generalizing in terms of the number of mechanisms which may be involved in
wet collection devices, the combined efficiency can be expressed as:
(Eq. 9-8)
In terms of the separation numbers discussed earlier, the combined efficiency is
then a function of many parameters.
(Eq. 9-9) i|o«*i—-1-0-
9-9
-f )(l-f )
-------
Depending on flow conditions, (laminar, turbulent, etc.) other combinations of
these parameters have been used to provide expressions for the efficiency of wet
collectors. Actually, the number of variables are so great and the interaction
between varying size distributions of both paniculate matter and water droplets is
so complex, that this type of theoretical approach is difficult to apply in practice.
This academic approach, although useful in understanding basic phenomena, is
commonly sidestepped for more empirical approaches. Correction factors obtained
from actual operating systems or pilot plant data are used to correct for complex
interactions. A newer concept expressing efficiency hi terms of power expended has
led to several empirical techniques for evaluating scrubber performance. The
subsequent sections of this chapter will describe some of these empirical methods.
The Johnstone Equation
Equations 9-8 and 9-9 represent the combined collection efficiency for a single
droplet. The combined efficiency equation includes the efficiency for impaction,
interception, and diffusion. A scrubber uses many droplets to collect paniculate
matter. This fact must be taken into consideration when developing a theory for
scrubber collection efficiency. Each droplet will have a combined efficiency i\e, for
collecting these particles.
In the first exposure to a drop:
Fraction of Fraction of
particles captured particles escaped
Exposure two to another drop:
Fraction captured Fraction escaped
Exposure three to a third drop:
Fraction captured Fraction escaped
1Je[l — 1}e ~ 1?e(l ~ lie)] 1 ~ 1/c ~ 1?e(l ~ f e) ~ l?e[l ~ *lc ~ *?e(l ~ »?e)]
or(l-Tj£)»
and so on.
If S. is the total number of exposures, then generalizing, the total fraction of
particles which escape can be written as:
9-10
-------
_s xs
[NOTE: the exponential function c~* can be expanded to e"*= 1 ~ x+ •§[ ~"f\ + "'
For small values of x, e~* can be approximated as e~*« 1 - x]
The overall collection efficiency TJ is then:
(Eq. 9-10) = l
S, depends on the type of scrubber and ije is the collection efficiency for a single
droplet. The terms in the exponent, ijeSe, are a function of the many variables
involved in the system. This exponent can be alternatively expressed a&f (system), a
function of the system. Therefore efficiency can be expressed as:
(Eq. 9-11) j.l-e-**—"'
Semrau (1952), using chemical engineering terminology for absorption phenomena,
expresses f(system) as N,, the number of transfer units. This makes sense if we con-
sider, for example, 100 particles moving through a gas stream. By impaction and
other mechanisms, a number of the particles will be "transferred" to or collected
by the first droplet they see. More will transfer to the second droplet, etc. The
number of transfers therefore indicates the degree of collection, as expressed in
Equation 9-11.
We will now look at a number of expressions for f(system) that have been used to
calculate collection efficiencies.
Johnstone (1951) developed an expression for a venturi scrubber, considering
only the predominant mechanism of inertial impaction.
The Johnstone equation is given as:
(Eq. 9-12) ij-l-e-'lj^
Where: Qj. = liquid flow rate (gal/min)
Qp - gas flow rate (ftVmin)
k= empirical constant
From Equation 9- 1 : V -- lg,
Where: d, = water droplet diameter (ft)
C/= Cunningham correction factor
v— gas stream velocity (ft/ sec)
d, = particle diameter (ft)
/t = gas viscosity (lb/ft»sec)
QP = particle density (lb/fts)
9-11
-------
Here k, an empirical constant, depends upon the geometry of the system and
on the operating conditions. Typical values are 0.1 to 0.2 gal/acfm.
The droplet size, d,, can be estimated by using the empirical relationship
developed by Nukiyama and Tanasawa (1938), for gas atomized sprays.
The Nukiyama Tanasawa equation is:
(Eq. 9.1S) a.- 15!2 (if + 597 (JLY- (lOOO ft)
vj, \c,/ Vwi/ V Qp/
Where: d, = water droplet diameter (fun)
Vj, = relative velocity of gas to liquid (ft/sec)
a, — liquid surface tension (dynes/ on)
g, = liquid density (g/cms)
/i, = liquid viscosity (lb/ft»sec)
Qf. = liquid flow rate (gal/min)
Qp = gas flow rate (ftVmin)
For air and water in a venturi scrubber, this expression reduces to:
Where: v* = relative velocity of gas to liquid at venturi throat (ft/sec)
It should be noted from the form of the impaction parameter $ given in Equa-
tion 9-11, that TJ is dependent upon dp, the particle size. The equation for the total
efficiency of the scrubber I\TOT must be solved for each particle size range of the
process. The sum of each TJ, (for each d,) multiplied times the distribution weight
of the particle range will give the total, efficiency of the scrubber denoted as IJTOT-
This discussion of the Johnstone equation is not intended to iimply that the equa-
tion is the best predictive equation available for venturi scrubber performance. It is
presented here because it demonstrates in a simple manner, the end result of a
number of theoretical concepts associated with wet collector design. Other expres-
sions, generally more complicated and also empirical in nature, have been
developed (Calvert, 1972). The various theories, however, have never been
adequately compared on a consistent basis.
The Cut Power Method
A generalized method of calculating the overall efficiency of a wet collector has
been developed by Calvert (1972, 1974, 1977). This method assumes that the
predominant collection mechanism is inertial impaction. It also requires a
knowledge of the particle size distribution as does the Johnstone equation.
9-12
-------
Several concepts previously discussed tie into the cut power method. The first
concept considers the general form of overall efficiency equation such as that given
in Equation 9-10. The second is cut size discussed hi Chapter 6. Equation 9-11,
gives overall efficiency expressed as:
Where: f(system) =some function of the system variables
The cut power method uses the concept of penetration, where penetration is
given as:
(Eq. 9-15) Pt=l-ij
Penetration is the fraction of paniculate matter which gets through a collector, i.e.
the opposite of the fraction collected.
In terms of penetration, then:
Pt=l-i?
=
(Eq. 9-16) .
Calvert has chosen to define f(system) as an empirical function dependent upon the
particle aerodynamic diameter dp.
(Eq. 9-17) . f(system) = Ac«d/c-'
Where: A«» = parameter characterizing the particle size distribution
Be*, = empirically determined constant dependent upon the type of
scrubber
Combining Equations 9-16 and 9-17, penetration is:
(Eq. 9-18) Pt = e-x~V~
[NOTE: this function is given only for one particle size. Integrate this over a par-
ticle size distribution to obtain the overall penetration, Ft for the wet collector.]
The cut diameter was discussed in Chapter 6. It is the diameter of the particles
collected by the control system at 50% efficiency. Since wet collectors and other
types of control equipment have limits to the size of particles they can collect, a
knowledge of a required cut diameter is helpful hi evaluating different scrubber
systems.
Calvert has developed a method of determining the cut diameter that is required
in order to achieve a given collection efficiency. By integrating Pt over a log nor-
mal distribution of particles and varying a^, (standard deviation of the distribution)
9-13
-------
and dym (geometric mean panicle diameter), Ft can be obtained as a function of
the required cut diameter [d,]^, divided by the geometric mean panicle diameter,
(Figure 9-6).
[NOTE: A log normal distribution of panicles can be statistically defined with only
two terms, the geometric mean particle diameter, d**, and the geometric stan-
dard deviation, o^,, of the distribution. From stack test data or particle size infor-
mation from a similar emission facility, the values of d^ and ofn can be deter-
mined using the techniques discussed in Chapter 4 of this manual.]
10
1.0
Overall
penetration, 0.1
Pt
0.01
0.001
6543 2
..///. /.../.. L:
0.001
0.01
0.1
1.0
Source: Calvert, 1977.
Figure 9-6. Penetration and the cut diameter.
Consider the following example: a particle size analysis indicated that d,*, = 12
/on and 0^, = 3.0. If a collection efficiency of 99% is required to meet emission
standards what would the cut diameter of the scrubber have to be?
First:
Pt=l-ij
= 1-0.99
= 0.01
From Figure 9-6, for Ft« 0.01 and 0,.= S.O
=0.063
9-14
-------
Since 6^ = 12 /un, then the scrubber must be able to collect particles of size
0.063 x 12 = 0.76 /tm with at least 50% efficiency to achieve an overall scrubber
efficiency of 99%. m
Figure 9-6 was developed using Equation 9-18, Pt=e~A'^dp *", where B= 2. For plate
towers, B = 2, but is only *2 for Venturis under certain conditions. For centrifugal
scrubbers B»0.7 and therefore Figure 9-6 should not be used as is. Further limita-
tions and models developed for specific devices using the cut power method are
discussed by Calvert (1972). The application of these models to actual operating
systems has been documented inadequately in the open literature.
The Contact Power Theory
Theoretical approaches based upon the dynamic interactions between particles and
water droplets are limited in applicability since actual scrubber systems are very
complex. For example, the Johnstone and cut power methods assume that the
water droplets are uniformly distributed over the cross section of the scrubber. The
actual turbulence and eddies existing in the contact zone are not accounted for,
except by using empirically determined constants (Beg, 1976).
A more general theory which avoids the details of how particles and droplets hit
each other, is the contact power theory. This theory is based upon a series of
experimental observations made by Lapple (1955). The fundamental assumption of
the theory is:
"When compared at the same power consumption, all scrubbers give
substantially the same degree of collection of a given dispersed dust,
regardless of the mechanism involved and regardless of whether the
pressure drop is obtained by high gas flow rates or high water flow
rates." (Lapple, 1955)
In other words, collection efficiency is a function of how much power is used,
and not upon details of design. This has a number of implications in the evalua-
tion and selection of wet collectors. Once it is realized that a certain amount of
power is needed for a required collection efficiency, the claims about specially
located nozzles, baffles, etc. can be evaluated more objectively. The choice between
two different scrubbers with the same power requirements may depend primarily
on ease of maintenance.
Semrau (1959, 1963), developed the contact power theory from the work of
Lapple and Kamack (1956). The theory as developed by Semrau, is empirical hi
approach and relates the total pressure loss, &r, of the system to the collection
efficiency.
9-15
-------
The total pressure loss is expressed in terms of the power expended to inject the
liquid into the scrubber plus the power needed to move the process gas through the
system.
(Eq. 9-19) #•- &,+ &i (hp/1000 acfm)
Where: ^r= total contacting power (units of hp/1000 acftn)
&*<} = power input from gas stream (hp/1000 acfm)
t^t = power input from liquid injection (hp/1000 adBtn)
[NOTE: the total pressure loss, 3r should not be confused with penetration, Pt
defined in the previous section. Penetration is the symbol used by Calven to express
the fraction of paniculate matter escaping from a collector.]
The power expended m moving the gas through the system, fr0t is expressed in
terms of the scrubber pressure drop:
(Eq. 9-20) &c = 0.1575 Ap (hp/1000 acfcn)
Where: Ap = pressure drop (in. of H±O)
The power expended in the liquid stream, SrL, is expressed as:
(Eq. 9-21) &L = 0.583pL (&} (hp/1000 acfm)
Where: pL = liquid inlet pressure (Ib/in.*)
Qj, = liquid feed rate (gal/min)
= gas flow rate (ft'/min)
The constants given hi the expressions for 3PC and ^ incorporate conversion fac-
tors to put the terms on a consistent basis.
The total power can therefore be expressed as:
(Eq. 9-22) &r= 0.1575 Ap + 0.583 pL
The problem now is to correlate this with scrubber efficiency.
9-16
-------
Equations 9-10 and 9-11 of this manual show that efficiency is an exponential
function of the system variables for most types of collectors.
(Eq. 9-23) i7=l-e-'">
Semrau defines:
(Eq. 9-24) f(s) = N, = a$V
Where: N, = number of transfer units
a and j8 = empirical constants which are determined from experiment and
are dependent upon the characteristics of the paniculate matter
The efficiency then becomes:
(Eq. 9-25) tj=l-e'a^r*
Table 9-1 gives values of a and /3 for different industries.
Table 9-1. Parameters a and 0 for the contact power theory.
Aerosol
Raw gas (lime dust and
soda fume)
Prewashed gas (soda fume)
Talc dust
Black liquor recovery
furnace fume
Cold scrubbing water
humid gases
Hot fume solution for
scrubbing (humid gases)
Hot black liquor for
scrubbing (dry gases)
Phosphoric acid mist
Foundry cupola dust
Open-hearth steel furnace fume
Talc dust
Copper sulfate
Ferrosilicon furnace fume
Odorous mist
Scrubber type
Venturi and cyclonic spray
Venturi, pipeline, and
cyclonic spray
Venturi
Orifice and pipeline
Venturi and cyclonic spray
Venturi, pipeline, and
cyclonic spray
Venturi evaporator
Venturi
Venturi
Venturi
Cyclone
Solivore (A) with
mechanical spray
generator
(B) with hydraulic nozzles
Venturi and cyclonic spray
Venturi
a
1.47
0.915
2.97
2.70
1.75
0.740
0.522
1.S3
1.35
1.26
1.16
0.590
0.562
0.870
0.565
0
1.05
1.05
O.S62
0.362
0.620
0.861
0.861
0.647
0.621
0.569
0.655
1.14
1.06
0.459
1.41
Source: Semrau, 1960.
9-17
-------
The contact power theory cannot predict efficiency from a given particle size
distribution as can the cut power and Johnstone theories. The contact power theory
gives a relationship which is independent of the size of the scrubber. With this
observation, a small pilot scrubber could first be used to determine the pressure
drop needed for the required collection efficiency. The full-scale scrubber design
could then be scaled up from the pilot information.
As an example, consider the following problem:
A wet scrubber is to be used to control paniculate emissions from a foundry
cupola. Stack test results reveal that the paniculate emissions must be reduced by
85% to meet emission standards. If a 100 acfm pilot unit is operated with a water
flow rate of 0.5 gal/min at a water pressure of 80 psi, what pressure drop (Ap)
would be needed across a 10,000 acfm scrubber unit?
Solution:
From Table 9-1 a = 1.35 and |8= 0.621
From Eq. 9-23 i\ = 1 - c~N*
„ , 1 1
1-ij 1-0.85
From Eq. 9-24 N, = a^/
1.896=1.35 ^r0-"1
' hi 6.66 =1.896
^r= 1.73 hp/1000 acfm
m
From Eq. 9-22 ^T= 0.1575 Ap+ 0.583 pL
1.73 = 0.1575 Ap + 0.583(80) ~
100 /
Ap=9.5in. H,O
9-18
-------
The basic principle of the contact power theory can be applied in ways similar to
Equation 9-25 developed by Semrau. For example, Kashdar (1979), in a regulatory
analysis of wet scrubbing applied to coal-fired utility boilers, developed the correla-
tion between outlet grain loading and power consumption shown in Figure 9-7.
2 S 4 5 6
Theoretical power consumption, hp/1,000 acfm
8 9 10
Figure 9-7. Correlation of tcrubber outlet dust loading with theoretical power consumption.
Here the correlation is not made in terms of transfer units as in Equation 9-24,
but directly with the particulate matter emission concentration. The good fit of the
data to the format y= 0.068 x'1-41 (where y=outlet grain loading (gr/dscf) and
x = theoretical power consumption calculated using Equation 9-22) is encouraging
support for the cut power theory.
9-19
-------
Similarly, Calvert (1974, 1977) has used this approach to develop a relationship
between the panicle cut diameter and scrubber power. Using data from scrubber
installations and relationships developed in the cut power approach, Calvert, in
Figure 9-8, shows a similar improvement in performance (i.e. lower [d,]^) with
increased power consumption.
0.1
0.4 0.5
Scrubber power, hp/1,000 mdtm
1.0 2345 10
20 SO
1. Packed column with 1-in. rings
or saddles
2. Fibrous packed bed with
0.012-in. diameter fiber—any
depth (experimental data and
mathematical model)
3. Gas-atomized spray
4. Mobile bed with 1 to S stages of
fluidized hollow plastic spheres
345 10 20 SO 40 50
Gas-phase pressure drop, in. HtO
Figure 9-8. Cut diameter [d,]^ as a function of gas pressure drop and power consumption.
The concept of the contact power theory does have limitations. It does not apply
to a number of new wet collecting systems where a combination of collecting
mechanisms are used, such as condensation scrubbers. Also, the theory applies best
when the power is applied in one scrubbing area (Mcllvane, 1977), such as in a
venturi scrubber. Multiple staged devices and packed towers will have collection
efficiencies varying from those of a venturi scrubber for a given power input.
9-20
-------
Pilot Methods
The semi-empirical theories previously discussed are useful for scrubber design and
evaluation exercises, since they can give qualitatively correct information. However,
they have a number of practical limitations. It is not common practice to choose
scrubber systems based only on this information. The uncertainties involved hi par-
ticle size determinations and the questions associated with using empirically deter-
mined parameters restrict the use of theoretical methods. Basically, there are too
many variables involved and it is too difficult to account for all of them in a simple
theory. The time and expense needed to obtain good input data for these methods
may be better spent in developing pilot plant information.
Scrubbers which work primarily through impaction mechanisms have certain
performance characteristics (such as efficiency and pressure drop) which are
independent of scale. This consequence of the contact power principle provides the
basis for using pilot systems. By using a small scale scrubber (100 to 1000 cfm) on
the exhaust gas stream, the effectiveness of the equipment for removing the actual
particles in the gas can be experimentally determined.
Pilot systems ranging from 100 cfm units to 1/10 size full-scale plants have been
developed in the past. Mcllvane (1977) has compared the effectiveness of the
various design methods. His work is summarized in Table 9-2.
Table 9-2. Methods for predicting Tenturi icrubber pressure requirements.
Description
Most reliable
Least reliable
1710 size full-scale plants
2000 cfm pilot units
100 cfm pilot units
Empirical curves based on
similar processes
Impactor in situ particle sizing
Expense
(relative scale)
100-1000
SO
5
0.2
2
Time
(months)
12-24
3-6
2-3
0.2
1
The design of a wet collector system for a paniculate emission problem requires
more than the application of a few design equations. The experience of scrubber
manufacturers with specific industry installations coupled with the use of pilot units
provides more reliable ways to determine the size of a system for a wide range of
operating conditions. In many cases, theoretical models can complement such
studies and provide qualitative data for wet collector evaluations.
9-21
-------
Wet Collector Systems
Although many unique wet collector systems are available, commercially designed
wet collectors use only a few basic components. Some of these components are:
spray nozzles
venturi constrictions
impingement surfaces
cyclonic openings
spray inducing orifices
plates
baffles
bubble caps
packing
mechanically driven rotors
The many possible combinations of these basic pieces of hardware has led to the
development of numerous types of wet collectors. This is an advantage for the
process engineer, who can specify a system in terms of size, cost, and collection effi-
ciency. However, the profusion of devices can also be a disadvantage, especially
when confronted with advertisements for such equipment. In this competitive
market, the various claims and counterclaims can be confusing. This part of
Chapter 9 will describe several commercially-marketed scrubbers;; with emphasis on
collection principles.
Although wet collection systems may vary greatly in complexity, most of them
follow a general rule; collection efficiency increases with increased power input.
Power is defined as energy applied per unit time. Energy from the gas stream,
energy from the liquid stream, or energy from a mechanically driven rotor is used
to bring gas stream particles into contact with the scrubbing liquor. The total
energy applied per unit time is the contacting power. Scrubbers can be categorized
in terms of how this power is applied in the system (Semrau, 1977). The categories
used are given hi Table 9-5.
Table 9-3. Categories of wet collector* and energy input.
Wet collector type
Gas phase contacting scrubbers
Liquid phase contacting scrubbers
Liquid phase/gas phase contacting scrubbers
Mechanically aided scrubbers
Scrubbers using energy from:
the gas stream
the liquid stream
the gas and liquid streams
a mechanically driven rotor
Scrubbers can also be classed by gas phase pressure drop: low energy scrubbers
having a pressure drop of less than 5 in. (12.7 cm) of water; medium energy scrub-
bers, 5 to 15 in. (12.7 to 38.1 cm) of water; and high energy scrubbers having a
pressure drop greater than 15 in. (38.1 cm) of water.
Gas Phase Contacting Scrubbers
Scrubbers using the process gas stream to provide the energy for particle-liquid
contact are known as gas phase contacting scrubbers. By moving the gas across or
through a liquid surface, the liquid is sheared to form small droplets. Paniculate
matter in the gas stream impacts on the larger droplets, which in turn are collected
by cyclonic action or other means. A number of methods are used to develop this
9-22
-------
shearing action in a scrubber. The gas can be forced through cascades of liquid
falling over flat plates. Holes can be punched in the plates and the gas can aspirate
the water flowing over the plate, or the gas can be forced through constricted
passages wetted with liquid such as in orifice and venturi scrubbers. We will discuss
three collectors which work primarily by this action: plate scrubbers, orifice scrub-
bers, and venturi scrubbers.
Plate Scrubbers
Plate scrubbers provide a simple means of interacting paniculate matter with a
liquid. A simple sieve plate scrubber is shown in Figure 9-9.
Clean gai
Plates
Dirty
Detail of plate
Figure 9-9. Sieve plate Krubber.
Water cascades down over the plates in the countercurrent system shown. The
process exhaust gas moves up through the scrubbing chamber and atomizes the
liquid at the edges of the holes. The plates (or trays) can contain from 600 to 5000
holes per square foot of surface. A sieve plate scrubber has two or three plates,
adjusted so that the holes are not aligned between plates.
Particles are collected as the gas atomizes the liquid flowing over the holes in the
plates. The droplets serve as impaction targets for the paniculate matter. Liquid
cascades from one tray to the other. In countercurrent flow, the cleanest liquid will
9-23
-------
contact the cleanest gas at the top plate. In contrast to gas absorption systems
where many plates may be used, systems designed for the collection of particles use
only two or three plates in the scrubbing column. For particles, collection efficiency
does not significantly increase by increasing the number of plates;.
Collection efficiency can be improved by decreasing the hole size and increasing
the number of holes per plate. More droplets of smaller size will be produced.
Impaction targets placed above each hole of the plate can also improve efficiency.
These targets can be of several types. In an impingement plate scrubber such as
that shown in Figure 9-10, stationary impingement baffles force the gas to change
direction.
dean gas
Mist eliminator
Impingement plate
Figure 9-10. Impingement plate scrubber.
Particles and water droplets are forced against the small impingement plates (or
baffle plates) with the consequent entrapment of particles in the liquid. Gas
velocities range from 12 to 20 ft/sec (170 to 610 cm/sec) through the holes which
may be from 1/8 (0.32 cm) to 1/4 inches (0.64 cm) in diameter. The change in
flow direction of the gas, 90 degrees from the plate, also aides in liquid atomiza-
tion and creates a turbulent froth on the plate surface. This provides an additional
contact zone to collect the paniculate matter (Figure 9-11).
9-24
-------
Gai
Plate
Figure 9-11. Detail of an impingement plate.
A problem that can occur with plate scrubbers is the weeping effect resulting from
low gas velocities. If exhaust gas flow rates into the scrubber are variable and
become too low, liquid can weep (drip) through the holes of the plate. Insufficient
gas flow rates reduce the impingement velocities, the amount of frothing, and con-
sequently, the collection efficiency. For cases of variable flow, bubble cap or valve-
type impingement surfaces can be used. Figure 9-12 shows the detail of caps used hi
a bubble cap plate scrubber.
Bubble cap
Gas stream
Figure 9-12. Detail of bubble caps.
9-25
-------
In a number of systems the caps rise above the plate support. The height that the
caps rise depends upon the gas stream velocity. Caps of different weights in alter-
nate rows provide the required flexibility in the system.
The atomization and frothing of liquid in plate scrubbers results from energy
supplied by the gas stream. Water sprays are sometimes added to wash over the
bottom of the plates and humidify incoming hot, dry gas. In these cases, the liquid
spray can also assist in the particle-droplet contacting process and will add to the
system power requirements.
Plate scrubbers are medium energy devices and have moderate collection effi-
ciencies. Problems arise if they are improperly applied to processes where heavy
particle loading, sticky particles, or scaling plug the small holes iin the plates. They
have been applied to processes where it is desired to both collect paniculate matter
and absorb gaseous pollutants.
A summary of operating characteristics is given for the general class of plate
scrubbers in Table 9-4.
Table 9-4. Operating characteristics of plate scrubbers.
Pressure
drop
(Ap)
1-8 in. of
water per tray
Liquid to
gas ratio
(L/C)
2-8 gpm/
1000 acfm
m
Liquid inlet
pressure
(PL)
<5psig
Cut
diameter
«4,U
>2.0^m
Applications
Coal driers
Copper roasting
Industrial boilers
Chemical process
{industries
Petroleum
: refineries
Incineration
processes
Note: gpm = gallon per minute
acfm = actual cubic feet per minute
psig= pounds per inch squared gage pressure
9-26
-------
Orifice Scrubbers
Orifice scrubbers have alternatively been called self-induced spray scrubbers,
impingement and entrainment scrubbers, submerged orifice or inertial orifice
scrubbers. Regardless of the term used, these scrubbers are designed so that the
process gas stream breaks through a pool of liquid. The gas moves through
restricted passages or orifices at velocities near 50 ft/sec (15.2 m/sec), to disperse
and atomize the water and entrain droplets in the gas stream as shown in
Figure 9-13.
Figure 9-13. Detail of orifice action.
9-27
-------
The larger panicles in the incoming gas stream impinge upon the surface of the
pool and are collected. Smaller panicles impact upon the droplets produced by the
high velocity gas skimming over the surface of the water. Baffles added in the
scrubber serve as impingement surfaces for droplet collection. Since the high
velocity gas enters into a large volume from the smaller orifice openings, the large
droplets lose momentum and are collected by gravity (Figure 9-14).
Clean gas
Dirty gas •
.. Dirty gas
Figure 9-14. Swirl orifice scrubber.
Paniculate matter is collected in the pool of scrubber liquid forming a sludge
which must periodically be disposed of. Removal of the sludge can cause problems
in the scrubber operation since the liquid level must be at a given height for
optimum performance. •
Orifice scrubbers are designed for specific gas stream velocities. A turn-down of
the system, reducing the gas velocity, will result in less atomization and larger
water droplets, with a consequent decrease in particle collection efficiency. The
relatively large orifice openings can accommodate gas streams with high loadings of
paniculate matter. Plugging by sticky or stringy material is not as great a problem
as with the sieve plate and impingement plate scrubbers. They are medium energy
devices with moderate collection efficiencies. Operational characteristics for orifice
scrubbers are given in Table 9-5.
9-28
-------
Table 9-5. Operating characteristics of orifice scrubbers.
Pressure
drop
(Ap)
2-10 in. of water
Liquid to
gas ratio
(L/C)
10-40 gpm/
1000 acfm;
V*-5gpm/
1000 acfm
with
^circulation
Liquid inlet
pressure
(pi)
not applicable
Cut
diameter
0.8-1 /un
Applications
Mining operations
Rock products
industries
Foundries
Pulp and paper
industries
Chemical process
industries
Venturi Scrubbers
Venturi scrubbers can give the highest collection efficiencies of any wet scrubbing
system. High pressure drops are needed to achieve high efficiencies, but the ver-
satility of venturi systems have made them attractive for many applications. One of
the simplest of the venturi designs is shown in Figure 9-15.
Venturi throat
Figure 9-15. Typical venturi scrubber.
9-29
-------
The venturi is designed to fan gas in and out of a constriction. Since the
volumetric flow rate of the gas (Q) must be the same throughout the system, the
velocity of the gas must increase at the throat of the venturi. That is, if
X^nrrone* =
VtAnMt "tl
Where: A= area
v= velocity
the velocity at the throat must increase in order to make up for the decrease in
area in the throat. Velocities at such a constriction can range from 200 to 800
ft/sec (61 to 244 m/sec). Now, if watei is introduced into the throat, the gas forced
to move at high velocity will shear the water into droplets. Particles in the gas
stream then impact onto the droplets produced. Moving a large volume of gas
through a small constriction gives a high velocity flow, but also a large pressure
drop across the system. Collection efficiency for small particles increases with
increased velocities (and corresponding increased pressure drops) since the water is
sheared into more and smaller droplets than at lower velocities. The large number
of small droplets combined with the turbulence in the throat section provides
numerous impaction targets for particle collection.
Water injection rate can govern the production of water droplets. Depending
upon design details, an increase in the liquid-to-gas ratio, (increase in liquid injec-
tion) can also improve collection efficiencies. There are, however, limits to the
liquid injection rate, since once enough liquid is present, further amounts of liquid
do not correspondingly increase efficiency.
The venturi design is merely a system for using the gas stream energy to atomize
liquid, thus the contact power comes primarily from the process gas flow. The
water droplets incorporating paniculate matter must be subsequently collected for
the system to be complete. The large bottom inlet cyclonic separators discussed in
Chapter 6 are often used for this purpose (Figure 6-2). Mist eliminator pads are
also used for this purpose. The droplets ranging from 50 to 500 fan in diameter are
easily collected by these systems or by combinations of cyclonic separators and mist
eliminators.
Many different venturi systems are commercially available. The primary dis-
tinctions between them are in method of water injection and in throat design.
Figure 9-16 shows a venturi with water swirling down from the top to wet the walls
before atomization at the throat.
9-30
-------
dean gas
Water
injection
nozzle
Dirty gas
' Figure 9-16. Swirl venturi scrubber.
9-51
-------
Venturis with wetted walls such as those shown in Figures 9-15 and 9-16 have an
advantage in preventing a buildup of wet paniculate matter at the wet-dry tran-
sition which could occur in a scrubber such as that shown in Figure 9-17.
Dirty gas
dean gas
Water sprays
Figure 9-17. Spray venniri scrubber.
The spray venturi scrubber (Figure 917) injects the water through nozzles at the
throat. Here, atomization can occur both by energy supplied by the gas stream and
by energy supplied from the liquid stream. In these applications, however, low
pressure sprays on the order of 5 to 15 psi (54.5 to 103 kPa) are normally used.
The greatest energy contribution for droplet formation therefore comes from the
gas stream. Spray injection can distribute liquid more uniformly over the throat
area for Venturis with large throat areas.
9-32
-------
A problem that can occur with venturi scrubbers is maintaining a given pressure
drop (and correspondingly, collection efficiency) under varying volumetric process
flow rates. A number of methods have been designed to vary the venturi throat
area under these conditions. Two of these designs are shown in Figure 9-18 and
Figure 9-19.
dean gas
Dirty gas
Water
injection
nozzle
Plunger
Figure 9-18. Variable throat venturi scrubber.
9-33
-------
Dirty gas
Clean gas
Water
injection
nozzle
Adjustable throat
Cyclone
Figure 9-19. Variable throat venturi scrubber.
In the scrubber shown in Figure 9-18 the throat area is varied by moving a
plunger up or down in the throat. Gas flows through the annular space to atomize
water sprayed over the plunger or swirled in from the top. The purpose of the
water spray is to continually wash collected matter from the top of the plunger
cone and to provide water for atomization. Figure 9-19 illustrates the use of a
movable plate to adjust the throat area with respect to process conditions. A water
wash spray is also employed here for much the same purpose as in Figure 9-18.
9-34
-------
A modification of the basic venturi design can be seen in the venturi-rod scrub-
ber. By merely placing a number of pipes parallel to each other, a series of
longitudinal venturi openings can be created as shown in Figure 9-20.
Dirty gai
Figure 9-20. Venturi-rod icrubber.
Water sprays serve to prevent clogging the openings from solids build-up. The
principle atomization of the liquid occurs at the rods, where the high velocity gas
moving through spacings creates the small droplets necessary for fine particle
collection.
Although venturi scrubbers have been used primarily for the collection of par-
ticulate matter, they have also been used to absorb gases. Lower gas rates and
higher liquid-to-gas ratios are necessary for efficient absorption. The advantage of
the venturi design for such an application is its relative freedom from scaling and
plugging from accumulated solids. The contact rime between gas and liquid is,
however, relatively shorter than in other type absorption systems and high removal
efficiencies for gases are more difficult to achieve. The basic design of the venturi
can lead to high abrasion and, consequently, high maintenance. The high gas
velocities can impel particles into the surfaces of the system and rapidly erode
them. For this reason, silicon carbide brick has been used to line the throat section.
The elbow at the bottom of the scrubber leading into the cyclonic separator can be
9-35
-------
flooded. Particles and droplets impact into the pool of liquid, reducing abrasion of
the scrubber itself. In designs using spray nozzles for liquid injection, recirculated
water can cause the nozzles to plug. Automatic or manual reamers are used to cor-
rect this problem.
The operating characteristics of venturi scrubbers vary greatly, depending upon
the application and specific design. An excellent and more detailed review of ven-
turi scrubber systems is given in Cheremisinoff, 1977. A range of values for various
performance parameters is given in Table 9-6.
Table SMi. Operating characteristics of venturi scrubbers.
Pressure
drop
(Ap)
5-100 in. water
(20-60 common)
Liquid to
gas ratio
(L/C)
3-20 gpm/
1000 acfm
(7-8 common)
<
Liquid inlet
pressure
(pi)
<1-15 prig
Cut
diameter
«*.U
0.2 /on
[dependent
upon Ap)
Applications
Pulp and paper
industry
Acid plants
Mining industries
E>ryers
Nonferrous metals
industry
Iron and steel
industry
Utility and indus-
trial boilers
Incinerators
Chemical industry
Liquid Phase Contacting Scrubbers
The previous section dealt with scrubbers which primarily used the process gas
stream to atomize liquid into collection droplets. Energy can also be applied to a
scrubbing system by injecting liquid at high pressure through specially designed
nozzles. Nozzles produce droplets which fan out into a scrubber chamber to impact
with paniculate matter contained in a polluted gas stream. Three types of nozzles
are shown in Figure 9-21 a-c; an impingement spray nozzle, a solid cone spray noz-
zle, and a helical cone spray nozzle.
9-36
-------
a. Impingement spray
nozzle
b. Solid cone spray nozzle
Figure 9-21. Types of spray nozzles.
c. Helical cone spray nozzle
In the impingement nozzle (Figure 9-21a), high pressure liquid strikes a plate or
pin to give a spray of uniformly sized droplets. The solid cone nozzle (Figure 9-21b)
directs the liquid through a central jet which strikes fluid rotating in from the side.
The two streams interact to break the liquid up into a droplet spray. The helically
shaped solid cone nozzle (Figure 9-2Ic) can provide a wide spray at high pressure
with fewer plugging problems.
Two scrubber designs which most clearly characterize the liquid phase contacting
class of wet collectors, are the spray towers and ejector Venturis. Other scrubber
designs use preformed sprays from nozzles to clean plates, packing surfaces, or
orifices as were shown in the case of gas phase contacting Venturis.
9-S7
-------
Spray Towers
Spray towers are alternatively called gravity spray towers, spray scrubbers, or spray
chambers. The basic design is simple. Liquid is sprayed into a cylindrical or rec-
tangular chamber using one nozzle or a series of nozzles as shown in Figure 9-22.
gas
Dirty gas
Figure 9-22. Simple spray chamber.
The gas flow is generally countercurrent to the liquid direction, moving up past the
downward spraying droplets. The open space of the scrubbing chamber allows for
few plugging or scaling problems. If the scrubber water is recirculated, however,
the small nozzle openings can become plugged or severely eroded by suspended
paniculate matter in the retirculating liquid. Spray scrubbers arc more successfully
applied to systems using relatively clean water.
Spray towers are low energy devices. Contacting power is much lower than in
venturi scrubbers and the gas pressure drop across such systems are generally less
than 1 in. (2.5 cm) of water. The collection efficiency for small particles is cor-
respondingly lower than more energy intensive devices. They are adequate for the
collection of coarse particles larger than 10-25 jun, although with increased liquid
inlet nozzle pressures, a cut diameter of 2.0 pm can be achieved. Higher liquid
pressures at the nozzle imply the formation of smaller droplets. The highest collec-
tion efficiencies are achieved when small droplets are produced and the difference
9-S8
-------
between the velocity of the droplet and the velocity of the upward moving particles
is high. Small droplets, however, have small terminal settling velocities, so there is
an optimum range of droplet sizes for scrubbers which work by this mechanism.
Stairmand (1956) found this range of droplet sizes to be between 500 and 1000 /tm
for gravity spray chambers. The injection of water at very high pressures, SOO to
450 psi (2070 to 3100 kPa), creates a fog of very fine droplets. Higher particle col-
lection efficiencies can be achieved hi such cases since collection mechanisms other
than inertial impaction occur (Bethea, 1978).
Exhaust gases entering a spray chamber, expand into the large volume and cool
down from both expansion and the water sprays. Large volumetric flows of hot
gases can be conditioned in this way before being exhausted or undergoing further
treatment in smaller systems. Gas velocities are generally low, on the order of 2 to
5 ft/sec (0.61 to 1.5 m/sec). Higher velocities will cause the smaller droplets to be
swept up and entrained in the exhaust gas stream. In such cases, water droplet
entrainment separators (mist eliminators) are needed to collect the escaping
droplets. Chevron baffle or fiber type eliminators such as those shown in Figure
9-23 are often used.
Wire mesh
•S" or Chevron curve
Figure 9-25. Mist eliminators.
These are placed at the top of the scrubber, before the gas exhausts into further
parts of the duct system.
Spray towers find wide application as inexpensive control devices for large par-
ticles. They often serve to condition the gas, reduce its volume, and cool and
humidify it before it is treated in a more efficient device. General characteristics of
spray towers are given in Table 9-7.
Table 9-7. Operating characteristics of spray towers.
Pressure
drop
(Ap)
Vi -3 in. of water
Liquid to
gas ratio
(L/C)
0.5-20 gpm/
1000 acfm
(5 normal; > 10
when using high
pressure sprays)
Liquid inlet
pressure
(pt)
10-400 pug
Cut
diameter
«d,U
2-8 /on
Applications
Mining industry
Chemical process
industry
Boilers and
incinerators
Iron and steel
industries
9-39
-------
Ejector Venturi Scrubbers
The ejector venturi scrubber uses a preformed spray as does the simple spray tower.
The difference is that only a single nozzle is used. This nozzle operates at
higher pressures and higher liquid injection rates than those in most spray
chambers. For example, liquid-to-gas ratios in such systems are on the order of
100 gpm/1000 acfm [(379 L/min)/(28 mVmin)] at liquid nozzle pressures near
100 psig (690 kPa). Figure 9-24 shows the ejector venturi design.
Water injection
nozzle
dean gas
Dirty gas
Figure 9-24. Ejector venturi scrubber.
The ejector venturi is unique among available scrubbing systems since it can
move the process gas without the aid of a blower or fan. The Liquid spray coming
from the nozzle creates a partial vacuum in the side duct of the scrubber. This has
the same effect as the water aspirator used in high school chemistry labs to pull a
small vacuum for filtering precipitated materials (this is all due to the Bernoulli
effect). This partial vacuum can be used to move the process g;as through the con-
trol device as well as the process system. In the case of explosive or extremely cor-
rosive atmospheres, the elimination of a fan in the system can avoid many potential
problems.
The energy for the formation of scrubbing droplets comes from the injected
liquid. Paniculate collection occurs primarily by impaction as the process gas
enters into the spray area from the side. The gas motion is initially in a crossflow
9-40
-------
direction with respect to the liquid, but then becomes cocurrent as it moves down
the venturi tube. The turbulence which occurs hi the throat area of the venturi
also aids in the collection of the paniculate matter. Since the flow is cocurrent
once the gas is in the system, the gas will remain in contact with the liquid longer
than hi some other systems. This longer rime improves absorption efficiency for
pollutant gases soluble hi the scrubbing liquor.
Very high liquid injection rates are used to provide the gas-moving capability
and higher efficiencies. As with other types of Venturis, a means of separating
entrained liquid from the gas stream must be applied. Ejector systems commonly
use a liquid sump, directing the gas flow at a right angle, to continuing duct work.
Mist eliminators are commonly used to remove remaining small droplets.
Ejector Venturis have an advantage hi being able to handle small, large, and
variable gas volumetric flows. In fact, 10 cfm units have been made using this prin-
ciple. Other design ranges are given hi Table 9-8.
Table 9-8. Operating characteristics of ejector Venturis.
Pressure
drop
(Ap)
V£-5 in. of water
Liquid to
gas ratio
(L/C)
50-100 gpm/
1000 acfm
Liquid inlet
pressure
(pr)
15-120 psig
Cut
diameter
-------
ducing the gas tangentially into a scrubber, a cyclonic motion can be given to the
gas (see Chapter 6). The resultant increased relative particle/droplet velocities, v*.
can lead to improved collection efficiency.
Gas phase contacting power was previously discussed in terms of the gas acting to
shear liquid into dispersed droplets in a venturi or orifice scrubber. In the case of a
cyclonic scrubber, energy is utilized to develop the cyclonic action which leads to
greater particle velocities. The pressure drops and energy requirements associated
with the gas flow are the same as the dry cyclone collectors discussed in Chapter 6.
By adding spray nozzles, a contribution is made from the energy of the liquid
stream to the collector. The total energy required to run the system is higher, but
the efficiency for particle collection is correspondingly improved.
Many combination devices use cyclonic action and liquid sprays. Modifying an
existing dry cyclone by adding spray nozzles at the top, as in Figure 9-25, promotes
significant improvements hi efficiency.
Clean gai
Din:y gas
Figure 9-25. Irrigated cyclone scrubber.
The liquid droplets serve as impaction targets and sweep up the smaller particles.
The larger droplets containing the particles are more easily collected hi the cyclone
and the additional feature of the sprays wetting the walls of the system reduces
9-42
-------
particle reentrainment. Reentrainment is a significant problem in dry cyclones
and the use of spray rings or other spray nozzle configurations to wet the walls is an
effective way of minimizing this problem.
A variation of the simple spray chamber directs the gas flow tangentially into the
chamber to induce cyclonic flow, thus improving efficiency. Figure 9-26 shows such
a system with spray nozzles mounted on a central post, directed toward the walls of
the chamber.
dean gas
Spray manifold
Dirty gas
Figure 9-26. Cyclonic spray scrubber.
9-43
-------
Other systems are designed with spray nozzles on the outside of the chamber. This
configuration allows for easier maintenance and nozzle replacement. Systems
have also been designed with directional vanes placed inside the chamber
(Figure 9-27).
dean gu
Mist eliminator
Vane cage
Water injection nozzle
Figure 9-27. Centrifield icrubber.
9-44
-------
These vanes can be used to accentuate the cyclonic motion and force the collecting
droplets to the walls for separation from the gas stream. A summary of operational
characteristics for the general class of cyclonic scrubbers is given in Table 9-9.
Table 9-9. Operating characteristics of cyclonic scrubbers.
Pressure
drop
(Ap)
1.5-10 in. of water
Liquid to
gas ratio
(L/C)
2-10 gpm/
1000 acfm
Liquid inlet
pressure
(PL)
40-400 psig
Cut
diameter
«a,U
2-3 «un
Applications
Mining operations
Drying operations .
Food process
Foundries
Fertilizer industry
Asphalt production
Cupolas
Moving Bed Scrubbers
Moving bed scrubbers utilize gas stream energy in a unique way to provide impac-
tion surfaces for particle collection. Figure 9-28 shows the basic components of the
moving bed or mobile bed scrubber.
dean gas
Mist eliminator
Spray nozzles
Mobile packing
Dirty gas
Figure 9-28. Moving bed scrubber.
9-45
-------
In one type of mobile bed scrubber, ihcfluidized bed scrubber, process gas is
injected at the bottom, keeping a bed of plastic spheres in constant motion. The
spheres, resembling ping pong balls, bounce between two perforated plates or
screens. Water is sprayed from nozzles over these moving spheres. Particles in the
gas stream impact upon the spheres, the liquid which runs over the spheres, and
the droplets directed down from the spray nozzles. The gas will also atomize liquid
circulating within the moving bed to create a turbulent zone for particle collection.
The continual movement of the bed combined with the washing effect of the water
sprays, prevents plugging by collected paniculate matter.
Energy is supplied by the gas stream to support the packing material and
generate the turbulent froth layer. Energy is also supplied by the liquid stream to
produce droplets in the water spray.
A similar device is commercially marketed which uses glass marbles in the
packing bed (flooded bed scrubber). Liquid is sprayed cocurrently into the bed
from the bottom. Fluidization is not as pronounced in this case, but the marbles
still jiggle by action of the gas stream.
These systems can also be used for gas absorption and in this sense have been
termed turbulent contact absorbers. They have found application in removing both
paniculate matter and SOt from coal-fired steam generator exhausts. General
characteristics of the moving bed scrubber are given in Table 9-10.
Table 9-10. Operating characteristics of moving bed scrubbers.
Pressure
drop
(Ap)
3-5 in. of water
per stage
Liquid to
gas ratio
. (L/C)
3-5 gpm/
1000 acfm
Liquid inlet
pressure
(PL)
Cut
diameter
Ud,U
2-3 pm
Applications
Mining operations
I*ulp mills
Utility boilers
Food industry
Baffle Spray Scrubbers
Baffles are commonly used in entrainment separators to remove droplets from the
outlet gas stream. Single baffles are placed in a chamber to redirect the gas flow,
or they can be arranged together in Chevron or parallel patterns as in mist
eliminators (shown in Figure 9-23). Water is sprayed on the plates to remove
accumulated paniculate matter and to aid hi collecting particles. A simple baffle
scrubber system is shown in Figure 9-29.
9-46
-------
dean gas
Baffle plates
'•••.Dirty gas
Figure 9-29. Baffle spray scrubber.
Liquid cascading over baffles can be atomized by the gas stream under
appropriate conditions (Figure 9-29). Depending upon the scrubber design, liquid
and gas streams will contribute to the collection of paniculate matter by varying
degrees. Pressure drops are normally low for these simple systems as are collection
efficiencies for small particles (Table 9-11).
Table 9-11. Operating characteristics of baffle spray scrubbers.
Pressure
drop
(Ap)
1-3 in. of water
Liquid to
gas ratio
(L/C)
1 gpm/
1000 acfm
Liquid inlet
pressure
(pi)
<15psig
Cut
diMi?***T
«d,U
• 10 pm
Applications
Mining
Incineration
Chemical proccw
industry
9-47
-------
Combination Devices
As mentioned previously, new wet collectors can be devised by modifying scrubber
geometry or adding hardware. The incorporation of a number of scrubbing prin-
ciples often improves the collection efficiency for small particles. The addition of
baffles, cyclonic motion, or spray nozzles generally raises the energy requirements
of the system. The improved efficiency would of course be consistent then with the
contact power principle. Two of these combination designs are shown in Figures
9-30 and 31. As an exercise, the reader can evaluate the mechanisms used for par-
ticle collection in each scrubber.
Dirty gas
dean gas
Spray nozzle
Spray nozzles
Impingement plate
Figure 9-30. Combination device A.
9-48
-------
.;..-J.;..'.. Dirty gas
dean gas
Water injector
Venturi throat
Mist eliminator
Spray ring
Mobile packing
Figure 9-31. Combination device B.
Mechanically Aided Scrubbers
Energy can be supplied externally to a scrubber system by using a motor. A motor
serves to drive a rotor or paddles to generate water droplets for particle impaction.
Systems designed in this manner have an advantage of requiring less space than
other scrubbers, but the overall power requirements tend to be higher than other
scrubbers of equivalent efficiency. This point might appear to contradict the con-
tact power principle, but it must be realized that significant power losses occur in
driving the rotor and is not expended to provide for particle-liquid contact.
9-49
-------
Mechanically aided scrubbers have higher maintenance costs than other wet col-
lector systems. The moving parts can be particularly susceptible 1:0 corrosion and
fouling. Caking or sticky materials should be collected by simpler devices.
Mechanical scrubbers are not normally used for absorbing gaseous pollutants since
residence times are short.
Fewer different mechanically aided scrubber systems are available than liquid
and gas phase contacting collectors. Two basic designs will be discussed here; the
centrifugal fan scrubbers and mechanically induced spray scrubbers.
Centrifugal Fan Scrubbers
A centrifugal fan scrubber can serve both as an air mover and a collection device.
Figure 9-32 shows such a system, where water is sprayed onto the fan blades cocur-
rently with the moving gas.
Spray nozzle
Dirty gas
dean gai
Figure 9-52. Centrifugal fan icrubber.
Droplets from the spray nozzle will initially collect some paniculate matter. These
will atomize when impacting the blades to create smaller droplets for additional
particle collection. The particles will also impact on the blades and be collected.
Liquid film on the blades, in conjunction with the centrifugal effects of the
9-50
-------
rotating blades, will force both paniculate matter and liquid together off the blade
ends. The particles and liquid fly off the blades with great acceleration, and are
subsequently collected.
Centrifugal fan collectors are the most compact of the wet scrubbers since the
fan and collector comprise a combined unit. No internal pressure loss occurs across
the scrubber, but a power loss equivalent to a pressure drop of 4 to 6 in. (10.2 to
15.2 cm) of water occurs due to a lower blower efficiency.
Mechanically Induced Spray Scrubbers
A whirling rotor submerged in a pool of liquid produces a fine droplet spray. By
moving the process gas through the spray, particles can subsequently be collected.
Figure 9-33 shows a vertical spray rotor which operates by this method.
dean gas
Mist eliminator
Dirty gas
Rotor
Spray zone
Figure 9-33. Vertical spray rotor scrubber.
The high radial droplet velocity provides good collection efficiencies for submicron
sized particles.
9-51
-------
Another system, the disintegrator scrubber, is used to treat blast furnace gas.
Water injected into a rotating apparatus is atomized into small droplets. This
scrubber operates well only at low grain loadings, has high operating costs, and
requires frequent maintenance. Further performance characteristics for
mechanically aided scrubbers are given hi Table 9-12.
Table 9-12. Operating characteristics of mechanically aided scrubbers.
Pressure
drop
4-6 in. of water
(nominal)
Liquid to
gas ratio
(L/C)
0.5-1.5 gpm/
lOOOacfm
(centrifugal)
4-5 (spray
rotor)
Liquid inlet
pressure
(Px>
20-60 psig
(centrifugal)
Gut
diameter
ffd,U
<1 pm
Applications
Mining operations
Food product
industries
(Chemical industry
Foundries and
steel mills
9-52
-------
Miscellaneous Devices
Many other systems also use a liquid to aid in the collection of particulate matter.
Wet film scrubbers, for example, are commonly used to remove gaseous substances
from exhaust streams, but under proper conditions, have been successfully used to
control particulate emissions. The introduction of new collection mechanisms such
as electrostatic charging or condensation has improved collection efficiencies for a
number of newer devices. As previously discussed, many collection mechanisms are
involved in some of the simplest appearing scrubbers. The generation of a fog or
foam, or the electrostatic charging of collecting droplets require extra increments
of energy. The efficiency relationships however, differ from those given by the
simple contact power theory discussed in this chapter. This section will first discuss
the use of wet film scrubbers for panicle control and will then briefly discuss some
of the new types of wet collection equipment.
Wet Film Scrubbers
A film of liquid will form by spraying or cascading water over packing material.
Packing materials are made of plastic or ceramic in many shapes. Figure 9-34
shows a number of forms commonly used. These convoluted forms of packing
material are designed so that a large wetted surface area is provided without
greatly impeding either the flow of gas or liquid.
Berl saddle
Raichig ring
Intaloz saddle
Pall ring
Tellerette
Figure 9-34. Common packing materials.
9-53
-------
Packing materials are literally packed in a column or in sectional compartments.
Liquid is sprayed on the material and allowed to flow through the system. Process
exhaust gas is introduced into this system and the particles in the gas stream move
through the packing or are collected. Figure 9-35 shows a packed tower with
countercurrent flow (gas and liquid moving in opposite directions). Figure 9-36
shows a packed tower with cocurrent flow (gas and liquid moving hi the same
direction). Cocurrent flow, provides better particle collection than countercurrent
flow since higher gas velocities can be used. With higher velocities, smaller particles
can be collected than in similar countercurrent scrubbers.
dean gas
Dirty gas
Mist eliminator
Water sprays
\'.-'.\Dirty gas
Packing
Clean gas
Figure 9-35. Countercurrent packed tower.
Figure 9-36. Cocurrent packed tower.
The particles impinge upon the liquid film covering the packing and are drained
off with the liquid as it flows through the system. Many changes of direction occur
as the gas winds through the openings of the packing materials. Larger particles
unable to follow the streamlines associated with this flow will consequently hit the
packing and be collected.
9-54
-------
The packed column or tower can have several problems -when not operated
under proper conditions. Heavy loadings of paniculate matter can plug beds of
packing material. Deep beds will provide high pressure drops and higher efficien-
cies, but can be more susceptible to plugging. However, gas channeling is more
likely to occur in thin beds. The gas flows through the path of least resistance and
may channel through a certain section of the bed if the bed is not thick enough. In
the countercurrent design, if the liquid rates are too high or the gas rates too low,
the tower will flood and begin to fill with liquid.
An alternative means of using packing material for particle collection is in a
crossflow design such as that shown in Figure 9-37. The water sprays can be posi-
tioned in front of the packing, above the packing, or behind the packing. This
configuration has an advantage since paniculate matter accumulated on the face of
the packing can be washed off by water sprays.
Water sprays
Water sprays
dean gai
Packing
Figure 9-37. Crottflow scrubber.
Another type of arrangement, the fiber bed scrubber, uses fibrous material such
as fiberglass or plastic in beds with water sprays that wash away collected material
(Figure 9-S8).
9-55
-------
Water sprays
dean gas
Sludge drain
Figure 9-38. Fiber bed scrubber.
Some operational characteristics of the general class of wet film scrubbers are given
in Table 9-IS.
Table 9-13. Operating characteristics of wet film scrubbers.
Pressure
drop
(Ap)
1.5-10 in. of water
(depending on
packing depth)
'Liquid to
gas ratio
(L/G)
1-15 gpm/
1000 acfm
(depending upon
counter current ,
cocurrent or
crossflow
operation)
Liquid inlet
pressure
(pt)
5-15 psig
Cut
diameter
«1.5/un
(depending
upon size of
packing)
Applications
Mainly used for
liquid aerosol or
gaseous pollutant
removal
Metal operations
Acid plants
Chemical process
industries
New Methods
Mechanisms other than impingement or diffusion can be used for the wet collection
of paniculate matter. Electrostatic effects, for example, are used to aid in droplet
collection. By charging droplets in a special nozzle or by charging particles in a
high voltage ionization section, a charged droplet scrubber can be produced.
Condensation effects can be used to grow droplets about particle nuclei. By
reducing the temperature of a hot, saturated gas stream, the condensation effect is
used to improve the collection of small particles.
Wet collectors have also been designed which utilize a foam consisting of
numerous bubbles. Long lasting bubbles in the foam can increase the likelihood of
capturing small particles by diffusional processes.
9-56
-------
Selecting a Scrubber
The large number of commercially available wet collector systems has often led
to confusion in selecting one for a given application. Vendors do, however, supply
extensive information and performance data on their own systems. Evaluations of
scrubber designs should be made carefully. Literature on possible systems should be
obtained from as many vendors as possible with a list of previous users. These
customers should then be contacted for information on actual performance
characteristics. Process industry trade associations should also be contacted to iden-
tify successful systems applied to processes similar to the one needing control.
References
American Petroleum Institute. 1974. Manual on Disposal of Refinery Wastes-
Volume on Atmospheric Emissions. API Publication 931, Chapter IS-Filters
and Wet Collectors for the Removal of Particulate Matter.
Beg, S. A. and Taheri, M. 1977. Test of Mathematical Modeling for the Design
of High Energy Scrubbers./. Atmos. Environ. 11:911-915.
Bethea, R. M. 1978. Air Pollution Control Technology—An Engineering Analysis
Point of View. New York: Van Nostrand Reinhold Co., pp. 253-328.
Brady, J. D. and Legatski, L. K. 1977. Venturi Scrubbers, Air Pollution Control
and Design Handbook, eds. P. N. Chereminisoff and R. A. Young, pp. 729-745.
New York: Marcel Dekker, Inc.
Busch, J. S., MacMath, W. E. and Lin, M. S. 1973. Part 1-The Basic Scrubber.
Pollut. Eng. Jan. 1973: 28-32.
Calvert, S., Lundgren, D. and Mehta, D. S. 1972. Venturi Scrubber Performance.
/ Air Poll. Control Assoc. 22:529-532.
Calvert, S. 1974. Engineering Design of Fine Particle Scrubbers./ Air Poll.
Control Assoc. 24:929-934.
Calvert, S. 1977. Scrubbing. In Air Pollution Vol. IV, ed. A. C. Stern. New
York: Academic Press, pp. 257-291.
Calvert, S. 1977. How to Choose a Particulate Scrubber. Chem. Eng. Aug. 1977:
54-68.
Calvert, S. 1977. Get Better Performance from Particulate Scrubbers. Chem. Eng.
Oct. 1977: 133-140.
Cheremisinoff, P. N. and Young, R. A. 1974. Wet Scrubbers-A Special Report.
Pollut. Eng. May 1974: 33-43.
Cheremisinoff, P. N. and Young, R. A. 1976. Control of Fine Particulate Air
Pollutants: Equipment Update Report. Pollut. Eng. Aug. 1976: 22-29.
9-57
-------
Cheremisinoff, P. N. and Young, R. A. 1977. Air Pollution Control and Design
Handbook. New York: Marcel Dekker, Inc.
Crews, B. D. 1978. The Selection of Wet Scrubbing Equipment, In Proceedings of
the International Clean Air Conference, ed. E. T. White, pp. 277-291. Ann
Arbor: Ann Arbor Science.
Dickie, L. 1967. All About Wet Collectors-Part I. Air Eng. Jan. 1967: 14-19.
Dickie, L. 1967. All About Wet Collectors-Part II. Air Eng. Feb. 1967: 24-27.
Environmental Protection Agency (EPA). 1969. Control Techniques for Paniculate
Air Pollutants. AP-51, pp. 50-81.
Environmental Protection Agency (EPA). 1972. Wet Scrubber System Study.
EPA-R2-72-118a.
Environmental Protection Agency (EPA). 1979. Design Guidelines for an Optimum
Scrubber System. EPA-600/7-79-018.
Environmental Protection Agency (EPA). 1978. Particle Collection by a Venturi
Scrubber Downstream from an Electrostatic Precipitator. EPA-600/7-78-19S.
Environmental Protection Agency (EPA). 1978. SR-52 Programmable Calculator
Programs for Venturi Scrubbers and Electrostatic Precipitators.
EPA-600/7-78-026.
Gardenier, H. E. 1974. Submicron Paniculate Scrubbing with a Two Phase Jet
Scrubber./. Air Poll. Control Assoc. 24:954-957.
Gilbert, N. 1961. Removal of Particulate Matter from Gaseous Wastes-Wet Col-
lectors. American Petroleum Institute.
Gilbert, W. 1977. Troubleshooting Wet Scrubbers. Chem. Eng. Oct. 1977:
140-144.
Gleason, T. G. 1977. Halt Corrosion in Particulate Scrubbers. Chem. Eng.
Oct. 1977: 145-148.
Hesketh, H. 1974. The Particle Collection Efficiency Related to Pressure Drop,
Scrubbant and Particle Properties, and Contact Mechanism./. Air Poll. Control
Assoc. 24:939-942.
Hesketh, H. E. 1979. Air Pollution Control. Ann Arbor: Ann Arbor Science,
pp. 217-261.
Imperato, N. F. 1968. Gas Scrubbers. Chem. Eng. Oct. 1968: 152-155.
Johnstone, H. F., and Roberts, M. H. 1949. Deposition of Aerosol Particles
from Moving Gas Streams. Ind. and Eng. Chem. 41:2417-2423.
Johnstone, H. F., Field, R. B., and Tassler, M. C. 1954. Gas Absorption and
Aerosol Collection in a Venturi Atomizer. Ind. and Eng. Chem. 46:1601-1607.
Klugman, W. and Sheppard, S. V. 1977. The Ionizing Wet Scrubber. Air
Pollution Control and Design Handbook, eds. P. N. Cheremisinoff and R. A.
Young, pp. 799-811. New York: Marcel Dekker, Inc.
9-58
-------
Krockta, H. and Lucas, R. L. 1972. Information Required for the Selection
and Performance Evaluation of Wet Scrubbers./ Air Poll. Control Assoc.
22:459-462.
Lapple, C. E. and Kamack, H. J. 1955. Performance of Wet Dust Scrubbers.
Chem. Eng. Prog. 51:110-121.
Lear, C. W., Krieve, W. F. and Cohen, E. 1975. Charged Droplet Scrubbing
for Fine Particle Control./. Air Poll. Control Assoc. 25:184-189.
Ucht, W. 1980. Air Pollution Control Engineering—Basic Calculations for Par-
ticulate Collection. New York: Marcel Dekker, Inc. pp. 355-S70.
MacDonald, J. W. 1977. Packed Wet Scrubbers. In Air Pollution Control and
Design Handbook, ed. P. N. Cheremisinoff and R. A. Young, pp. 699-720. New
York: Marcel Dekker, Inc.
Marchello, J. M. 1976. Control of Air Pollution Sources. New York: Marcel
Dekker, Inc., pp. 187-207.
McCarthy, J. E. 1980. Scrubber Types and Selection Criteria. Chem. Eng. Prog.
May 1980: 58-62.
Mcllvaine, R. W. 1977. When to Pilot and When to Use Theoretical Pre-
dictions of Required Venturi Pressure Drop. Air Poll. Control Assoc. Meeting
Paper, Toronto: 77-17.1.
Nukiyama, S. and Tanasawa, Y. 1983. An Experiment on Atomization of
Liquid by Means of Air Stream (in Japanese) Trans. Soc. Mech. Eng. Japan:
4, 86.
Onnen, J. H. 1972. Wet Scrubbers Tackle Pollution. Environ. Set. Technol.
6:994-998.
Ostojc, N., Yankura, E. S. and Buonicore, J. A. 1977. Energy Savings with the
Centripetal Vortex Scrubber. Air Poll. Control Assoc. Annual Meeting Paper
77-17-8. Toronto.
Perry, J. H. and Chilton, C. H. (eds). 1973. Chemical Engineers Handbook
5th ed. New York, McGraw-Hill Book Co., pp. 20-94 to 20-104.
Rimberg, D. and Peng, Y. M. 1977. Aerosol Collection by Falling Droplets. Air
Pollution Control and Design Handbook, eds. P. N. Cheremisinoff and R. A.
Young, pp. 747-777. New York: Marcel Dekker, Inc.
Rimberg, D. B. 1979. Tips and Techniques on Air Pollution Control Equipment
O & M. PoUut. Eng. March 1979: 32-35.
Semrau, K. T. 1960. Correlation of Dust Scrubber Efficiency./ Air Poll.
Control Assoc. 10:200-207.
Semrau, K. T. 1963. Dust Scrubber Design—A Critique on the State of the
Art./ Air Poll. Control Assoc. 13:587-593.
9-59
-------
Semrau, K. T. 1977. Practical Process Design of Paniculate Scrubbers. Chem.
Eng. Sept. 1972: 87-91.
Sparks, L. E., McCain, J. D. and Smith, W. B. 1974. Performance of a Steam
Ejector Scrubber. / Air Poll. Control Assoc. 24:958-960.
Stairmand, C. J. 1956. The Design and Performance of Modern Gas-cleaning
Equipment./ Inst. Fuel. 29:58-81.
Strauss, W. 1975. Industrial Gas Cleaning. Oxford: Pergamon Press, pp.
367-408.
Theodore, L. and Buonicore, A. J. 1976. Industrial Air Pollution Control
Equipment for Particulates. Cleveland: CRC Press, pp. 191-250.
9-60
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Appendix A
Common SI Units
Quantity (1)
area
volume
•peed or velocity
acceleration
rotational frequency
mass (5)
density
force
movement of force (6)
pressure (or vacuum)
Kress
viscosity (dynamic)
viscosity (kinematic)
energy, work, or
quantity of heat
power, or heat flow
rate
temperature, or tem-
perature interval
Same common units
kilometer
meter
centimeter
millimeter
micrometer
square kilometer
square hectometer
square meter
square centimeter
square millimeter
cubic meter
cubic decimeter
cubic centimeter
meter per second (12)
kilometer per hour (4)
meter per second squared
revolution per second
revolution per minute (4)
megagram
kilogram
gram
milligram
kilogram per cubic meter
kilonewton
newton
newton meter
kilopascal
pascal
megapascal
millipascal second (7)
pascal second
square millimeter per
second (8)
joule (9)
kilowatt hour (10)
kilowatt
wan
kervm
degree Celsius (11)
Symbol
km
m
cm
trim
pm
km'
hm'
m>
cm'
nun*
m'
dm'
cm1
m/s
km/h
m/s'
r/s
r/min
Mg
k*
8
mg
kg/m»
kN
N
N«m
kPa
Pa
MPa
mP»M
P».»
mm'/s
J
kW.h
kW
W
K
°C
Equivalent
hectare (2)
liter (S)
milliliter (S)
metric ton
gram per liter
kilogram
meter
per second
squared
Newton per
meter
squared
Newton
meter
kflowatthour
Symbol
ha
L
mL
t
g'L
kg«m/s>
N/mf
N-m
kWb
NOTES
(1) Any measurable prop-
erty (such as length, area,
temperature) is called a
quantity. Listed in same
sequence as ISO 1000 and
ISO 31, except plane angle.
(2) For land or water area
only.
(S) To be used only for
fluids (both gases and
liquids), and for dry ingre-
dient* in recipes, or for
volumetric capacities. Do
not use any prefix with liter
except mtZii.
(4) The symbols for
minute, hour, and day are
min, h, and d, respectively.
(5) Commonly called
weight.
(6) Torque or bending
movement.
(7) 1 mPaM'l cP(cen-
tipoise, which is obsolete).
(8) 1 mm'/s- 1 cSt (cen-
tistokes, which is obsolete).
(9) The unit-multiples
kilojoule (kj) and mega-
joule (MJ) are also com-
monly used.
(10) To be abandoned
eventually. 1 kW-h-3.6
MJ.
(11) The degree mark ° is
always used in °C to avoid
confusion with coulomb
(C), but never with K for
kdvin.
(12) Second is denoted by s
in SI units.
Source: The American National Metric Council, 1978.
A-l
------- An error occurred while trying to OCR this image.
-------
Appendix B
Conversion Factors
Length
1 inch =2.54 cm
1 m= 3.048 ft
1 ft = O.S05m
Mass
1 lb = 453.6g
1 lb=7000 grains
lkg=2.21b
Pressure
1 atm= 101,825 Pa
= 760mrnHg(0°C)
= 14.7 psia
Force
1 N=l kgTA/S*
1 N = 0.225 lb,
Energy
lcal = 4.184 J
1 J = 9.48x10-* Btu
1 Btu = 252.2 cal
Kinematic viscosity
1 mVs= 10* stokes
lmVs=3.875 10* ftVhr
Power
1 W=lJ/s
1 W = 3.414 Btu/hr
1 W=l.S41XlO-»hp
1 hp= S3,479 Btu/hr
Area
1m* =10.764 ft*
1 cm* = 0.155 in1
1 m'= 1.196 yd1
Volume
lm' = 35.31 ft'
1 cm* = 0.061 in3
lm' = 264gal(US)
1 ft8 = 28.317 L
1 barrel (oil) = 42 gal
1 ft8 = 7.48 gal
Density
1 kg/ms = 0.0624 lb/fts
Dynamic viscosity
1 Pa»s= 1 N«m/s= 1000 centipoise
1 cp= 0.000672 lb/ffsec .
Volume flow
lms/s=35.3ftVsec
1 ms/min = 35.S ftVmin
1 scfm=1.7mVh
1 gpm= 0.227 mVh
Velocity
1 m/s= 3.28 ft/sec
1 m/s=196.85 ft/min
1 mi/hr= 0.447 m/s
Geometry
area of cu:cle=Tr1
circumference of circle = 2 TT
surface area of sphere = 4 TT*
volume of sphere = 4/3 irr3
area of cylinder = 2 mrh
B-l
-------
Appendix C
Constants and Useful Information
Gas constants
R= 0.0821 atm liter/g mol»K
= 83.14x10* g»ons/sl»g mol»K
- 8.314 N»m/gmol»K
= 0.7302 atm-ftVlb mol»°R
= 1.987 g»cal/g mol»K or Btu/lb mol«°R
Acceleration of gravity
g=32.17 ft/sec1 = 980.7 cm/5^9.8 m/s1
Newton's conversion constant
&= 32.17 (lb—)(ft)/(lb>rB.)(sec«)
1 Ib mol= 359 fts of ideal gas at STP (S2°F and 14.7 psia)
1 g mol= 22.4 L of ideal gas at STP (0°C and 760 mm Hg)
C, for water*! Btu/lb»?R*1 cal/g °C (at 20°C and 1 atm)
Cp for air*0.26 Btu/lb'"°R*0.26 cal/g °C
viscosity of water, ji= 1 cp = 0.01 g/cm»s (at 20°C and 1 atm)
viscosity of air, ^, = 4.1 x 10'7 lb^sec/ft' = 2 x 10'5 N»»/m« (at 20°C and 1 atm)
density of air= 1.29 kg/ms= 7.49X 10"1 lb/ft8 (at 20°C and 1 atm)
density of water= 1 g/cms = 62.4 lb/fts (at 4°C and 1 atm)
1 cubic foot of air weighs 34.11 g
conversion from ppm to g/ms at STP (273.15 K and 1 atm)
/ a \
ppm x MW [—*—r
** Vgmol/
^m -^^™^«*i»p««i^-^^i^^^^i^i—•™""^^"^"^"i^^^"*p^^"^"^^~—™"™"™
' /293-15K\ I
! \27TT5KJ
dscm _ .,. liters m» /293.15 K\ 1X 10« ppm
C-l
-------
Appendix D
Capital and Operating Cost Estimations
This appendix contains generalized cost data for air pollution control systems
described throughout this manual. These data should be used only as an estimate
to determine systems costs. In some cases the cost of the control device may repre-
sent only a very small portion (<20 percent) of the total installed cost; in other
cases it may represent the total cost. Variations in the total cost can be attributed
to a number of variable factors such as cost of auxiliary equipment, new or retro-
fitted installation, local labor costs, engineering overhead, location and accessibility
of plant site, and type of installation (factory or field assembled).
This cost estimation data, included in this appendix, first appeared hi an EPA
publication (EPA, 1976) and then in a series of articles published in the Journal of
the Air Pollution Control Association (JAPCA, 1978). The reader should refer to
these publications for additional information concerning this subject. These estima-
tions represent equipment costs based on a reference date of December 1977 and
are estimated to be accurate to within ± 20 percent on a component basis, except
where noted (JAPCA, 1978).
Electrostatic Precipitators
The cost of a basic electrostatic precipitator is a function of the collection area
which is specified by the desired collection efficiency. Dust resistivity varies with the
temperature and moisture content of the exhaust gas stream. For proper operation,
the gas may therefore require some type of conditioning prior to entering the ESP.
High moisture contents combined with low operating temperatures will necessitate
insulating the ESP to prevent corrosion of precipitator components. Figure D-l
gives cost data for dry precipitators utilizing mechanical or vibrator rappers. This
figure has curves for both insulated and uninsulated precipitators.
The collection plate area can be estimated using the Deutsch- Anderson equation :
(Eq. 7-3) i,= l-
The equation can be rearranged to give the plate area:
A=-Qln(l-ij)/w
Where: y = collection efficiency
w = migration velocity, ft/sec (m/sec)
A = plate area, ft* (m*)
Q= exhaust gas flow rate, ft'/sec (m'/sec)
D-l
-------
Example:
For dust generated by a coal fired industrial boiler the migration velocity of a
dust particle is approximately 0.25 ft/sec. If a 99.5 percent collection efficiency is
required with a flow rate of 50,000 cfm into the precipitator, the net plate area is
calculated as follows:
A = (- 50,000 cfm)ln(l - 0.995)/(0.25 ft/secX60 sec/mm)
= 17,661 ft2
The price of an uninsulated precipitator (Figure D-l) would be approximately
$138,500.00. For an insulated precipitator the price would be approximately
$212,000.00
10
Uninsulated electrostatic
precipitator price
Px 86,400+ 2.95A
Note: Sources: EPA. 1976;
revised JAPCA. 1978.*
Insulated electrostatic
precipitator price
P« 134,150 + 4.42A
10*
10« 10*
A, net plate area, sq ft
Figure D-l. Dry type electrostatic precipitator purchase prices versus plate area.
Data valid for December 1977.
Venturi Scrubbers
The cost of a venturi scrubber and separator (mist eliminator) depends upon the
volumetric flow rate, operating pressure and materials of construction. The size of
the scrubber, separator, and elbows are determined from the actual inlet gas flow
rate (acfm) and priced accordingly for a scrubber with a plate thickness of 1/8 in.
Additional cost factors are provided for pricing scrubbers with different metal
thicknesses, stainless steel construction, rubber and Fiberglas liners, and manual
and automatic variable throats (Figure D-2). Prices for venturi scrubbers are con-
tained in Figure D-2 through D-6. To estimate the scrubber <»sts using these
curves, use the following steps:
"A. Determine the gas volume entering the venturi section and
read the price for 1/8 in. thick carbon steel scrubber from
Figure D-2. For example, at 100,000 acfm the price is
approximately $39,000.
•All figures in Appendix D (Figures D-l through D-18) are from EPA, 1976;
revised JAPCA, 1978.
D-2
-------
B. Determine the pressure drop across the scrubber required
to obtain the desired efficiency and find the required
metal thickness for the design inlet volume from Figure
D-3. For 100,000 acfm and 30 in. the required metal
thickness is Vi in. plate (always round up to the next stan-
dard plate thickness).
C. From Figure D-4, find the price adjustment factor for the
design inlet volume and the material thickness found in
Step B. For 100,000 acfm and V4 in. plate, the factor is
approximately 1.6. Thus, the carbon steel scrubber price
is now $39,000 x 1.6 = $62,400.
D. If stainless steel construction, rubber or fiberglas lining,
or variable venturi section is to be included, refer to
Figure D-2 and adjust price accordingly. For 304 stainless
steel construction, the adjusted price would be
$62,400 x 2.3 = $143,520. If rubber linings are required,
refer to Figure D-5 to determine total square footage.
E. If an internal gas cooler is to be used, determine the
number of trays that can be fitted into the separator
(from separator height, Figure D-5), and determine the
diameter of each tray (from separator diameter, Figure
D-5). Read price for one tray from Figure 6. For 100,000
acfm the separator diameter is approximately 13.5 ft.
Thus the price for one tray is about $14,800." (JAPCA,
1978)
D-3
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50,000
45,000
40,000
,; 35,000
2
~
g 25,000
so.ooo
a 20,000
fc
15,000
10,000
5000
0
I I I I
Price for one
. P«25.9rD*
5 10 15 20 25 30
D, separator diameter, ft
Figure D-6. Internal gas cooler bubble tray con versus separator diameter.
Data valid for December 1977.
Fabric Filters
' ~ •» *iv-.
The ccst data for fabric filters is based on the net cloth area. The net cloth area is
the total filter area available for on-stream filtration. This would not include the
isolated compartment being cleaned in the case of an intermittently cleaned
baghouse. For intermittently cleaned baghouses requiring an off-line compartment,
the total cloth area must be calculated as the gross cloth area. The gross cloth area
is the net cloth area of the baghouse plus the cloth area for an extra compartment.
The gross cloth area can be calculated for various values of net cloth area from
Table D-l.
D-6
-------
Table D-l. Approximate guide to estimate grow cloth area.
Net cloth area
(ft*)
1-4000
4401-12000
12001-24000
24001-36000
S6001 -48000
48001-60000
60001-72000
72001-84000
84001-96000
96001-108000
108001-152000
1S2001- 180000
180001 on up
Gross cloth area
(ft')
Multiply by 2
1.5
1.25
1.17
1.125
1.11
1.10
1.09
1.08
1.07
1.06
1.05
1.04
The cost for various type baghouses—shaker, reverse air, or pulse jet units—are
listed in Figures D-7 through D-11. These figures include curves of additional
prices for stainless steel construction, insulation, suction baghouses, and standard
or custom designed units. Suction baghouses are negative pressure systems with the
fan located on the clean side of the baghouse. Standard baghouses are predesigned
and built as modules which can be operated singly or combined to form units for
larger capacity. Custom baghouses are designed for a specific application, are
erected in the field, and are used most often for large capacity applications. The
cost of the baghouse units in Figures D-7 through D-11 are for the baghouse only •
(bags are not included). -The costs for bags using various fabrics can be calculated
from Table D-2.
F I
Stainless steel add on «
2740+ 1.12 (net cloth area)
* ^
Note: Baghouse price*$3351 + 1.84
times the net cloth area. All price
add-ons in Figure D-7 through D-11
are calculated in this manner.
^
Insulation add on •
2035 + 0.84 (net cloth area)
| J | j | | | i | i__L__t___L—
"0 2 ^T 6 8 10 12 14 16 18 20
Net cloth area, 1000 sq ft
Figure D-7. Intermittent, pressure, mechanical shaker baghouse prices versus net cloth area.
Data valid for December 1977.
D-7
-------
I
140
120
100
80
60
40
20
0
1 I ' I ' I ' I ' I '
Basic baghouse price*
~ 5370 + 7.6 (net cloth area)
(bags not included)
Stainless itteel add on *
1650 + 5.0 (net cloth area)
i I
I . I
„ Insulation add on •
4910 + 2.4 (net doth area)
I t I i i i "I
46 8 10 12 14 16 18
Net doth area, 1000 sq ft
Figure D-8. Continuous, suction or pressure, pulse jet baghouse prices versus net doth area.
Data valid for December 1977.
1401
I
1 I ' I '/I ' I ' I VI ' I ' I ' 1
ic baghouse price * / ~\
• i
Basic baghouse price _
-6660 + 3.5 (net doth areaj^/'stainless steel'add on =
(bags not included)
m t
WW****«*MB W**r^>* •»**«* V«* —
7340+1.9 (net cloth area)
Insulation add on *
2280+ 1.77 (net doth area)
Suction baghouse add on «
2260 + 0.25 (net cloth area)
•
201
JKr-t-T"] i I i I i I t ! . I . I i I i '
0 10 20 30 40 50 60 70 80 90 100
Net doth area, 1000 sq ft
Figure D-9. Continuous, pressure, mechanical shaker baghouse prices versus net cloth area.
Data valid for December 1977.
D-8
-------
Insulation add on •
11,200 + 1.66 (net cloth area)
steel add on «
79 (net doth area)
10 20 SO 40 50 60 70 80 90 100
Net cloth area, 1000 iq ft
Figure D-10. Continuous, pressure, revene air baghouse prices versus net cloth area.
Data valid for December 1977.
300
—I 1 T 1 1
Baghouse price •
1 11,700 + S.I (net cloth area)
(bags not included)
Stainless steel add on •
63,900+ 1.8 (net cloth area)
Insulation add on •
44,950 +1.66 (net cloth area)
.' ' ' I L
50 100 150 200 250 300 350 400 450
Net fabric area, 1000 tq ft
Figure D-11. Custom pressure or suction baghouse prices versus net cloth area.
Data valid for December 1977.
D-9
-------
Table D-2. Bag prices ($/fts). Data valid for December 1977.
Class
Standard
Custom
Type
Mechanical
< 20,000 ft1
Mechanical
> 20,000 ft1
Pulse jet*
Reverse air
Mechanical
Reverse air
shaker.
shaker,
shaker
Dacron
0
0
0
0
0
0
.36
.31
.57
.31
.21
.21
Orion
0.62
0.57
0.93
0.57
0.31
0.31
Nylon
0.73
0.67
0.67
0.42
0.42
Nomez
1.14
1.04
1.30
1.04
0.62
0.62
Glass
0.47
0.42
0.42
0.26
0.26
Polypropylene
0
0
0
0
0
0
.62
.52
.67
.52
.31
.31
Cotton
0.43
0.38
0.38
0.38
0.38
*For heavy felt, multiply price by 1.5.
Sources: EPA, 1976; revised JAPCA, 1978.
Example:
A baghouse is used to clean the exhaust from an industrial boiler with a flow
rate of 50,000 cfm. The baghouse is a reverse air unit, suction type, using glass
bags and an air-to-cloth ratio of 1.5. The baghouse should include an extra com-
partment for off-line cleaning.
Net cloth area =50.000
1.5
= 33,333 ft'
From Table D-l:
Gross cloth area = 33,333 x 1.125
= 37,500 ft1
The price of the unit is:
Baghouse $138,180 (Figure D-10)
Suction 13,690 (Figure D-10)
Insulation 73,470 (Figure D-10)
Bags 15.750 (Table D-2)
Total 241,090
Cyclones
Cyclones are used to remove larger sized paniculate matter from gas exhaust
streams (usually >20 ftrn diameter panicles). They are not as efficient as scrub-
bers, electrostatic precipitators, or baghouses and consequently are cheaper to
install. The size of a cyclone is usually based on an inlet velocity of approximately
S600 ft/min, and therefore the cost of the cyclone is based on the inlet area size.
The inlet area size (ft1) can be determined from an exhaust gas capacity for various
pressure drops by using Figure D-12. An inlet area can also be estimated from a
critical panicle size, [dp]erft, (panicle size collected with 100 percent efficiency), by
using Figure D-13. Figure D-14 gives cyclone prices for carbon steel construction
D-10
-------
and Figure D-15 gives prices for stainless steel construction. Support, hopper, and
scroll outlet price additions can be detennined from Figures D-16, D-17 and D-18
respectively.
"0 1 2 3 4 5 6 7 8 91011121314
A, collector inlet area, >q ft
Figure D-12. Capacity estimates for cyclones.
D-ll
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4000
3000-
2000 -
1000-
10GA carbon steel
P»U8 +
024 6 8 10 12 14
A, collector inlet area, sq ft
Figure D-17. Cyclone dust hopper prices for carbon and stainless steel construction
versus collector inlet area. Data valid for December 1977.
I I I I I I I I I I I I
CUITC Equations
Steel Thickness Equation
Stainless 3/16" P » 513 + 618A - 12.9A*
Stainless 10 GA P a 462 + 432A - 12.0A1
Stainless 14 GA P« 352+ 307A- 9.1 A1
Carbon 3/16* P« 310 +214A- 4.1 A1
Carbon 10 GA P.290+165A- S.OA1
Carbon 14 GA P.269+I43A- 2.2A1
2 4 6 8 10 12 14
A, collector inlet area, sq ft
Figure D-18. Cyclone scroll outlet prices for carbon and stainless steel construction
versus collector inlet area. Data valid for December 1977.
D-14
-------
Example:
A cyclone is used to clean the exhaust from an industrial process with a flow rate
of 25,000 cfm. The cyclone should be constructed of S/16 in. carbon steel and the
price should include a hopper, support, and a scroll outlet. The pressure drop
across the cyclone is 4 in. HjO.
From Figure D-12: Inlet area = 9.5 ft*
The price of the cyclone:
Cyclone $ 6,600 (Figure D-14)
Support 3,100 (Figure D-16)
Hopper 1,050 (Figure D-17)
Scroll outlet 2.000 (Figure D-18)
Total $12,750
The figures in Appendix D must only be used as an estimate for determining the
cost of a control device. As previously mentioned, these costs are based on 1977
dollars and do not include ancilliary equipment (hoods, fans, ducting, stacks),
engineering overhead, labor erection cost and others. The reader should consult
vendors of air pollution control equipment when an actual price quote is needed. A
listing of air pollution control equipment vendors can be obtained by consulting
the following sources:
Journal of the Air Pollution Control Association. March 1981. 31:205-326.
Pollution Equipment News. November 1980. 13: No. 6.
Pollution Engineering. December 1980.
D-15
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Appendix F
Industry Pollutant Sources and
Typical Control Devices
Industry
Brick manufacturing
Capable refractories
day refractories
Coal-fired boilers
Conical incinerator!
Cotton ginning
Detergent
manufacturing
Feed mills
Ferroalloy plants
a. HC Fe Mn
b. 50% Fe Si
c. HC Fe Cr
Oast manufamjr™»g
Grey iron foundries
Source
. Tunnel kiln
!. Crusher, mill
\. Dryer
4. Periodic kiln
;. Electric arc
!. Crusher, mill
1. Dryer
4. Mold and ihakeout
1. Shuttle kiln
!. Calciner
\ Dryer
4. Crusher, mill
1 . Steam generator
1. Incinerator
1. Incinerator
1. Spray dryer
1. Storage bins
2. Mills/grinders
S. Flash dryer
4. Conveyors
1. Submerged arc
furnace (open)
2. Submerged arc
furnace (closed)
S. Tap fume
1. Regenerative tank
furnace
2. Weight hoppers and
mixen
1. Cupola
2. Electric arc furnace
3. Core oven
4. Shakeout
Control syatem
Scrubber, baghouse
precipitator
Baghouse, scrubber
. Same as I
. Same as 1
. Baghouse, scrubber
. Same as 1
. Same as 1
. Same as 1
. Baghouse, precipi-
tator, scrubber
. Same as 1
. Same as 1
. Baghouse,
precipitator
. Precipitator,
scrubber,
baghouse
1. Scrubber
1. Scrubber
1. Scrubber, baghouse
. Baghouse, scrubber
!. Same as 1
S. Same as 1
4. Same as 1
1. Scrubber, baghouse
precipitator
2. Scrubber
3. Scrubber or
baghouse
1. Baghouse, scrubber
precipitator
2. Same as 1
1. Afterburner-bag -
house for doted
cap; afterburner-
precipitator for
closed cap; scrubbe
2. Baghouse, scrubber
predpitator
3. Afterburner
4. Baghouse
Capture device
. Direct tap
. Canopy hood
. Same as 1
. Same as 1
. Direct tap
:. Canopy hood
. Direct tap
. Canopy hood
. Direct tap
:. Same as 1
i. Same as 1
>. Canopy hood
.. Direct tap
1. Direct tap
1. Direct tap
1. Direct tap
1 . Direct tap
2. Canopy hood
S. Direct tap
4. Canopy hood
1. Full or canopy hooc
2. Direct tap
3. Canopy
1. Direct tap
2. Canopy
1. Direct tap
2. Direct tap, full/side
draft hood
3. Direct tap
4. Full/side draft hooc
Typical gas Dow
design rate
. Combustion air
fan capacity
. 250 fpm hood face
. Same as 1
. Same as 1
. Infiltrated air
:. 250 fpm hood face
. Fan capacity
. Same as 2
. Fan capacity
. Same as 1
. Same a> 1
4. 250 fpm hood face
. Induced draft fan
capacity
1. Combustion air rate
.. Combustion air rate
1 . Fan capacity
1. 250 fpm canopy
hood face velocity
2. Same as 1
S. Air heater flow
rate (dryer)
4. Same as 1
1. 2500-5500 scfm/
MW with scrubber
2. a. 220
b. 180 scfm/MW
c. 190
3. 200 fpm/ft'
1. Fan capacity
2. 200 fpm/ft1
1. Tuyere air
•f infiltrated door an
+ afterburner
second air
2. 2000 fpm/ft1 hood
3. Fan capacity
4. 200-500 cfm/ft1
hood
Typical gas
icuipcnuurc
. 200-600°F
. 70°F mill
. 250 °F
4. Same as 1
. 3000-4000°F
2. 70°F
. 300 °F
4. 150eF
. 150-800°F kiln
2. Same as 1
S. 2508F
4. 70°F
1. 300 °F
1. 400-700T
1. 500-700°F
1. 180-250"F
1. 70°F
2. 70eF
3. 170-250°F
4. 70eF
1. 400-500T open arc
2. 1000-1200°F dosed
arc
3. 150°Fhood
1. 600-850°F furnace
2. 100T misers
1. 1200-2200T
2. -2500eF direct tap
- 400 °F hood
3. 150"F
4. -150°F
F-l
-------
Industry
Iron and steel
(sintering)
Kraft recovery
furnaces
Lime kilns
Municipal incinerator
Petroleum catalytic
cracking
Phosphate rock
crushing
Polyvinyl chloride
production
Pulp and paper
Secondary aluminum
Secondary copper
smelters
Source
I . Sinter machine
a. Sinter bed
b. Ignition fee.
c. Wind boxes
2. a. Sinter crusher
b. Conveyors
c. Feeders
1. Recovery furnace
and direct con-
tact evaporator
1. Vertical kilns
2. Rotary sludge kite
1. Incinerator
1 . Catalyst regenerator
1. Crusher and screens
2. Conveyor
S. Elevators
4. Fluidized bed
calriner *
1. Process equipment
vents
1. Fluidized bed reaaor
1. Reverbatory furnace
2. Electric
induction furnace
S. Crucible furnace
4. Chlorinating station
5. Drost processing
6. Sweating furnace
1. Reverbatory furnace
2. Crucible furnace
3. Cupola and blast
furnaces
4. Converters
5. Electric
induction furnace
Control system
. Precipitator, bag-
house, scrubber
'.. Baghouse, scrubber
. Precipitator,
scrubber
1. Baghouse, scrubber,
precipitator
2. Scrubber,
preopttatoT
1. Scrubber, pretipi-
tator, baghouse.
afterburner
I. Precipitator,
(boiler)-precipi-
tator, scrubber
I. Baghouse, scrubber,
precipitator
2. Same as 1
S. Same as 1
4. Same as 1
1. Adsorbers, after-
burners,
precipitators
1. Scrubber
1 . Scrubber (low
energy) + baghouse
precipitator
2. Same as 1
3. Same as 1
4. Same as 1
5. Same as 1
6. Same as 1
1. Baghouse, scrubber
precipitator
2. Same as 1
3. Same as 1
4. Same as 1
5. Same as 1
Capture device
1 . Down draft hood
2. Canopy hood
1. Direct tap
1. Direct tap
2. Direct tap
1. Direct tap
1. Direct tap
a. High pressure
b. Low pressure
1. Canopy hood
2. Same as 1
3. Same as 1
4. Same as 1
1. Direct tap
1. Direct tap
1. Canopy hood
(hearths), direct tap
2. Same as 1
3. Same as 1
4. Same as 1
5. Same as 1
6. Same as 1
1. Direct tap, canopy
hood, full hood
2. Same as 1
3. Same as 1
4. Same as 1
5. Same as 1
Typical gas flow
design me
I. Based on bed size
2, 250 fpm hood face
1. Primary and sec-
ondary air supply
capacity
1. Combustion air rate
2. Combustion air rate
1. Combustion air far
applicable
1. Regeneration air
rat e-f boiler
combustion air
1. 350 rfm/ft belt
width at speeds
-£00 fpm
503 cfin/ft belt
width at speeds
-200 fpm
2. ICO dm/ft of casing
cross-section
(elevator)
150 cfm/ft of screen
area
3. Combustion air rate
4. Blower rate
1 . Process gas stream
rate
1 . Combustion air rate
1. Max. plume vol.
4 20% (hearths)
2. Infiltrated air
3. Same as 2
4. Siame as 2
5. Same as 2
6. Siame as 2
1. !!00 fpm/ ft* canopy
hood
2. Max. plume vol.
+ 20%
3. 1800 fpm infil-
trated air (full
hood)
4. Based on type
i rapture
!>. Same as 4
Typical gas
temperature
1. 150-400°F sinter
machine
2. 70°F conveyors
1. 350°F
1. 200-1200eF
2. 200-1200T
1. 500-700°F
capacity where
1. 11 00 °F regenerator,
500 °F from boiler
1. 70"F hoods
2. Same as 1
3. Same as 1
4. 600-1500°F calciner
1. -15tolSO°F
1. 600-1500°F
1. 1 600 °F fluxing,
600 "F holding
hearth
2. Based on type
capture
3. Same as 2
4. Same as 2
5. Same as 2
6. Same as 2
1. 2500°F direct tap
2. Based on type
capture
3. Same as 2
4. Same as 2
S. Same as 2
F-2
-------
Sewage sludge
incinerators
Surface coatings—
cpray booths
Portland cement
Basic oxygen
Electric arc furnaces
Phosphate fertilizer
1 . Multiple hearth
incinerator
2. Fluidized bed
incinerator
1. Spray booth
1. Rotary kiln
a. Wet
2. Crushers and
conveyors
5. Dryers
1. Basic oxygen
furnace
2. Charging hood
1. Arc furnace
2. Charging and
tapping
1. Digester vent air
2. Filters
3. Sumps
Control system
. Scrubber
. Same as 1
. Adsorber
. Precipitators.
baghouses
2. Baghouses
1. Precipiiaton,
baghouses
1. Precipitator,
scrubber, baghouse
2. Same as 1
1. Baghouse, scrubber
precipitator
2. Same as 1
1. Scrubber, baghouse
2. Same as 1
S. Same as 1
Capture device
2. Same as 1
I. Canopy hood
1. Direct tap
2. Canopy hoods
S. Direct tap
1. Full-canopy hood
2. Canopy hood
1. Direct tap, full/side
draft hood
2. Canopy hood
1. Hood
2. Same as 1
3. Same as 1
Typical gm» flow
design rate
blower capacity
2. Same as 1
1. 150 fpm/ft' hood.
100 fpm booth
face velocity
1. Combustion air rate
where applicable
2. 250 fpm hood face
S. Same as 1
1 . Function of lance
rate and hood
design— up to
1, 000,000 acfm
2. 5000 fpm hood face
1 . Function of lance
rate and hood
design— up to
200,000 acfm
2. 250 fpm hood face
1. Process stream rate
2. Same as 1
3. Same as 1
temperature
1. 600 to 1500°F
2. Same as 1
1. 70°F
1. 150-850eF kilns
2. 70 °F crushers and
conveyui's
S. 200eF dryers
1. 3500-4000 CF
2. 150-400eF
1. 3500 °F (direct tap)
2. 150CF (canopy)
1. 150°F
2. Same as 1
3. Same as 1
Source: EPA, 1976.
F-S
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-------
Appendix G
Characteristics of Particles
(I W>
Parade diameter.
• w
>
0.00
Technical
definition*
Typical panicles
and
ga» dispenoidi
Methods (or
particle sue
analysis
Types of
pi cleaning
equipment
Terminal
gravitational
settling
[for spheres.
-p. F. I 0!
Soil
In air
atlS-C
1 atm.
0.
01 0.01
urrberi or Interns
Reynolds
Settling veloeitv
envsec
i mill
0001 0.
II
9! 0.
iional Sid OuMfic
•*—
k*-X-rav
M
• n lo-.. ,(
10-
i mini
> J 45* 1
Ml 0
"•»>
)1 0
lion Smem
IM4 *"
M C
«• Tobac
*t Carbon bL
>^Lt-Zmc oxide ft
H* M
CoUoida) ulica
AH lien nuclei"
A
f^Sea u
..gCombuKionB*
nuclei
Ret
H*-
^ uluatemniu
.ffraction* *
Ad*orj
I 1
Clay
tallurgKal dusti ;
r Ammonium chl
"1.1
sulfunc
« Paint pigi
me-tX H»lni
•—
•* Spray i
^ Alkali f
•"
intoiphenc dust •
Ji nuclei tx
Ijan
blood cell diaim
1 — s=d
•« Impini
t mitiuatapc
f Omnfufe"
11
»|< Sat-
10
0 1.01
. 1 1 IIIUI
Spray
s> 1 satuarse san
J*fenUi«r ground Umotorie-tf
»ndea»l< — Cel
concentrator mu
menu ^|
«,,CKle duM,»J
—— Ground tafc
nedmilk •>-
me— H i
« MiUed
|
neni dust 1>
r *
:ilverued coal
Flotation ore
.Plzni^j
spores^
^-PoHem— •>
flour •"•
- a»d
4 Nebulizer drop* »H ^«—
.Lung i^. i^ Pneumatic •>
aging dust jnoiile drop*
in (adulti) 7S«±03^ Humi
f" »<* uevei *"l* —
— *4*
H
Dentation -
Utuaaomct
[very limucd tnduatrial applKatioi
1
-"" ' ta J ^s*
l) *^^
L Crntnfugal aeparaiori
I —11
1 !••} - - Common
Thermal peecuMtation 1 M4
lined only lor sampling) |
•• 10- H
J i
i mm
t S « i* 1
01
y* \tr* Mr* ID** io~* io~j
? 1 * 1 i 1 » I ? 1 I
10"
lit 1 13 »
i mm
t 3 < S* 1
)-' 10"
1 i js i »;
Timm
I J 4 S* 1
1
IT filien »
s»i
0" lO" 10"
10" 10'
1 »?» 1 »?
1 3 < St 1
0 1
1 Beach sand
•Hydraulic ooufc
n
Sieving-
. ^^
X HI."
IJJJJ11I
d»^i Ot»vt
drop.--,*.
~M
)
« FurflMhei avrraft panicle
dmnetrt b%» no we
daKnbution
** SIM diKnbution nuy be
•ktanied by ipecul
calibration
., £— '
Machine tool* (n
1
rmeni aeparatorv
raiori •>
10' 101
. ? 1 ?
1 3
080
o»
Pinidt
Source: Stanford Research Institute Journal, 1961.
G-l
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-------
Appendix References
The American National Metric Council. 1978. Metric Editorial Guide. 3rd ed.
Environmental Protection Agency (EPA). 1976. Capital and Operating Costs of
Selected Air Pollution Control Systems. EPA 450/3-76-014. PB 258, 484.
Neveril, R. B., Price, J. U. and Engdahl, K. L. 1978. Capital and Operating Costs of
Selected Air Pollution Control Systems-l.J. Air Pott. Control Assoc.
28:829-836.
Neveril, R. B., Price, J. U. and Engdahl, K. L. 1978. Capital and Operating Costs of
Selected Air Pollution Control Systems-1./. Air Poll. Control Assoc.
28:963-968.
Bethea, R. M. 1978. Air Pollution Control Technology—an Engineering Analysis
Point of View. New York: Van Nostrand Reinhold Company.
Vandegrift, A. E., Shannon, L. J., Gorman, P. G., Lawless, E. W., Sallee, E. E.
and Reichel, M. 1971. Particulate Pollutant Systems Study I, Mass Emissions.
Report PB 203, 128. Midwest Research Institute. Kansas City, MO.
Stanford Research Institute Journal. 1961. Memo Park, CA: SRI International.
H-l
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TECHNICAL REPORT DATA
(Please read ImsBuctioru on the reverse before completing)
^ «« Be i"
REPORT NO.
EPA 450/2-80-066
2.
I. RECIPIENT'S ACCESSIOI
TITLE AND SUBTITLE
APTI Course 413
Control of Particulate Emissions
Student Manual
REPORT DATE
.1981
PERFORMING ORGANIZATION CODE
AUTHOR(S)
J. PERFORMING Ol
David S. Beachler, James A. Jahnke
PERFORMING ORGANIZATION NAME AND ADDRESS
Northrop Services Inc.
P.O. Box 12313
Research Triangle Park, NC 27709
10. PROGRAM ELEMEI
818A2C
11.C6NTRACTVGRANTN6.
68-02-2374
2. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency
Manpower and Technical Information Branch
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
St-udent Manual
14. SPONSORING AGENCY CODE
5. SUPPLEMENTARY NOTES
EPA Project Officer for this Student Manual is R.
MD-20, Research Triangle Park, NC 27711
6. ABSTRACT
E. Townsend, EPA-ERC,
The Student Manual is to be used in conducting APTI Course 413 "Control of
Particulate Emissions". This manual supplements the course lecture
material, presenting detailed discussions on particulate emission control
equipment. The major topics include: Basic Gas Properties, Particle
Dynamics, Particle Sizing, Settling Chambers, Cyclones, Electrostatic
Precipitators, Fabric Filters, and Wet Collectors. This manual will
assist the reader in evaluating plans for particulate emission control
systems and in conducting plan reviews.
This guide is intended for use in conjunction with the Instructor's Guide
(EPA 450/2-80-068) and the Student Workbook (EPA 450/2-80-067) for APTI
Course 413.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution Control
Particulate Emission Control
Training Manual
Student Manual
13B
51
68A
[18. DISTRIBUTION STATEMENT
Available from National Technical
Information Service, 5285 Port Royal
IB. SECURITY CLASS (ThuRtport)
Unclassified
21. NO. OF PAGES
264
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
CPA Perm 2220-1 (f-73)
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