Modern Electrical Precipitation HARRY J. WHITE Research-Coffrell, Inc., 465 Wesf Union Ave., Bound Brook, N. J.
The main objective of the paper is to present an integrated picture of the basic principles of modern electrical precipitation. Equations are derived for particle collection efficiency, including the important cases of steady and pulsating voltage wave forms, uniform and nonuniform particle size distributions, and Dipe- and duct-type precipitators. Laboratory investigations are described which confirm the validity of the fundamental theory, and engineering applications to industrial electrical precipitation are discussed. Finally, a summary and analysis of the important factors for optimum precipitator performance are included.
A
LTHOUGH the fundamental idea of electrically separating particles from gases was demonstrated on a laboratory scale as early as 1824 (4), the development of adequate equipment and sound technology for coping with large scale industrial gas cleaning problems has required many years of effort and is still in progress. The pioneer work of Lodge (6), Cottrell ( 2 ) , and others in the period from 1880 to 1915 was essentially empirical and intuitive in nature. Fundamental investigation of the process on a limited scale began about 1920 and was carried forward primarily in America and in Germany throughout the thirties. However, greatest activity has occurred since the war coincident with the rapidly expanding fields of application, with fundamental research and development being conducted in a number of laboratories both in this country and abroad. As a result, electrostatic precipitation has been advanced from an essentially empirical process or art t o a moderately well understood technology. The objects of this paper are to present the modern theory of the electrical precipitation process, including some heretofore unpub!ished results; to describe laboratory investigations which confirm the validity of the fundamental theory; and to discuss the engineering application to industrial gas cleaning problems, with emphasis on the factors essential for ensuring optimum performance.
1906 indicated the great superiority of negative corona, and experience since then has confirmed this result. The only exception to the use of negative corona is in the field of cleaning ventilating air, where positive corona is generally used because of its lower ozone generation, as indicated by the work of Penney (7). Corona Discharge. I n electrical precipitation, the corona usually takes the form of a unipolar gas discharge emanating from a wire or series of wires designated as emitting or discharge electrodes, and terminating on pipes or plates designated as collecting electrodes (Figure I). The discharge electrodes are usually round wires of 0.050- to 0.150-inch diameter, or square wires of 0.125- to 0.375-inch side. Collecting pipes range in size from about 6inches in diameter by6feet inlength to lainches in diameter by 15 feet in length. Plate-type precipitators usually have plate spacings from 6 to 12 inches and plate heights to about 20 feet. DISCHARGE
v (A)
Physical Basis of Process In the electrostatic precipitation process, suspended particles in a gas are first electrically charged end then driven t o collecting electrodes under the force of the electrical field. The process is distinguished from all mechanical or filtering processes in that the separating forces are exerted directly on the particles themselves rather than on the gas as a whole. From a practical standpoint, this means that effective separation of particles can be achieved with low power expenditure, with negligible draft loss, and with little or no effect on the gas. Thus, the processis inherently efficient and economical in energy requirements. Calculation of the electric separating forces requires knowledge of the electric charges on the particles and of the electric field to which they are subjected. Many types of dispersoids are naturally charged as a result of their method of formation. However, these natural charges usually are inadequate for effective removal of the particles on a commercial basis, so that in practice i t is necessary to provide positive means for charging the particles. Of the possible charging methods, corona discharge is the most practical and is universally used in electrical precipitation practice. Cottrell’s earliest work on electrostatic precipitation in
932
f
/’
COLLECTING ELECTRODES
m)
Figure 1 . Schematic illustration of electrode systems for electrical precipitators A. 6.
Pipe type Plate or duct type
The high voltage power required for the corona is usually obtained from rectifier sets, using synchronous mechanical switches, electron tubes, or metallic-type rectifier elements. Voltages range from about 25 to 100 kv., while currents range from less than 0.1 ampere for small precipitators of a few thousand cubic feet per minute capacity to 1 or 2 amperes for very l u g e precipitators of the order of 1,000,000 cubic feet per minute capacity. I n the basic mechanism of the negative corona, electrons are released from the wire surface by positive ion impact or by photoelectric emission and travel toward the collecting electrodes. These electrons in moving through the strong field near the wire generate many new electrons and positive ions by molecular impact. The electron swarm thus formed quickly moves to the lower field region where the electrons attach to gas molecules to form negative ions. The positive ions produced by the impact of
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41,No. 5
AIR POLLUTION the electrons on gas molecules are accelerated t o the wire and produce at, least some of the necessary new electrons b y positive ion impact a t t h e wire surface. Further electrons also are produced a t the wire surface by ultraviolet radiation from the corona glow.
but results thus far obtained in general agree with the theoretical values. Basic theoretical equations for the charging of small spherical particles in a unipolar ion field were first developed by Rohmann ( 8 ) , and later in somewhat more detail by Pauthenier and MoreauHanot (6). The problem is essentially one in electrostatics. Gas ions traveling along the field lines intercepted by a particle will strike the particle and impart charge to it b y attachment. This charge in turn produces a repulsive force which alters the field configuration near the particle and thereby reduces the rate of charging. Ultimately, sufficient charge is received by the particle to counteract the external field completely; the electric flux entering the particle is reduced to zero, and charging ceases. The particle-charge equation for the case of a spherical conducting particle is
VOLTAGE
-
Figure 2. Variations in corona current-voltage characteristics for several typical gases
I n the meantime, the negative ions in the lower field region are moving to the collecting electrode and form a dense cloud of unipolar ions which substantially fills the interelectrode space. The space-charge thus formed serves to stabilize the corona discharge and provides an effective charging and precipitation medium for suspended particles present in the gas. Electron attachment coefficients and negative ion mobilities vary greatly from gas t o gas, and this together with gradations which occur in the ionization coefficients accounts for the wide range of corona characteristics met with in different gases (Figure 2). I n electrical precipitation practice, the very steep characteristic of the nitrogen type seldom if ever occurs, but large variations such as represented by the differences between oxygen and methyl chloride are common. Ion space-charge densities in the corona typically range between 107 and 108 ions per cubic centimeter. The great stability of the negative corona may be explained in terms of an anode spark breakdown process. I n such a process, the ion multiplication and generation of breakdown fields occur near the anode. I n the positive corona, the breakdown would start from the wire in a naturally high field region and sparking occurs at relatively low voltage. I n t h e negative corona, on the other hand, the breakdown would start from the lower field region near t h e pipe or plate surfaces and, therefore, requires a higher voltage than in the case of the positive corona. I n electric precipitators the difference in sparking voltage between negative and positive corona may be as high as a factor of two or more. Particle Charging. Theoretical studies (11)indicate that two particle charging mechanisms are present in the corona discharge: 1. Bombardment of the suspended particles b y ions under the force of the electric field in the region between the corona wires and collecting electrodes 2. Attachment of ions to the particles b y ion diffusion I n general, process 1 is of primary importance in electrostatic precipitation, while process 2 is of limited interest and is import a n t only for particles smaller than about 0.2-micron diameter. Laboratory studies of particle charging in the corona ionization field have been made by a number of investigators, and the results agree with theoretical values. Field studies of particle charging in large precipitator installations are more difficult because of inherent lack of control of the experimental factors and t h e difficulty in obtaining small samples which are representative of the large gas flows and weights of dispersoids treated,
May 1955
[electrostatic centimeter-gram-second (system) units]
(1)
where
n Eo
particle charge in electrons electric field to which the particle is exposed a particle radius t time particle is exposed to charging field N o = ion density in charging zone e = charge on electron = 4.8 X electrostatic units K = ion mobility =
= = =
The limiting or saturation charge approached for large values of time t is n = 3Eoaz/e. The term l/rrNoeK may be regarded as the particle-charging time constant. One half the final charge is reached a t a time equal to one time constant, and 91% of the final charge at a time equal t o 10 time constants. Under electrical precipitation conditions the particle-charging time constant is of the order of 10-8 second, so that charging is substantially complete in about second, or in the first few inches of gas flow path in the precipitator field. Values of t h e particle charge under representative precipitator conditions are given in Table I. Small particles of the order of 1 micron in diameter receive charges of the order of several hundred electrons, while large particles of the order of 100 microns in diameter receive charges of the order of several million electrons.
Table 1. Calculated Values of Particle Charge for Representative Precipitator Conditions Charging Fieldn Eo, Kv./Cm. 5 5 5
10 10 10
Particle Diameterb 2a, Microns 1 10 100 1 10
Particle Charge in Electrons 260 2 . 626,000 X 106
520 52,000
5 . 2 x 10% 100 a An average charging field condition for a n industrial precipitator is 5 kv./cm.; the maximum attainable under optimum laboratory conditions is 10 kv./cni. b 1 micron represents fine particle size: 10 microns, medium particle size: and 100 microns, coarse particle size.
The conclusion from the theoretical and experimental studies of particle charging is that all particles become highly charged in a few hundredths of a second or almost immediately on entering the precipitator field. Hence, particle charging under normal conditions is fundamentally an efficient and rapid process. Separation Velocities of Charged Particles. The electric separating forces acting on particles in a precipitator increase rapidly with the strength of the electric field to which the particles are exposed. The electiic field in turn. is determined primarily by electrode geometry and by space-charge effects of the ionic
INDUSTRIAL AND ENGINEERING CHEMISTRY
933
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Q\I
c
1.2
The motion of a charged suspended particle in an electric field is determined basically by the coulomb force which drives the particle and by the frictional retarding force of the gas. For particle size and conditions usually encountered in electrical precipitation, the resistive force for spherical partirles is given by Stokes law
-I
\\
D
=
6 d a w centimeter-gram-second (system) units
(2)
where
D = resistive force on particles = gas viscosity a = particle radius w = particle velocity relative to gas
5
s 0
B ncL
"'"I
0'4
O2
\
\YFr-
Stokes formula under most precipitator conditions is valid for particle sizes between 1 and 40 microns. Particles smaller than 1-micron diameter require a correction by the Stokes-Cunningham formula
i t
0
PIPE WALL
(3) 2
4
6
DISTANCE FROM WIRE
8
IO
where, in addition to previously defined symbols,
- CM.
Figure 3. Electric field distribution in 8inch pipe precipitator in air showing effect of corona current in raising field strength in region near pipe surfuce
A
and pressure The coulomb force, F,, on the particle is given by equation Fc = qEp
current flow between the electrodes. Electric field distribution for a typical pipe precipitator is shown in Figure 3 The field is seen to be relatively constant over most of the space except near the wire, where it is much higher. The field distribution in the case of plate-type precipitators is somewhat similar, although more complicated because of the two-dimensional geometry of the electrode system. I n both cases the average electric field in a practical precipitator as limited by spark-over usually is of the order of 4 to 5 kv. per centimeter. Suspended fine particles in a gas are characterized by very low settling velocities under the force of gravity. Therefore, such particles tend to remain in suspension for long periods of time and are difficult to separate from a gas by mechanical means. B y contrast, the velocities of fine particles in the electric field of a precipitator can be made relatively high, and the particles, therefore, can be efficiently separated from the gas in practical size equipment.
Table II.
934
where q = charge on particle E p = magnitude of collecting field
Equating the driving and resistive forces on the particle using Stokes law and introducing the particle charge previously calculated, g = 3&a2, there results the equation for particle velocity
w = -a E 8 , 2 TI9 [electrostatic centimeter-gram-second (system) units]
(4)
C.
The quantity w is also frequently referred to as the precipitation rate. The formula shows that w is directly proportional t o particle size, to charging field EO, to precipitating field E,, and inversely proportional to the gas viscosity 8. For the cases of nonspherical and dielectric particles, the equation must be multiplied by appropriate factors which will change the numerical values somewhat but not the form of the equation. Typical values of w calculated from the formula for representative precipitator conditions are given in Table 11. I n using the formula the electric fields must be expressed in electrostatic units, the particle radius in centimeters, and the viscosity in poises. The value of w is then obtained in centimeters per second, but for convenience in engineering calculations it is usually expressed in feet per second. Table I1 indicates that calculated values of w for particle sizes most important in electricalprecipitation-Le., about 0.5 to 20 microns in diameter-range from approximately 0.1 to 2.4 feet per second.
C.
Efficiency Equations
Calculated Particle Separation Velocities w for Representative Precipitator Conditions
Particle Charging CollectDiameter, Field, ing Field, Gas . 2a, Eo E Viscosity, w, Microns KvJkm. Kv.&m. Poises Ft./Seo. Comments 1 5 4 1.8x 10-4 0 . 3 2 Fine particles Av. electric field Normal air 1 5 4 3.0 X 10-4 0.19 Fine particles Av. electric field Hot gas, about 350' 10 5 4 3 . 0 X 10-4 1 . 9 4 Medium particles Av. electric field Hot gas, about 350' 100 5 4 3 . 0 X 10-4 1 9 . 4 Coarse particles Av. electric field Hot gas, about 350' 10 10 8 3.0 X 10-4 7 . 7 6 Medium particles Strong electric field Hot gas, about 350' 0.5 5 4 3.0 x 1 0 - 4 0.10 Very fine perticlea Av. electnc field Hot gas 20 6 4 2 . 0 x 10-4 3.88 Medium coarse particles Av. electric field Hot gas
= mean-free-path between coIlisions of gas molecules % 10-5 cm. for air a t normal temperature and pressure = a constant = 0.86 for air a t normal temperature
C. C.
Experiments by Anderson (1) in 1919 showed that the collection efficiency of a precipitator is an exponential function of the time the gas is exposed to the electric field, A few years later Deutsch (3) derived a theoretical formula in general agreement with the Anderson results although somewhat different in form. The Deutsch formula is limited in scope to uniform particle sizes and also rests on somewhat arbitrary assumptions. I n this paper the formula for collection efficiency is derived on a more general basis, and experimental investigations are described which are in agreement with the formula.
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
Vol. 47, No. 5
AIR POLLUTION The efficiency equation may be developed b y considering the case of a single particle following through a precipitation zone. For this purpose i t is important to note that the separation velocities of fine particles in the electric field are substantially smaller than the precipitator gas velocities. Therefore, the path of a particle through the precipitator is determined primarily b y gas flow and only secondarily by the electric field. The type of gas flow in the precipitator is also important. With streamline gas flow i t is easy to show that 100% collection efficiency may be achieved in a precipitator of a certain length. I n this cme all particles would follow smooth paths toward the collecting surfaces and the time of collection is easily calculated. However, in a practical precipitator gas flow is always turbulent because of the flow conditions and electric wind effects. Hence, the actual path of a suspended particle is erratic and may be described in terms of an average forward velocity on which is superposed random to-and-fro motions in all three dimensions. At any given instant in its passage through the precipitator a particle is as likely to be found in one small region of t h e cross section as another. To calculate the probability A P of a particle's being captured in a small time interval At, the particle must be within a distance 6 = wAt from the collecting surface to be caught. The chance of the particle's being within this distance is the ratio of the area SS to the total cross-sectional area A,, where S = periphery of collecting electrode surface. Hence,
diameter even for relatively dense concentrations. Particle reentrainment effects and disturbances caused by high resistivity particles also are neglected. Equation 6, then, is the basic precipitator efficiency equation and has the exponential form shown in Figure 4. q=1--&=1--6
-_Av"
T h e equation shows that precipitator efficiency is determined by three basic quantities 1. Efficiency increases with collecting electrode area A 2. Efficiency increases with the drift velocity w produced by the electric forces acting on the particles 3. Efficiency decreases with gas flow rate V Drift velocity w is determined by Equation 4 and increases directly with particle diameter and with the product of the charging and collecting fields E&',. It is important to observe that w is particularly sensitive to electrical conditions in the predipitator, and small increases in operating voltages produce relatively large changes in collection efficiency.
ae
'
O
O
i
2. 0
z
w
u
LL
LL
W
T h e chance of the particle escaping capture in this time is
a
t-
SwAt
l - A P = l - -
a a
Hc
If the time required for the particle to pass through the precipitator is t, and there are n time intervals of Ai, the chance of the particle's escaping capture in these n intervals is
EXPONENT
A yw
Figure 4.
Variation of precipitator efficiency with exponent ( A / V ) w in Equation 6
If At becomes infinitesimal, n will approach infinity in such a way that nAt = t. Then the limit is
This is the chance of the particle's escaping capture in passing through the entire length of the precipitator. Denote this chance b y Q, so that
-S w t Ac
& = E
If the length of t h e precipitator is L and the gas velocity is u, then L = ut. Further, the rate of gas flow through the precipitator is V = A,v, and the total collecting surface A is A = LS.
Hence, the relation St
LS
Case of Fluctuating Voltage Wave Form. The preceding derivation of the efficiency equation is based on steady or direct current voltage. In actual practice most industrial precipitators use pulsating voltage wave forms of the half-wave or full-wave types. This causes w to become a function of time. The charges imparted to the particles under these conditions will be governed by the peak value of t h e voltage. Since the frequency of the voltage normally is 60 cycles per second, and the treatment time of the gas in the precipitator is usually several seconds, the particles still receive their maximum charge almost immediately on entering the precipitator. However, the collecting field acting on t h e charged particles will vary in time in accordance with the fluctuations of the rectified voltage. T h e efficiency equation for pulsating voltages is identical with that previously derived except that the average value of drift velocity, which may be denoted b y w,,,must be used
A
x=x=v and
where & = e
A --
vw
If now, instead of a single particle, a large number of particles having a common value of w are considered, Q represents t h e fraction escaping capture or the fractional loss of the precipitator. This result is based on the assumption of negligible interaction between t h e suspended particles, which follows because the average distance between particles is many times the particle May 1955
T
= period of rectified voltage wave
( E p ) a v= time-average value of collecting field Nonuniform Particle Size Distribution. When considering a gas containing a range of particle sizes, the efficiency may be calculated b y integration methods. For discrete particle sizes this leads to a simple sum, while in the more general case of continuous distributions i t leads to an integral. Only the latter case occurs in normal practice and is therefore the only one treated here.
INDUSTRIAL AND ENGINEERING CHEMISTRY
935
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT Let ? ( a ) ,be the distribution function; that is, ?(a,)Aa represents the fraction by weight of the particles having radii between a, and a, Aa. Then the precipitator loss Q, for particles in this size range is
+
=
Qr
- $ w (ar) E
Y(ar)Aar
where w(a,) is a function of particle size. Summing for all size ranges, one gets
ai
T
For convenience in carrying through this procedure the particle size function Z ( a ) may be plotted on log probability paper, combined with the factor k , and integrated numerically according to Equation 12. Unfortunately, this formula is rather cumbersome for use in actual practice and, therefore, is limited primarily to research and development work. Over the range of precipitator efficiency of interest in most practical applications, t h e simpler Equation 6 may be used with sufficient accuracy for most engineering applications.
= ai
Passing to limit,
where
c
r(a)da
=
Figure 5. Schematic diagram for pipe- and duct-type precipitators showing dimensions
1 b y definition
al = smallest particle size a2 = largest particle size Equation 8 is the required result. It involves the particle size distribution function ? ( a ) , which in general must be determined by experiment. If the particles are approximately of uniform size, the distribution function becomes very large in that region and zero elsewhere, so t h a t the equation reduces to formula 5, valid for uniform particle size. Equation 8 may be expressed in a more specific form by noting that
Comparison of Pipe and Duct Precipitators. The fundamental Equation 6 may be applied specifically to both pipe- and ducttype precipitators (Figure 5). For a pipe of radius R and length L operating at a gas velocity u
A = collecting area = 2 r R L V = gas flow rate
= xR2v
Hence
A
v =2rRL a = 2Ls
and precipitator efficiency is given by where Correspondingly, for a duct precipitator of length L, height h, and wire-to-plate spacing s and is a constant for any given precipitator operating under a given set of conditions, Using this substitution, me may write Equation 8 as y(a)a-lca da
Q =
Integrating Equation 10 by parts and introducing the function Z ( a ) leads to
+k
sa:p
Z ( a ) e-ka da
(12)
Since the cumulative particle size function Z ( a ) may be determined directly by experiment, Equation 12 permits calculation of precipitator efficiency to be carried out in any given case b y numerical or graphical integration.
936
2Lh
L
v=ashv=sv
and
(10)
The problem of calculating precipitator loss is, therefore, reduced to that of determining the particle size distribution function $a). There are a number of experimental methods for determining particle size distributions. However, these methods usually do not give the function ?(a)directly, but rather its cumulative or integral form. For this reason it is convenient to use the integral of the distribution function which may be denoted by Z(a).
Q = e - b
A
These formulas lead to the basic conclusion that for a given collection efficiency a pipe precipitator may be operated a t twice the gas velocity as a duct precipitator of equal length and electrode spacing. This results from t h e presence of the factor of 2 in t h e exponent of the equation for the pipe precipitator. The fundamental precipitation Equation 6 shows that t h e efficiency of a duct operating at a given gas flow V i s independent of duct spacing, other factors being equal. On the other hand, the efficiency of a pipe a t a given gas flow will increase with pipe diameter. Further, for a given efficiency, the gas flow which can be treated in a duct is independent of duct spacing, but for a pipe increases directly with pipe diameter. Experimental Verification of Basic Efficiency Equation
A program of carefully controlled experiments was carried o u t by the author to test the validity of the basic equation on a Bornewhat broader basis than had been done b y earlier investigators. I n this work a single-stage pipe-type precipitator was used with interchangeable pipes of 4-,6-, 9-, and 12-inch diameters. T h e pipe length was 8 feet in all cases and t h e discharge wire had a 0.014-inch diameter. T h e dispersoid was produced by an oil fume generator which used an electric heater to vaporize the oil,
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 5
AIR POLLUTION and'a controlled air blast to cool and disperse the vapor. A high voltage tube rectifier set was used to supply direct current voltages up to 100 kv. for the corona discharge. T h e experimental apparatus is shown in schematic form in Figure 6. I n these experiments the collecting surface area A was varied b y changing pipe diameter, gas flow V b y changing gas velocity through the precipitator, and particle drift velocity w b y changing the applied voltage, which varies Eo and E,. However, the charging field EOand the collecting or precipitating field E, cannot be measured readily. The precipitation field, Ep, which is the field a t the pipe surface, can be calculated with reasonable accuracy from the formula E, = (electrostatic units)
\
\\'\
y;, \
\ ' \ \ \ \
0 .I
0 In In
3 L
c
n
df
a
k
\
'
0.01 -
6,
,".
where
i
'\
-%\ \
= =
corona current per centimeter of wire mobility of gas ions
0
T h e charging field Eo, on the other hand, is much less sharply defined because particles will receive their maximum charge at points where the field is highest. However, not all particles are subjected to the maximum field. I n practice, a reasonable approximation is to use the average value of the field across the pipes as representing the charging field. The effect of varying gas flow V for each pipe size used is shown in Figure 7 . The results are plotted on semilog graph paper i n terms of the treatment time t which is proportional to 1 I n agreement with the theory, the points for each of the pipes
0.001
4
II.o e $$ \
\\
I I.5\
\
2.0 1
\
:
\,
5
Figure 7. Experimental curves for precipitator loss as a function of treatment time and pipe diameter Single-pipe precipitator, 8 ft. long, d.c. voltage, corona adjusted to 5 ma. for all pipe sizes
are seen to lie quite accurately on straight lines passing through or close to the point t = 0, Q = 1. I n obtaining these results, the voltage for each pipe size was adjusted to give a corona current of 5 ma. This corresponds to a collecting field at the pipe surfaces of 4.1kv. per centimeter. The effect of varying collecting surface area A is shown in Figure 8. Again, in accordance with theory, the experimental points lie closely on straight lines passing through the origin. As further predicted by theory, the gas flow for each pipe size at 90% efficiency is approximately double that for 99% efficiency. Variations in particle drift velocity w were obtained by varying the applied voltage. From Equation 4,w should be proportional t o the product Eo X E, when particle size and gas viscosity are
f
0.5 I\$ \I
held constant.
Figure 9 shows the results obtained by plotting
v
1
the experimental values of w calculated from w = 2 log - against
Q
the product E a p , The curves are seen to be linear over most of the range of EoE,, although curved near the origin. I n view of t h e uncertainty in the actual values of the charging field EO, these results must be regarded as illustrative only, but do show that w is roughly proportional to EoEpas the theory indicates. I n summary, these experiments are in good agreement with the theoretical precipitation equation as far as the effects of collecting surface area A and gas flow rate V are concerned, and are in rough agreement for the effect of particle migration velocity w. However, the actual values of the latter factor are in themselves somewhat uncertain because of the uncertainty of the effective particle charging field. The usefulness of the equation as a basis for precipitator design and for analysis of precipitator performance may be considered established not only by the experiments described, but also by a wide range of field experience.
TO HIGH POTENTIAL SOURCE
OUTLET
Practical Application
GAS ME7'ER
,
I
I
I ELECTROSTATI G FILTER
I I
I I
!+-CORONA
WIRE
DAMPER
I I I4 I
'
--
GENERATOR
t
BLOWER
Figure 6.
May 1955
Experimental apparatus for testing validity of theoretical precipitation equation
The fundamental precipitation equations apply with reasonable accuracy under controlled laboratory conditions. However, calculated values' of the precipitation rate w based on performance measurements for representative field precipitators give values two or three times those actually obtained in practice. The discrepancy is explained by the presence of uneven gas flow, re-entrainment of collected particles, and other factors which cannot he included in the idealized theory. Therefore, in engineering design it is practical t o use Equation 6 with modified values of These modified or pracprecipitation rate w tical values of w are determined from actual
INDUSTRIAL AND ENGINEERING CHEMISTRY
937
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT 400
1.4
I
I
I
I
I
3 00
r
5 I g 200 A LL v)
4 (3
100
Figure 9. Increase in particle separation velocity w with increasing precipitator fields
PIPE DIAMETER-INCHES Figure 8. Experimental curves for gas-treating capacity' of a single-pipe precipitator as a function of pipe diameter Pipe length 8 ff., d.c. voltage, constant electrical fields, charging tleld = 5.6 kv./cm., precipitation field = 4.1 kv./cm.
field experience with a wide variety of precipitator installations, and may be used as a basis for new designs or applications. Where experience is limited, designs are usually established by means of preliminary experimental. and pilot plant tests. I n essence, the theoretical equation is used as the basis for analyzing field precipitator performance and for calculating new designs where previous experience exists for determining practical values of the precipitation constant, w. The situation is similar to that existing in many other fields of technology where theoretical equations are used with modified practical values of some of the constants in order to take account of departures from idealized conditions never achieved under practical conditions. The general range of practical values for precipitation rate w for most industrial precipitators is approximately 0.1 to 1 foot per second depending on particle and gas characteristics and on operating conditions. I n addition to performance analysis and design applications, the basic formulas are valuable in advancing precipitation technology. Knowledge of the relative importance of the fundamental factors has indicated the fruitful phases for research investigations, and many important advances have resulted from these studies. Factors for Optimum Precipitator Performance
Precipitator performance is determined basically by particle velocity Equation 4 and precipitation efficiency Equation 6, and by the degree t o which the subsidiary conditions of uniform gas flow, retention of collected particles, and uniform corona discharge are satisfied. The object of this section is to summarize and correlate the effect of these factors. Electrical Conditions. Electrical conditions are of major importance in determining precipitator performance. This is evident from Equation 4, which shows that precipitation rate w 938
€0 = charging field in kv./cm. Ep = collecting field in kv./cm.
increases both with the charging field Eo and the collecting field E,. Thus u1 might be expected to increase with the square of the applied voltage. Actually, because of space-charge effects and the nonlinear nature of precipitator fields, the increase in w is, in many cases, closer to the third or fourth power of the applied voltage. For this reason it is essential for best performance that voltages be maintained a t the highest possible levels. Factors which tend to limit operating voltage and, therefore, to reduce performance are: 1. Electrode misalignment. This may be kept small by proper mechanical design and good construction practices. 2. High dust resistivity. This has been discussed in a number of papers (9, IO) and is important for certain types of dusts and fumes. I n some cases conditioning of the gases by moisture or by trace quantities of certain chemicals is effective in improving operation. Other measures include use of half-wave voltage, increased sectionalization of the discharge electrodes, and conservative choice of precipitator size.
Usually optimum precipitator collection efficiency is obtained when the voltage is set high enough t o cause some sparking, typically in the range of 10 to 100 sparks per minute per high tension section. The use of automatic voltage control systems to maintain sparking rates a t optimum levels has been found valuable for precipitator installations where gas and particle characteristics vary with time due t o variations in process conditions or raw materials used. Voltage Wave Forms. Pure direct current voltage is seldom used in energizing electric precipitators for industrial applications because of its inherent instability. Instead, a choice is made between full-wave and half-wave rectified alternating current voltage. Figure 10 shows typical wave forms for half-wave and full-wave rectified voltages taken in the field on a fly ash precipitator. With half-wave voltage, precipitator sparks are usually extinguished in less than one cycle and electrical operation is relatively smooth. With full-wave voltage there is much greater tendency for precipitator sparks to develop into high current arcs, and electrical operation tends to be rougher than for halfwave. Large precipitators, high dust concentration, and high resistivity particles usually indicate the use of half-wave voltage.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 5
load some portions and underload other portions, with resultant loss in performance. Excessive turbulence, eddies, swirls, and pulsations also lower precipitator efficiency. Therefore, in order to retain the advantages of low draft loss and to ensure maximum collection efficiency, provisions for good gas flow should be made in the original inlet flue design. Retention of Collected Material. I n deriving the fundamental equations i t is assumed that all particles collected are retained on the electrodes and successfully transferred to the hoppers without losses due to re-entrainment. However, these conditions are only partially satisfied for precipitators collecting dry particles. For these precipitators, which comprise the majority of applications, it is essential to make provision for limiting particle re-entrainment. Measures which have proved effective in practice include
A
c-, I
SEC*
4 4'KV.
+I
6 Figure 10. Typical wave forms for half-wave and full-wave rectified voltages taken in field on fly-ash precipitator A. B.
Half-wave Full-wave
Pulse energization (12)is a recent development and represents a basically new system of supplying high voltage power to precipitators. This system comprises a high voltage, high power pulse generator capable of supplying high voltage pulses of the brder of 100 microseconds in duration a t a frequency of several hundred pulses per second. T h e pulse output may be commutated to as many as four or six precipitator sections. Advantages of the pulse method include higher peak voltages, inherent current-limiting action on spark surges, and higher precipitator collection efficiency. Corona Discharge. Best operation requires uniform corona discharge throughout the precipitator at the highest possible voltage. This, in turn, requires that the discharge electrodes be kept relatively free of dust deposits, and that deposits on the collecting electrodes be limited to reasonable amounts of not more than about 10 to 15% of the electrode spacing. For drytype precipitators these requirements imply an efficient rapping Bystem which will maintain the necessary cleanliness of the electrodes without causing excessive dust loss due to particle reentrainment. Gas Flow. A precipitator ie inherently low draft loss equipment and will not correct major unbalances or disturbances in gas flow. Uneven gas flow through a precipitator will tend to over-
May 1955
1. Limiting gas velocities in treatment zones to approximately 10 feet per second 2. Use of shielded-type collecting electrodes 3. Use of properly designed rapper systems which may be adjusted in intensity and frequency to suit individual conditions 4. Use of adequate and properly designed electrical energization equipment 5. Adequate baffling:of the precipitator to prevent gas sneakage and eddies 6. Maintenance of adequate hopper-emptying schedules Conclusion
Basic laboratory and field investigations, particularly those in recent years, have gone far in increasing our knowledge and understanding of the phenomena and laws which govern the electrostatic precipitation process. Much of this newer knowledge has been translated into useful engineering form and is being applied in producing improved and more efficient electrostatic gas-cleaning equipment. However, important problems await complete solution, and new problems continually occur, so t h a t research and development activity in this field continues a t an accelerated pace. literature Cited (1) Anderson, Evald, Chemical Engineers' Handbook, John H. Perry, editor-in-chief, 1st ed., p. 1548, MoGraw-Hill Book Co., New York, 1935. (2) Cottrell, F. G., U. S. Patent 895,729 (1908). (3) Deutsch, W., Ann. Phpsik, 62, 343 (1922). (4) Hohlfeld, M., Kustner Archiv Naturlehre, 2, 205-6 (1824). ( 5 ) Lodge. 0. J.. J . SOC.Chem. Ind.. 5 . 572 (1886). -,(Sj Pauthenier, M . M., and Moreau-Hanot, M., J . phys. radium, Ser. 7, 3, 590-613 (1932). (7) Penney, G. W., Elec. Eng., 56, 159-63 (January 1937). (8) Rohmann. H., 2 . Physik, 17, 253-65 (1923). (9) Schmidt, W. A , , IND.ENG.CIIEM.,41, 2428 (1949). (IO) White, H. J., Air Repair, 3 , No. 2, 79-87 (November 1953). (11) White, H. J., Trune. Am. I m t . Elec. Engrs., 70 (pt. II), 1186-91 (1951). (12) Ibid., 71 (pt. I), 326-30 (Xovember 1952).
.~
~
for review October 6,1954. RECEIVED
ACCEPTED March 9, 1955.
INDUSTRIAL AND ENGINEERING CHEMISTRY
939
