AN INVESTIGATION OF AEROSOL COAGULATION BY MEANS OF AIR-JET GENERATED ULTRASONIC VIBRATIONS

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AN INVESTIGATION OF AEROSOL COAGULATION MEANS OF AIR-JET GENERATED ULTRASONIC VIBRATIONS

a thesis submitted to the faculty of Purdue University

by Robert Marshall Fryar

In Partial Fulfillment of the Requirements of the Degree

of

Doctor of Philosopy

August, 1950

P U R D U E LW’I V E K S I T Y

TH IS

by

IS T O

C E R T IFY

T H A T T11 K T I I K S I S

I ' l i K I ’A H K I )

I N D K K

MY

S l'I'E H V IS lO X

R obert Marshall Fryar

An Investigation of Aerosol Coagulation

e n t it l e d

by Me ans of Air-Jet Generated Ultrasonic V ibrations

C O M 1*1*1 K S

W ITH

TH E

I'M Y K IiS IT Y

A M ) I S A I T ’K O Y K I ) BY M E A S

FOR

T H E

I)EC; R E E

HKIM

F IL F IL I.IX O

l.A T IO X S

T H IS

ON

O K A D I'A T IO X

P A K T O F T H K

TH K SES

H K Q ITK K M EX TS

O F

Doctor of Philosophy

P

H

k o fk m n o r

e a d

o f

in

O

h a b o e

s c h o o l

o h

D

o f

T

h k h is

e p a r t m e n t

T O T H E I . I B R A R I A V : -----

SC T H IS

TH ESIS

IS

NOT TO

B E R E O A If I) E l ) A S

C O N FID EN TIA L.

FROFERHOH

B B O I8 T B A R

FORM

lO —7 . * 7 —' ! «

IX

C lU R C iE

ACKNOWLEDGMENT

The writer is grateful for the helpful advice of Dr. R. C. Binder, at whose suggestion and under whose guidance this work was carried out.

ABSTRACT

The present day methods of air cleaning are described and compared, and it is concluded the combination of the ultrasonic coagulator and the cyclone precipitator has ad­ vantages not found in any other method or combination of methods of air cleaning equipment.

A study is made of the air-jet ultrasonic generator and it is found that when the generator is operated at op­ timum conditions, which are determined, it has certain ad­ vantages over the

siren-type generator in that it produces

waves of comparable intensities with higher efficiency, lower weight per unit of power output, greater simplicity of design, and smaller cost of manufacture.

A study of the reflection characteristics of the ultra­ sonic beam reveals that at frequencies as low as 12.5 kc/sec the beam is reflected from solid objects in the same manner as light and with negligible loss.

This allows the air-jet

generator’s spherical output to be converted into more anoli cable plane waves for use in the coagulation process.

A particle size study reviews the methods available to measure the particle sizes of smokes and dusts, lists the particle sizes of common industrial dusts, and makes

a study of the frequency ranges for various oarticle size ranges.

In addition, an analysis of the growth of particle

sizes as a result of the action of ultrasonic waves is made and it is demonstrated that small size smoke particles may be sufficiently Increased in size so as to be efficiently removed by the cyclone precipitator.

TABLE OF CONTENTS

Page I N T R O D U C T I O N ...............................................

1

O B J E C T I V E ..................................................

2

BRIEF REVIEW OF THE L I T E R A T U R E ............................

4

TYPES OF AEROSOL P R E C I P I T A T O R S ............................

4

Settling Chambers .......................................

6

Bag C o l l e c t o r s .........................................

7

Scrubbing Devices .......................................

9

Electrostatio Precipitators ...........................

11

C y c l o n e s ................................................ 18 Venturi Scrubber..........................................24 Cottrell Precipitator and Cyclone .....................

26

Ultrasonic Coagulator ..................................

26

The Siren-Type Ultrasonic Generator ...................

30

The Ultrasonic Coagulator and the C y c l o n e ............. 37 CHARACTERISTICS OF THE AIR-JET G E N E R A T O R .................. 41 Instrumentation

............................» •

42

The Experimental Air-J et Generator..................

46

Jet Segment Length....................................... 49 Generator Acoustical Intensity.........................

53

Generator F r e q u e n c y ..................................... 68 Intensity Distribution and Power O u t p u t ................83 Generator Efficiency..................................... 93 ULTRASONIC COAGULATION CHAMBER DESIGN...................... 99

Page Intensity Distribution of the Reflected Wave

. . . .

109

PARTICLE SIZE S T U D Y ................................... 114 Parti ole Size Measurement M e t h o d s .................. 115 The Particle Sizes of Industrial Smokes and

Dusts • . 1E8

The Particle Size Range of Present A i r Cleaning A p p a r a t u s ....................................... 133 Frequency Range for Various Particle Sizes. . . . . . Particle Size G r o w t h ...............................147 C O N C L U S I O N S ............................................161 B I B L I O G R A P H Y ............................................166 V I T A .................................................... 171

136

TABLE OF FIGURES

Figure

Page

1.

Terminal Velocities for Various Particle Sizes and Various Specific Gravities...............7

2.

Schematic Diagram of a Commercial Multibag Air Filter........................................... 8

3.

Schematic Diagram of a Non-Mechanical Dust Washer .............................10

4.

Schematic Diagram of a Cottrell Precipitator . . .

5.

Effect of Moisture and Temperature on the Apparent Resistivity of Cement D u s t .............. 16

6.

Schematic Diagram of a Commercial Cyclone Tube . . .20

7.

Schematic Diagram of a Multltube C y c l o n e ............ 21

8.

Collection Efficiency for a Commercial Multltube C y c l o n e ................................. 23

9.

Schematic Diagram of a Venturi S c r u b b e r ............ 25

10.

Schematic Diagram of a Laboratory Model Siren G e n e r a t o r ..........................................33

11.

Sketch of a Commercial Siren-Type Ultrasonic G e n e r a t o r ..........................................35

12.

Schematic Diagram of a Commercial Ultrasonic Coagulator-Cyclone Separator...................... 38

13.

Schematic Diagram of the Wave A n a l y z e r ...............44

14.

The Experimental Air-Jet G e n e r a t o r ................... 47

15.

Values for "d" corresponding to the actual Segment Length for various Pressures............. 52

16.

Variation of Intensity with Cup Volume for Different Distances, 40 p s i g .................... 55

17.

Variation of Intensity with Cup Volume for Different Distances, 40 p s i g .................... 56

18.

Variation of Intensity with Cup Volume f o r Different Distances, 50 p s i g .................... 57

.13

Page

Figure 19.

Variation of Intensity with Cup Volume for Different Distances, 60 p s i g .................... 58

20.

Variation of Intensity with Cup Volume for Different Distances, 70 p s i g .................... 59

21.

Volumes giving Maximum Intensities for Various Cup Positions in the Jet Segment L e n g t h ......... 62

22.

Variation of Maximum Intensity with Cup Position for Various Pressures .............

..64

23.

Variation of Intensity with Cup Position for Constant Cup Volume and for Various P r e s s u r e s ..................................... 65

24.

Variation of Maximum Intensity with Nozzle Pressure for Various Cup Positions with Optimum Cup Volumes.............................67

25.

Variation of Frequency with Cup Volume for Various Distances, 40 psig....................... .70

26.

Variation of Frequenoy with Cup Volume for Various Distances, 40 psig........................ 71

27.

Variation of Frequenoy with Cup Volume for Various Distances, 50 psig........................ 72

28.

Variation of Frequency with Cup Volume for Various Distances, 60 psig........................ 73

29.

Variation of Frequency with Cup Volume for Various Distances, 70 psig........................ 74

30.

Looatlon of Frequenoy DIscontunlty Terminal Points for Various Cup Volumes. ................76

31.

Variation of Frequency with Cup Position for Various Cup V o l u m e s ............................... 78

32.

Variation of Frequenoy with Cup Volume at Zero Jet Segment Length .................... 80

33.

Typical Intensity-Volume and Frequency—Volume Curves ...................... 82

34.

Spherical Intensity Survey N o . l ..................... 86

35.

Spherical Intensity Survey No, 2 ............

....87

Figure

Page

56.

Spherical Intensity Survey No. 3 .....................88

37.

Spherical Intensity Survey No. 4 .................

38.

Spherical Intensity Survey No. 5 .....................90

39.

Spherical Intensity Survey No. 6 .....................91

40.

Defining Diagram for Spherical Survey showing Microphone in Position to measure I .............. 92

.89

41.Defining Diagram, Power Input Calculations . . . .

.95

42.

Maximum Air-Jet Generator Efficiency as a .................... 98 Function of Nozzle Pressure

43.

Schematic Arrangement of Air-Jet Generator in the Parabolic Reflector and Reflector E x t e n s i o n ................................. 108

44.

Intensity Survey across Coagulation Chamber, Section I . ................................... 110

45.

Intensity Survey across Coagulation Chamber, Section II ..............................

112

46.

Intensity Survey across Coagulation Chamber, Section I I I ...................................... 113

47.

Schematic Diagram of Roller’s Air Elutriation Apparatus .................................. 117

48.

Welgner’s Apparatus for measuring Sedimentation by Pressure Change............

120

49.

Balance Method Apparatus for Determining the Sedimentation Rate................................ 122

50.

Graphloal Representation of the Particle Sizes of Industrial Smokes.........................

. 135

51.

Plot of Frequenoy against Particle Diameter for Amplitude ratio of 1/2....................... 140

52.

Plot of Amplitude ratio of Various Particle vibrating in air for a frequency of 20 kc/sec . 141

53.

Plot of the Periodic Force Ratio of the Sound Radiation Pressure......................... 145

Figure

Page

54.

Particle Diameter Growth for Ammonium Chloride S m o k e ................................ • 154

55.

Particle Diameter Growth for Ammonium ................................. 155 Chloride Smoke

56.

Particle Diameter Growth for Titanium Tetrachloride Smoke ............................

158

Particle Diameter Growth for Titanium Tetrachloride Smoke ............................

159

57.

TABLE OF TABLES

Table

Page

1.

Measured Segment Lengths of Free Air Jet from a 0.125 inch N o z z l e ............................... 50

2.

Tabulated Results and Diagram of Equipment for Single Plane Wall Reflection Test .............

104

3.

Tabulated Results and Diagram of Equipment for Two Wall Reflection T e s t ....................... 106

4.

Table of Some Common Industrial Smokes and Dusts and their Particle Size R a n g e ............ 131

5.

Tabulated Results of a Typical Particle Size Count for Titanium Tetrachloride Smoke.......... 132

6.

Approximate Particle Size Range of Present Air Cleaning Apparatus ............................

134

INTRODUCTION

For several hundred years the problem of Injury to plant and animal life resulting from air pollution has been recognized both in the laboratory and in legislative halls. But it was not until the turn of the present century, that the seriousness of atmospheric contamination in the United States was recognized, and steps taken to prevent and com­ bat it.

A high degree of Industrialization, with the accompaning large outputs of smokes, fogs, and dusts has given impetus to the development of several types of air cleaners based upon gravitational, centrifugal inertia, filtration, scrubbing, and electrostatic effects, or upon a combina­ tion of these effects. successful.

None of these tvoes are completely

Recent Investigations tend to indicate that

there is another type of air cleaner which is of nractical use.

It has been shown that when high-intensity highfrequency sonic vibrations are Impressed upon a smoke or fog, the particles coagulate and increase to such a size that It is possible to precipitate them raoidly by siranle means.

This process of ultrasonic coagulation appears to

possess certain advantages not found In the other

?

jipitation processes.

The utility of ultrasonic smoke coagulation has been demonstrated by Industrial installations employing a sirentvie generator.

There is another tyne of ultrasonic genera­

tor, the so-called "Hartmaan" sir-jet generator, which arrears to hove certain advantages of simrlicity of construction, rr'^edness of design, end low initial and maintenance costs. Th-se qualities make the air-jet

generator seem desirable.

However, there is a real lack of basic information otout the characteristics of tie air-jet generator and the application of this generator to smoke and fog coagulation.

OBJECTIVE

In view of the apparent practicability of aerosol coa­ gulation by means of air-jet generated ultrasonic vibrations; the lack of basic information about the operating charact­ eristics of air— jet generators and the application of this generator to the coagulation nrocess, the objective of this thesis will be to get data and Information useful in design­ ing equipment for coagulating aerosols by reans of an air— jet ultrasonic generator.

3 This objective will be accomplished byr (1) A review of present tvoes of nrecioitators to deterrir.t. ♦be limitations, advantages, and disadvantages of each, and to determine if any of the present types have desirable features Wtaich might be incorporated wlth ultrasonic equip­ ment to produce a superior type precipitator.

(?)

A study of the operating characteristics of the

air-jet generator to obtain the relationships between inten­ sity, frequency, nozzle pressure, the cylindrical cun dimen­ and position of the cup in the Jet; and to determine

sions,

the intensity distribution around the generator, the power output, and the efficiency of the generator.

(3)

A study of the behavior of the ultrasonic beam

to obtain information useful in designing optimum chambers for ultrasonic coagulation.

(4)

A particle size study to determine the methods

available for measuring particle sizes, the range of parti­ cle sizes of industrial smokes and dusts, the ability of ultrasonic vibrations to produce successful coagulation in this range, and a study of the growth of the average oorticle size as a result of the action of the ultrasonic asves

.

BRIEF REVIEW OF THE LITERATURE

* In an earlier investigation (1)

a review was made

of the literature existing at the beginning of 1948.

Since that time some work of an engineering nature has appeared in the literature covering the work done in the field of aerosol coagulation by means of ultrasonic vibrations.

A rather comprehensive survey of the aerosol

precipitation problem in generel was presented in a sym­ posium (2) in November, 1949.

Monson (3) presented a

detailed discussion of the instrumentation required for the analyzing of ultrasonic waves and some work on the ultrasonic coagulation of titanium tetrachloride smoke.

Because of the nature of this thesis, since there are several topics Involved, the pertaining literature will be cited in the various sections.

TYPES OF AEROSOL PRECIPITATORS

It is interesting to note that in 1941 Anderson (4) lis ted o n l y f i v e * Numbers

different

In p a r e n t h e s e s

types

refer

of p r actical

to B i b l i o g r a p h y

systems

5 foi separating particles front gas^s.

chambers, gravitational method;

settling

fi *ration method; (:5 J

Cottrell

liter at ur e

(a)

(b) bag collectors,

(c) scrubbing devices, washing method;

precipitators,

electrostatic m e t h o d ;

centrifugal inertia method.

cyclones,

an d

(e)

The m o r e r e c e n t

introduces a new type o f air cleaner, t h e u l t r a ­

sonic coagulator;

the development o f the combination o f the

scrubb ing method and the Cottrell

These five were:

cyclone, the c o m b i n a t i o n o f the

process and the cyclone, a n d t h o c o m b i n a t i o n of

t'-.e ultrasonic

coagulator and the c y c l o n e .

Although commercial units of all the above listed tyres of eir cleaners,

including an ultrasonic coagulator powered

by a siren-t^me ultrasonic generator, are available, none of those units are completely satisfactory.

The suitability

of application of each of the different types or combina­ tion of

different types, primarily depends on the size

of the particles present

In the air which is to be cleaned.

It is possible to have four different types of parti­ cles suspended in air to form different types of aerosols. These types of aerosols aret

(a) smoke or fume, composed

of solid particles generally considered to have a particle size range of from 0.01

to 3 microns; (b) mist or fog, com­

posed of liquid particles with a particle size range corresD o n d i n g to that of smoke;

(c) dusts, composed of solid

^articles with a particle size range of from 3 to 50 microns

t:.r; (d) spray, composed of liquid particles with a particle range corresponding to that of dust.

Settling Chambers

Settling chambers utilize the fact that gravitational force will cause smokes and dusts to settle out of suspen­ sion.

The time required for various particle sizes to settle

e given distance can be calculated if the particles are ssruited to be spheres and the flow is laminar.

The termin­

al velocity of the particles can be found from Stokes* lav; as

where d is the particle diameter, g is the gravitational d e c e l e r a t i o n , i s the particle d e n s i t y , ^ is the density and

is the viscosity of the medium.

An insight to the size of settling chambers required can he obtained if the terminal velocities of various parti­ cle diameters and for various specific gravities of the particles are plotted as is shown on Figure 1.

From this

figure, it is seen that the terminal velocity of a parti­ cle with a diameter of 50 microns and with a specific grav­ ity of 1.0 is approximately 180 inches per minute.

This

7

50 o

5 ID 50 100 Terminal Velocity - in/min Fig. 1

? 00

Terminal Velocities for Various particle Sizes and Various Specific Gravities

::>eens that for smokes, which not only have much smaller parti­ cle diameters, but also may have lower srecific gravities; the size of any settling chamber would be prohibitive.

It then appears that settling chambers are of practi­ cal Importance and are reasonably efficient only when the size of the aerosol particles is large and the amount of aerosol to be cleaned is relatively small.

Bag Collectors

Bag collectors and bag houses, which utilize the

Clean air Outlet pipe

Dust laden air inlet pipe

LIU

Dust Hopper

Fig. 2

Schematic Diagram of a Commercial Multibag Air Filter

filtration effect, can be built with very high efficiencies for nearly all sized particles, costs and operating costs

but not only are the first

high, but this type collector is

limited by the temperature and nature of the aerosol. conanercial multibag air filter is shown in ifigure 2.

A In the

filter type air cleaner, the gas temperature must be below 180 F for cotton filters and below 225 F for wool filters, °:.d even with low temperatures,

under the action of certain

^cid or base type aerosols, the filters disintegrate very

rapidly*

Use of spun glass type filters has been suggested,

but at present the cost of this type filter is prohibi­

In addition to the above mentioned limitations, the

tive.

dust removal problem increases the operating

costs of this

type cleaner•

It may be concluded that bag or filter type cleaners are satisfactory air cleaners for low temperature, type aerosols,

neutral

but are expensive not only in first costs,

but also in maintenance costs.

Scrubbing Devices

Scrubbing devices operate on the principle that water particles will coalesce with solid or liquid particles of an aerosol and the dust and smoke particles will be carried

out as sludge in the washing water.

Scrubbing devices may

be in the form of scrubbing towers, non-mechenlcal washers, cr mechanical washers.

Scrubbing tow e r s are generally of simple onen construc­ tion.

The wash water enters at the top of the tower and

cascades over a series of baffles.

The dust laden air

enters at the bottom and is intermingled with the water by the baffles.

Since the water-air mixing is relatively

inefficient, this type of device is only of practical

10

Clean Air Outlet

Fntrainment Separator

Water Inlet

Dust laden Air Inlet*"

Water and Sludge Outlet Fig. 3

Schematic Diagram of a Non-Mechanical Dust Washer

importance with coarse dusts.

Non—mechanical washers are constructed similarly to towers, hut are generally on a much smaller scale.

Figure

3 shows a sketch of a commercial non—mechanical washer. The water—air mixing efficiency is increased by improving the design of the baffles,

so that larger quantities of the

aerosol can be handled with less wash water end less space. This device is relatively efficient

small size particles,

but rust be operated at a temperature below the boiling point of water.

Mechanical washers are used to increase the cleansing efficiency for smaller sized particles (down to around e.f microns) by increasing the number of spray unrticles, dccrc sing the spray particle size and by providing larger surfaces of contact between spray and dust. cal washers, derend upon

But mechani­

especially of the disintegrator tyne, which high speeds and fairly close clearances to

accomplish the violent mixing required for small parti­ cles, have both high first and operating costs and have a restricted

field of application since high dust loading,

ebrasive and corrosive particles are undesirable.

Electrostatic Precipitators

One of the earliest recognized phenomena leading to aerosol Drecipitation was the electrostatic attraction between charged aerosol parti cles and oppositely charged electrodes.

Beceria, an Italian, recorded observations

on electrical discharges in 1771, but it was not until after the time of Faraday, Maxwell, and Lord Kelvin that scientific observations of practical importance were made. The development of the electrostatic precipitator is due primarily to the work of Cottrell (5, 6, 7). trical precipitator, which bears his name,

His elec­

is still the

basis of design for this tyne of equipment.

The theory, development, and or8ctlce of electrostatic

precipitators Cottrell

Here been covered by many investigators since

made the greatest single contribution when he

rp:csmized that the electrical method requires two simul­ taneous or successive physical phenomena. steps

The necessary

are: (a) the electrical charging of the aerosol

oarticle, and (b) the migration of the charged particle to an electrode of opposite polarity caused by a suffi­ ciently strong electrostatic field.

Electrical precipi­

tators are constructed in several forms, but basically they may be classed as being of the electrode-in-cylinder or the electrode-and-plete types.

In the common electrode-

in-cylinder type it is the usual practice to have an arrange­ ment whereby the aerosol passes around a negative electrode with a high angle of curvsture (fine wire), and within a positively charged electrode with a low angle of curva­ ture (pipe) such as is shown schematically on Figure 4. The process depends upon high-voltage unidirectional current which simultaneously charges and plates out the aerosol particles.

Xt is therefore necessary to have some means

of producing direct current such as a high-tension trans­ former and a direct— current rectifier if it is desired to use commercial low-voltage current.

Deutsch (8) was able to show that the collection efficiency of such a precipitator could be expressed as

High Voltage D C Line

Clean Air

AC-DC Rectifier

Outlet

Collecting Electrode

Weight Dust Laden Air Inlet Precipitate collects on ^Electrode and drops into Hopper

Fig. 4

Schematic Diagram of a Cottrell Precipitator

wr»r° v is the drift velocity of the particle toward the positive electrode, L Is the length of the collector, R is :>, iistance between electrodes, and v_ is the velocity D of ":ie aerosol through the precipitator. By assuming HtukeS* law, it is possible to show that the drift velo­ city will be given by

3 rrft d

where E is the voltage gradient, Q is the charge on the particle, d is the particle diameter, and ft is the viscos itv of the medium.

By combining these two relationships,

the efficiency is found to be

-E Q L 1 - e

This relationship shows that the collection efficiency for this precipitator depends upon the mechanical design of the precipitator, the velocity of the aerosol, and urnn the electrical characteristics of the aerosol particles.

Since the fundamental phenomena of electrical precipi­ tation is based on mechanical movement of the electrically charged particles by the force existent in the electrostatic field, it is essential that the particles be charged.

This

15 charging, the Q, term, under certain conditions, although app­ arently not limited by the smallness of the particles, is diificult to accomplish. aerosol,

The apnarent resistivity of the

which serves as an index of the resistance of the

particles to take a charge, is shown by Schmidt (9) to depend uron the nature of the aerosol, the temperature of the aero­ sol, and the percentage of moisture in the aerosol.

For

example, on Figure 5 is plotted the apparent resistivity of Portland cement for various temperatures and for various percentages of moisture.

It will be noted that the apparent

resistivity increases and then decreases rapidly, reaching a maximum somewhere from 290 F to 400 F depending upon the moisture content, and that for increasing percentages of moisture, the apparent resistivity decreases.

This experi­

mental work of Schmidt coincides with field tests on indus­ trial installations, which show that a marked decrease in collection efficiency is suffered as the aerosol temperature drops from 700 F to 400 F.

In order to Improve the collection efficiency use can he made of the decrease in apparent resistivity with increase of moisture.

It is believed the presence of the mist

increases the conductivity around and through the aerosol particles.

Wolcott (10) showed that water mist and free

acid were good conditioning agents for some aerosols, and Schmidt (11) states that ammonia and triethylamine are affective for certain other type aerosols.

While fog and

16

10 10

13

Percentage Moisture -1.? /5.0

^ < 10.0

>>

/ / 20.0

5 ioM

10

10 Temperature —

F

. of Moisture and Temperature on the Apparent Resistivity of Cement Dust

water spray are probably more widely used because of their availability and cheapness, It has been found that oil mist is particularly effective (12) as a conditioning agent in special oases.

It Is essential for all type aerosols that

the electrloal resistivity be maintained below a certain critical value in order to prevent arcing, the so-called "back discharge.**

In addition to the effect on the efficiency of

17 ci a c t i o n ,

the velocity of the aerosol throughout the precipi­

tator must be maintained wlth^ln limits.

That is, most pre­

cipitated materials adhere to the collecting electrode up to p certain critical velocity.

Above this point the materiel

begins to shift and be torn loose and can be resuspended and carried out In the aerosol.

The critloal velocity for carbon

black is less than two feet per second, and the critical velocity for tightly adhering liquid films is less than 20 feet per second for smooth surfaces.

It Is possible to

increase the critical velocities by introducing electrodes with collection pockets, but these Dockets increase the pre­ cipitate removal problem.

In all cases there is a maximum

value for the velocity for any given precipitator, and this maximum value must be maintained below the critical value for a given aerosol.

In addition to the effect of the charge and the gas velocity, examination of the collection efficiency equation reveals that the precipitator voltage* the length and radius of the collector, the viscosity of the aerosol medium, and the diameter of the particles all effect the efficiency.

Interpretation of the equations shows that for

each specific set of conditions —

that is, type of aerosol,

Particle size, temperature of the aerosol —

there is a

definite optimum nrecipitator design and definite opera­ ting conditions which will give maximum collection efficien­ cy*

Any change in any of the factors can greatly reduce

18 t e f f i c i e n c y of precioitation.

Summing up, the advantage of the Cottrell method Is th*-.t this method appears aerosol particles, particles.

effective on nearly any sized

and particularly effective on small sized

Its disadvantages are: high initial and opera­

ting costs, minimum efficiency in the temperature usua ll y encountered in smoke

ing many types of aerosols,

range

stacks, necessity of condition­ critical optimum operating

conditions, and difficulty of removing the p r e c i p i t a t e a f t e r it has

collected on the collecting °lectrode.

Cyclones

If dust laden air is subjected to high angular accel­ eration, the centrifugal forces cause the dust particles, because of their higher density, to be thrown outward. This angular acceleration

can be quite easily caused by

having the aerosol follow arcxmd the inside of a cylinder. Such a device Is called a cyclone.

The relationships be­

tween the size and density of the dust particles and the necessary

angle of c u r v a t u r e of the cyclone, and t h e p r e s ­

sure drop

through the cyclone have been extensively p r e s e n t e d

in the literature.

Many investigators (13, 14, 15, 16) have investigated

t r.n theoretically and experimentally, removal

the problem of lust

by the cyclone and have arrived at the following

relationship for the minimum diameter particle which will be separated In a cyclone as being 1 d

=

T______ 9 /> D_____ "1 1 [lT

tt

t’

.yj

)J

wh e r e d is the diameter of the particle,

Is the viscos­

ity of the aerosol medium, D is the width of the gas stream, N

is the number of revolutions made b y the gas

st re a m In the cyclone,

the gas particles, and

v^ is the tangential velocity of 0 ~^o ) is "the difference in

densities of the particles and the medium.

Investigations

(17, 18, 19) of the pressure drop through the cyclone have shown that the pressure drop, P, measured In terms of the inlet velocity heads is a function of the outlet pipe diameter D_ to the cyclone diameter D ratio, the 3 2 produot of the angular width of the cyclone B timeB the inlet duct height H to the square of the diameter of the cyclone, and Reynolds number.

The relationship which

holds for vaneless cyclones can be expressed as

20

Clean A5

utlet

Dust Laden Air Inlet

Dust Laden Air Inlet

Dust Outlet Fig. 6

Schematic Diagram of a Commercial Cyclone Tube

Sheppard and Lapple (19) made an experimental study of

the

losses by altering the dimensions of the cyclone and meas­ uring the velocity distribution within the cyclone and were able to evaluate the above functional equation in terms of a semi-empirical equation.

Gardner (17) evaluated the

functional relationship in terms of certain design constants. While the above mentioned relationships are useful in design­ ing proper proportioned cyclones, much of the fundamental theory of the device is unknown.

The present commercial cyclones are very efficient on moderate and large size dust particles when operating under

Dust Laden Air Inlet

C l e a n Air Outlet

■Dust Hopper

Fig. 7

Schematic Diagram of a Multitube Cyclone

design conditions. lones.

Basically there are two designs for cyc­

The cyclone may either consist of one large chamber,

or of a series of small chambers. the large chamber design

The main disadvantage of

is that changes in t h e flow rate

can cause a decrease In the collection efficiency.

That is,

while the large chamber may be very effective in cleansing toe aerosol when

the flow rate through the cyclone corres­

ponds to the design flow rate, changing the actual flow "ate can greatly effect the efficiency of collection of the ^articles.

The disadvantage of t h e single chamber design

h j been

circumvented by r e p l a c i n g t h e l a r g e c y c l o n e with,

a 'eries of small

cyclones as is s h o w n in F i g u r e s 6 and 7.

this a r r a n g e m e n t , t h e n u m b e r o f c y c l o n e s in s e r v i c e can be

adjusted

to

correspond

to

the

flow r a t e o f t h e d u s t -

laden air.

High temperature and abrasive—type aerosols tend to cause excessive wear In metal cyclones.

However, a recent

improvement, the introduction of the fused quartz cyclone, appears to have eliminated some of these difficulties.

Figure 8 shows the efficiency of a commercial multitube cyclone as reported by Pegg (20).

This curve is

based on the particle count per unit volume, and according to the manufacturer,

is practically Independent of the

density of the particles.

An analysis of this curve shows

that the cyclone type precipitator is 1 0 0 percent

effi­

cient on particles with a size of 22 microns or larger, but

inefficient for particles In the size range of smokes

or mists.

The advantages of the cyclone type separator are its low Initial and low maintenance costs, low resistance to the flow of aerosol, high efficiency on larger-sized parti­ cles, and ease of removing the collected particles.

Its

disadvantage Is its very low efficiency on small— sized dust and smoke— sized particles.

23

80

60

Collection

Efficiency

- percent

100 j-

40

20

10

20

Particle Diameter

Pig. 8

30 microns

Collection Efficiency for a Commercial Multltube Cyclone

Venturi Scrubber

The venturi scrubber is a combination of the scrubbing •ini cyclone methods.

It is based upon the principle

tV>t high velocity gas streams will atomize liquids into small particles and these small particles will coalesce wi^h the aerosol particles.

The aerosol, which is to be

cleaned, is put through a venturi.

In the throat section

of the venturi, the high velocity aerosol collides with a

curtain of water and the water is violently accelerated

and disrupted.

In the diverging section of the venturi,

the aerosol is decelerated and the mist formed wets the aerosol particles.

This action results In such an Increase

in the aerosol particle size that even sub-micron sized aerosol particles may be removed by the cyclone after wetting.

Figure 9 shows the schematic arrangement of

the venturi scrubber.

The efficiency of cleansing of this type of apparatus depends upon the velocity of the aerosol in the venturi throat, the ratio of the liquid absorbed per unit volume of aerosol, and the distribution of the liquid in the throat.

Although a well designed venturi will give about

^5 percent pressure recovery to the aerosol, the Intro­ duction of the curtain of water of zero velocity in the throat requires that this water be accelerated In the diverging section and this causes a considerable pressure

25

Clean Air Outlet

Cyclone Make-up Water ~

Water Inlet Dust Laden Air Inlet

Sludge

Settling Tank^ Pig. 9

Schematic Diagram of* a Venturi Scrubber

drop which must be overcome by external power input.

Jones (21) reports this method, while relatively new, gives indications of practicability.

However, this type

of installation requires relatively large-sized

equipment,

low temperatures; and the pressure drop encountered in the venturi requires considerable power addition which results in Increased operating costs.

The advantage of this system

appears to be its ability to handle small particle sizes. T h e disadvantages are erature

its large-sized equipment, low temp­

range, and high operating costs.

26 Cottrell Precipitator and Cyclone

The combination of the centrifugal and electrostatic r recipitation

methods is an attempt to utilize the advan­

tages of each and nullify the disadvantages of each method to effect a higher cleansing efficiency.

It is common practice to precede the Cottrell precipi­ tator with a cyclone so that the cyclone can remove the larger sized particles and thus reduce the dust removal problem in the electrostatic precipitator.

A commer­

cial manufacturerof such a combination air cleanser reports successful results, though no factual data, on such an arrangement •

While this ocmtination system has advantages over either of the separate systems, in that the particle size range is increased, it nevertheless possesses the disadvantages associated with the electrostatic method.

Ultrasonic Coagulator

The exact mechanism of coagulation by action of ultra­ sonic vibrations is extremely complicated and has not been fully explained at the present time.

However, It Is believed

the rapid coagulation of the aerosol particles occurs as

. result of three different types of forces acting.

The

res are (a) the covibration of the aerosol particles on i'l'erent amplitudes,

caused by the ani sodi spersi ty — the

v-rifltion of the aerosol particle size and/or the variation the density of the aerosol particles— of the particles, which

increases the possibility of collision and agglomera­

tion, (b) the Bernoulli forces, or the hydrodynamic forces of attraction and repulsion, which are caused by the con­ striction of the medium between adjacent particles and results in a difference in static pressure, and which enures, when the spheres are properly

aligned, the partl-

les to agglomerate, and (c) the sound radiation pressure which Is caused by the difference In momentum on onposite sides of the particle.

The radiation nressure Is zero at

the nodes and antinodes of the ultrasonic wave, has a m a x ­ imum value midway between the node and antinode, and Is of opposite sign on different sides of the antinodes (see Figure 53).

Thus the sound radiation nressure tends to

concentrate the aerosol particles In the antinodal plane and thus causes agglomeration of the particles.

St. Clair

(22) presents the most comprehensive analysis of the mech­ anism of coagulation available to hate.

St. Clair performed laboratory experiments on ammon­ ium chloride smoke, which has a particle size range of from 0.3 to 0.5 microns, using an electromagnetic ultrasonic generator.

He was able to coagulate the particles so that

t're final diameters of these particles were of the order magnitude of 100 microns.

He

be obtained with ultrasonic ,vatt per square centimeter,

reported that coagulation intensities of about one

but rapid coagulation requires

intensities of several watts per square centimeter. and Neumann

Denser

(23) state that noticeable coagulation in a

reasonable time occurs with sonic intensities of 1 4 0 deci­ bels

(0.1 w / s q cm), but upwards of 1 5 0 decibels ( 1 . 0 w/sa cm)

are required for reasonable industrial practice.

Danser and Neumann also point out that temperature, with in reasonable limits, does not Impose restrictions on the use of

sonic coagulation.

That is, there is little

noticeable change In the coagulation characteristics as the temperature is varied from 0 F to 1000 F. the coagulation cal

In addition,

effect appears independent o f the electri­

charge of the aerosol particles.

Monson (24), St. Clair (25), Danser and Neumann (26), and Fryar (27) all found that the coagulation of the aero­ sol depended upon the concentration of the aerosol.

That

is, these investigators found that the coagulation effect varied with the concentration; the higher the concentra­ tion, the greater the coagulation effect.

The optimum

time of exposure appears to be somewhere between 3 and 15 seconds.

29 Monson (28)

found the rate of coagulation Increased

ver.- rapidly for titanium tetrachloride sroke down to -: .at 10 kilocycles per second.

He recommends a frequency

r *r.ge of from 8 to 1ft kc/sec for optimum coagulation. Danser and Neumann (29)

state that experimental results

indicate that the best frequency range may extend down to 1,0 kc/sec for particles with diameters of 10 microns, and that somewhat higher frequencies are required for parti­ cles as small as 0.01 microns.

St. Clair experimented in

the frequency range of from 4.9 to 8.8 kc/sec with ammon­ ium chloride smoke and found no essential difference in the coagulation results as the frequency was varied.

He

does not state any particularly desirable frequency range.

From the literature available,

it appears that this

system of coagulation to increase the particle size so thut the particles may be easily removed by the centrifugal method, has certain very definite advantages over any of the present systems of combinations of systems.

The ultra­

sonic coagulator has the advantages of independence of aerosol temperature, of the electrical charge of the aerosol particles, and of composition, that is, whether the aerosol is composed of solid or liquid particles.

Tt has

the further advantage in that coagulation can be effectively accomplished with a reasonable time of treatment— tire of exposure to the vibrations—— which allows large quant"! ties of aerosol to be treated with reasonably sized equipment.

30

The Siren—Type Ultrasonic Generator

The first necessity for successful ultrasonic coa­ gulation is the availability of a generator capable of producing high intensity vibrations.

Although there are

several methods of ultrasonic generation available based piezoelectric crystal, magnetostrlctive, and electro— magnetic effects; it appears there are only two practical venerators —

because of the large amounts of power required—

available for commercial application.

The two generators

are the sir«n-type generator and the air-jet generator. The oresent day industrial installations all utilize the siren—type generator.

The generation of ultrasonic vibrations in the sirentype generator depends upon the chopping off of slugs of compressed air by a high speed rotor.

That is, the high

frequency interruption of the flow of air through a port will cause pressure waves to be formed, and these pressure or sonic waves will have the some frequency as the flow interruntion•

Therefore, the essential components of any

siren—type generator are a source of air, a rotor which interrupts the air flow at the frequency of sound desired, and ports in a stator through which the air escapes.

Since all previous siren-type acoustical generators had efficiencies of one or two percent, Jones (30)

31 un rjrtook a study of the theoretical considerations for t-r proper design of siren-tyne generators. ■n s primarily interested in sonic generators,

Although he and his work

to the development of a generator with a frequency o..put of 500 cycles per second, the princinles he set forth nay be applied to the design of a siren for ultrasonic wave production.

His development which utilized equivalent

electrical circuits, is based, for the most part, on the customary linear equations for sound propagation, but the non-linear nature of the ports is taken into account. According to Jones* development, the maximum efficiency of wave production is obtained when (a) the time of open­ ing and closing of the ports is small in comparison to the period of one cycle,

(b) the excess pressure of the

sir supply is small in comparison to the absolute pressure of the atmosphere,

and (c) the orifice of thegenerator

is provided with a

suitable acoustical horn.

Jones verified

the accuracy of his theory and design

criteria in the development of a fifty

horsepower siren

'■hich operated at 500 cycles per second and with an “ffici^ncy of 72 percent.

Allen and Rudnick (31), utilizing the theoretical work of Jones, developed a laboratory model siren-type generator •••'rich has a useful frequency range of from 3 to 33 kc/sec. T his siren was driven by a 2/3 horsepower variable speed

oric motor and is supplied with 60 cubic feet per minute ir at variable pressure by an external compressor.

The

:or is provided with 100 conically shaped ports (0.094 x

.108 inches)

equally spaced on a 6 inch diameter circle.

"T'e rotor has 100 teeth, which are slightly wider than the orts to assure complete closing of the ports.

The rotor

iu mounted integrally with the motor as is shown in Figure 1 .

The adjacent surfaces of the rotor and stator are

-round and lapped so that the clearance is around 0.001 of an inch.

A wooden exponential horn is mounted on the

siren to increase its efficiency.

Tests run on this siren indicated a relatively uni­ form power output of 125 watts and an efficiency of 30 percent over the frequency range of the siren.

The siren

acts as a ring source with a directivity pattern closely resembling the expected theoretical results with the maximum intensity in the central lobe end with a value exceeding 160 decibels.

Tn order to increase the acoustical output, Allen und Fudnick made certein structural changes which increased the air flow through the siren.

Although little data

ere reported, it is stated that the redesigned siren has a oower output of over 2 kilowatts and has an efficiency of about 20 percent. Commercial siren-type generators (32) are available

Adjusting Screw \

Wooden Exponential Horn

Outlet Port Stator Rotor

Air Deflecting Vane Compressed T Air J Tnlet

Driving Motor

I Pig. 10

Schematic Diagram of a Laboratory Model Siren Generator

h've been demonstrated as being suitable for Indus•rlal aerosol coagulation.

The commercial units are

!: ilar in design to the laboratory model of Allen and 7 ';:nick.

Figure 11 shows a schematic diagram of a commer­

cial ultrasonic generator. V;out pounds

The generator diagrammed used

T25 cubic feet per minute of air compressed to 8 per square inch gage and has an output acoustical

intensity of about 150 decibels.

It is reported (33) that

the commercial generators convert from 40 to 60 percent of the energy in the compressed gas into useful ultrasonic vi> rations.

The siren-type ultrasonic generator is suitable for use in industrial

aerosol coagulating systems and has the

advantage in that the intensity of the output wave can be controlled by regulating the flow of air

through the norts

f;nd the frequency may be independently regulated by adjust­ ing the speed of the rotor driving motor.

This flexibility

of control allows a wide variation of intensities for any given frequency.

However, the siren generator has certain

disadvantages, among which is the disadvantage of the high speed rotor and the disadvantage of the high cost of manufacture as compared with the air— jet generator.

Assuming the stator and rotor are equipped with 100 oorts and 100 teeth, the rotor must make 10 revolutions per kilocycle per second, or

the rotor must turn at an

35

Driving Motor for Kotor

Storage Chamber for Compressed Air \

Kotor Bearing Stator

Vibration Porta Exponential Horn

Fig. 11

Kotor Rotor Bearing

Sketch of a Conmercial Siren-Type Ultrasonic Generator

4 uilsr speed of 10,000 revolutions per minute to oroduce

itput wave with a frequency of 30 kc/sec. > -riting speeds,

stresses become important.

At high Tt is there—

f re necessary to use high tensile strength material for the rutor, and to reduce the inertia force, it is desirable v' u 3 e a rotor shape which will give a uniform stress distri­ bution.

While the desired shape of such a constant stress

rotor is easily obtainable from any good text on mechanics of materials,

and can be expressed as

where t is the thickness, r is the radius, and A and B are constants depending upon the material used.

This parti­

cular exponential shape is difficult to machine, and there­ fore expensive to manufacture. speeds, the

dynamic

Tn addition, at these high

balance of the rotor is difficult to

obtain and requires that the machining of the rotor teeth be exact.

Besides the accurate and expensive machining re­

quired, the rotor must be equipped with sufficient high speed bearing surfaces to hold the rotor in extremely accur­ ate alignment at all times since the operating principle of the generator requires that the air flow through the rorts be completely interrupted by the rotor if any rea~ nable value of efficiency is desired.

Allen end Budnick report their leboratory irodel of » - .^iren-type generator weighs about 55 pounds without •

p^ponentiel horn.

i:. this thesis,

As will be demonstrated leter

the experimental air-jet generator which

•ei