NATIONAL BUREAU OF STANDARDS REPORT

4157

PERFORMANCE OF THE FARR ROTONAMIC AIR CLEANER

by

C, W, Coblentz

T. W. Watson

P, R, Achenbach

Report to

Engineering and Development Branch

Office of the Chief of Transportation

Department of the Army

Washington, D. C,

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1003-20-4838 June 16, 1955 4157

PERFORMANCE OF THE FARR ROTONAMIC AIR CLEANER

by

C. W, Coblentz

T, W, Watson

P, R, Achenbach

Heating and Air Conditioning Section

Building Technology Division

To

Engineering and Development Branch

Office of the Chief of Transportation

Department of the Army

Washington, D. C

<^b|>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publloitlon, rei unleu perinliilon li 25|D.Ci Such per

cally prepared If tl

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

In part, la prohibited

indardiiWaahlngton

porthai beenepecIA*

iport for III own uie,

PERFORMANCE OF THE FARR ROTONAMIC AIR CLEANER

by

C, Wo Coblentz, T* Wo Watson and P. R. Achenbach

Abstract

The air cleaning efficiency and the pressure drop of a specimen Rotonamic air

cleaner manufactured by the Farr Company

were determined using a half-and-half mixture of coarse and fine test dust furnished

by the Ao C, Spark Plug Division of General

Motors, Corp, The specimen was comprised

of nine cyclones and was tested over a range of air flow rate from ^0 to 280 cfm.

Efficiency measurements were made simultaneously by the gravimetric and dust-spot

methods. Different numerical values of

efficiency were obtained by the two methods

and the reason for the difference is explained. The gravimetric efficiency ranged

from about 70 percent at an air flow rate

of 50 cfm to about 9I4 percent at an air

flow rate of 280 cfm. The pressure drop

across the air cleaner ranged from 0.63

in W. G. to 16.9 in W, G. over the same

range of air flow rate. The gravimetric

efficiency was affected only slightly by

the rate of bleed-off of dust-laden air

in the range from 7 l/2 to 15 percent of

the filtered air flow rate. A microscopic

examination of the filtered air showed

that the air cleaner removed nearly all of

the particles larger than three microns in

diameter. The Important advantages of the

Rotonamic air cleaner are that it requires

no cleaning, oiling, or servicing and its

efficiency remains constant indefinitely for

any given duty. The principal disadvantage

of the device is the magnitude of its pressure drop. It appears likely, however, that

the Rotonamic air cleaner can be applied to

helicopters without exceeding the three percent total power loss usually considered as

a maximum that can be permitted as a result

of applying an air cleaning system.

1. INTRODUCTION

As a part of the research project "Air Filter Systems

for Army Aircraft" the performance characteristics of a Rotonamic air cleaner manufactured by the Farr Company

were determined with a view to employing this type of filter as an Induction air cleaner for helicopter or other

small aircraft engines.

2. DESCRIPTION OF TEST SPECIMEN

The test specimen was manufactured by the Parr

Company of Los Angeles, California, and is one of a series

of different sizes of induction air cleaners designed for

internal combustion engines that operate in very dusty

locations and for industrial air cleanlngo

Figure 1 is a photograph of the test specimen showing

the air inlet side with the turbulator vanes of the nine

cyclones employed in this model. The specimen was made

entirely of steel and was of welded construction without

movable or detachable components. The main body of the

unit measured 7 l/2 x 7 l/2 x I4 l/2 Inches and had a two

inch steel pipe attached to the bottom through which the

dust that was extracted from the air was removed. The

total weight was eight lbs. two oz. The small tube shown

at the top of the unit in Figure 1 was installed in this

laboratory to observe the vacuum required to withdraw the

desired bleed-off air. The manufacturer recommended filtered air flow rates in the range from 135 cfm to 200 cfm

and a bleeu-off rate of 10 percent of the inlet air flow

rate for this size unit.

The Kotonamic device is a cyclonic type air cleaner

in which the solid particles are separated from the air

by centrifugal force. The dust separating efficiency of

such cyclones, therefore, depends to a large extent on

the angular speed of the air in the cyclone tube. This

circular motion or vortex, is usually produced by turbulator vanes at the tube inlet and the velocity is a

function of the pressure drop across these vanes. In

order to obtain the same dust separating efficiency in

a cyclone of larger diameter the linear air velocity

must be increased, requiring a greater pressure drop

across the deflecting vanes than for a cyclone of smaller

diameter. Except for large stationary installations,

where a separate blower could be used to overcome its

pressure drop, the cyclone type air cleaner has not previously been considered practical for combustion engines.

Fig. 1

By employing a larger number ol‘ small cyclone tubes

arranged in parallel Instead of using the conventional

single large tube the Rotonamic has attained a given

angular velocity of the air vjith a pressure loss better

suited to reciprocating combustion engines.

Figure 2 is a schematic drawing of one cyclone tube

showing how the inlet air, after the vjhirling motion isimparted to it by the turbulator vanes, centrifuges the

dust against the wall of the outer cylinder. This whirling motion is reported to persist in the entire space between the discharge tube and the outer cylinder with its

conical extension. As the axial air velocity decreases

near the outer cylinder towards the open end of the cone

it is to be expected that there would be a reverse axial

flow towards the entering end of the inner cylinder, as

shown by the inner spiral in Figure 2, which would augment the dust separating effect of the device. As the

dust is centrifuged out of the air it moves along the

inside wall of the outer cylinder towards the open end

of the cone. The dust concentration at this point has

Increased to such an extent that it is possible to bleedoff more than 90 percent of the total dust by weight with

only 10 percent of the air under certain conditions. The

dust-laden bleed-off air from each cyclone flows into the

common enclosure which serves as the dust bin and is continuously removed from this space by means of an aspirator

installed in the engine exhaust pipe or some other suction

devic e

.

3, TEST APPARATUS

For test purposes the filter was clamped tight between two adapters near the middle of a 20 feet length

of five inch duct. The edges were sealed with foam

rubber strips to prevent outside air leakage. The air was

drawn through this duct by an exhaust blower and a second

blower was used for withdrawing the dusty bleed-off air.

The air flow rates could be adjusted within a wide range

by means of valves at the inlets of the two blowers. The

air flow rates were measured with orifice flow meters designed in accordance with the A,S.M.E, Research Publication, "Fluid Meters, Their Theory and Application",

Each orifice flow meter was equipped with a U-tube manometer and an inclined gage connected in parallel. The

FARR AIR CLEANER "ROTONAMIG".

SOHEKATIC •DIAGHA^^ OF CYCLONE OPERATION.

D

A PEFLEGTINC ViVNES

B DUST BIN

G DUSTY INLET .-.JR

D FILTERED <n^

-LEED-OFF

o

- 3 “

latter was used for small air flow rates and closed off

at higher rates when readings were taken on the U-tube

manometer.

For each test run a measured amount of dust was placed

in a small hopper which fed the groove of a turntable to a constant level. This turntable was mounted on a variable

speed Graham transmission and the dust was picked up from

the groove by a high pressure aspirator which broke up

most of the agglomerations and supplied the dust to the

open inlet of the test duct. By changing the speed of

the turntable the dust feed rate could be ijaf^inihely

varied from zero to 30 grams per minute to provide the

desired dust concentration.

The dust used was classified air cleaner test dust

produced by the A. C. Spark Plug Division of General

Motors Corporation. All tests were conducted with a mixture of ^0 percent each "fine" and "coarse" dusts.

For comparison purposes, two test runs were made with

Cottrell percipitate.

The efficiency of the test specimen was determined

by sampling the air upstream and downstream of the cleaner with identical sampling nozzles installed in the

center of the duct. The velocity of the sampling air

at the nozzle inlets was maintained approximately equal

to the air velocity in the duct in order to obtain air

samples with a representative amount of dust. The dust

drawn through the sampling nozzle was collected on glass

fiber paper' whose smallest fibers were about 0,3 micron

in diameter. Tests of the air cleaning efficiency of

similar paper by the Atomic Energy Commission indicate

that such paper will retain more than 99.99 percent of

all particles 0,3 micron and larger and it can, therefore,

be considered an absolute filter for these tests.

The air flow rate through the two saraplers was measured with Identical orifice flow meters which were calibrated with a gas meter. The manometers connected to

these flow meters were mounted on either side of a graduated rule to facilitate the adjustments for maintaining

equal flow through the two samplers during each test.

The filter efficiency was calculated from the formula

» l4 »

E. = (1 - » ) X 100^

^ U

where Eq = gravimetric efficiency^ percent , D = weight increase of downstream sampler,

U = wei^t increase of upstream smapler.

Another method was employed for determining the

efficiency of the filter based on the discoloration caused

by the dust. This method is known as the "National Bureau

of Standards Dust-Spot Method" and is described in the

paper, "A Test Method for Air Filters" by R. S. Dill,

ASHVE Transactions, Vol, p. 339, 1938® this

method, equal air samples from upstream and downstream

of the filter are passed through known areas of Whatman

#[|1 filter paper. The areas of the upstream and downstream

filter papers are selected by repeated trials to obtain

an approximately equal change of light transmission through

the upstream and downstream filter papers during a given

test while a known amount of dust is fed into the air

stream approaching the filter.

i|. TEST PROCEDURE AND OBSERVATIONS

The range of air flow rate associated with normal

operation of a typical aircraft engine in a helicopter

is wider than that recommended by the manufacturer for the

Rotonamic air cleaner. Accordingly, the test specimen was

operated at filtered air flow rates ranging from 50 cfm

to 280 cfm' instead of the recommended range from 135 cfm

to 200 cfm. A special series of tests was made with ll^O

cfm filtered air flow rate to determine the effect on the

filtering efficiency of varying the bleed-off rate from

five percent to 20 percent of the filtered air flow rate.

Table 1 shows a summary of the results at 10 different test conditions with one to three individual tests

at each condition. It will be noted that 150II4I4 grams

of dust were introduced into the duct during each test

run; Although the dust concentrations ranged from 7.U

to 25.6 mg/cu,ft. during tests lA, IB, and 1C there was

less than three percent change in the filter efficiency

over this range of dust concentrations. Figure 3 shows

the efficiency of the test specimen plotted against the

filtered air flow rate for both the gravimetric and dustspot methods. It will be seen that the gravimetric efficiency increased from about 90 percent at I30 cfm air



Tabl* 1 - Sunmiary of Teat Reaulta

Farr Botonanlc Air Oleoner with Hlne Cyclonoi

T 1 S T

Obaerved Values Unit lA IB

Filtered Air Flow Hate OFM 50 50 Bleed-Off Air Plow Rate

Weight of Duet Used, Half

CFH 5 5

conrae. Half Fine a 15.144 15.144

Duration of Teat Min 11.21 10.65

Upatraan Sampler Wgt Increase

Downetream Sampler Wgt InMO 47.0 46.2

creaee

Oooputad Values

MO 14.6 13.5

Inlet Air View Rate

Inlet Flow Hate Equivalent

OFM 55 55

to 6U Cyclones

Bleed-Off In ^ of Filtered

CFM 395 395

Air Flow 10 10

Duet Concentration K0/Ft3 24.5 25.8

Efficiency - Gravimetric

Average Efficiency -

69.

C

70.8

Gravimetric $ 69.2

Efficiency - Duat Spot 65.5 45.0

10

T E S

2A

T

2B

T E S

3A

T

3B

Teat

4a

50 90 90 l4o i4o i4o

5 9 9 7 7 10.5

15.144 15.144 15.144 15.144 15.144 15.144

il']? 7.93 6.26 10.73 10.72 6.4a

45.0 71.2 bl.fa 66.4 67.9 56.2

l4.4 9.2 9.1 7.8 7.1 5.1

55 99 99 147 147 150.5

395 710 710 1070 1070 1070

10 10 10 5 5 7.5

19.3 24.4 9.6 9.6 15.7

68.0 87.1 85.1 88.3 89.5 90.8

53.3

86.

64.4

1

62.2

88.9

60.0 60.3

90.8

59.4

•OottreXl Precipitate used na Teat Duat

TBS T

T :E S T

7A

TEST T I S T T 1 8 T T 1 II T

5* 5B 6a 6b 2_ I£ 8A 8B 9A 9B lOA lOB

l4o l4o

l40 l4o i4o l4o l40 200 200 280 280 200 200

i4 14 21 21 28 28 28 20 20 28 28 20 20

15.144 15.144

15.144 15.144 15.144 15.144 15.144 15.144 15.144 15.144 15.144 15.144* 15.144*

11.78

73.9

11. 78

69; 6

6.32

65.8

6.4o

56.7

9.60

58.7

10.83

54.2

10.50

63.3

6.22

87.6

6.15

90.7

4.97

191.5

4.88

81.0

10.17

95.3

6.43

98.2

6.0 7.0

4.0 6.2 2.6 5.0 4.6 7.2 6.0 6.9 7.0 5.2 4.4

154 154

161 161 168 168 168 220 220 3O8 308 220 220

1070 1070 1070 1070 1070 1070 1070 I58O I58O 2210 2210 1580 1580

10 10 20 20 20 10 10 10 10 10 10

8.3 8.U 14,9 14.7 9.4 8.3 8.6 11.1 11.2 10.9 11.1 6.8 10,7

91.9 90.0 93.9 89.1 95.6 90.8 92.7 91.8 93.4 96.3 91.4 94.6 95.5

91.0

65.1 57.0

91.5

58.0 ’ 60,2 66.5

93.0

70.9 56.6

92.6

74.8 71.3

93.9

80.7 79.4

95.

75.7

,1

81.3

“ 5 “ flow rate to about 9h percent at 280 cfm air flow rate In

nearly linear relation but dropped rapidly at air flow

rates below I 3 O cfirio The gravimetric efficiency for the

range of air flow rates recommended by the manufacturer,

135 cfm to 200 cfm, averaged about 92 percent* The discoloration of dust-spot method showed a nearly linear increase of efficiency from 55 percent at 50 cfm to 80 percent at 280 cfm based on the smooth curve of least mean

distances from the plotted points shown in Figure 3» The

third significant figure shown for the gravimetric and

dust-spot efficiences on Figure 3 £^nd Table 1 is considered uncertain.

The difference in the numerical efficiency values

obtained by the gravimetric method and the dust-spot

method can be explained qualitatively by the different

principles on which the two measurements are based. The

gravimetric efficiency is related to the ratio of the

weights of dust on the downstream and upstream filter

papers, the weight of each particle being proportional to

the cube of the effective diameter, and the efficiency is

sensitive, therefore, to the capture of large particles.

The discoloration efficiency is presumably related to the

ratio of the projected areas of all the dust particles

on the downstream and upstream filter papers, the projected area of each particle being proportional to the

square of the effective diameter. The discoloration

efficiency is therefore less sensitive to the capture of

large particles than the gravimetric efficiency but more

sensitive than a count method. For any air cleaner that

is more effective in removing large particles than small

ones, the gravimetric efficiency is higher than the dustspot efficiency. This is illustrated by an example under

Discussion and Conclusions.

Test 10, summarized in Table 1, was made with Cottrell

precipitate, a dust often used for determining the efficiency of dust filters for heating and ventilating systems.

It was found that the average efficiency by the gravimetric

method was 95 percent during this test or about 2 l/2 percent higher than it was under the same conditions with

A. C. Spark Plug Division dust. The efficiency determined

by the dust-spot method, under the same conditions increased about five percent as compared to that with A, C,

Spark Plug Co dust, from 73 percent to 78.5 percent.

FARR GO. AIR CLEANER "ROTONAMIC"

» 6 “ Table 2

Pressure Loss of Rotonamlc Air Cleaner

TBleed°-Off , 10% of Filtered Air Flow )

Filtered Air Flow Rate Pressure Difference in W.G.

Inlet to Inlet to

CFM Outlet Dust Bin

50 0.63 0,20

90 1,69 0.55

140 4 <.10 lo33

200 8,4 2.57

280 16,9 5o5

Table 2 shows the pressure loss of the Rotonamlc air

cleaner at different air flow rates. The pressure loss between inlet and outlet for the observed range from ^0 cfm

to 280 cfm increased from 0,63 inch W.G. to 16,9 inch W.G.

and that between inlet and dust bln Increased over the same

range from 0,20 inch W.G, to 5®5 inch W.G. Figure 4 Pi’S"

sents the values in Table 2 graphically and shows the two

curves as nearly straight and parallel lines on log-log

paper.

Table 3

Effect of Bleed-Off Rate on Filter Efficiency

and Bleed-Off Pressure at lUO cfm Filtered

Bleed-Off t

Air Plow Hate

Filter Efficiency, io Bleed-OffPressure

of Filtered Gravimetric Dust-Spot (Inlet to Dust

Air Bin), In. W.G.

5 88,9 60,1 1,00

7.5 90,8 59.14 1.18

10 91.0 61,1 1.26

15 9i<.5 59.1 1.34

20 93oO 68.7 2.87

'.y

[FLOW

r lidJ'-rrlf ~t

AIRFLOY/ flATS

r7

{ Table 3 shows the effect of changing the bleed-off

rate at a constant rate of flow of filtered air. The

intermediate filtered air flow rate of II4O cfm was

selected for this series of tests. The bleed-off rate

was varied between ^0 percent and 200 percent of its

normal rate and the efficiency is shown as determined

by both the gravimetric and the dust-spot methods. The

values of efficiency observed by the two tests methods

bear approximately the same relationship to each other

as that indicated by Figure 3« Figure 5 shows the gravimetric efficiency and the bleed-off pressure plotted

against the bleed-off rate. The pressure drop from inlet

to dust bin was reasonably constant for bleed-off rates

from five percent to 15 percent, but increased sharply

above a 15 percent bleed-off rate. Under the conditions

of the tests the observed gravimetric efficiency increased

from 90.8 percent at 7 l/2 percent bleed-off to 91»5 percent at 15 percent bleed-off rate, corresponding to an

Increase in efficiency of less than 0.1 percent for each

one percent increase in bleed-off rate within this range.

Due to a misunderstanding, the 10 percent bleed-off

rate used for the evaluation of the test specimen was

calculated from the filtered air flow whereas the manufacturer recommended that it be based on the inlet air

flow. The 10 percent bleed-off with reference to the

filtered air represents only 9.1 percent bleed-off of the

inlet air flow. It is evident from Figure 5 that for

the range used in these tests one percent change of bleedoff caused less than 0,1 percent change in the efficiency.

Therefore, if the test had been conducted with 10 percent

bleed-off based on the inlet air instead of the filtered

air the gravimetric efficiency values reported in Table

1 would' have been higher than those reported by about 0,1

percent.

5. DISCUSSION AND CONCLUSIONS

Important advantages of the Rotonamic air cleaner

are that it requires no cleaning, oiling, or servicing

and its efficiency remains constant Indefinitely for any

given duty. The efficiency was found to exceed 90 percent for air flow rates of 129 cfm and above when a bleedoff rate of approximately 10 percent was used. These results were obtained with a half-and-half mixture of classified ’’coarse" and "fine" test dusts prepared by the A. C.

FARR GO. AIR GLEANER "ROTONAMIG" WITH 9 GYGLONES

- 8 -

Spark Plug Division and stated to have the particle size

distribution shown below# Presuraably, the Rotonarnic filter would have a higher efficiency than that observed

during these tests with a dust of higher average mass per

particle and conversely a lower efficiency with a dust of

lower average mass per particle.

Particle Size Distribution of A.C. Spark Plug Division

Coarse and Fine Test Dusts

Coarse Fine

Size Range Microns Perc ent Size Range, Microns Perc ent

0 -5 12±2 0 -5 39 ±2

5 -10 12±3 5 -10 18±3

10 -20 ll4±3 10 -20 16±3

20 -40 23±3 20 -40 18±3

40 -80 30±3 40 -80 9±3

80 -200 9±3

The principal disadvantage of the Rotonamlc air cleaner

is its pressure drop. The pressure drop of the test specimen at the recommended maximum air flow rate of 200 cfm

was 8.4 inch VT.G, In spite of this high pressure drop it

appears likely that the Rotonamlc air cleaner can be adapted

to helicopter use without exceeding the three percent total

power loss usually considered a maximum that can be tolerated for the air cleaning system or device.

Information obtained here on the relation of power loss

to absolute pressure reduction at the carburetor Intake of

reciprocating engines Indicates 1 l/2 percent reduction in

power for each five inch W.G, reduction in pressure. This

corresponds to a 2 l/l\ percent reduction in power for a 7.5 inch W.G. pressure drop. The air cleaner under test

had an air flow capacity of about 190 cfm for 7.5 inch W.G.

pressure drop. Assuming an air induction requirement of

1 1/3 cfm per brake horsepower the test specimen could

supply air for an engine with 143 HP under these conditions.

If the remaining 3/4 percent of the total of three percent

permissible power loss were available for lifting the air

cleaning system, the air cleaner and the aspirating device

in the exhaust pipe could weigh 10.7 lb. assuming 10 lb.

lift per horsepower for the engine. The test specimen alone

weighed 8 lbs. 2 oz. made of lb-gage steel. The weight of

the test specimen could probably be reduced by half using
)

1
„ 9

“

aluminum for fabrication thus allowing some margin for

the aspirating device required to remove the dust and bleedoff air*

It cannot logically be claimed that either the gravimetric or dust-spot efficiency of an air cleaner is a true

measure of the protection it provides to an aircraft

engine against wear caused by dust in the indue cion air.

Investigations by Watson, Hanly, and Burchell^ indicate

that the piston ring wear is a constant per unit weight

of dust of a given size distribution entering the induction air system for a wide range of concentrations.

However, they also found that the wear per unit weight

of dust varied with particle size reaching a maximum for

particles about 20 microns in diameter and decreasing for

either larger or smaller particles. Studies by Pavia^ at

the Australian Aeronautical Research Laboratories led him

to conclude that wear at the top of a cylinder and of the

top compression rings was proportional to the amount of

dust fed into the induction system. He also found that

the maximum wear per unit weight of dust occurred with a

dust size of about 15 microns. Therefore, an air cleaner

that removed particles in the range 10 to ^0 microns more

efficiently than small ones would presumably be more

effective in reducing engine wear than an air cleaner of

the same overall efficiency having the same efficiency on

all sizes. The Rotonamic air cleaner was of the former

type, A microscopic examination of the dust that passed

through it revealed that most of the particles were less

than three microns In diameter with an occasional 20 to 30

micron particle. The characteristics of the Rotonamic

air cleaner with respect to particle size appear to be

favorable in view of the above Information.

Assuming that the coarse and fine test dusts of the

A. C. Spark Plug Division actually had the median size

distribution cited earlier in this report, a half- and

half mixture of the two kinds would have the size distribution shown below:

1, Abrasive Wear of Piston Rings, by C,E, Watson, F. J.

Hanly, and R,W. Burchell, Presented before Annual

. Meeting of SAE, Jan 1955 • • • >

• 2. An Investigation into Engine Wear Caused by Dusts, by

R.W. Pavia, Dept of Supply and Development, Aeronautical Research Laboratory, Melbourne, Australia, 1950

» 10»

Size Distribution of a Half- and -Half Mixture

of Ao Co Spark Plug Division Coarse and Fine

Test Dusts

Effective Diameter, d

Microns

Percent of Total

by weip:hto p

0

5 -10

10 -20

20 -i|0

I|0 -80

80 -200

25 1/2

15

20 1/2

19 1/2

h 1/2

The gravimetric and dust spot efficiences of an air

cleaner can be determined by the following equations if

the size distribution of the test dust and the incremental

efficiency of the air cleaner for different sizes of dust are

known.

Gravimetric efficiency = ®2^2 + ®n^n

Dust Spot efficiency

®lPl ^ ®2^2 n-^n

•n

^1

£2

dp

n

‘n

where is the efficiency of the air cleaner for dust

particles of the effective diameter dj^, making up pj^

percentage of the total weight. The efficiency, e^, for

particles all of the same size would have the same

numerical value for the gravimetric, dust-spot, and count

methods of measurement.

For Illustrative purposes, assume that the Kotonamic

air cleaner removed all particles larger than five microns

with an efficiency of 100 percent and those from 0-5

microns with a 50 percent efficiency, since the microscopic examination indicated that most of the particles

passing through the device were less than three microns

in diameter.

= 11

On this basis the gravimetric efficiency would be?

( 50x25. 5 )^-( 100x15) +(iooxi5)-f( 100x20, 5) +(100x19. 5) -<-(iQQxl4o5 ) 100

= 87.25^

Assuming all particles of the test dust In a given size range

had the mean diameter of the range, the dust-spot efficiency

would be?

50x25o 5+100x15+100x15+100x20. 5+100x19 o 5+lOOxU.

5

275^ 7,5 l5 30 bO ““140 ^+ li + li

2o5 7o5 15

+ 20^

30

+ 19

foO

^ + ^

lijO = 6hro2fo

This example illustrates the difference that might be

expected in the numerical values of the gravimetric and

dust-spot efficiences for a particular set of conditions.

The relation of the computed values of efficiency shoivn

above for the two methods is very nearly the sarae as the

relation in observed values plotted in Figure 3 for an

air flow rate of 100 cfm. However, the theoretical

equations shown above do not contain a variable to account

for the effect of different air flow rates on the efficiency. Presumably, changing the air flow rate would

change the values of ep, 02 , etc. for particles of selected

si zes cThe reduction of wear effected in an engine by the use

of an air cleaner can be expressed by the following ratio?

Wear Reduction

e^flPl + egfgpg f

flPl + f2P2 + f„Pn

where is the efficiency of the air cleaner for dust

particles of the effective diameter dj^ making up pj^ percentage of the total weight of dust and having a specific

wear ratlc of in mass of metal worn off per unit mass

of dust introduced into a reciprocating engine.

12 > Data on particle size distribution for tes,t dusts

can be obtained using the Whitby sedimentation apparatus3

or other known methods and the report by Watson et al cited

above provides information on the relation of wear rate to

particle size in a reciprocating engine for Arizona road

dust and some other air contaminants. However ^ no test

apparatus has yet been reported for determining the specific

efficiencies of an air cleaner for particles of specific

size ranges,, If information on specific efficiencies were

available the wear reduction to be expected from an air

cleaner could be determined.

If the data used in the previous example are used in

the formula for wear reduction together with the wear

factors reported by Watson the wear reduction is shown to

be about 96 percent for the air cleaner efficiencies and

dust composition assumed. It can be shown that the percentage wear reduction effected by an air cleaner whose

efficiency on particles larger than 10 microns is greater

than for smaller particles will be greater than the percentage gravimetric efficiency for the A, C, Spark Plug

Division test dust and other dusts of similar nature.

3* Measurement of Size Distribution of Small Particles, by

K. T. Whitby, ASHAE Journal Section, Heating, Piping and

Air Conditioning, Jan 1955*

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