I

NATIONAL BUREAU OF STANDARDS REPORT

PERFORMANCE TESTS OF AN AAF CO.

MULTI-DUTY SELF-CLEANING AIR FILTER

Henry E. Robinson

Thomas W, Watson

Report to

General Services Administration

Public Buildings Service

Washington 25, D.C.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

3730

by
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

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^Office of Basic Instrumentation ^Office of Weights and Measures.
NATIONAL BUREAU OF STANDARDS REPORT

mS PROJECT NBS REPORT

1000-30-4&30 October 11, 1954 3730

PERFORMANCE TESTS OF AN AAF CO.

MULTI-DUTY SELF-CLEANING AUTOMATIC

AIR FILTER MODEL B (TYPE MS PANELS)

by

Henry E* Robinson

Thomas W. Watson

Heating and Air Conditioning Section

Building Technology Division

to

General Services Administration

Public Buildings Service

Washington 25, D.C.

<NBS>

U. $. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, rep

unless permission Isi

25, D, C, Such pern

cal ly prepared If thi

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015.

n part, Is prohibited

ndards, Washington

ort has been speclfiiort for its own use.

PERFORMANCE TESTS OF AN AAF CO„

MODEL B SELF-CLEANING AUTOMATIC AIR FILTER

1. INTRODUCTION

At the request of the Public Buildings Service, General

Services Administration, the performance characteristics of

self“Cleaning automatic oil-type air filters were determined

to provide information to assist in the preparation of new air

filter specifications,,

The test results presented herein were obtained on a

specimen automatic oil-type air filter submitted by its manufacturer at the request of the Public Buildings Service and

included determinations of dust arresting efficiency with two

aerosols (atmospheric air and Cottrell precipitate), pressure

drop, dirt load and cleanability of the specimen,,

2 o DESCRIPTION OF THE FILTER SPEC B EN

The filter was manufactured by the American Air Filter Co,,

Inc. , Louisville % Kentucky, and was of the self-cleaning

automatic oil-type. It was identified by nameplate as an AAF

Multi-Duty Self-Cleaning Air Filter Miodel B, size 30 x 54%

Capacity 19$0 efm. The drive motor current characteristics

were as follows: 110 volts; 3«4 amps; 60 eye ; 1 ph.

The test unit had a housing with actual outside dimensions

of 31 inches in width, 54 inches in height and 15 inches in

length. The exposed face of the curtain media on the upstream

side was 26-3/4 inches wide by 21-5/16 inches high (3«96 sq, ft,

face area). The unit was rated at 19$0 cfm air delivery at

500 fpm face velocity. Special upstream and downstream transitions, 11-1/3 inches and 6-1/2 inches in length, respectively,

with flanges 30 inches square matching those of the duct of the

test apparatus, were used to adapt the unit for test. The

openings formed by these transitions at the flanges were 24 inches

square both upstream and downstream. The downstream curtain

face had an exposed transverse area 26-3/4 inches wide and 25-3/4

inches high (4.73 sq. ft.)

The filter consisted of Type MS' panel sections 29-1/4 inches

in width and about $-1/2 inches in height, overlapped about 2 inches

and attached to two endless chains approximately 7$ inches in

length, forming an endless curtain that rotated over sprockets

on horizontal shafts located in the top and reservoir assemblies.

The curtain rotated upward on the upstream face and downward on

the downstream face, and in passing around the lower sprocket

dipped into a reservoir of oil for cleaning and re-oiling the

media.
.

‘

•

;
The endless curtain was shifted intermittently by a timercontrolled motor and gear box, The synchronous electric timer

was set by the factory to actuate the drive motor for a period

of 12 seconds every 12 minutes, resulting in a curtain movement

of approximately 4 inches per hour. For the dirt-loading tests

the timer was re-adjusted for a curtain movement of 2-1/2 inches

per shift, or 12-1/2 inches per hour.

The manufacturer furnished an adhesive designated as

"Viscosine 3H". Approximately 22 gallons were required to

fill the reservoir to the indicated level.

3o TEST METHOD AND PROCEDURE

Efficiency determinations were made by the NBS "Dust-Spot

Method" using the following aerosols: (a) outdoor atmospheric

air drawn through the laboratory without addition of other dust

or contaminant; and (b) Cottrell precipitate, dispersed in the

outdoor atmospheric air. The test method is described in the

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

Transactions, Vol, 44, P. 379* 193$) « The test duct and arrangement are shown in Figure 1, A baffle made of two 3 “inch wide

slats at right angles to each other was located in the duct

about 3-1/2 ft. downstream of the test assembly to intermix the

air discharged from it.

For these tests, the unit was installed in the test duct

and its exterior housing was carefully sealed to prevent inleakage of air. The desired rate of air flow through the air cleaner

was established and samples of air were drawn from the center of

the test duct, at points one foot upstream and eight feet downstream of the air cleaner assembly, at equal rates for equal

times, and passed through known areas of Whatman No, 41 filter

paper. The filter papers used in the upstream and downstream

positions were selected to have the same light transmission readings when clean, as determined by means of a photometer using

transmitted light. Using filter paper sampling areas downstream

equal to 30 percent of the filter paper area upstream, an efficiency

of 70 percent would be indicated if the upstream and downstream

dust- spots on the papers had the same opacity, as indicated by the

change in the light transmissions of the dust-spot areas before

and after the sample was drawn. If the opacities of the dustspots differed, the efficiency was calculated by means of the

formula _

£2 O2

A1

” 0i

Efficiency, percent = 100
,

.
- 3 -

where Ax and A2 were the areas of the dust-spots upstream and

downstream, respectively, and Ox and O2 were the opacities of

the dust-spots upstream and downstream respectively.

In testing the unit, it was desirable to subject it to a

dirt-loading condition that would simulate a service condition

and that would allow the results to be applicable for a wide

range of unit sizes.

In service, a unit having an upstream exposed curtain

height of H ft, and receiving air containing a dust concentration of C grams per 1000 cf, at face velocity V, shifts its

curtain periodically or uniformly at an average rate of S inches

per hour. Under these conditions, the curtain, when it leaves

the air stream entering the front face of the unit, will have

received a "burden" of

12H x 60V x C

S x 1000

or 0.72 HVC

S grams of dust

per square foot. The average pressure drop of a unit and to

some extent its efficiency, regardless of its size provided the

face velocity is the same, depend on the average dirtiness of its

curtain, and therefore on the magnitude of the "burden" as defined above. On this basis, a test of a small-sized unit can be

expected to be reasonably representative for a much larger unit,

if the "burdens" are the same.

To evaluate a burden reasonably representative of what might

be experienced in actual service, the following in-service conditions were assumed: H = 7 ft ; S = 7 inches/hour; C = O.O65 grams

per 1000 cf (1 grain per 1000 cf)

;

V = 500 fpm. Using these values,

a representative service burden was computed to be

B 0.72 x 7 x 500 x 0.65

7

23.4 grams/sq.ft®

In selecting conditions for the test it was desirable t© Increase the curtain shifting speed S to 12.5 inches/hr, to shorten

the time required for several revolutions of the curtain. Assuming

that the test should be conducted with a burden B = 23.4 grams/sq .ft .

,

and that for the unit tested the height of exposed face = 1.7$ ft.;

'width = 2.23 ft.; curtain shifting speed S = 12.5 inches/hour; and

velocity V = 500 fpm: then

23c4- 0.72 HVC

23.4 x 12.5

0.72 x 1.73 x 500 or C - 0.456 gram per 1000 cf
,

*

.

• V,

f:
- 4

-

The air flow capacity of the unit was 1.7$ x 2.23 x 500 = 19&0 cfm,

and the rate of dust feed to the unit should therefore be 0.90 gram

per minute. Actually, 0.95 gram per minute was fed to compensate

for a slight settlement of dust in the upstream duct of the test

apparatus.

The following procedure was employed in these tests. The

clean unit was installed in the test duct, its oil reservoir was

filled to the indicated level, and all discoverable air leaks

into its housing were sealed. The timer control was adjusted to

shift the curtain 12.5 inches per hour. During a period of l£ hours

immediately prior to the tests the curtain was allowed to shift

automatical 13?- for a total of approximately 3 revolutions through

the oil reservoir, with no air flow, to saturate the curtain media.

The initial resistance of the clean filter at various rates of air

flow was then measured. Next, three determinations of the efficiency

of the clean unit were made at rated velocity, using as the aerosol

outdoor air drawn into the test duct through a nearby open window,

Following these, three efficiency determinations were made at

rated velocity, using as the aerosol outdoor air in which was disparsed Cottrell precipitate at a concentration of approximate^

G.$3 gram per thousand cubic feet of air, i.e., a net dust feed

rate of 1.65 grams per minute. The sampling times for the efficiency

test with Cottrell precipitate as the aerosol were 12 minutes each

and the curtain made one shift during each test. When these had been

obtained, the process was begun of loading the unit with a mixture

of 4 percent cotton lint, and 96 percent Cottrell precipitate, by

weight, separately dispersed into the air stream. The lint used for

this purpose was No. 7 cotton linters previously ground in a Wiley

mill with a 4 - millimeter screen; the lint was dispersed into the

air stream every 12 minutes (immediately after each curtain movement) through an aspirator operating at approximately 35 psi inlet

air pressure. At suitable periods as loading progressed, the

efficiency of the unit was determined using 100 percent Cottrell

precipitate in outdoor air. The pressure drop was recorded at

intervals during the tests.

In order that the dirt-loading process simulate continuous

operation of the unit in service at the desired burden, the curtainshifting timer mechanism was deenergized during all periods when the

air flow through the unit was stopped e.g., overnight, or at

lunch periods. Similarly, the increased rate of dust feed occurring

during efficiency determinations was in effect reduced to the desired

average net rate of feed (C .90 gram per minute) b3r operating the unit

without dust feed for ten minutes following each 1

2

-minute efficiency

determination

.
1 > . • .
- 5 -

The dirt-loading process was continued until the curtain

had made two or more revolutions under the imposed dust burden

conditions. Although some variation of pressure drop occurred

as a result of the periodic changes of curtain position, an

equilibrium of efficiency and pressure drop appeared to have

been substantially attained at this stage of the test.

At the conclusion of the dirt-loading tests the efficiency

was determined again using laboratory air as the aerosol. In

order to ascertain the self-cleaning performance of the unit,

the timer control was then adjusted to give a curtain movement

of 4 inches per hour and left running intermittently for a

period of 6S hours with no air flow through the unit. The

resistance of the cleaned filter at various air flows was then

observed.

4„ TEST RESULTS

Tabulated in Table 1 are measurements, at four different

face velocities, of the pressure drops through the filter when

it was clean, and when it hah been allowed to clean itself after

the dirt-loading test. In each case, the observed values apply

for a particular position of the curtain. As may be seen from

the pressure drop data in Table 2 for the clean unit tested with

atmospheric air (which could have caused no significant change

in curtain resistance) the pressure drop of the clean unit was

observed to vary from 0.394 to 0.431 inch W.G., depending upon

curtain position.

The operating performance of the unit at 500 fpm face velocity

is summarized in Table 2 0 The table gives data on the efficiency

of the unit in arresting Cottrell precipitate, and on its pressure

drop, as the curtain was progressively loaded, from a clean condition to an operating eouilibrium condition corresponding to a

"dirt burden" of 24.7 grams per sq. ft., with a mixture of approximately 4 percent lint and 96 percent Cottrell precipitate by weight.

It is believed that the unit was operating substantially at the

equilibrium condition for this burden after the curtain had turned

more than 1.5 revolutions. This is indicated by the trend of the

efficiency and pressure drop curves plotted from the data of

Table 2, as shown in Figure 2. Also, the efficiency of the unit

in filtering atmospheric air is given for the clean unit, and for

the unit when its media was loaded with a dirt deposit corresponding to a "dirt burden" of 24.7 grams per sq. ft.
\

•* ' .
- 6 -

5.. SUMMARY

A. Performance

Due to the self- cleaning feature of the unit, which occurs

periodically during operation, the performance of the unit depends on, and must be related to, the dirt deposit on the curtain,

expressed in this report by means of the "dirt burden" on the

curtain as it leaves the airstream on the upstream side. On

this basis, at 500 fpm face velocity, the efficiency of the unit

in arresting Cottrell precipitate was about £3 percent for equilibrium operation at a dirt burden of 24.7 grams per sq. ft. The

corresponding pressure drop averaged 0.50 inch W.G., with a plus

or minus variation of about 0.03 inch depending upon the curtain

position.

The effect of the magnitude of the burden is shown by the

corresponding figures obtained when the curtain was practically

clean, even though the rate of dirt feed was the same, i.e., at

the start of the dirt-loading test. The efficiency and pressure

drop of the unit at this stage were about 75 percent and 0.42 inch

W. G. , respectively. As the deposit on the curtain increased to

the equilibrium condition, the efficiency and pressure drop rose

to the values given in the preceding paragraph.

The efficiency of the unit in arresting the dust in the atmospheric air at the test laboratory was on the order of 10 percent,

individual values ranging from 7 to 14 percent. Its efficiency

appeared to be approximately the same with the curtain clean, and

with a dirt burden of 24.7 grams per sq. ft.

At conclusion of the tests, examination of the test duct

downstream of the unit showed no evidence of oil carry-over, even

though the unit had been operated at approximately three times the

shift speed recommended by the manufacturer. Early in the tests,

before significant dust had been received on the curtain, a few

droplets of oil were caught on a cellophane tape stretched across

the test duct just downstream of the unit; no droplets were caught

later in the tests when the curtain had received a moderate dirt

deposit. From this evidence, it is concluded that oil carry-over

would be negligible for a unit operated at the recommended curtain

shift speed (4 inches per hour).

It was observed during the dirt-loading test that a few large

wads of oil-wet lint and dirt were dislodged from the curtain on
x > •
- 7

-

the downstream face as it passed the top and bottom seals outlining the exposed curtain area. These were all too heavy to be

carried in the air stream* and were all found deposited on the

bottom of the transition immediately adjacent to the downstream

face. It is considered that such wads are not of importance as

regards the performance of the unit, except that their removal

occasionally may be desirable.

B. Self-Cleaning

Automatic self-cleaning of the curtain by its intermittent

motion through the oil reservoir appeared to be effective. At

the conclusion of the dirt-loading test, the curtain was allowed

to move intermittently through the oil at a rate of 4 inches per

hour for 6# hours with no air flow, for a total of 3.5 revolutions.

The pressure drop of the unit, at 500 fpm face velocity, was reduced from its value when dirty (about 0 . 50 ) to a value of 0.412

inch W. G., as compared with the initial average value of 0,409

for the clean filter as indicated in Table 2. On inspection of the

unit after the test and this self-cleaning operation, the curtain

appeared to be quite clean except that at the outer vertical edges

of the curtain, approximately in line with the edge-seals, some

dirt deposits remained on the armor strips of the media. The wads

of lint and dust previously referred to were also observed clinging

to parts of the horizontal seals.* Neither of these types of deposit seemed of a kind to add to the pressure drop of the curtain,

and they are considered to be of little practical importance.

The lint and dust removed from the filter curtain settled

to the bottom of the oil reservoir as a heavy sludge, leaving the

oil substantially clear. The sludge was readily removed from the

reservoir using a suitable scraper.

C. General

The following observations were made concerning particulate

matter downstream of the test unit.

(1) Cellophane tapes stretched across the duct 15 n downstream

of the unit with the adhesive side facing upstream and ocposed for

several hours each during the dirt-loading test, showed on examination under the microscope that many particles from 15 to 5 microns,

and smaller, in size were caught^ Relatively few larger particles,
•

.

.

.

;
ranging from 20 to 200 microns, were found on the tapes. No

lint fibres were observed; early in the tests 4 or 5 small

droplets of what appeared to be oil were found on a tape, but

none were observed on later tapes as the loading process was

continued.

(2) A cellophane tape exposed at the downstream dust

sampling station $ ft. from the test unit throughout the

entire test showed many fine particles, and relatively few

large particles of dust; no lint was observed.

(3) Examination by microscope of the downstream filter

papers from efficiency determinations showed that almost all of

the particles on the paper were under 20 microns, with most

under 5 microns; a few up to 50 microns were observed.

(4) Examination of the downstream duct of the test

apparatus after the tests indicated no observable oiliness of

the duct. Upon careful sweeping of the downstream duct, no

observable quantity of dust was obtained from it. A few wads

of* oil-wet dirt were found just downstream of the test unit,

as mentioned previously.

A cloth was wetted with the oil submitted with the unit,

and taken outdoors and ignited with a wooden safety match.

The cloth did not ignite readily, but once ignited it burned

strongly with a smokey flame.

The curtain-shifting timer adjustment of the unit was

"sealed" at the factory-set position when received, by means

of a plastic cement. When the seal was broken, no difficulty

was experienced in setting the adjustment as desired.. The

intermittent shifting action of the curtain occurred smoothly

and without objectionable noise.
*

'
Table 1

Face

Velocity

fpm

Air Pressure

Flow Drop*

efin inch W.G.

Pressure

Drop**

inch W.G.

253 1000 0.117

379 1500 .242

500 1930 .393

632 2500 .610

0.126

.253

.412

.623

*Initial resistance of the clean filter

**Resistance of the unit after the dirt-loading test and

63 hours of intermittent self-cleaning action (approximately 3.5 turns) without air flow.

Table 2

Face Inlet Curtain Total PresVelo- Aero- Total Move- Dirt sure Efficcity sol* Time ment Fed** Drop iency

fpm Minutes Revolutions Grams inch WG Percent

500 A 0 0.394 7

- - 0 .403 14

- - 0 .431 12

0,409 avg «

500 C 22 0.06 20 .429 74.7

44 .12 42 .420 75.8

66 .Id 62 .424 76.8

174 0 50 174 .4-71 77.7 (PM)

206 .56 195 .458 77,5 (AM)

302 .32 279 .467 -- 444 1.20 424 ,508 79.2 (PM)

466 1.26 444 .471 81.3 (AM)

560 1.51 522 .515 72„6

654 1.77 624 .499 72,7

772 2.09 738 .530 73 0 2 (PM)

A — 738 .475 11 (AM)

<zm 738 ,466 7

*A = Particulate matter in atmospheric air at NBS.

C = Cottrell precipitate in atmospheric air.

** Average mixture

3.9 percent lint, 96 .1 percent Cottrell

precipitate bv weight. (The dirt fed corresponds to a

n dirt burden” on the curtain of 24.7 grams per sq. ft, on

leaving the airstream) e

I INT

Up Stream Duct Down Stream Duct

Side elevation

Filter Frame

rjfr» . .t.i.n.l.Fffl £* San

Tu

s

ipling

be

[if— w*

, _ i _ it 12- 0

' 4-0"

£

>

Exhaust f

Down Stream Duct

VERTICAL SECTION

Air Filler Test Apparatus

Figure I

CM

3b)f)SS3dd

TOTAL

DIRT

FED,

GRAMS

FIG.

THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

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