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NATIONAL BUREAU OF STANDARDS REPORT

(<433

PERFORMANCE TESTS OF AN AAF CO. " ROLL -0 -MAT IC”

AUTOMATIC RENEWABLE MEDIA AIR FILTER

by

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

COPY. DO NOT rc
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

IOOO-3 O-I483O December 5, 1955 hk33

PERFORMANCE TESTS OF AN AAF CO. "ROLL-O-MATIC”

AUTOMATIC RENEWABLE MEDIA AIR FILTER

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. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publicatic

unless permiss

25, D,C. Sue

cal ly prepare'

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 201 5.

Je or In part, is prohibited

of Standards, Washington

n a report has been speclfilar report for its own use,

PERFORMANCE TESTS OF AN AAF CO.

*ROLL-0-MATIC" AUTOMATIC RENEWABLE MEDIA AIR FILTER

1. INTRODUCTION*

At the request of the Public Buildings Service, General

Services Administration, the performance characteristics of

automatic renewable media 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 renewable media 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 renewability of the specimen.

2. DESCRIPTION OF THE FILTER SPECIMEN

The filter was manufactured by the American Air Filter

Co., Inc., Louisville 8, Kentucky, and was of the automatic

renewable media type. It was identified by nameplate as

an AAF ”Roll-0-Matic n Air Filter, Size 3-50 (3 ft. wide

x 5.0 ft. high) Capacity 2000 cfm at 500 fpm face velocity.

The test unit had a housing with actual outside dimensions of 36 inches in width, 52 inches in height and ]q9

inches in length. The exposed face of the curtain media

on the upstream side was 2I4 inches wide x 26 inches high,

but the media was supported on the downstream side by a

strip of metal across the top which reduced the actual area

through which air could flow to 24 inches x 24 inches

4.0 sq. ft.). The unit was rated at 2000 cfm air delivery

at 500 fpm face velocity. Special upstream and downstream

transitions, with flanges 30 inches square matching those

of the duct of the test apparatus, were used to adapt the

unit for test. The openings farmed by these transitions

at the flanges were 24 inches square both upstream and downstream.

*This report is submitted for information only, and is not

released for use in connection with advertising or sales

promotion.
,

‘

.

-
- 2 -

The Roll-O-Matic filter is designed, to move a long blanket

or strip of filtering media across the air flow opening in the

housing, in a manner similar to that in which strip film is

moved in a box camera. A roll of the clean blanket media is

held at the top of the housing, from which the media is unrolled as needed by a wind-up roller or spool at the bottom

of the housing driven through a friction clutch by an electric

motor. In passing from the top to the bottom roller, the

media is supported on the downstream side against the pressure drop of the air passing through it by an endless curtain

of small horizontal rods, which turns on sprockets at the top

and bottom of the housing. The bottom sprocket of the curtain,

and the lower media wind-up spool are turned by the same drive

motor and speed reducer mechanism. The drive motor characteristics are given as l/6 H.P., 115 volts, 60 cycle, 1 phase.

The filtering media, designated as "Roll- 0 -Mat" , consists

of a long strip or blanket (2 inches thick when uncompressed)

of interlaced matted fine glass fibers bonded together at

points of contact by a thermo-setting resin. The fibers are

adhesively-coated with a tricresyl-phosphate thixotropic gel

designated as "TCP- jell Roll-0 -Mat Adhesive" before shipment.

As furnished on a spool, the blanket is tightly rolled and

compressed in thickness, but expands upon being unrolled for

use.

The unit is designed so that the shift drive mechanism

is intermittently energized by means of either an adjustable

timer-clock or a differential pressure switch actuated by

the pressure drop through the filtering media. As initially

submitted for these tests, the unit was equipped with a timer control. It was found however that because of variability in the operation of the friction-clutch, the media

failed to move satisfactorily in accordance with the schedule

imposed by the timer mechanism. Consequently, a new friction

clutch was installed, and at the same time, a Dwyer differential pressure control was installed in place of the timer,

for the tests reported here. The pressure control was adjusted to energize the drive motor when the pressure drop

reached a value of about O.I48 inches W.G. and to open the

drive motor circuit when the movement of clean media into

the air stream lowered the pressure drop to about O.I4I inches

¥. G. Motion of the media during periods of shift occurred

at the rate of about 83 ft. per hour.

- 3 -

3. TEST METHOD AND PROCEDURE

Efficiency determinations were made by the NBS "DustSpot 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. kk, 379, 1938).

The test duct and arrangement are shown in Figure 1. A

baffle made of two 3“i nch 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. Jql 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 dust-spots differed, the efficiency was

calculated by means of the formula

Efficiency, percent = 100

where A1 and A2 were the areas of the dust-spots upstream

and downstream, respectively, and 0^ and O 2 were the opacities of the dust-spots upstream and downstream respectively.

An air filter of this kind equipped with a differential

pressure switch to actuate the media shift mechanism operates

at a sort of "steady-state" condition, in which the pressure

drop slowly increases to an upper limit as dust is received,
. \

,

*

•

'
- 4 -

and is then rapidly reduced to a somewhat lower limit as

clean media is moved into the air stream due to action of

the pressure switch. An average pressure drop is thus maintained by intermittent shifting of the media as the pressure

drop through the filter changes within these limits. It is

evident that the dirtiness of the filter media as it is moved

out of the air stream is dependent on the magnitude of the

average pressure drop at which the unit is adjusted to operate,, and that the rate of usage of media therefore depends

on it. Further, since dust-arresting efficiency of this

type of media increases slightly as the media becomes choked

with dirt, the efficiency of the unit may be affected by the

operating pressure drop. In order that the tests represent

conditions reasonably appropriate for service applications,

the differential pressure control was adjusted to yield an

average pressure drop of the magnitude commonly experienced

with automatic self-cleaning air filters (about 0.45 inch

W . G) . The following procedure was used in these tests. The

clean unit was installed in tandem between the upstream

and downstream sections of the test duct, and all discoverable air leaks were sealed. The differential pressure control was adjusted to actuate the media shift drive-motor when

the pressure drop across the media increased to O .48 inch

W.G.; its operating differential was such that the control

opened the drive-motor circuit when the pressure drop decreased to 0.4l inch W.G.

With the filtering media in the air stream all clean,

the pressure drop was measured a‘t several face velocities,

and the efficiency of the filter on atmospheric air was

determined at rated face velocity. Next, the efficiency

was determined with 100% Cottrell precipitate. The process

of dirt-loading the filter at rated face velocity was then

begun, using a mixture of 96% Cottrell precipitate and 4%

cotton lint by weight, separately dispersed into the air

stream at a concentration of 1.0 gram of mixture per 1000

cf of air. The lint used for this purpose was No. 7 cotton

linters previously ground in a Wiley mill with a 4~millime ter

screen; the required weight of lint was dispersed into the

air stream every 12 minutes through an aspirator operating

at approximately 35 psi gage inlet air pressure.

When the pressure drop of the filter reached the upper

limit of the differential pressure switch, and thereafter

9 fluctuated about the average value established by the control

- 5 -

setting, the filter operation became substantially " steadystate”'. Readings of pressure drop just before and after the

shifts of the media, and of the media travel as indicated

by a mark on the curtain, were recorded from this point on;

occasional measurements of dust-arresting efficiency were

also made as this condition was continued. Some of these

determinations were made so that a shift of the media occurred during the air-sampling period, and some were made without such a shift.

After the filter had been operated at 1.0 gram per

1000 cf dirt concentration for a period in which approximately I4OO grams per foot width of filter were fed at

substantially "steady-state” condtions, the efficiency of

the unit was determined with atmospheric air as the aerosol

,

Following this test, the dirt-loading process was resumed, but with the 96% Cottrell precipitate l\% cotton lint

mixture fed at a concentration of 0.5 gram per 1000 cf, with

occasional efficiency measurements being made with Cottrell

precipitate. This was continued until an additional 280

grams of mixture per foot width of filter had been fed, at

which time the test was terminated, in part because of increasing and erratic slippage of the friction clutch driven

by the shift mechanism.

During the tests, at appropriate times, strips of 3/4

inch cellophane adhesive tape were stretched across the

test duct with the adhesive side facing upstream to catch

samples of dust particles approaching in the air stream.

The tapes were placed in the duct at a position 15 inches

downstream of the downstream flange of the test unit, or

about 40 inches downstream of the filtering media of the

unit.

4 . TEST RESULTS AND DISCUSSION

Tabulated in Table 1 are measurements, at four face

velocities, of the pressure drop through the filter when

the media was clean, and ayerage values of the efficiency

of the unit on atmospheric air and on Cottrell precipitate

for the filter when clean and when dirt-loaded to the

"steady-state” condition corresponding to an average pressure drop of about 0.45 inch W.G. at rated face velocity.
.
- 6 -

The operating performance of the unit at $00 fpm face

velocity is summarized in Table 2 and in Figure 2, Referring

to Figure 2, it is seen that the pressure drop increased

gradually from its initial value , as dirt was fed to the filter, to a value of about 0.485 inch W.G. At this point the

differential pressure control caused about five inches of

new clean media to be shifted into the air stream, which

lowered the pressure drop to about 0.41 inch W.G. Thereafter, the unit operated automatically in a "steady-state”

condition with the pressure drop varying between maximum

and minimum values of 0.49 and 0.4l inches W.G. respectively.

The travel of the media as this process continued is also

shown; it is seen that the plotted points describe a substantially straight line having constant slope, i.e., that

the use of media per gram of dust received per foot of filter width was substantially constant. The efficiency of

the filter on Cottrell precipitate in atmospheric air is

shown at the top of Figure 2. It increased from an initial

value of 75% to an approximately steady value of 79%> for

the "steady-state" operating condition.

The results above were obtained with the filter being

loaded with a dirt mixture at a concentration of 1.0 gram

per 1000 cf of air. When the test was continued with the

dirt concentration reduced to 0.5 gram per 1000 cf, substantially the same results were obtained, as shown on the

right-hand portion of Figure 2, although the time required

to feed a given quantity of dirt was doubled because of the

halved concentration. During this later test, the friction

clutch through which the media wind-up spool was driven

failed at times to transmit enough torque to turn the spool

when the differential pressure switch called for a shift of

the media. (The low value of pressure drop occurring at

about 650 grams of dust per foot of filter width was caused

accidentally when a window in the side of the test duct was

opened to allow the wind-up spool to be manually started.

)

Although the cyclic operation of the unit was not as regular

in this test as in the earlier test because of the erratic

action of the friction clutch, the overall results, as

shown in Figure 2:, were practically the same.

For a filter of this type, with a filter curtain H

feet from top to bottom of the air-passing area, receiving

at a face velocity of V fpm an aerosol having a dust concentration of C grams per 1000 cf, the rate of dirt-loading,

S, is given by
.
- 7

-

_ 60VHC

1000

0.06 VHC grams/hr per foot of media width (a)

If T is the average rate of travel of media in feet per hour,

the ’'burden 1 ’ , B, or weight of dust received by a square foot

of the media by the time it leaves the air stream, is given

by

60VHC _ n n

/ VHC _ S

1000T ° T T

(b)

For the " steady-state" operating condition shown in

Figure 2, S/T is given in effect by the inverse slope of

the line showing media travel versus total dust received per

foot of media width, in appropriate units. Table 2 gives

the values of S, T, and B derived from the data plotted in

Figure 2 and from the elapsed-time records of the tests,

for the " steady-state" operating condition for both aerosol

dirt concentrations „ It is seen that the value of the burden

B is very nearly the same for the tests at the two concentrations.

It would be expected on general grounds that for the same

filtering media, face velocity, and aerosol, the burden B

would be affected chiefly by the pressure drop at which the

unit was operated. However, the fact that almost identical

burdens resulted for two dirt concentrations gives support

for the use of a relation like equation (b) for making estimates of media usage for various possible values of the dirt

concentration C or for units of various heights of exposed

filtering blanket. It should be recognized however that B

(here about 106 grams/sq. ft.) may have a quite different

value for some other operating pressure drop or face velocity,

or for some other aerosol.

The results show plainly that for a unit equipped with

a differential pressure switch controlling the media shifting

mechanism, the use of media is proportional to the concentration or amount of dirt being received. For a unit equipped

with a timer-clock, the rate of use of media would be constant

and unresponsive to dirt loading rates, and the pressure drop

would rise and fall in magnitude with increase or decrease in

dirt concentration. It should be noted that excessive slippage

of the shift mechanism friction clutch is more serious, as

regards proper shifting of the media, with a timer-clock

control than with a differential pressure control, since the

latter will keep the shift mechanism drive motor energized

unt.il the required movement of media results.

- 8 - The efficiency of the unit appeared to be unaffected by

movement of the media during an efficiency measurement,, After

the unit had been removed from the test ductj the section of

the duct five feet long downstream of the flanges^ and upstream of a 3/I4 inch thick wood strip fastened flat across

the bottom of the test duct s was carefully swept out with a

fine brush. No observable amount of material was obtained from

the duct by this sweeping.

Cellophane tapes 9 stretched across the test duct with

the adhesive side facing upstream,, indicated upon visual and

microscopic examination after exposure to the air stream

that some particles of sizes up to approximately 12£ microns

had passed through the filter during the dirt-loading tests.

Particles smaller than five microns were observed in quantity

by microscopic examination of the downstream filter papers

obtained in tests with both aerosols. No lint was observed

on the tapes during these tests.

The intermittent shifting action of the media occurred

without significant noise.

TABLE 1

Initial and Final Performance of Air Filter

Face Air Pressure Average Efficiency

Velocity Flow Drop (1) Atmospheric Air Cottrell Precipitate

fpm cfm inch W.G, percent percent

200 1000 0.075

375 1500 .121

500 2000 .188 11 75

625 2500 .273

o4 2) 13

2) 500 2000 .45 ± *

CM

0- O'

1 ) 2 ) Resistance of the clean filter.

Values for the filter at " steady-state'* condition

loaded with a mixture of 96% Cottrell precipitate

lint by weight.

when dirtand i|%

TABLE 2

Summary of Performance with Cottrell Precipitat

Condition

Duration, hours

Aerosol cone, (c)

gram/1000 cf

Avg. efficiency, percent

Avg. pressure drop, inch

W.G.

Pressure drop, avg. max/

avg. min., inch W.G.

Rate of dirt ldg. (S),

grams/hr. per ft of media

width

Rate of travel of media

(T), ft/hr

"Steady-state” burden on

media (B) grams/ft2

Initial "Steady State"

3.2 6.9

1.0 1.0

74.6 79 <>2 0.188 0.445

- . 485/.407

- 60

- 0.555

=. 107.5

at 500 fpm

"Steady State"

9.3

0.5

79.14

0.452

.4 89/. 416

30

0.285

105.3

Up Stream Duct Down Stream Duct

Side elevation

_ o

m I CVS

JL

CJ 5

N | JO NI

Filter Frame

VERTICAL SECTION

Air Filter Test Apparatus <nbs>

Figure I

DUST

CONCENTRATION

GRAM/IOOO

CF

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

for making measurements consistent with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials,

devices, and structures; advisory services to Government Agencies on scientific and technical

problems; invention and development of devices to serve special needs of the Government; and the

development of standard practices, codes, and specifications. The work includes basic and applied

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Commission. The scope of activities is suggested by the listing of divisions and sections on the

inside of the front cover.

Reports and Publications

The results of the Bureau’s work take the form of either actual equipment and devices or published papers and reports. Reports are issued to the sponsoring agency of a particular project

or program. Published papers appear either in the Bureau’s own series of publications or in the

journals of professional and scientific societies. The Bureau itself publishes three monthly periodicals, available from the Government Printing Office: The Journal of Research, which presents

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publications should be addressed to the Office of Scientific Publications, National Bureau of

Standards, Washington 25, D. C.
I