NATIONAL BUREAU OF STANDARDS REPORT

4521

PERFORMANCE TESTS OF A TRION AUTOMATIC TRAVELING

CURTAIN SELF -CLEANING AIR FILTER

by

Henry E 0 Robinson

Thomas W, Watson

Report to

General Services Administration

Public Buildings Service

Washington 25, D 0 C,

<NBS>

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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Qr-/n?f

NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

1000-30-14830 February 6, 1956

NBS REPORT

4521

PERFORMANCE TESTS OF A TRION AUTOMATIC TRAVELING

CURTAIN SELF -CLEANING AIR FILTER

(SPECIAL SIZE MODEL T 2-2)

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 publication, repi

unless permission Iso

25, D. C, Such perm

cal ly prepared If th:

part, is prohibited Approved for public release by the

ldardSiWashington Director of the National Institute of , rt has bee n spec*. Standards and Technology (NIST) ort for its own use, on October 9, 201 5.

PERFORMANCE TESTS OF A TRION MODEL T 2-2

AUTOMATIC SELF-CLEANING AIR FILTER

by

Henry E. Robinson and Thomas W. Watson

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 . DESCRIPTION OF THE FILTER SPECIMEN

The filter was manufactured by Trion, Inc., McKees Rocks

Pennsylvania, and was stated by the factory representative to

be a special-sized model of 1750 cfm air capacity designed to

fit the laboratory test equipment. Except for size, the unit

designated as Model T 2 -2 , was stated to be representative of

the standard models produced by the company.

The test unit had a housing with actual outside dimensions of 32 inches in width, 32 inches in height and 12

inches in length. The exposed face of the curtain media on

the upstream side was 2lq inches square (I4.O sq. ft. gross

face area). The unit was rated at 1750 cfm at a face velocity of 520 fpm based on the net air-passing exposed area

of the upstream curtain ( 3 . 37 sq. ft.). Special upstream

and downstream transitions, II4 inches in length with flanges

26 inches square were used to connect the unit to the test

ducts. The openings formed by these transitions at the duct

flanges were 2 l\ inches square upstream and downstream. The

downstream curtain face had an exposed gross area 2 l\ inches

square (5.0 sq. ft.)

ttThis report is submitted for information only, and is not

released for use in connection with advertising or sales

promotion

.
.«

Si .
- 2 -

The filter consisted of Trion MAF panel sections 25 l/4

inches in width, 8 inches in height and 2 inches thick, consisting of thin perforated and formed metal strips installed

in vertical planes parallel to the direction of air flow,

and stacked side by side to form 6 layers per inch of horizontal width, between which the air flows in sinuous or

irregular paths.

Each panel had a 5/8-inch surrounding edge frame which

reduced the net face area of the upstream curtain to Ql\% of

the gross area. The panels were hung and pivoted at the

ends on two heavy roller conveyor chains approximately 80

inches in length, thus 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,

with the panels riding in Ferris-wheel fashion presenting

the same face to incoming air at all times. The panel in

passing around the lower sprocket dipped into and passed

through a reservoir of oil for cleaning and re-oiling the

media.

The endless curtain was shifted intermittently by a timer-controlled motor and gear box. The synchronous

electric timer was set by the factory to actuate the drive

motor for a period of approximately 23 seconds every 12

minutes, resulting in an average curtain movement of about

67/8 inches per hour. For the tests herein the timer was

re-adjusted so that it actuated the drive motor for 37«6

seconds every 12 minutes, yielding a curtain movement of

2 I/I4 inches per shift, or 11 I/I4 inches per hour. The

drive motor current characteristics were stated as follows;

1/6 HP; 1 PH; 60 CY; 115 volts; 3.4 Amps; 1725 RPM.

The manufacturer furnished an adhesive designated

as "Flowrex 100" . , Approximately 18 gallons were required

to fill the reservoir to the indicated level.

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. I4

I4 , p. 379, 1938). The test duct and arrangement are shown

- 3 -

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 l/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.

J4 I 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 a filter paper

sampling area 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 r 1

Ap Op

Efficiency, percent = 100 1 - — * Ai °1 . where Aq and A2 were the areas of the dust-spots upstream

and downstream, respectively, and 0q 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 or 0.72 HVC grams of dust per square ft.

S' x 1000
-

,

..

'

.
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 inservice conditions were assumed? H = 7 ft; S = 7 inches/hr;

C = 0.065 gram 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 0.065 = 23.4 grams/sq. ft.

7

In selecting conditions for the test it was desirable to

increase the curtain shifting speed about 12. inches/hr. to

shorten the time required for several revolutions of tne curtain. It was assumed that the test should be conducted with

a burden B = 23,4 grams/sq. ft., and that for the unit as

tested the height H of the exposed face = 2.0 ft.; width

= 2.0 ft.; curtain shifting speed = 12 inches/hr; and the

face velocity V = 520 x 0.84 fpm (based on gross face area),

so that

23.

b

= s

or C = A-JLfL-— = 0.447 gram per 1000 cf.

0.72 x 2.0 x 520 x 0. 84

Since the air flow capacity of the unit was 2.0 x 2.0 x 520 x 0.84 = 1750 cfm, appropriate average rate of dirt feed

to the unit was therefore estimated to be O .783 gram per

minute consisting of 96/ of Cottrell precipitate and of

cotton lint by weight. During the test, however, the actual

average rate of curtain travel proved to be 11 1/4 inches

per hour, and as a result the burden to which the unit was

subjected in these tests was actually about 25 grams/sq. ft.

instead of the intended value of 23„4 grams/sq. ft.

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 so that it shifted the curtain at an

average rate of 11 I/I4 inches per hour. During a period of

17 hours immediately prior to the tests the curtain was allowed to shift automatically for a total of approximately

2 .I4 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, two 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, two efficiency determinations were

made at rated velocity, using as the aerosol outdoor air in

which was dispersed Cottrell precipitate at a concentration

of approximately 1.0 gram per thousand cubic feet of air,

i.e., a net dust feed rate of 1.75 grams per minute. The

sampling times for the efficiency tests with Cottrell precipitate as the aerosol were 10 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 four 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 (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 curtain-shifting timer mechanism was deenergized during

all periods when the air flow through the unit was stopped - e .

g

e , 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 (0.75 gram of dust per minute) by operating

the unit without dust feed for about 13 minutes following

each 10-minute efficiency determination.

The dirt-loading process was continued until the curtain

had made two or more revolutions under the imposed dust burden conditions.

- 6 -

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 curtain was left running intermittently through the oil

for a period of 60 hours (eight revolutions) 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

net face velocities, of the pressure drops through the filter

when it was initially clean,, and when it had been allowed to

clean itself after completion of the dirt-loading test. The

pressure drop value 0.558 inch W.G. obtained at 520 fpm net

face velocity after self-cleaning is higher than the initial

clean value (0.537) ° It should be noted, however, that the

pressure drop of the unit varied markedly, as shown in Table

2, not in response to the dirt load upon its media, apparently,

so much as to the portion of the curtain receiving the air

stream, i. e., the various filter cells making up the curtain

apparently differed measurably In air flow resistance. It is

believed that the greater pressure drop after self-cleaning is

a result of the variability of cell resistance, rather than an

indication of failure to clean the media thoroughly.

The operating performance of the unit at 520 fpm net

face velocity is summarized in Table 2. 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 to a "dirt burden" of 25°

7

grams/sq. ft. of a deposit of

approximately 4$ lint and 96$ Cottrell precipitate by weight.

The plotted data on pressure drop exhibit variability, as

mentioned above, but in general the pressure drop did not tend

to increase measurably as dirt was accumulated on the media in

the amount corresponding to the above dirt burden. The efficiency of the unit in arresting Cottrell precipitate appeared

to be substantially constant throughout the test, irrespective

of the dirt burden deposited on the media.

The efficiency of the unit in filtering atmospheric air

is also given in Table 2, both for the unit when clean, and

for the unit with dirt on its media at the end of the dirtloading test.
,« .
- 7

-

5» SUMMARY

A. Performance

The operating performance of the unit appeared to be

practically unaffected by the dirt upon its media, up to the

amount attained after several hours of steady operation at a

dirt burden of 25»7 grams/sq. ft „ Neither efficiency nor pressure drop showed any steady change as this burden was imposed,

the average values over the duration of the dirt-loading test

being approximately 61$ and 0.51 inch W,G., respectively, at

520 fpm net face velocity. Variations in pressure drop from

Ooii73 to 0.^85 inch W.G. were observed as the test progressed,

but these are believed to have been due to differences in the

air flow resistances of the several cells composing the filter

curtain, rather than to the effect of dirt deposits on the

media.

The efficiency of the clean unit in arresting the dust

in the atmospheric air at the test laboratory averaged about

11$, individual values ranging from 11 to 17%« Its atmospheric

air efficiency with the curtain loaded with dust was measured

as 10% 0

At the conclusion of the tests, examination of the test

duct downstream of the unit showed evidence of slight oil

carry-over. It is believed that oil carry-over would be

negligible for a unit operated at curtain shift speeds not

greater than 11 inches per hour.

B. Self -Cleaning

Automatic self -cleaning of the curtain by its passage

through the oil reservoir appeared to be effective. No dust

or lint was visible on either face of the media, or in the

media so far as could be seen without disassembling the filter cells, upon inspection after the curtain had been allowed

to rotate eight times through the oil bath at the rate of

11 l/k inches per hour, with no air flow through the filter.

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 could be readily removed from the reservoir by means of the catch pan provided.

- 8 -

Co General

The following observations were made concerning particulate matter downstream of the test unit c

(1) Cellophane tapes stretched across the duct 15"

downstream of the unit with the adhesive side facing upstream

and exposed for several hours each during the dirt-loading

test, showed on examination under the microscope that many

particles from l£ to 5 microns, and smaller, in size were

caught. Relatively few larger particles, ranging from 20 to

150 microns, were found on the tapes. No lint fibres were

observed; a few small droplets of what appeared to be oil

were found on a tape, but it is believed this was of little

importance since little was observed in the downstream duct

at the end of the tests.

(2) 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 30 microns were observed.

(3) Examination of the downstream duct of the test

apparatus after the tests indicated a slight observable

oiliness of the duct. Upon careful sweeping of the downstream duct, no observable quantity of dust was obtained

from it.

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 smoky flame.

No difficulty was experienced in setting the adjustment

of the curtain-shifting timer. The intermittent shifting

action of the curtain occurred smoothly and without objection

able noise.
,* . , *•
TABLE 1

Net Face Air Pressure Pressure

Velocity Flow Drop*”* Drop-55-*"*

f pm cfm inch W.G. inch W.G

5914 2000 0 0 7 0 0 0.722

520 1750 .537 .558

1900 «393 .lllO

297 1000 .181 .190

^Initial pressure drop of the clear filter,,

-5HS- Pressure drop of the unit after the dirt-loading test and

60 hours of intermittent self-cleaning action (approximately

eight turns) without air flow.
>
TABLE 2

Inlet Total Average

ace Aero- Total Curtain Dirt Pressure Efficelocity sol* Time Movement Fed** Drop iency

fpm minutes revolution s grams inch W. G. percent

520 A <«> 0 0.516 11

- - 0 .510 17

520 C 11 0.03 18 .506 62

33 .08 35 .479 6U

154 .36 113 • C 65 6I4 I4OO .94 322 .493 65

578 1 ,36 I466 .507 64

696 1.63 561 .543 65

886 2 o 08 7ll| .503 6I4 520 A - - 7iU .533 10

* A = Particulate matter in atmospheric air at KBS,

C = Cottrell precipitate in atmospheric air.

•** Average proportions; I| percent lint, 96 percent Cottrell

precipitate by weight. (The dirt fed corresponds to a "dirt burden" on the curtain of 25»7 grams per square ft

on leaving the air stream)

.

LINT

> DUST

//7///;//s/S////V//////777/7/77/y7/7/77//7/77/7777/77/7///'

To

Bi owe

6- It" 2'-

O'"

lip Stream Duct Down Stream Duct

Side elevation

T

o

-*

CM

I

y

u_

N I CD sI

I f«

Up Stream Duct

VERTICAL SECTION

Filter Frame

VERTICAL SECTION

Air Filter Test Apparatus <nbs>

Figure I

00

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li_

lN30U3d ‘A0N3I0IJJ3 9*M HONI ‘dOUQ 3dflSS3dd

TOTAL

DIRT

FED,

GRAMS

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

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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

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Information on the Bureau’s publications can be found in NBS Circular 460, Publications of

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

Standards, Washington 25, D. C.