NATIONAL BUREAU OF STANDARDS REPORT

3319

PERFORMANCE TESTS OF AN ELECTROMAZE

NO. ES-1195 ELECTRIC AIR FILTER

by

Henry E. Robinson

Thomas W. Watson

Report to

General Services Administration

Public Buildings Service

Washington 25, D. 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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^Office of Basic Instrumentation ^Office of Weights and Measures.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1000-30-4^37 May 26, 1954 3319

PERFORMANCE TESTS OF AN ELECTROMAZE

N0 o ES-1195 ELECTRIC AIR FILTER

Heating and Air Conditioning Section

Building Technology Division

to

General Services Administration

Public Buildings Service

Washington 25, D 0 C»

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, reprint Approved for public release by the l,,< is prohibited

unless permission is ob t£ Director 0f the National Institute of '^- W^hington

by

Henry E. Robinson

Thomas W0 Watson

25, D, C, Such permlss

cal ly prepared If that < Standards and Technology (NIST)

on October 9, 2015.

has been speclfifor Its own use.

PERFORMANCE TESTS OF AN ELECTROMAZE

NO, ES-1195 ELECTRIC AIR FILTER

lo INTRODUCTION

At the request of the Public Buildings Service, General

Services Administration, the performance characteristics of

electrostatic air cleaners were determined to provide information to assist in the preparation of new air filter specifications o The test results presented herein were obtained on a

specimen electrostatic filter unit submitted by its manufacturer at the request of the Public Buildings Service and

included determinations of dust arresting efficiency with

three aerosols (atmospheric air, kerosene lamp smoke, and

Cottrell precipitate), pressure drop, specific dirt load

and cleanability of the specimen 0

2 o DESCRIPTION OF THE FILTER SPECIMEN

The cleaner was manufactured by the Air-Maze Corporation,

Cleveland 5, Ohio, and was of the electrostatic tvpe 0 It

was identified as an Air-Maze tT Electromaze Electric Air Filter”

Number ES-1195 with four ES-130 cells, rated velocity 333

feet per minute (1332 CFM total) 0 The power pack had the

following nameplate data:

Power Pack

Model ES370-030 Serial RC11-3

Input

120 Volts 60 cycles 3S VA

Output

10 KV 1 MA

The power pack had on its front panel three indicating

lamps, one of which was connected in the ionizer or plate

circuit o This was a green lamp appearing as a ring of green

light with a dark center when voltage was normal ( 10 0 3 kv)

,

- 2 -

The test unit had a housing with actual outside da.-

mensions of 24= 1 /4x24-1 A inches and was 32 - 1/2 inches long.

It was rated as having 4 sq.ft, of transverse area, i.e.

,

1332 CFM air delivery at 333 fpm velocity 0

The upstream and downstream faces had flanges 30 inches

square

*1 matching those of the duct of test apparatus

n^rMe^aapted

tMrunirwL

n

r29-3/8°243/3x7/«-inch "Kleenflo" primped

screen wire cleanable impingement type air filter

factured by the Air-Maze Corporation

There were four similar filter ceils in )h e housinf,

P o 0h having: actual outside dimensions of 11-3/4 incnes m

e ach navi

g height and 21 - 1/2 inches m length,

and each containing 23 aluminum plates spaced approximately

7/l 6!inch apart, presenting a total surface area of approximately 52 square feet for each cell.

The manufacturer furnished an adhesive (a water-emulsifying oil) designated as "Filterkote E Adhesive for oiling t

collecting plates of the cells by spraying them copiously,

before^ inserting the cells into the ^housing .The afterfilter was oiled in preparation for the test by spraying t

media from the upstream and downstream faces with the same

adhesive

„

The power pack, connected to a 120-volt 60-cycle supply,

was adjusted by the manufacturer’s representative to recommended settings prior to the tests; the 10^^

e ^ f

a^

d

h

? h re „ vnltaees that resulted were measured by means oi a nign

ITstlnte voltmeter which was compared with an accurate electrostatic voltmeter

o

3 0 TEST METHOD AND PROCEDURE

Efficiency determinations were made by the NBS Dust-Spot

Method" using the following aerosols: (a) outdoor air drawn

through the laboratory without addition of other dust or contaminant; (b) kerosene lamp smoke and (c) Cftrel! precipitate.

The test method is described m the paper "A Test Method for

Air Filters” by Ro S„ Dill (ASHVE Transactions, Vol. 44, F °3 7 ., 193 g ) 0 The test duct and arrangement are shown m figure ±.

- 3 -

A baffle made of two 3-inch wide slats was located in the

duct about 3=1/2 ft0 downstream of the test unit to intermix the air discharged from it Q For these tests, the unit was installed in the test

duct and the housing was carefully sealed to prevent inleakage of air Q The desired rate of air flow through the

air cleaner was established and samples of air were drawn

from the center points of the test duct one foot upstream

and eight feet downstream of the air cleaner at equal rates

and passed through known areas of Whatman No 0 41 filter

paper. For the atmospheric air and oil lamp smoke tests,

the samples were drawn at equal rates through equal areas

of filter paper (3/4=inch diameter spots) 0 The downstream

sample was drawn continuously during the test; the upstream sample was drawn intermittently in a number of oneminute periods uniformly distributed over the duration of

the test, aggregating one-tenth of the downstream sampling

period. Under these conditions an efficiency of 90 percent

would be indicated if the upstream and downstream dustspots on the filter papers had the same opacity, as indicated by the change in the light transmissions of the dustspot areas before and after the sample was drawn, which were

determined by means of a photometer using transmitted light.

The filter papers used in the upstream and downstream positions were selected to have the same light transmission

readings when clean. If the opacities of the dust-spots

differed, the efficiency was calculated by means of the

formula

where Op and O2 were the opacities of the dust-spots upstream

and downstream respectively, and tp and t2 were the aggregate times during which the upstream and downstream samples,

respectively, were drawn.

For the efficiency tests with Cottrell precipitate as

the aerosol the samples upstream and downstream were drawn

at equal rates and for equal times but unequal dust-spot

areas were used to obtain opacities that were approximately

/

Efficiency, percent = 100 1-

- 4 -

equalo If the opacities of the dust-spots differed, the

value of the efficiency was calculated by means of the

formula above, with the ratio A2 substituted for the

ratios t-j_ . , where A2 and A-j_ werex the areas of the dustspots downstream and upstream, respectively 0

The following procedure was employed in these tests 0 After the clean and oiled unit had been installed in the

test duct, and all discoverable air leaks into its housing

had been sealed, its input and output voltages were adjusted to recommended values by a representative of the manufacturer (input 120 volts'; ionizer and plates 10 o 3 kv) 0 Three determinations of the efficiency of the clean unit

were made at the rated velocity, using as the aerosol outdoor air drawn into the test duct through a nearby open

window., A determination of efficiency with the unit not

energized was also made., Following these, single determinations were made, using outdoor air, at velocities 20

percent greater and 20 percent less than the rated velocity

Next, three efficiency determinations were made at the

rated velocity, using as the aerosol outdoor air with the

addition of kerosene smoke generated by an open lamp flame

near the inlet to the test duct e Following these, three efficiency determinations were

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

which was dispersed Cottrell precipitate at a concentration

of one gram per thousand cubic feet of air 0 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 0 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 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 B In these

tests, and during the loading process, the rate of feed of

the dispersant was one gram per thousand cubic feet of air 0 The pressure drop and the ionizer and plate voltages of the

unit were recorded at intervals during the tests., The dirt

loading process was continued until about 900 grams of the
• .
- 5 -

lint and Cottrell precipitate mixture had been fed (i 0 e 0

, 2/3 gram per CFM of unit rating)

„

At suitable periods as the dirt-loading process progressed, strips of transparent cellulose adhesive tape

(3/4 inch wide) were stretched vertically across the test

duct near its axis, with the adhesive side facing the air

stream c Tapes were located at three positions (1) 12 inches

upstream, (2) 15 inches downstream, and (3) $ feet downstream, of the test unit; the tapes at stations (1) and

(3) were in the same longitudinal positions in the test

duct as the inlets to the upstream and downstream sampling

tubes o The adhesive surface of such a tape captured a sample of the particulate matter in the air flowing past

it, and after suitable times of exposure to the aerosol,

scrutiny of the tapes by eye and with a microscope afforded considerable information as to the vertical distribution,

the nature, number, and size, of the particles caught at

the various stations «, Photographic enlargements (10X) by

transmitted light were made of sections of the tapes corresponding to a position at mid-height in the test duct

At the end of the tests, one of the filter cells was

removed from the test unit and cleaned by means of a stream

of cold water from a high pressure hose nozzle, directed

at and into the cell plates from both ends of the cello

4o TEST RESULTS

A summary of the test data, giving efficiencies in percent with the three aerosols and the pressure drop of the

complete unit including the after-filter, in inch ¥0 G 0

, at

rates of air flow corresponding to various face velocities,

is given in Table 1 G Also, a summary of the test data obtained in the dirt-loading test conducted at the rated face

velocity of 333 fpm is given in Table 2 0 Photographs No 0 1, 2 and 3 included in this report are

10X enlargements of the center 3/4 x 1-inch sections of

cellophane tapes located at the three similarly numbered

stations described under Test Method and Procedure „ Tapes

N0S0 2 and 3 were exposed at their respective stations

simultaneously during the dirt-loading test for a period

- 6 -

in which 124 grams of mixture were fed to the test unite

This exposure was during the interval in which the dirtload increased from 41 to 165 grams, as shown in Table 2 0 Photograph No c 1 shows a tape exposed at station No 0 1

for an equal length of time Q After exposure, the tapes

were carefully removed from the test duct for photographing and microscopic study Q Throughout the tests with atmospheric air and oil

lamp smoke electrical sparking or flashing in the unit

audible to the ear occurred one to two times per hour Q However, during the Cottrell and lint loading test electrical sparking or flashing occurred fifty to sixty times per

minute at the start and throughout the test except during

times when the ionizer and plate voltage were low, as evidenced by the voltmeter or by the appearance of the green

short-circuit-indicating lamp„ Sparking or flashing did

not occur when the short-circuit lamp was very dim or completely extinguished

5o SUMMARY

A 0 Performance

The efficiency of the air cleaner in arresting the

particulate matter existent in atmospheric air drawn through

the unit varied considerably with the face velo;cit3

r at which

it was onerated, as shown in Table I Q At the rated velocity

(333 fpm) , the average efficiency on atmospheric air (90„2

percent) and that on oil lamp smoke ( 90 0 4 percent) were very

nearly the same Q The efficiencies are reported to three

significant figures obtained from the test data c In reporting thus, however, it is considered desirable to point out

that an uncertainty on the order of one or two percent is

possible in determining efficiencies, although in these results the differences between comparable efficiency values

were consistently of a lesser magnitude

„

As recorded in Table 2, the ionizer and plate voltage

of the unit did not remain steadily at the adjusted value

during the course of the dirt-loading testo When the dirtload had increased to 37$ grams, the ionizer and plate voltage was found to have decreased to $ o 0 kv from the adjusted

value of 10o3 kv 0 Throughout the remainder of that test-day,

- 7

-

the voltage varied as shown , the first value in each bracket

being that at the start of the efficiency determination, and

the second that at its end D Voltages during the efficiency

determinations were not recorded in detail, consequently the

average value through each determination was not ascertained.

However, it is evident that the efficiency of the unit in

arresting Cottrell precipitate was reduced to some extent by

the lowering of ionizer and plate voltage.

During the first day of the loading test, the voltage

occasionally dropped below, and later re-attained, its

initial adjusted value, A complete recovery of this kind

occurred during the over-night shut-down before the second

day of loading. However, as loading progressed on the

second day, the voltage decreased, until finally just at

the end of the loading test, the voltage on the ionizers

and plates fell to zero, indicating a complete short-circuit,

and thereafter did not rise of itself. On re-energizing

the unit on the next day, the voltage rose momentarily to

10,3 kv, several sparkings occurred, and the ionizer and

plate voltage thereafter remained at about 6,3 kv. However,

it was apparent that the power-pack had not been damaged by

the short-circuits experienced. In the unit tested, the

green short-circuit light remained illuminated when the

ionizer and plate voltages were not lower than about 9«0 kv.

The particular causes of the voltage decrease were not

determined, although it was evidently due to the dirt-load

on the unit. The electrical insulators were observed to be

heavily coated with dust and lint, which may have decreased

their resistance markedly; also, as described later under

"General”, lint fibers bridging the gaps between alternate

collecting plates may have assisted in shorting the circuits.

The greater part of the pressure drop through the complete unit was due to the resistance of the after-filter.

It is noted in Table 2 that in the dirt-loading test, the

pressure drop of the complete unit increased by 0,032 inch

W»G, for a total dirt-load of 901 grams. This rise was due

chiefly to an increase in the pressure drop of the afterfilter, as a result of an accumulation of cotton lint and

of comparatively large particles of Cottrell precipitate

on its media.

Bo Cleanability

After the loading test, one of the four filter cells

was removed from the housing and subjected to the cleaning

process described under Test Method and Procedure, No diffi

culty was experienced in thoroughly cleaning the ionizer and

collector sections of the unit, using moderate care. The

cleanability of the after-filter was not determined, however

filters of this general type have been found in other NBS

tests to be readily cleanable by the same methodo

C„ General

On completion of the dirt-loading test, the filter cell

were removed from the unit and examined. The perforated upstream grille, the collector plate surfaces and rods, the

insulators and the ionizer supports were all well-covered

with Cottrell precipitate and lint deposits. The dust deposits were about 1/32 inch in thickness overall, and were

slightly thicker near the leading edges of the collector

plates; lint deposits were heavier over the aft areas of the

plates. In several areas, the spaces between adjacent collector plates were crossed by bridges of lint fibers; such

bridging occurred most extensively in the plate spaces in

line downstream from the ionizer bars, and in the Spaces

bounded on one side by the cell walls. Some lint fibers

extended downstream for as much as l/2 inch from the aft

edges of the collector plates.

The upstream face of the after-filter revealed considerable deposits of lint, somewhat concentrated in seven

vertical strips about 2-1/2 inches wide, each apparently

centered downstream of one of the ionizer wires or bars.

The after-filter media was approximately uniformly darkened

by a dust deposit on its upstream face, and slightly darkened by dust visible on its downstream face. It was evident

that many of the dust particles were relatively large, but

because fine particles are less readily observed, it cannot

be stated that many fine particles were not also present.

The dirt (dust) deposits on the electrostatic unit,

and on the after-filter, appeared to be well-saturated with

oil. After the unit had been removed from the test duct,

the section downstream of the unit was carefully swept out

with a fine brush. The amount of material obtained from

the duct by this sweeping was less than one gram.

- 9

-

The cellophane tape samples shown in Photographs

Nos. 1, 2 and 3, corresponding to stations (1), (2) and

(3), respectively, indicate in a general way the performance of the complete unit at the rated ionizer and plate

voltage of 10.3 kv. The tape shown in Photograph No. 1

was obtained at a different time than those shown in

Photographs No. 2 and 3, but was exposed for an equal

length of time, and may be considered as representative

for the upstream station. Photograph No. 1 shows many

particles under 5 microns in actual size, and a distribution of larger particles up to a few as large as 400

microns, as well as many fibers of lint. (In these photographs, l/l6 inch corresponds to an actual dimension of

about 160 microns.) Photograph No. 2, for the tape 15

inches downstream of the unit, shows several large particles

up to about 150 microns in size, and a few lint fibers, but

very few fine dust particles considering their number upstream of the unit. Photograph No. 3, for the tape 8 ft

downstream of the unit, shows an absence of the larger

particles of Photograph 2, which apparently settled out

of the air stream, although a few fibers of lint remained.

Comparison of the numbers of particles on the upstream

and downstream tapes indicates, in an obvious way, a high

order of efficiency for the unit in arresting Cottrell

precipitate, as is also indicated by the discoloration test

results presented in Table 2. The latter results show a

considerably higher efficiency for the unit when Cottrell

precipitate was being received in the air stream than when

the aerosol was outdoor air or kerosene lamp smoke. The

overall efficiency of the unit on particles of the sizes

found in Cottrell precipitate appears therefore to be better than on the finer particles in outdoor air or kerosene

smoke. Nevertheless, the downstream tapes, and the deposits on the after-filter, show that many quite large

particles of dust and lint escaped beyond the electrostatic

unit. Whether the large particles were passed through the

unit because they were not arrested at all, or were caught

and later dislodged from the collector plates by electrical

sparking, is not known from these tests.

- 10 -

As the photographs show, many large particles and

some lint passed unarrested through the after-filter.

Assuming that one of the functions of the after-filter

is to arrest as much as possible of the material escaping the electrostatic unit, the arrestance characteristics

of the after-filter are of major importance in determining the presence, in the air leaving the complete unit,

of such particulate matter as is shown in the downstream

tape photographs.

No evidence of oil droplet entrainment in the effluent

air was observed, either by microscopic examination of the

downstream cellophane tapes, or by examination of the test

duct downstream of the unit after the tests.

TABLE 1

Ionizer

Face

Velocity

Inlet

Aerosol*

and Plate

Voltage

Pressure

Drop

Duration

of Test Efficiency

fpm kv inch Wo G„ minutes percent

333 A 0 0.120 15 11 . 1**

333 A 10.3 .120 130 90.0

333 A 10.3 .120 130 90.6

333 A 10.4 .120 130 89.9

266 A 10.3 .035 130 94.1

400 A 10.3 .172 130 37.0

333 S 10.2 .120 30 90.5

333 S 10.2 .122 20 90.5

333 s 10.3 .122 20 90.3

333 c 10.3 .122 3 97.1

333 c 10.3 .123 10 97.5

333 c 10.3 .123 10 97.1

* A = Particulate matter in atmospheric air at NBS.

S = Kerosene lamp smoke in atmospheric air.

C = Cottrell precipitate in atmospheric air

(1 gram/1000 C.F.

** Since unit was not energized, the efficiency was chiefly

that of the after~filter

TABLE 2

Face

Velocity

Dirt

Load*

Ionizer and

Plate Voltage

Pressure

Drop Efficiency**

fpm grams kv inch W0 G 0 percent

333 41 10o3 ol23 97.2 (avg)

165 => d24 97o4

335 10 o 2 ..127 97o 5

373 8,0 ,128 -

392 0

0

toO

1

—

,128 96.2

406 (10 o 2

( 8 0 2

,128 97.2

476 ( 7.3

t 9.2

,129 92.1

2 nd day) 476 10o3 ,129 -

618 10o 5 d34 97 0 0

703 ( 7.5

(10.4

ol41 93 0 9

816 60 3 0 H8 89.0

901 ( 7o2

( 6.3

,155 88 e 0

0

* Average mixture: 3„9% lint 3 96 0 Vf> Cottrell

precipitate by weight*

** Efficiency determined with 100% Cottrell Precipitate,

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Photograph No. 1

Photograph No. 2

Photograph No. 3

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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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

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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.
1