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NATIONAL BUREAU OF STANDARDS REPORT

6908

PERFORMANCE CHARACTERISTICS OF

SOME VISCOUS-IMPINGEMENT AND THROW-AWAY

TYPES OF AIR FILTERS

by

Carl W. Coblentz and Paul R. Achenbach

Report to

Office of the Chief of Engineers

Bureau of Yards and Docks

Headquarters,, U. S. Air Force

Washington 25* D. C.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

for mm K?r

iht
Till-: NATIONAL MlltKAU OF STANDARDS

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• < i I ».

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maintenance of I hr national standards of measurement and tin- prm ision ol means and method.-

for making measurements consistent with these standards; ifie determination ol pin -deal constants and properties ol materials; tin- development of methods and instruments for testing material',

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NATIONAL BUREAU OF STANDARDS REPORT

NESS PROJECT N8S REPORT

1003-12-10431 July 22, I960 6908

PERFORMANCE CHARACTERISTICS OF

SOME VISCOUS-IMPINGEMENT AND THROW-AWAY

TYPES OF AIR FILTERS

by

Carl W. Coblentz and Paul R. Achenbach

Air Conditioning, Heating, and Refrigeration Section

Building Technology Division

to

Office of the Chief of Engineers

Bureau of Yards and Docks

Headquarters, U. S. Air Force

Washington 25., D. C.

IMPORTANT NOTICE

NATIONAL BUREAU OF S7

Intended for use yvithin the

to additional evaluation and

listing of this Report, either the Office of the Director, N

however, by the Government

to reproduce additional copi

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

progress accounting documents

rmally published it is subjected

, reproduction, or open-literature

ision is obtained in writing from

Such permission is not needed,

prepared if that agency wishes

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

V

Performance Characteristics of

Some Viscous-Impingement and Throw-awayTypes of Air Filters

by

Carl W. Coblentz and Paul R. Achenbach

1 . Introduction

The performance of a group of panel air filters of the cleanable viscous-impingement and throw-away types of 1- and 2-inch

nominal thickness was determined to provide information for preparation of new specifications for air filters, to assist in the

proper application of these devices, and to permit an economic

comparison of cleanable and disposable types of equipment. This

investigation was carried out as a part of the Tri-Service research

and development program at the National Bureau of Standards.

The performance of the individual filters with respect to arrestance, pressure drop, dust-holding capacity, and cleanability

has been described previously in separate reports. This report

summarizes and compares the data obtained on 16 viscous-impingement

type and 6 throw-away type filters.

2 . Description of Test Specimens

The test specimens were designated as types A through L and

marked with the suffix 1 or 2 to indicate their nominal thickness

in inches. The actual thickness of the nominal 1-inch filters

ranged from 7/8 inch to 1 inch, whereas that for the nominal 2-inch

filters ranged from 1 13/16 inches to 2 inches. The cleanable

viscous-impingement filters were manufactured and furnished by 6

different manufacturers and the throw-away filters were manufactured and furnished by 4 different manufacturers.

The eight types of cleanable viscous-impingement filters were

made of corrosion-resisting material, either aluminum or galvanized steel, with the frames and filter media always composed of the

same metal. The steel filters had media made of wire whereas the

aluminum filters used layers of perforated sheets. The four types

of throw-away filters had glass fiber media and cardboard frames.

The adhesives used on the cleanable filters were furnished

by the respective manufacturers and were soluble in water. The

throw-away type filters were furnished with an adhesive, believed

to be Tricresyl phosphate.

- 2 -

3 . Test Method and Procedure

All of the cleanable filters and a part of the throw-away filters were tested at both 360 ft/min and 5^0 ft/min face velocity.

The air flow rates were determined as the product of the net face

area and the face velocity. Since all filters tested had a nominal size of 20 x 20 inches, the actual air flow rates corresponding

to the two values of face velocity were all close to 800 cfm and

1200 cfm, respectively.

The cleanable type filters were immersed in the adhesive furnished by the manufacturer and left to dry in the laboratory at

least 16 hours before being weighed and installed in the test apparatus. The initial pressure drop of each filter was determined

for both face velocities after which the arrestance of the clean

filter at the air velocity desired for that test was determined

with the NBS "Dust Spot Method" as described in the paper, "A

Test Method for Air Filters" by R. S. Dill (ASHVE Transactions,

Vol. 44, p. 379, 1938).

The arrestance determinations were made with laboratory air

into which Cottrell precipitate had been injected and diffused at

a ratio of 1 gram per 1000 cu ft of air. The sampling air was

drawn from the center points of the test duct one foot upstream

and eight feet downstream of the filter at equal flow rates and

passed through known areas of Whatman No. 4l filter paper. The

change of opacity of these areas was determined with a photometer

which measured the light transmission of a part of the area on

each sampling paper before and after the sampling period. The

two sampling papers used for each arrestance measurement were selected to have the same light transmission readings when clean.

In order to obtain similar increases of opacity on both samplers,

different size areas were used upstream and downstream of the

filter. The arrestance, A (in percent), was calculated by the

following formula:

where Sp and Sp are the downstream and upstream areas, and AD

and AU the observed changes in the opacity of the downstream

and upstream sampling areas, respectively.

Whereas the arrestance determinations were made with Cottrell precipitate only, cotton lint was added during the loading

process in a ratio of 4 parts by weight to every 96 parts of

Cottrell precipitate, including that amount used for arrestance

measurements. Arrestance determinations were made at the beginning and at the end of the loading period of each filter and

at several intermediate load conditions.
-
-3-

The pressure drop across the filter under test was recorded

after each increment of 20 grams of dust had been introduced into

the test duct, and the test was terminated when the pressure drop

reached 0.5 in. W.G. at the lower air flow rate and 0.8 in. W.G.

at the higher air flow rate.

The cleanable/t, viscous-impingement type filters were washed

with hot water using a hose until the effluent water appeared

clean and then were allowed to dry. After drying, the filter

was re-oiled, weighed and installed in the test apparatus for

determining any change in pressure drop and for a second loading, after which the cleaning procedure and the determination

of the pressure drop were repeated.

4. Test Results

The data obtained on arrestance, dust holding capacity,

pressure drop, and cleanability for the two types of air filters are summarized in Tables 1 to 4 and a part of the data

is shown graphically in Fig. 1 to 4.

The "Dust Load" shown in these tables and graphs is the

dust holding capacity of the filters, or the dust received by

the filter, at the selected final pressure drops of 0.5 and

0.8 in. W.G. for face velocities of 360 and 540 ft/min, respectively. It is the weight of Cottrell precipitate and lint

introduced into the test apparatus divided by the net face area

of the filter and diminished by the percentage of dust fallout

upstream of the filter. This fallout was determined by weight

at the conclusion of the test by sweeping out the test duct.

The "Average Arrestance" in the tables and graphs is the

average of the arrestance values obtained during the period in

which the capacity dust load was accumulated. It was determined graphically from a curve drawn through the plotted values

of arrestance versus dust load for each filter test.

The values shown in Tables 1 to 4 for weight increase and

pressure drop increase after the second cleaning are cumulative,*

that is, they are changes from the initial values for the clean

filter.

Tables 1 and 2 show that the average arrestance of the oneinch cleanable filters ranged from 47 to 69 percent at a face

velocity of 360 ft/min with an average of 56 percent for all

specimens, and ranged from 47 to 73 percent at a face velocity

of 540 ft/min with an average of 59 percent for all specimens.
•

.

.
-4-

The arrestance did not increase for all of the filters at the

higher air velocity; it remained constant for two filters and

decreased for one other. The average arrestance of one of the

one-inch throw-away filters decreased significantly, from 58

to 46 percent , and that of a n o them remained about constant

as the face velocity was increased from 360 to 540 ft/min. The dust load of the one-inch cleanable filters ranged

from 85 to 362 g/sq ft at the lower face velocity and from j8

to 690 g/sq ft at the higher face velocity, indicating a wide

disparity among the eight different models. The variation in

dust load for the specimens of the throw-away type was much

less than for the cleanable filters. The average dust load of

the two types of filters was comparable for the 1-inch thick

media, but in the 2-inch filters the cleanable type had a higher

dust-holding capacity.

A graph of average arrestance versus dust load for the

eight models of cleanable filters shows a general inverse relationship between these two variables. However, the correlation is not good enough to warrant developing a mathematical expression for it.

Tables 1 and 2 show that the initial pressure drop increased

less than 0.01 in. W.G. after two cleanings for most of the

viscous-impingement filters and several of the filters showed

no measurable increase in pressure drop after two cleanings at

the lower face velocity. The weight change after two cleanings

ranged from a loss of 12 grams to a gain of 123 grams, with an

average gain in weight of 34 grams for all specimens. This

average weight gain was about 6 percent of the average total

dust load on the filters. In the worst case, that of specimen

E-l, the weight increase after two cleanings was about 27 percent of the dust load for the lower face velocity and 31 percent

of the dust load for the higher face velocity.

Tables 3 and 4 show that the average arrestance of the twoinch cleanable filters ranged from 58 to 66 percent with an

average of 60 percent for all specimens at a face velocity of

360 ft/min, and ranged from 57 to 74 percent with an average of

$5 percent at a face velocity of 540 ft/min. All except one of

the cleanable filters increased in average arrestance as the

face velocity was increased. The average arrestance of the

one 2-inch throw-away filter tested at both face velocities decreased from 74 to 71 percent as the face velocity was increased

from 360 to 540 ft/min.
>• •
-5-

The dust load of the two-inch cleanable filters ranged from

110 to 468 g/sq ft at the lower face velocity and from 92 to 775

g/sq ft at the higher face velocity. The ratio of the maximum

to the minimum values of dust load for the eight specimens was

about the same for the two-inch filters as for the one-inch

specimens

.

The arrestance values obtained for the two-inch throw-away

filters was appreciably higher than for the cleanable filters.

Tables 3 and 4 show that the initial pressure drop increased

0.021 in. W.G. at the lower face velocity and 0.035 in. W.G. at

the higher face velocity,, on the average, after two cleanings.

One filter showed a decrease in pressure drop after two cleanings

and two filters, specimens B-2 and E-2, showed an increase much

higher than the average. The weight increase after two cleanings

ranged from 0 to 385 grams, with an average of 145 grams for all

specimens. This average weight gain was about 12 percent of the

average total dust load on the filters. In the two worst cases,

specimens B-2 and E-2, the weight increases after two cleanings

were 64 and 49 percent of the total dust loads for the two filters, respectively.

Small variations in weight, in either direction, would be

expected after cleaning and re-oiling as a result of variations

in oil drainage, but this does not account for the large changes

in weight shown in Tables 1 to 4. Variations in oil drainage

probably accounts for the decreases in weight shown in the tables

as compared to the initial weight of the specimens.

The average values of the dust load and arrestance for the

two types and two thicknesses of filters are summarized in Table

5 for face velocities of 360 and 540 ft/min. This summary shows

that the average arrestance of the 2-inch cleanable filters was

4 to 6 percent higher than that of the 1-inch filters of the same

type, whereas the average arrestance of the 2-inch throw-away filters was 11 to 13 percent higher than that of the 1-inch filters.

Table 5 also shows that the arrestance of the cleanable filters

increased, on the average, with increased face velocity, whereas

it decreased for the throw-away types with increased face velocity, It should be noted that the number of throw-away filters

tested was considerably less than the number of cleanable filters, so the average values obtained on the former type are perhaps less representative of all available models than for the

latter type.

- 6 -

Table 5

Summary of Average Performance

Nominal

Thickness

of Filters,

Face

Velocity, Dust Load , g/sq ft Mean Arrestance, $

inches ft/min Cleanable Throw-away Cleanable Throw-away

1 360 200 200 56 65

1 540 254 249 59 60

2 360 279 187 60 78

2 540 325 264 65 71

Table 6 was prepared to show the general nature of the arrestance curve plotted against dust load for each filter tested.

Filters A, D, E, F, G, and J showed a continuous rise in arrestance from the beginning to the end of the loading period for both

face velocities and both filter thicknesses, whereas filter B

showed this same characteristic in the 1-inch thickness, filters

K and L showed this rising characteristic in the 2-inch thickness,

and filter C had this characteristic at a face velocity of 360

ft/min for both thicknesses. Filter C revealed minimum arrestance values at:. 20 and 30 percent of full dust load for the 1- and

2-inch thicknesses, respectively. Filter H had a rather flat arrestance curve at the higher face velocity, but the curve drooped

a little near full dust load. The arrestance curve for filter I,

in the 1-inch thickness, sloped downward continuously throughout

the loading of the filter.

The current Federal Specification for cleanable viscousimpingement type air filters, F-F-300, dated October 1957^ specifies a 2-inch thick filter with the following performance characteristics at a face velocity of 350 ft/min;

fa) Initial arrestance, 45$ minimum

(b) Dust load at 0.4 in. W.G. pressure drop,

minimum

(c) Pressure drop increase after 1 cleaning,

maximum.

Table 6 indicates that all of the filters, A to H inclusive, had

a minimum arrestance in excess of 45$ at 360 ft/min face velocity.

Figure 3 shows that the dust load of filters C, E, G, and H was

in excess of 250 g/sq ft at a pressure drop of 0.5 in. W.G. An

examination of the curves for these filters showed that the dust

load requirement was also met at a pressure drop of 0.4 in. W.G.

250 g/sq ft

0.01 in. W.G.

-7-

The other filters of this type did not meet the dust load requirement. Table 3 shows that filters A, D, and E exceeded the specified maximum increase in pressure drop of 0.01 in. W.G. after one

cleaning. Thus, of the eight specimens tested, only specimens

C, G, and H met all three requirements of Federal Specification

F-F-300

.

The current Federal Specification for throw-away type filters, F-F-310, dated October 1957^ specifies one-inch filters or

two-inch filters with the following characteristics at a face

velocity of 370 ft/min.

Nominal Filter Thickness 1-inch 2-inch

Arrestance, Minimum 60% J0%

Dust Load at 0.5 in. W.G., Minimum 115 g/sq ft 115 g/sq ft

Figure 1 shows that only filter J met both requirements in a 1-

inch thick filter, and all four specimens, I through L, met both

requirements in a 2-inch thick filter.

5 . Discussion and Conclusions

It seems more logical to specify the arrestance of air filters

on the basis of the average value during the loading of the filter

rather than the initial value for a clean filter, since most filters improve in arrestance as the load increases and the change of

arrestance with load is gradual. Furthermore, where a considerable number of suppliers is available and the performance of the

different models covers a wide range, it should not be necessary

to set the requirements so low that all can comply.

The results of this series of tests indicate that, on the

average, a 2-inch thick filter of the cleanable viscous-impingement

type had about 5 percent higher arrestance and from 25 to 30%

greater dust-holding capacity than a 1-inch thick filter of the

same type in the range of face velocity from 360 to 5^-0 ft/min.

These average values do not apply to every model tested, because

some 2-inch thick filters had lower arrestance than the corresponding 1-inch filter.

The two models of 1-inch throw-away type filters used in

these tests revealed arrestances and dust-holding capacities

generally comparable with those obtained for the cleanable filters of the same thickness. The 2-inch throw-away type filters

had higher arrestances than any of the cleanable filters and a lower dust-holding capacity, on the average, than the cleanable

filters at a face velocity of 360 ft/min, but the same comparison was not true at the higher face velocity. The principal
,

*
- 8 -

difference between the two types of filters was the relation of

arrestance to face velocity. An increase in face velocity was

accompanied by an increase in arrestance, on the average, for

the cleanable filters, but the reverse was true for the throwaway filters in two out of three cases.

The data obtained on cleanable viscous-impingement filters

during this series of tests are considered to be sufficiently

comprehensive to serve as a basis for revising the Federal

Specification requirements and providing requirements for both

1-inch and 2-inch filters. Estimates could probably be made

from these data on the relative merits of 1-inch and 2-inch

cleanable filters for various applications. Data on additional

models of throw-away filters are needed before any revision of

the Federal Specification on this type of filter could be suggested.

At the same time, it should be remembered that either type

of filter can readily be modified by the manufacturer with respect to media density, pressure drop, thickness, and type of

adhesive, accompanied by a change in performance. Thus, it

cannot be assumed that the performance data observed during a given series of tests will be applicable over a long period of

time unless the design of the filter is known to be unchanged.

For example, a cleanable filter made for similar duty by the

same manufacturer as one of the present test specimens, but

with a different model number, was tested at the National Bureau

of Standards about a year prior to this series of tests. In the

present series, this filter had a very low dust-holding capacity,

but in the earlier test the dust-holding capacity was comparable

to other models. A careful examination of the construction of

the filters revealed that a small change in the arrangement of

the filter media had increased the arrestance somewhat, but had

very considerably reduced the dust-holding capacity. Conversely,

an earlier design of one of the throw-away type filters reported

herein, was tested less than a year ago and found to have a very

low arrestance. The filter was redesigned and retested with the

resulting performance being incorporated in this summary.

The determination of acceptable cleanability should probably be based on more than one loading and cleaning of a filter, and probably should be based on weight gain rather than

increase in pressure drop. Changes in pressure drop of a few

thousandths of an inch of water cannot always be determined

with certainty in such laboratory procedures . It will be noted

In Table 1 that filter E-l gained 65 grams in weight, which

was approximately of the total dust load, after the first

cleaning, accompanied by a pressure drop increase of only

- 9

-

0.005 in. W.G.; and in Table 3, filter C-2 gained 212 grams in

weight , which was about 26 $ of the total dust load, after the

first cleaning, accompanied by a pressure drop increase of only

0.003 in. W.G. Experience with the specimens tested indicates

that a more reliable indication of cleanability would be established after two or three cleanings than after one cleaning.

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-14-

Table 6

Maximum and Minimum Arrestance Values and

the Percent of Dust Load at Which They Occurred

Thickness of Filter 1 inch 2 inches

Face Velocity, ft/min 360 540 360 540

Filter

Specimen Arrestance A L A L A L A L

A max. 70 100 78 100 76 100 72 100

min. 52 0 64 0 55 0 64 0

B max. 63 100 66 100 68 100 79 60

min. 50 0 56 0 52 0 64 0

C max. 50 100 61 100 65 90 70 100

min. 47 0 41 20 53 0 57 30

D max. 62 100 63 100 65 100 65 100

min. 51 0 51 0 53 0 56 0

E max. 71 100 81 100 71 100 81 100

min. 50 0 51 0 52 0 57 0

F max. 77 100 56 100 75 100 65 100

min

.

42 0 43 0 49 0 50 0

G max. 79 100 80 100 79 100 81 100

min. 60 0 62 0 59 0 60 0

H max. 53 70 48 50 62 100 59 90

min

.

38 0 45 100 48 0 56 100

I max. 60 0 55 0 86 100 - - min. 54 100 38 100 73 0 — —

J max. 78 100 79 100 78 100 77 100

min

.

58 0 63 0 61 0 59 0

K max. _ __ 85 100 - - min. - - - - 74 0 - —

L max. _ 82 100 - - min. — - - - 69 0 - -

A is the arrestance of Cottrell precipitate.

L is the percent of full dust load at which the maximum or minimum

arrestance occurred.

USCOMM-NBS-DC

FILTERS



;

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UiS. DEPARTMENT OF COMMERCE

Frederick H. Mueller, Secretary NATIONAL BUREAU OF STANDARDS

A. V. Aaiin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work.. In general, each section carries out specialized research, development, and engineering in the field indicated by

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• Office of Basic Instrumentation. • Office of Weights and Measures. BOULDER, COLORADO

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Ilpdio CommismieuSion and Systems. Low Frequency and Very Low Frequency Research. High Fre- quency and Very High Frequency Research. Ullru High Frequency andSupcr High Frequency Research. Modulation Research. Antenna Research. Navigation Systems. Systems Analysis. Field Operations.
)