NATIONAL BUREAU OF STANDARDS REPORT

5715

OPERATING CHARACTERISTICS OP A PROTOTYPE MULTI-LAYER

CLOTH AIR FILTER FOR ARMY AIRCRAFT

by

C. W. Coblentz

and

P. R. Achenbach

Report to

Aviation Division

Transportation Research and Engineering Command

Fort EustiSj Virginia

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1003-20-4838 January 6, 1958 5715

OPERATING CHARACTERISTICS OF A PROTOTYPE MULTI-LAYER

CLOTH AIR FILTER FOR ARMY AIRCRAFT

by

C„ Wo Coblentz

and

P 0 R„ Achenbach

Air Conditioning, Heating, and Refrigeration Section

Building Technology Division

to

Aviation Division

Transportation Research and Engineering Command

Fort Eustis, Virginia

IMPORTANT NOTICE

NATIONAL BUREAU OF STAN'

Intended for use within the Gc

to additional evaluation and rev

listing of this Report) either In the Office of the Director) Nati< however) by the Government a§ to reproduce additional copies

Approved for public release by the

Director of the National Institute of

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

)gress accounting documents

tally published It Is sub|ected

•production or open-literature

>n Is obtained In writing from

uch permission Is not needed,

repared If that agency wishes

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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OPERATING CHARACTERISTICS OF A PROTOTYPE MULTI-LAYER

CLOTH AIR FILTER FOR ARMY AIRCRAFT

by

C. W. Coblentz and P. R. Achenbach

ABSTRACT

Earlier investigations indicated that heavily-napped

cloth fabrics have promising characteristics as media

for air filters, having high efficiency and moderate

pressure drop. As a part of the development program

on better air cleaners for helicopters and small aircraft being conducted for the Transportation Research

and Engineering Command of the U. S. Army, a prototype multi-layer dry cloth air filter was designed

and tested. Two specimens were furnished for flight

testing. Tests of an experimental model and one of

the prototypes composed of four layers of an Orion

fleece revealed that i-n excess of 99 percent of the

dust in an air stream could be removed under velocity

and pressure drop conditions compatible with the

requirements of the H-13 and H-23 model helicopters.

Although a dust load of 200 to 400 grams of the

standard AC Spark Plug Co. test dusts, fine and

coarse, used for the tests, increased the pressure

drop of the filter from an initial value of 1 to 2

in. W.G. to a final value in the range from 4 to 9

in. WoG., it is probable that a dust load In excess

of 2 lb can be accommodated on most airfields because

analyses show that the average particle size is

larger under actual operating conditions than for the

test dusts. The complete prototype filter weighs 3

lb. The cloth media can be readily cleaned and re-used

many times and are not adversely affected by being

wetted with water.

- 2 -

Int roduction

By request of the Aviation Division, Transportation

Research and Engineering Command, the development of an oilbath air cleaner was discontinued in order to complete the

development and testing of a prototype dry cloth air cleaner

for use on the H-13 and H-23 helicopters 0 It was indicated

that support for this investigation would be terminated at

the close of calendar year 1957-

Two specimens of the prototype dry cloth air filter have

already been furnished to the Transportation Research and

Engineering Command for flight testing. The design of these

air filters and the performance of a third specimen in the

laboratory are described in this report together with the

development work done on a preliminary experimental model.

This report completed the investigation of air cleaners

and air cleaning methods by the National Bureau of Standards

for the Transportation Research and Engineering Command.

Development of an Experimental Multi-Layer

Dry Cloth Air Cleaner

Earlier investigations of a number of fabrics indicated

that heavily-napped cloth showed promising characteristics as

a medium for air cleaners. However, the filtering efficiency

of all single layer fabrics which had a reasonable dust

holding capacity was lower than desired and the air velocities

through the media had to be kept so low that the physical

dimensions of a single layer air cleaner exceeded those practicable for aircraft application. The results of these tests

were shown in the Progress Report for calendar year 1956,

pages 16 and lj a For determining the performance of multiple layers of

cloth an experimental air cleaner was designed and constructed.

A photograph of this experimental air cleaner is shown in

fig. 1. It consisted of a plywood box, the upper part of

which was detachable and had a metal hood as an air intake.

Three deep triangular notches were cut in the front and rear

walls of the upper part of the box to match corresponding

notches in the lower portion of the box, leaving a quarterinch space between matching edges. The lower part was

Pig

-3

-

equipped with an adapter to connect it to the air duct

leading to the exhaust blower, A 16-mesh wire screen was

installed in the lower notched section to support the fabric

that was pleated along the cut-outs and clamped between the

two halves when the box was closed. The free area of cloth

was two sq ft and the angle of the pleats was 20°. The

important feature of this apparatus was that it permitted

determination of the amount of dust retained in each of

several layers of filter cloth by measuring the weight

increase caused by the introduction of a known weight of

dust during each test.

Filters consisting of two to five layers of fabric were

investigated. In some cases all layers were of the same

fabric, in others different fabrics were used together,

whereas in still other tests the backing of some layers was

installed facing towards the air inlet. These tests indicated that, in most combinations, more than 60 percent of

all dust was retained by the first layer which resulted in a

more rapid increase in pressure drop for this layer. Many

kinds of fabrics were tested trying to obtain a more nearly

equal deposit of dust on the various cloth layers. But it

was determined that any combination of materials that would

produce approximately equal dust accumulations on two or more

layers with one of the test dusts would not perform the same

with the other one. Since the difference in size of the dusts

from different air fields was found to be much greater than

the difference between the AC Spark Plug dusts "fine” and

"coarse" used in these tests, it was felt that It would not

be possible to find any combination of fabrics which would

retain on several successive layers equal amounts of dust

over as wide a range of particle sizes as would be expected to

occur during actual use of the air cleaner in flight and on

the ground. In military application, the use of only one

kind of fabric would eliminate the possibility of mixing up

the sequence of the several layers, when reassembling the air

cleaner after cleaning.

Since Orion fleece produced the best over-all results of

all materials tested, including some experimental fabrics, a

series of performance tests was made In which the weight

increase of each layer was determined for a dust load of lOOg.

Table 1 shows the results observed for five different air flow

rates ranging from 175 cfm to 300 cfm, corresponding to face

velocities ranging from 87.5 ft/min. to 150 ft/min„ using AC

Spark Plug Co. coarse and fine dusts and a mixture of 50 percent each of the two dusts. These dusts had the following

particle size distribution by weight.

-4-

Table 1

Weight Increase

in

Layer 175

of Individual Layers of Orion Fleece

Experimental Air Cleaner

Air Flow Rate., CFM

200 225 250 300

Coarse Dust

1 86. 01 76.94 76.38 73.65 73.45

2 12.13 18.62 18.30 20.04 19.09

3 1.64 3.44 3.91 4.72 5.61

4 0.07 0.41 0.74 1.31 1.65

Total 99.95 99. -41 99.33 99.72 99.80

50^ Coarse and 50^ Fine

1 92„ 46 91.13 89.52 88.71 88.62

2 4.69 6.84 8.20 9.03 8.83

3 1 o73 1.27 1.24 1.46 1 0 38

4 00.73 0.60 0 0 68 0.58 0.50

Total 99.61 99.44 99.64 99.78 98.93

Fine Dust

1 92.97 93.10 92.43 91.75 90.15

2 3.81 4.11 4.38 5.38 6.20

3 1.21 1.07 0.98 l.ll 1.24

4 0.55 0.57 0.57 0.52 0.49

Total 98.54 98.85 98.36 98.76 98.08

“5"

Size Range Coarse 50/50 Pine

0-5 microns 12± 2 % 25.5±2$ 39±2$

5-10 15±3 $ l8±3$

10-20 " 14±3^ 15±3 $ l8±3$

20-40 " 23+3$ 20„5±3$ l8±3$

40-80 " 30+3$ 19o5±3$ 9±3$

80-200 " 9±3$ 4„5±3$ --

It will be noted in Table 1 that the amount of dust

retained on the first layers decreased with increasing air

velocity and also with increasing fineness of the dust. The

weight gain of the second layers increased appreciably with

an air flow rate up to 250 cfm and decreased slightly at

300 cfm air flow during the tests with coarse and 50/50 dust

but kept on increasing when fine dust was used. The gain of

the third and fourth layers increased with increasing air

velocity only when coarse dust was used but was very little

affected by the air velocity with finer dusts.

The fact that the weight increase of the first layers

represented up to 93 percent of the total fine dust introduced explains the fact that the pressure loss increased more

with fine dust than with coarse dust where a maximum of 86

percent was retained in the first layers. The total amount

of dust collected on the four layers of cloth from lOOg of

dust introduced into the air cleaner was very close to the

efficiency values observed on this experimental model by

means of the sampling method. Since the effect of dust load

and air velocity on efficiency and pressure loss are shown

later for the prototype model., they are omitted here for the

experimental model.

A microscopic examination of the dust particles that had

passed through the filter media of the experimental model at

200 cfm air flow rate showed very little difference in size

distribution regardless of which of the three test dusts had

been used, Some particles up to 80 microns in diameter were

found to have passed through the filter media when coarse and

mixed coarse-and-fine dusts were used„ The largest size particles in the samples taken downstream of the filter when

fine dust was used were about 50 microns in diameter. There

were few particles in any sample below 25-micron and above 50- micron size. This appears to indicate that the small dust

particles adhered to the fibers of the fabric whereas all

particles larger than 80 microns and most of those larger

than 50 microns were retained by the backing of the cloth.

Design and Performance of a Prototype Dry Cloth

Air Cleaner for Helicopters

The results observed with the experimental model indicated that the over-all characteristics of an air cleaner

with four layers of Orion fleece should meet nearly all the

requirements of size., weight, efficiency and pressure drop

set forth at the beginning of the project. A prototype air

cleaner of this type was, therefore, designed and constructed

of light weight metal in which one piece of fabric was

wrapped continuously in four layers around a cylindrical

screen. Fig. 2 shows the prototype assembled and fig. 3 is

a view of the component parts. In the latter photograph

the core with the heavy aluminum screen cylinder welded

between two flanges is at the upper left. The flanges are

used to support the air cleaner in the aircraft. One of

the flanges is closed and the other one is fitted with a

two-step adapter which can be connected to either 3-in. I.D.

or 4-in„ I.D. air intake hoses. The Orion fleece, wrapped

around this core, is held on each end with a stainless steel

clamp, and a 16-mesh aluminum window screen protects the

fabric from excessive erosion due to wind. The screen is

held by four small coil springs. The over-all length of the

air cleaner is 20 in., the diameter 6 in., and its weight is

3 lb, 1 oz (l,400g) o Inasmuch as the maximum air requirement of the engines

used on the H-13 and H-23 helicopters will not exceed 230

cfm, the performance tests of this prototype air cleaner

were conducted at 250 cfm, 200 cfm and 150 cfm, the latter

being the approximate air requirement at idling speed.. For

testing purposes, the air cleaner was mounted inside a

cylindrical container which left a 2-in. annular air space

between the outside of the air cleaner and the container.

The air was introduced through the top pipe as shown in

fig. 4, where the container is shown to the left of the

filter itself. This photograph also shows the location of

the pressure taps and the upstream sampler holder. The

lower flange of the air cleaner was tightly sealed against
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the bottom plate of the container and the container itself

could be closed so that no air could leak into it. All air

going through the outlet pipe at the bottom of the container

had to come through the duct inlet where the test dust was

introduced. A series of tests was conducted at the three

air flow rates with classified test dust of three particle

size ranges, namely AC SparkPlug Co. dust "coarse," ’’fine”

and a mixture of 50 percent of each.

A dust load of 50g was introduced during each test run

and the pressure drop and filtering efficiency were determined

with the apparatus described in previous reports. Table 2

shows the values observed for the pressure drop and indicates that the finer the dust, the greater was the increase

of pressure loss for an equal amount of dust introduced.

This phenomenon had been observed on most of the impingement type air cleaners that have been examined at this

laboratory. At an air flow rate of 250 cfm the pressure

loss increased from 2.32 in. W,G, with a clean cloth to

8.90 in, W,G. when 200 g of fine dust were introduced,

whereas it increased to only 5-31 in. W,G. with 400 g of

coarse dust.

Table 2

Pressure Loss of Prototype Cloth Air Filter,

in. W. G„

250 CFM 200 CFM 150 CFM

Dust Load, g C 50/50 4 C 50/50 F C

' 5 O/50 ~~F

0 2.32 2.32 2,32 1.54 1.54 1,54 1. 14 1.14 1.14

50 2,44 2.75 3.07 1.73 1.93 2,16 1.18 1.34 1.58

100 2,60 3.34 4.37 1.93 2.52 3.30 1.34 1.77 2,68

150 2.72 4,33 6,50 2,17 3. 58 4,92 l. 6 l 2,52 4.25

200 2,98 5.70 8.90 2,52 4.92 7,01 2.01 3.58 6,50

250 3.35 7.37 3,03 6 , 62 2.60 5, 04 9.42

300 3.86 3.58 3.19 6.70

350 4.53 3.78 8.86

400 5.31

- 8

-

Table 3

Efficiency of Prototype Cloth Air Filter,

Percent

250 CFM 200 CFM 150 CFM

Dust Load, g C 5O/5O F C 50/50 E

1 C 50/50 T

50 99.7 98.7 98.4 99.8 98.8 98.1 99.6 98.9 97.1

100 99.6 99.2 99.1 99.8 99.4 99.1 99.7 99.2 99.0

150 99.8 99.4 99.6 99.7 99.5 99.5 99.7 99.6 99.4

200 99.8 99.6 99.6 99.6 99.6 99.6 99.8 99.6 99.5

250 99.8 99.5 99.5 99.8 99.9 99.7 99.6

300 99.6 99.4 99.8 99.7

350 99.6 99.8 99.7

400 99.5

Average 99.7 99.3 99.2 99.6 99.4 99.1 99.8 99.5 98.9

A summary of the efficiencies determined from these

tests is contained in Table 3 . The average efficiencies were

above 99 percent except for the series of tests with fine dust

at an air flow rate of 150 cfm. The lowest value observed

for a single test was 97*1 percent for the first 50 g of

fine dust introduced into a clean air cleaner at 150 cfm„

The data in Tables 2 and 3 are shown graphically in fig. 5

and 6 , respectively.

No change in the efficiency or pressure loss could be

observed when certain tests made with a: new cloth were

repeated after the cloth had been cleaned more than twenty

times and also had been soaked with water and dried several

times

.

Conclusions

It has been determined from dust analyses made of dusts

from numerous air fields that the classified test dusts used

in these tests have a much smaller mean particle size than

the dust that will probably be accumulated in an air cleaner

during actual operation. Therefore, it is estimated that

during usage of this type air cleaner on typical air fields,

a pressure loss of 8 in. W.G. will not occur until the dust

load is in excess of 2 lb. It is also probable that a

portion of the dust accumulating on the outside layer of

cloth will be blown off by the down-wash of the rotor. The

Pressure Drop versus Dust Load of the Prototype Cloth

Air Filter at Three Air Flow Rates

DUST LOAD, g

Pig. 5

Efficiency versus Dust Load of the Prototype Cloth

Air Filter at Three Air Flow Rates

DjJST; LOftD,

Fig. 6

- 9

-

cleaning of the prototype air cleaner requires no special

equipment; the cloth is taken off and the dust can be shaken

out. No replacement of the Orion fleece nor any other component of this air cleaner will be required under ordinary

circumstances. The weight of the complete cleaner of 3 lb

1 oz compares favorably with that of any other air cleaner

plus its enclosure, and its filtering efficiency at least

equals that of any other air cleaner tested in this laboratory.

It is, therefore, recommended that appropriate flight

tests be made with this prototype multi-layer dry cloth

air cleaner.

USCOMM-NBS-DC
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U. S. DEPARTMENT OF COMMERCE

Sinclair Week*, Secretory

NATIONAL Ill'REAL' OF STANDARDS

A. V. Astin. Director HUS NATIONAL BUREAU OF STANDARDS

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