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NATIONAL BUREAU OF STANDARDS REPORT

8013

SUPPLEMENTARY INVESTIGATION OP TEST PROCEDURES FOR

HEATING AND AIR CONDITIONING DUCTS

by

Selden D. Cole and Paul R. Achenbach

Report to

Federal Housing Administration

Washington, D.C.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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 019, 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. Tin*

work includes basic and applied research, development, engineering, instrumentation, testing,

evaluation, calibration services, and various consultation and information services. Research

projects are also performed for other government agencies when the work relates to and supplements the basic program of the Bureau or when the Bureau’s unique competence is required.

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

the back cover.

Publications

The results of the Bureau’s research are published either in the Bureau’s own series of

publications or in the journals of professional and scientific societies. The Bureau publishes

three periodicals available from the Government Printing Office: The Journal of Research,

published in four separate sections, presents complete scientific and technical papers; the Technical News Bulletin presents summary and preliminary reports on work in progress; and the

Central Radio Propagation Laboratory Ionospheric Predictions provides data for determining

the best frequencies to use for radio communications throughout the world. There are also

five series ol nonperiodical publications: Monographs, Applied Mathematics Scries, Handbooks,

Miscellaneous Publications, and Technical Notes.

A complete listing of the Bureau’s publications can be found in National Bureau of Standards ( 'ircular 460, Publications of the National Bureau of Standards, 1901 to June 1947 ($1.25),

and the Supplement to National Bureau of Standards Circular 460, July 1947 to June 1957

($1.50), and Miscellaneous Publication 240, July 1957 to June 1960 (includes Titles of Papers

Published in Outside Journals 1950 to 1959) ($2.25); available from the Superintendent of

Documents, Government Printing Office, Washington 25, D.C.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1003-20-10437 June 28 , 1963 8013

SUPPLEMENTARY INVESTIGATION OF TEST PROCEDURES FOR

HEATING AND AIR CONDITIONING DUCTS

t>y

Selden D. Cole and Paul R. Achenbach

Mechanical Systems Section

Building Research Division

to

Federal Housing Administration

Washington, D.C.

IMPORTANT NOTICE NATIONAL BUREAU OF ST

for use within the Government, and review. For this reason, th whole or in part, is not authoi Bureau of Standards, Washingt

the Report has been specifically

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

ss accounting documents intended

subjected to additional evaluation

i listing of this Report, either in e Office of the Director, National

i the Government agency for which

:opies for its own use.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

SUPPLEMENTARY INVESTIGATION OF TEST PROCEDURES FOR

HEATING AND AIR CONDITIONING DUCTS

by

Selden D. Cole and Paul R. Achenbach

1.0 INTRODUCTION

At the request of the Federal Housing Administration

task groups appointed by the Building Research Advisory

Board of the National Academy of Sciences - National

Research Council identified what were considered to be

the essential properties of fabricated ducts for residential heating and air-conditioning systems. The reports

of these task groups to the Federal Housing Administration

were issued as NAS-NRC Publication 651 (1959) entitled

"Criteria for Ducts to be Used In Residential Warm Air

Heating and Air Conditioning Systems," and NAS-NRC

Publication 838 ( 1961 ) entitled "Heating and Air Conditioning Ducts Encased In and Under Concrete Slabs-onGround," respectively.

Most of the test procedures recommended and described

in these reports for evaluating the properties and

performance of ducts were new. Subsequently, the National

Bureau of Standards, at the request of the Federal Housing

Administration, investigated the suitability of ten test

procedures described in NAS-NRC Publication 651* applicable

to heating and air-conditioning ducts that were not encased

in or installed under concrete slabs-on-ground . This

investigation covered test procedures for

1. Resistance to Mold Growth

2. Odor Emission

3. Deformation at High Humidity

4. Deterioration at High Humidity and High Temperature

5. Bending Strength and Permanent Deformation after

Loading
- 2 -

6 . Impact Resistance

7. Erosion

8 . Resistance to Rupture Due to Internal Pressure

9. Resistance to Collapse Due to Negative Internal

Pressure

10.

Airtightness

National Bureau of Standards Report 7362, issued

under date of October 31* 1961, contained the results of

this investigation and recommended the following revisions

of the test procedures described in NAS-NRC Publication 651

(a) A non-comparative type of odor test with a

simplified apparatus

(b) A modified erosion test with both weight and

visual evaluation of the eroded material.

(c) A combined deformation and deterioration test

with alternating exposure to high humidity

and high temperature comprising 7 cycles of

24 hours each. This test would combine

Tests 3 and 4 in the originial recommendations.

The proposed test procedure for evaluation of Bending

Strength and Resistance to Crushing from Live Construction

Loads, identified as Test No. 3 in NAS-NRC Publication 838 , covering ducts encased in and under concrete slabs-onground was also Investigated.

In addition, consideration was given to the logical

requirements to be applied to flexible ducts in above-thefloor installations. It was concluded that all of the

test procedures recommended for rigid or semi-rigid ducts

should be used for flexible ducts except Test 5* Bending

Strength and Permanent Deformation after Loading. A

modified crushing test is proposed for evaluating the

resistance of flexible ducts to excessive change in crosssection area as a result of limited loads that would be
-3

imposed by their own weight or by light loads that might be

laid on a horizontal run of flexible duct supported under

the first floor joists in a basement. In designing this test it

was assumed that a horizontal run of flexible duct would be

supported at closer intervals than a rigid duct to prevent

excessive sagging between supports. In other words,, some

sagging between supports should be expected when flexible

ducts are deliberately chosen to gain the advantage of

easier accommodation to change in direction.

This supplementary report uescribes test results

obtained for several duct materials when employing the

revised test procedures for rigid and semi-rigid ducts_,

the proposed bending strength and resistance to crushing

test for ducts encased in or under concrete slabs-onground and for the proposed test for resistance to crushing

for flexible ducts.

2.0 REVISED TEST PROCEDURES FOR RIGID OR

SEMI-RIGID DUCTS SUPPORTED UNDER FLOOR JOISTS

2.1 Odor Emission

Purpose

The purpose of this test was to determine whether or

not a duct or duct material will emit excessive objectionable or non-ob jectionable odor during prolonged use.

Test Specimens

A section of duct of each of the following materials

was used as a test specimen:
-4-

Supplier Material

Gust in Bacon Mfg. Co.

International Oil Burner Co.

Flexible Tubing Corp.

Sonoco Products Co.

Johns-Manville Co.

Sonoco Products Co.

Semi-rigid glass fiber, plastic

covered

Laminated paper fiber, foil

lined

Plastic over wire (flexible)

Laminated paper fiber, foil

lined

Cement asbestos

Paper fiber impregnated with

high-temperature asphalt

Each specimen was a 6 -foot length of duct with 4-inch

nominal diameter except for the last material listed above 0 In this case, the specimen consisted of a strip 6 inches

wide cut from a 3 -foot length of duct with nominal 8 -inch

diameter.

Other familiar sources of odor were used, as follows:

(l) a spoonful of ground coffee in a can, ( 2 ) the vapors

emitted through the small opening of an ammonia bottle,

(3) a cut apple, M) a dash of ethyl alcohol, ( 5 ) a squirt

of luminating gas, ( 6 ) a stream of fresh air, ( 7 ) crushed

citrus fruits, and ( 8 ) crushed pine needles 0 Preconditioning of the Specimens

The duct specimens were preconditioned before test

by heating them at 200°F for 8 hours and then alternating

the temperature between 70°F and 200 °F for two cycles.

Test Apparatus and Procedure

The apparatus used for the odor test consisted of

one-half the duct system shown In Figure 1 of NBS Report

7362 which was used for the earlier odor tests. The

present system consisted of an 8 -inch galvanized duct for

holding the specimen connected at one end to a source of
-5

outdoor air and at the other end to a chamber or box where

odor detection was made. A parallel 8-inch duct containing

electric heating elements was connected to the specimenholder with tees and incorporated a small blower for

recirculation of heated air. Dampers permitted circulation

of air through the heater duct and the specimen-holder

as a closed loop, or the circulation of outdoor air through

the specimen-holder to the odor-detection chamber, using

two different blowers. The odor-detection chamber consisted

of 3-foot cubical box having one-half of the lower side

removed, and the vertical side opposite the duct inlet

loosely covered with heavy cloth to partly confine the

odors and to restrict the entry of ambient air by natural

convection.

In conducting the test, the specimen was heated to a

temperature of 200°F, as indicated by a thermocouple at

a station downstream from the heater and near the end of

the specimen. When the specimen had attained the desired

temperature, outdoor air at a velocity of 500 fpm, was

passed over the specimen and to the odor-detection chamber

for individual evaluation by a panel of men and women.

The panel consisted mainly of employees of the

National Bureau of Standards. When evaluating the air

passing over the heated specimen, each panel member was

instructed that a duct specimen was being evaluated for

odor. When the panel members f sense of odor detection

was being evaluated with well-known products, they were

not Instructed as to what the odor source would be.

The panel members recorded answers to the following

questions on a prepared form:

1. Was an odor observed? Yes or no.

2. Was the odor objectionable? Yes or no.

3. Was the odor weak or strong?

4. Remarks, not required.
- 6 -

Basis of Acceptance

To be acceptable an objectionable odor shall not be

recorded by more than 5 of 15 panel members,, or a just

perceptible (weak), non-ob jectionable odor by more than

10 of 15 panel members.

Test Results and Discussion

The observations of the odor panel are summarized in

Table 1„ The total number of observations of each duct

specimen exceeded 15 by one or more to eliminate marginal

evaluations at exactly 1/3 or 2/3 of the panel. Not all

of the specimen ducts were evaluated by each person used

on the panel, but each member was exposed to nine determinations including a minimum of five duct specimens.

The data in Table 1 indicates that the panel members

were nearly unanimous in detecting that well-known odors

were present, but there was substantial disagreement on

their strength, and whether or not they were objectionable.

The results of odor detection of the duct materials

indicates that none of the ducts would be rejected by the

criteria cited above. More than 1/3 of the panel members

detected an odor from the Gustin Bacon material, but less

than 1/3 of the panel found it objectionable, and less than

2/3 detected any odor. Two members of a panel of 16

detected an odor from cement asbestos duct, indicating

that this material could not serve as a reference material

for odors. In the previous investigation, described in

NBS Report 7362, a higher percentage of the panel members

used for that investigation detected an odor from four of

the same ducts or the same materials, and the Gustin

Bacon duct would have been unacceptable by the same

criteria, on the basis of their odor perception.
Results

of

OdorDetection

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- 7-

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The results of this supplementary investigation of

odor perception together with the previous findings,

indicate that judgment will need to be exercised in the

use of the criteria for acceptance, especially since each

determination of the presence of an odor, its character,

and intensity by each panel member represents an exercise

of judgment.

2 . 2 Deformation at High Humidity and High Temperature

Purpose

The purpose of this test was to provide an accelerated

means of determining the ability of a duct to maintain its

character under exposure to moisture-saturated air and

temperature at or above maximum use temperature.

Test Specimen

Sections of duct 6 feet long and 8 inches inside

diameter of the following materials were used as test

specimens

Supplier

NBS Sheet Metal Shop

Sonoco Products Co.

International Oil Burner Co.

Johns-Manville Co.

Gustin Bacon Mfg. Co.

Flexible Tubing Corp.

Material

Galvanized sheet metal, 26

gage

Laminated paper, foil lined

Laminated paper, foil lined

Cement asbestos

Semi-rigid glass fiber

plastic covered

Plastic over wire, flexible
- 9

-

Test Apparatus and Procedure

The specimens were suspended in a fog room by metal

straps about 6 inches from each end for a period of 19

hours. They were then removed and inserted into the

supply duct of a unit heater, and air at a velocity of

1000 fpm and a temperature of 250°F was recirculated

through the specimen for a period of 4 hours. This

alternating cycle of exposure to saturated air and high

temperature was repeated each 24 hours for 7 consecutive

cycles. About an hour was required for installing the

specimen in the fog room and the unit heater system

during each cycle.

The air temperature and velocity was measured in the

supply duct of the unit heater just ahead of the test

specimen.

The distance from the bottom surface of the duct at

midpoint between hangers and a reference straight edge

drawn between hangers was measured before exposure to

conditions in the fog room, and at the conclusion of the

seventh cycle. The outside diameter of each duct in a vertical direction was measured at midlength and at each

end before the start of the test and at the conclusion,

with large outside calipers. These two measurements

required replacing of each duct in exactly the same

position and orientation for each cycle.

The ducts were examined at the end of the

7 cycles for delamination, cracking, corrosion

fractures, and other deterioration.

Basis for Acceptance

The change in vertical diameter, based on measurements

before and after exposure to the test conditions described

above, shall not exceed 5 percent of the initial vertical

dimension at any of the three planes of measurement. The
- 10-

sag at the bottom of the duct midway between hangars shall

not increase more than 0.25 inch as a result of this test.

There shall be no evidence of delamination,, cracks,

corrosion, fractures or other deterioration of the duct at

the end of the test.

Test Results

The dimensional data on vertical diameter and sag

observed for the duct specimens are summarized in Table 2.

The change in vertical diameter during the 7 exposure

cycles ranged from none for the cement asbestos and

flexible duct to a maximum of 2.6 percent of the initial

outside diameter for the glass fiber duct. The increase in

sag at midlength of the specimens ranged from none for the

sheet metal, cement asbestos, and glass fiber ducts to

0.156 inch for the International paper fiber duct. Sag

was not evaluated for the flexible duct specimen.

The aluminum foil lining on the inside of the International paper fiber duct showed some small blisters at

the conclusion of the fifth cycle, and was separated from

the duct over about 2/3 of the inside area after the

seventh cycle. No delamination, cracks, corrosion, or

fractures were observed for the other specimens at the end

of the test.

Discussion

Duct specimens 6 feet long and 8 Inches Inside

diameter appear to be of suitable size for observed

dimensional changes resulting from environmental conditions.

The combined test for deformation from high humidity and

high temperature appears to be satisfactory as an accelerated

test. The seven 24-hour cycles of alternate exposure to

saturated air and heat appear to be long enough to reveal

the effects of absorption of moisture and the drying of

the specimen at a temperature which would produce a vapor

pressure In the material well above atmospheric pressure,

if significant amounts of moisture were present. Rusting

of unprotected ferrous metal would also be revealed by this

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- 12 -

2 . 3 Erosion

Purpose

The purpose of this test Is to provide a method for

measuring the erosion of a duct under accelerated conditions . Test Specimens

Each test specimen consisted of two 6-foot lengths

of 8~inch duct connected by a galvanized sheet metal elbow.

The following materials were tested.

Supplier

Sonoco Products Co.

Gustin Bacon Mfg. Co.

Johns-Manville Co.

International Oil Burner Co.

Flexible Tubing Corp.

Material

Laminated paper fiber,, foil

lined

Semi-rigid glass fiber,

plastic covered

Cement asbestos

Paper fiber, foil lined

Plastic over wire, flexible

Test Apparatus and Procedure

The apparatus used for the erosion test is shown in

Figures 1 and 2. The inlet to the blower was enclosed

with a plenum chamber measuring 2 ’ x 2

f x 2*6". Each of

the three vertical sides of the plenum was fitted with a

20” x 20" glass fiber filter, 2 inches thick. The blower

had sufficient capacity to provide an average air velocity

in an 8-inch duct of 4000 fprn. The collection box at the

discharge end of the duct system measured 36 inches in

each dimension, and the side opposite the duct opening was

covered with a 200-mesh screen. A 2’ x 2* metal baffle

was centered in the box 6 inches from the duct opening,

as indicated diagrammatically in Figure 2.
-13-

Prior to the erosion test, air was passed through the

system for 10 minutes without the mesh screen in place. The

air velocity in the test duct was maintained at 4000 fpm

during this purging period as determined by a pitot tube

measurement. After purging, the collection box was

carefully wiped clean with a damp cloth. One strip of

cellophane tape was attached to the baffle opposite the

duct opening; and another strip of tape was stretched

from top to bottom of the collection box near the screen

opening. Both cellophane tapes had the adhesive side

facing the air stream. A third strip of tape was fastened

over the blower exit upstream of the test duct.

With the tapes and screen in place, air was passed

through the test duct at a velocity of 4000 fpm for four

hours, after which the screen and tapes were examined for

arrested particles and the collection box was brushed out

with a soft brush to collect fallen particles.

This procedure was followed with each different

duct material.

A Basis for Acceptance

No particles of duct material should be visible to

the eye on the screen, and none visible to the eye should

be collected by brushing out the collection box. The two

tapes removed from the collection box should not show more

foreign matter on their adhesive surfaces than the tape

removed from the blower outlet position. The three

cellophane tapes should be compared under a microscope.

This comparison should reveal no deposit on the two tapes

taken from the collection box that is attributable to

erosion of the duct specimen.
-14-

Test Results

No particles were visible on the screen and none

were collected by brushing out the inside of the

collection box. The three tapes were indistinguishable

from each other,, and there were no macroscopic particles

observable on the tapes. A microscopic examination

revealed a widely spaced collection of unidentified airborne dust of about the same character and density at

the inlet and outlet of the test duct.

Discussion

Dust particles or eroded material that could be

arrested by a 200-mesh screen would appear as a deposit

on floors and furniture in an occupied space, but would

be of little significance as an inhalation hazard.

However, much smaller particles of eroded material would

remain air-borne for longer periods and could be inhaled

by the occupants of the building. Thus an identifiable

amount of duct erosion products of any size that was

collected during a 4-hour test period would be undesirable.

A high efficiency panel filter should probably be used at

the inlet to the blower instead of the low efficiency

filter used in these tests, so any material eroded from

the duct during the test period would not be as readily

obscured by the background dust concentration in the

circulated air.
-15-

3.0 TEST PROCEDURES FOR DUCTS ENCASED IN AND

UNDER CONCRETE SLABS-ON- GROUND

NAS-NRC Bulletin 838 entitled ’’Heating and AirConditioning Ducts Encased in and under Concrete Slabson-Ground” describes test procedures for evaluating ducts

with respect to (l) Resistance to mold growth, ( 2 ) Odor

emission, ( 3 ) Bending strength and resistance to crushing

from live construction loads, (4) Airtightness, ( 5 ) Deteri

oration at high temperature and high humidity, and

(6) Erosion. Since these test procedures, except for the

test for bending strength and resistance to crushing from

live constructions loads, were essentially the same as

for the ducts covered by NAS-NRC Bulletin 6^1j they were

not re-evaluated except for the revisions discussed in

Section 2 of this report.

The test procedure for bending strength and

resistance to crushing from live construction loads

described as Test 3 in NAS-NRC Bulletin 838 was investigated and tried out on five duct materials.

3 . 1 Bending Strength and Resistance to Crushing From

Live Construction Loads

Purpose

To determine the susceptibility of heating or air

conditioning ducts to damage before or during installation

in concrete slabs.

Test Specimens

Fabricated ducts 6 feet long and 8 inches in diameter

of the following materials, were used for these tests.
- 16 -

Supplier

Johns-Manvllle Co.

Sonoco Products Co.

International Oil Burner Co.

Gustin Bacon Mfg. Co.

NBS Sheet Metal Shop

Material

Cement asbestos

Laminated paper fiber (foil

lined)

Laminated paper fiber (foil

lined)

Semi-rigid glass fiber

(plastic covered)

Galvanized sheet metal (26

gage)

Test Apparatus and Procedure

An apparatus essentially like that shown in Figure 2*

page 33 j> of NAS-NRC Publication 838 was used for the tests.

The increments of load applied to the duct specimens were

determined by placing the duct and its supports on the

bed of a platform scale having a weighing arm graduated

in 1/4-lb. divisions. The load was applied to the top center

of the duct by means of a screw jack,, placed on top of the

loading block* and exerting force downward on the block

and upward against a stiff member extending to the ceiling.

A nominal 4” x 4" timber 6-feet long was placed on the

scale platform to serve as a continuous support for the

duct specimen during the compression test. For the bending

tests,, supports of 1 1/2-Inch pipe 8 inches long were laid

crosswise on the 4" x 4" timber and spaced 5 feet apart.

These round supports had a tendency to be pressed into

the bottom side of the ducts during the bending tests*

giving a false indication of the bending deflection at

midlength. The pipe supports were replaced with wooden

supports 8 Inches long and with a cross section of 1 3/4

x 1 5/8 Inches with slightly rounded corners at the top.

The flat supports eliminated the problem of indentation.
-17-

During the bending tests* load was applied in increments

of about 5 lb. in the range up to 20 lb. and thereafter in

increments of 10 lb. to the final load of 160 lb. The

deflection of the bottom of the duct at midlength was determined by measuring the vertical distance between the

bottom of the duct and the 4

n x 4" timber serving as a bed.

The deformation of the top of the duct was determined by

measuring the distance from the top of the loading block

to a reference point on the stiff member above the screw

jack. The load was terminated at 160 lb. or when the

deflection of the bottom surface reached 1 inch* which ever

occurred first. At the conclusion of the loading period*

the load was removed and the top and bottom deformation

were measured 5 minutes later.

During the crushing tests* the duct specimen was

placed in contact with the 4” x 4” timber for the full length

of the specimen. Load was applied in increments of 10 lb.

by extending the screw jack* to a final load of 160 lb.

or a depression of the upper duct surface of 3 inches*

which ever occurred first. The deformation of the upper

surface of the duct was determined in the same manner as for

the bending test. At the conclusion of the loading period*

the load was removed and the residual deformation was

measured immediately and again 15 minutes later.

Basis for Acceptance

In the bending test a duct shall be able to withstand a load of 160 lb. without deflecting more than 1

inch at midlength on the bottom or deforming more than 3

inches at the station of load application on the top.

Permanent deformation after load removal shall not

exceed 2 percent of the original outside diameter of the

duct at the bottom or 5 percent at the top.
-18-

In the crushing test a duct shall be able to withstand a load of 160 lb. without deforming more than 3

inches at the station of load application on the top.

Permanent deformation at the top after load removal shall

not exceed 5 percent of the original outside diameter of

the duct

Results and Discussion

The results of the Bending and Crushing tests are

given in Table 3 and Table 4, respectively.

Table 3 contains data obtained from four rigid and

one semi-rigid type ducts randomly positioned, except

for the metal duct which had the seam positioned at the

top.

It was found that a round support at the ends of

the duct could not be used to indicate bending, as the

round supports had a tendency to be pressed into the duct

surface with increase in load. This deformation at the

supports tended to lower the entire duct and introduced

an error in the measurements of bending deflection.

All of the rigid ducts tested supported the full

load of 160 lb. with no more than 1/8 (0.125) inch

deflection of the bottom surface. The top surface

deformation was considerably more than the bottom

deflection, but recovery after load removal was almost

complete. A semi-rigid glass fiber duct reached a depression of more than 3 inches in the top surface and

the full 1 5/8 inch depression permitted by the apparatus

on the bottom with a load of 13 lb. The recovery of the

fiber glass duct was complete 5 minutes after removal

of the load. The top deformation of the 26 gage galvanized duct was 4 to 5 times as great as that for the

two paper fiber specimens. The cement asbestos specimen

deformed very little for a load of 160 lb.
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In Table 4 the deformation is reported for two

different orientations of the Sonoco paper fiber duct 90

degrees apart and for three different positions of the

crimped seam of the 26 -gage galvanized sheet steel duct.

The data show that the deformation was essentially the

same for the two positions of the fiber duct. However,

the deformation of the galvanized sheet steel duct was

consistently somewhat greater with the seam on top than

for the other two positions. The deformation of the

sheet metal duct was several times as large as for either

of the two paper fiber ducts at the final loading of 160 lb.

The glass fiber duct deformed 3 inches with a 40 lb. load,

but returned to its original condition when the load was

removed

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A small Increase in load completely flattened the

glass fiber duct, but it suffered no permanent deformation

on removal of the load. The permanent deformation of the

galvanized sheet steel duct ranged from 0.062 to 0.093

inch for the different positions of the seam.

The results shown In Tables 3 and 4 for the glass

fiber duct indicate that it would not be suitable for

encasement In or under a concrete slab-on-ground.

4.0 CRUSHING TEST FOR FLEXIBLE DUCTS

Purpose

The purpose of this test was to evaluate the

crushing resistance of flexible ducts as a basis for

recommending criteria for this characteristic of this

type of duct.

Test Specimens

Sections of duct 8 inches in diameter and 6 feet in

length of the following materials were used as test

specimens.
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Supplier Material

Wiremold Co. Spiral metal and plastic

type laminate

Flexible Tubing Co. Plastic over plastic

coated spiral of wire

Gustin Bacon Mfg. Co

Semi-rigid glass fiber,

plastic covered.

Test Apparatus and Procedure

The apparatus used for this test was the same as

that described in Section 3 for the crushing and bending

tests of rigid ducts.

The duct supplied by the Flexible Tubing Co. was

tested with the material allowed to assume its natural

degree of elongation and a second time with the duct

compressed longitudinally as much as possible when

placing it on the bed of the apparatus, and then allowed

to expand naturally. These two methods of placing the

specimen on the bed allowed a different number of wires

to be under the loading block.

The flexible duct supplied by the Wiremold Co. was

tested only in an extended condition. The semi-rigid

glass fiber duct was included because it appeared to have

limited flexibility.

The specimens were placed in contact with the 4" x 4”

timber as a bed, for the full length of the test specimens.

Load was applied in 5 It. increments by means of the screw

jack, until a 3-inch deformation occurred, or until the

duct collapsed.

Flexible duct shall be able to withstand a crushing

load of 40 lb., using the test procedure described, without deforming more than 3 inches or fracturing any of the

component materials of the duct. Permanent deformation

at the top after load removal shall not exceed 5 percent

of the original outside diameter of the duct.

Basis of Acceptance
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- 24-

Test Results

Table 5 summarizes the deformation of the test

specimens during the crushing tests. The maximum loads

reported in the table for the two flexible ducts produced

the maximum deformation that could be tolerated without

elastic failure of the metal ribbing 0 Greater loads

produced serious bending of the metal ribbing from which

no recovery occurred after removal of the load* On the

other hand* the fiber glass duct exhibited complete

recovery after a deformation of 3 inches 0 Discussion

Neither flexible duct or glass fiber duct will

support much of a load without excessive permanent

deformation or collapse 0 For this reason these types

would probably not be suited for encasement in or under

concrete slabs-on-grade . Flexible ducts could probably

be used in horizontal runs under first floor joists in a basement* provided they could be supported at close

intervals and provided a little sagging between supports

could be tolerated 0 The principal application for flexible

ducts appears to be for making offsets or unusual turns

in a duct system in locations where adequate support can

be provided.


FILTERS

<roo

SCREEN

U. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A.*V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

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front cover. WASHINGTON, D. C.

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