NATIONAL BUREAU OF STANDARDS REPORT

22bk

AN APPARATUS FOR THE STUDY OF ADIABATIC

SELF-HEATING OF SOLIDS AND LIQUIDS

by

A. F. Robertson

and

W. Raskin

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 THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards is suggested in the following listing of

the divisions and sections engaged in technical work. In general, each section is engaged in specialized research, development, and engineering in the field indicated by its title. A brief description

of the activities, and of the resultant reports and publications, appears on the inside of the back

cover of this report.

Electricity. Resistance Measurements. Inductance and Capacitance. Electrical Instruments. Magnetic Measurements. Applied Electricity. Electrochemistry.

Optics and Metrology. Photometry and Colorimetry. Optical Instruments. Photographic

Technology. Length. Gage.

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Mineral Products. Porcelain and Pottery. Glass. Refractories. Enameled Metals. Concreting Materials. Constitution and Microstructure. Chemistry of Mineral Products.

Building Technology. Structural Engineering. Fire Protection. Heating and Air Conditioning. Floor, Roof, and Wall Coverings. Codes and Specifications.

Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Machine

Development.

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Instrumentation.

Radio Propagation. Upper Atmosphere Research. Ionospheric Research. Regular Propagation Services. Frequency Utilization Research. Tropospheric Propagation Research. High

Frequency Standards. Microwave Standards.

Ordnance Development. These three divisions are engaged in a broad program of research

Electromechanical Ordnance, and development in advanced ordnance. Activities include

Ordnance Electronics. basic and applied research, engineering, pilot production, field testing, and evaluation of a wide variety of ordnance materiel. Special skills and facilities of other

NBS divisions also contribute to this program. The activity is sponsored by the Department of

Defense.

Missile Development. Missile research and development: engineering, dynamics, intelligence,

instrumentation, evaluation. Combustion in jet engines. These activities are sponsored by the

Department of Defense. • Office of Basic Instrumentation • Office of Weights and Measures.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1002-L0-V720 February 4, 1953 2244

AN APPARATUS FOR THE STUDY OF ADIABATIC

SELF-HEATING OF SOLIDS AND LIQUIDS

by

A. F. Robertson

and

W. Raskin

The publication, reprinting, or reproduct|on!of thlslreport In anylform, either In wholeJor In part, Is prohibited

unless permission Is obtained In writing from theOffice of the Director, National Bureau of Standards, Washington

25, D, C, Such permission Is not needed, however, by a Government agency for which a report has been specifically prepared If that agency wishes to!reproduce additional copies of that particular report for Its own use,

AN APPARATUS FOR THE STUDY OF ADIABATIC

SELF-HEATING OF SOLIDS AND LIQUIDS

1. INTRODUCTION

An investigation of the thermal decomposition of

materials has led to the study of self-heating of specimens

under conditions which resemble an adiabatic process and

result in only a small heat exchange between the specimen

and the surroundings.

Previous study in Sweden-'- and a study undertaken at

the National Bureau of Standards^, carried out under constant

ambient temperature conditions, involved a study of selfheating and self-ignition in wood fiberboards . In this

process, the specimen was supported in an oven maintained

at constant temperature. The temperature distribution through

the specimen was monitored by means of imbedded thermocouple So

Using octagonal specimens, the heights and widths of which

were equal, it was found that the center of the specimen

self-heated to temperatures higher than at any other point.

In cases where the ambient temperature was sufficiently high,

the self-heating continued to ignition of the specimen, even

though the surface of the specimen was only slightly warmer

than the ambient air at the time ignition occurred. This

resulted from the fact that, as the specimen self-heated,

heat was lost to the surroundings; however, since the center

of the specimen was insulated to a greater extent, due to the

greater amount of material surrounding it, the peak temperatures developed at this point. Such a system permits study

of self-heating behavior of specimens of a given size and

shape, at a given ambient temperature. A large number of

tests is required to survey the field of specimen sizes,

shapes, and ambient temperatures.

To overcome these limitations of the constant temperature method a new furnace has been devised. It was constructed for the purpose of obtaining data on the self-heating
2

characteristics of materials in the temperature range from

30 to 500° C, in such a way that their size and shape would

not appreciably affect the results obtained. This is done

by first bringing the specimen and furnace air to equilibrium at some preselected temperature, and then allowing

the specimen to heat throughout its volume in such a way

that the temperature gradient within it is nearly zero.

This is accomplished by a control device which keeps the

furnace temperature very closely equal, at all times, to

the temperature of the center of the specimen. In this

manner, the entire volume of the specimen heats at the same

rate, providing the process is not controlled by the rate

of gaseous diffusion in the material, producing a timetemperature curve that may be characteristic of the material

being tested.

Somewhat similar types of apparatus 3 A have been

constructed for the study of spontaneous heating of coal.

In both cases, however, the furnaces had a comparatively

large thermal inertia, and as a result could not follow

rapid changes of specimen temperature.

2. DESCRIPTION OF APPARATUS

The furnace and its related control equipment are

shown in figures 1, 2, and 3 • The apparatus consists of a

furnace and its components, within the chamber of which the

specimen is supported on a wire attached to an electric strain

gage dynamometer arranged to record continuously the weight of

the sample. The associated equipment includes a pyrometer

controller, a twelve-point pyrometer recorder and controller,

a servo-amplifier and a servo-motor driving a variable voltage

autotransformer which supplies power to the furnace heating

coil, a galvanometer type relay, .and two latching relays for

automatic control of the operation of the furnace. A cathode

ray oscillograph is useful for monitoring controller operation

A block diagram of the system is shown in figure

3. METHOD OF OPERATION

In the performance of a typical test, the specimen is

suspended in the furnace by means of a wire secured to a

strain gage dynamometer. Thermocouples are mounted at the

center and near the surface of the specimen, as well as in
- 3

-

the air within the furnace. Air is then supplied to the

furnace at any desired rate through the tube provided.

During the initial warm-up period, a constant selected

furnace temperature is maintained by the pyrometer controller

which provides an "on-off" heating cycle. When the interior

of the specimen reaches, and due to self-heating slightly

exceeds, the air temperature within the furnace, a galvanometer relay is actuated and by means of a latching relay

automatically disconnects the thermostatic controller 0 The

latter relay also initiates operation of a servo-controller

which supplies heat to the furnace as needed to maintain the

smallest feasible temperature difference between the interior

of the sample and that of the gases within the furnace . The

sensing element for this controller is a differential thermocouple responsive to the temperature difference between the

center of the specimen, and that of the surrounding furnace

gases. Continuous records are provided by means of a twelve-point recorder of the furnace atmosphere temperature,

and two or more temperatures within the specimen, as well as

the specimen weight. A limit switch on the pyrometer recorder

serves to disconnect power to the furnace and its related

equipment when the furnace and/or specimen temperature exceeds

a selected value. A circuit diagram of the electrical connections of the various parts of the instrument is shown in

figure

4. FURNACE .CONSTRUCTION DETAILS

The furnace itself has been designed to have small heat

capacity and losses and thus permits rapid changes in its

temperature with moderate power input. Figures 2 and 3 show

construction details of the furnace. The major portion of

the furnace chamber is formed by a one-gallon Dewar flask.

A plug of diatomaceous silica three inches thick forms the

top closure. This plug, together with the interior furnace

structure and blower drive, are mounted on an asbestos-cement

sheet, held by an external supporting device. The interior

of the furnace is provided with a stainless steel baffle which

directs air circulation around the specimen, shields the

specimen from radiant heating, and supports mica strips carrying the nichrome heater winding. This winding is designed to

dissipate a maximum of 700 watts at 135 volts. The blower

rotor is a standard model 3 in. in diameter and having a face

two in. wide, and is driven by an external electric motor.

The blower rotor is mounted on a hollow shaft through which

the wire which supports the specimen passes with only slight

rubbing friction. The shaft passes up through the refractory
- If -

plug and is supported by sealed ball bearings. Power for

driving the blower is furnished from a d-c motor through

a rubber belt.

The parts of the temperature controlling and recording

system and related equipment are of standard commercial construction as listed in figure 5* One exception to this is

the servo-motor which has been modified to reduce its speed

from the nominal 27 r.p.m. to lA r.p.m. This comparatively

slow speed has been used to slow the response rate of the

system but nevertheless allows the furnace to follow closely

the temperatures developed by the self-heating of the

materials studied.

5. FURNACE OPERATION

This furnace has been in operation for more than a year

and has provided reliable performance with need for only

minor repair. The typical set of test data shown in figure

6 was obtained during a test of a laminated wood fiberboard

specimen 3 in. in diameter and 2-5/8 in. long. The starting

temperature for this test was set at a value considerably

above the lowest known temperature at which self-heating would

occur in order to shorten the period required for the completion

of the reaction. During this test, air was supplied to the

interior of the furnace at the rate of 200 ml per min; this

was equivalent to 0.06 air change per minute. It is evident

from figure 6 that the furnace temperature followed the specimen temperature very closely. The maximum temperature difference between the center of the specimen and the furnace

atmosphere after automatic operation commenced appears from

the record to have been less than 1°C. The actual temperature

weight relationships shown on this curve are the result of

complex processes and a discussion of the relationships is

beyond the scope of the present paper. An example of this is

the final temperature rise at constant weight seen near the

end of the test.

Another method for evaluating the performance of this

type of furnace is to conduct a test on a specimen formed of

inert material. If there is actually no heat exchange between

furnace gases and the specimen, the specimen temperature should

remain constant and the furnace atmosphere should remain constant at very nearly the same temperature. Such a test was

performed on a specimen of fire brick with the results

shown in table 1„ These results indicated that there was a
- 5 -

small heat flow between the specimen and the surrounding

air which is considered not serious for the studies now

under way with this apparatus.

6. CONCLUSIONS

1. An apparatus has been constructed for the study

of the self-heating of combustible materials under conditions such that the heat exchange to or from the exterior

of the specimen is kept at a low value.

2. Tests with this device for study of the self-heating

of specimens of wood fiberboard have demonstrated that the

apparatus performed in the manner desired.

3. Tests with inert material indicate that the

stability of the system in the temperature range from

100 to 300° C is such that the rate of temperature change

of the specimen can be kept at or below 2°C per hour.
- 6 -

Table 1 - Temperature Drift of the System when

Operated with an Inert Specimen

Furnace and

Specimen

Temrerature

Rate of

Temperature

Change

Duration

of Test

°C ° C/hr hr

100 +1.0 3

200 -0.5 2

300 -2.0 2

References

1. Bjorn Holmgren, Self-Ignition of Porous Wood

Fiberboard, Svensk Papporstidning , V.51, No. 10,

May 31, 1948.

2. Nolan D. Mitchell, New Light on Self-Ignition,

Quarterly of the National Fire Protection Association, Volume 45, No. 2, Oct. 1951*

3. J. Davis and J.F. Byrne, Journal, American

Ceramic Society 7, 809, 1924.

4. J.L. Elder, L.D. Schmidt, W.A. Steiner and

J.D. Davis, U.S. Bureau of Mines Technical

Paper 68l , 1945*


Constant

Tempozature,

ZoNTR-OLLTfE

STRAIN

GuflQe: DYNAMOMETER,

Fig. 4 Block Diagram or Control System

Legend

SL

- limit switch

S^ - controller relay switch

C - 40 uf. 2?0v

S^ , S^, - DPDT toggle switch

S^ , - SPST toggle switch

- SPDT plate circuit relay 5000 ohms

R

2

- DPDT A.C. latching relay

L - Neon indicator lamp

Recorder - Minneapolis Honeywell 12-point

Electronik Recorder Controller

Controller - Minneapolis Honeywell Electronik

Indicator Controller

Galvanometer Relay - G-M Laboratories Inc. Galvanometer Relay

Servo Motor - Minneapolis Honeywell Servo Motor Amplifier - Minneapolis Honeywell Servo Motor Amplifier

Transformer - Powerstat 7-5 ampere autotransformer

Fig. 5 Wiring Diagram of Control System

(strain gage circuit not included)

ONE

HOUR

Fig 6 Typical Time-Temperature Curve of Wood Fiberboard

THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act

of Congress, March 3, 1901, as amended by Congress in Public Law 619, 1950.

These include the development and maintenance of the national standards of

measurement and the provision of means and methods for making measurements

consistent with these standards; the determination of physical constants and

properties of materials; the development of methods and instruments for testing

materials, devices, and structures; advisory services to Government Agencies on

scientific and technical problems; invention and development of devices to serve

special needs of the Government; and the development of standard practices,

codes, and specifications. The work includes basic and applied research, development, engineering, instrumentation, testing, evaluation, calibration services

and various consultation and information services. A major portion of the

Bureau’s work is performed for other Government Agencies, particularly the

Department of Defense and the Atomic Energy Commission. The scope of activities is suggested by the fisting of divisions and sections on the inside of the

front cover. Reports and Publications

The results of the Bureau’s work take the form of either actual equipment and

devices or published papers and reports. Reports are issued to the sponsoring

agency of a particular project or program. Published papers appear either in the

Bureau’s own series of publications or in the journals of professional and scientific

societies. The Bureau itself publishes three monthly periodicals, available from

the Government Printing Office: The Journal of Research, which presents complete papers reporting technical investigations; the Technical News Bulletin, which

presents summary and preliminary reports on work in progress; and Basic Radio

Propagation Predictions, which provides data for determining the best frequencies

to use for radio communications throughout the world. There are also five series

of nonperiodical publications: The Applied Mathematics Series, Circulars, Handbooks, Building Materials and Structures Reports, and Miscellaneous Publications.

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

Publications of the National Bureau of Standards ($1.00). Information on cali- bration services and fees can be found in NBS Circular 483, Testing by the National

Bureau of Standards (25 cents). Both are available from the Government Printing Office. Inquiries regarding the Bureau’s reports and publications should be

addressed to the Office of Scientific Publications, National Bureau of Standards,

Washington 25, D. C.
))