Reference

NBS

PUBLICATIONS

NAT'L INST. OF STAND & TECH

1110b 3 M b A 3

NBSIR 85-3122

Self-Heating to Ignition

Measurements and Computation

of Critical Size for Solar Energy

Collector Materials

Joseph J. Loftus

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Fire Research

Gaithersburg, MD 20899

March 1985

Final Report

Prepared for:

-QC

100

, U56

85-3122

1985

U.S. Department of Energy

-Office of Solar Heat Technologies

Active Heating and Cooling Division

Washington, DC 20585

NBSIR 85-3122

} pC SELF-HEATING TO IGNITION

MEASUREMENTS AND COMPUTATION

OF CRITICAL SIZE FOR SOLAR ENERGY

COLLECTOR MATERIALS

RATIONAL BUREAU OF STANDARDS

LIBRARY

CitlO

, UCL

m. h ^

-

Joseph J. Loftus

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Fire Research

Gaithersburg, MD 20899

March 1 985

Final Report

Prepared for: U.S. Department of Energy

Office of Solar Heat Technologies

Active Heating and Cooling Division

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director

TABLE OF CONTENTS

Page

List of Tables . iv

List of Figures v

Abstract . 1

1. INTRODUCTION 1

2. DESCRIPTION OF A COLLECTOR AND SOLAR ENERGY SYSTEM 2

3. CAUSE OF THE FIRE 4

4. SELF-HEATING 5

5. CRITICAL SIZE 6

6. TEST APPARATUS 8

7. TEST MATERIALS 9

8. MATERIAL CONDITIONING 10

9. TEST RESULTS 10

10. OBSERVATIONS 11

11. DISCUSSION 13

12. CONCLUSIONS AND RECOMMENDATIONS 14

13. ACOOWLEDGEMENTS 15

14. REFERENCES 16

iii
LIST OF TABLES

Page

Table 1. Solar Collector Materials 18

Table 2. Physical and Kinetic Properties 19

Table 3. Retardant Treated Plywoood 20

Table 4. Plywood 21

Table 5. Polyurethane Foams 22

iv
LIST OF FIGURES

PAGE

Figure 1. Collector Section 23

Figure 2. Roof Section . 23

Figure 3. Adiabatic Furnace Test Apparatus 24

Figure 4. Plywood: Zero Days Conditioning at Room Conditions. Rate

of Temperature Change Versus Reciprocal Temperature K .... 25

Figure 5. Retardant Treated Plywood: Zero Days Conditioning at Room

Conditions. Rate of Temperature Change Versus Reciprocal

Temperature K 26

Figure 6. Plywood - Computed Critical Half Thickness (cm) of a Slab

Versus Critical Surface Temperature °C 27

Figure 7. Retardant Treated Plywood - Computed Critical Half

Thickness (cm) of a Slab Versus Critical Surface

Temperature °C 28

Figure 8. Computed Critical Half Thickness (cm) of a Slab for

Urethane Foam C-32 and C-42 Versus Critical Surface

Temperature °C 29

Figure 9. Computed Critical Half Thickness (cm) of a Slab for

Urethane Foam F-32 and F-42 Versus Critical Surface

Temperature °C 30

Figure 10. Computed Critical Half Thickness (cm) of a Slab for

Urethane Foam G-32 and G-42 Versus Critical Surface

Temperature °C 31

Figure 11. Computed Critical Half Thickness (cm) of a Slab for

Urethane Foam H-32 and H-42 Versus Critical Surface

Temperature °C 32

v

SELF-HEATING TO IGNITION MEASUREMENTS AND COMPUTATION OF

CRITICAL SIZE FOR SOLAR ENERGY COLLECTOR MATERIALS

Joseph Loftus

Abstract •

Kinetic constants of the self-heating reaction were determined for plywood, a

retardant treated plywood, and eight samples of polyurethane foam

representing possibly two different kinds of foam materials. Under the

assumption that self-heating follows a first order reaction, these constants

were used to calculate the critical half thickness of slabs of these

materials for surface temperatures likely to be experienced during long term

use in solar energy collectors. Based on these calculations, estimates are

provided on the self-heating or ignition hazards associated with the size and

use of these materials in solar energy systems.

Key words: Critical temperature; ignition hazards; polyurethane foams;

plywood; reaction rate; self-heating; solar collectors; wood.

1. Introduction

In May of 1980 a solar energy collector fire incident occurred in Boulder,

Colorado in an unoccupied single family dwelling. It was reported that the

house was part of a Federal residential solar heating and cooling

demonstration program and that damage due to the fire was limited to the

collector and adjacent roofing and framing materials. Further investigations

revealed that the solar collector itself was the only possible source of he it

1
for ignition and that the most likely point of ignition was the plywood base

or backing of the collector. Identical collectors on three other dwellings

at the same site were found to have backing material or plywood deck in

various stages of thermal degradation. This may have ultimately led to

similar fire incidents.

In July 1980, the Department of Energy (DOE) requested that the National

Bureau of Standards (NBS) review the factors which may have led to the

ignition of the Colorado collector system and to provide recommendations as

to how similar events may be prevented in the future. The Solar Technology

Group, Building Equipment Division, Center for Building Technology at the

Bureau, gave this task to the Center for Fire Research and requested that the

ignition characteristics of polyurethane foam insulation materials be

determined.

2. Description of a Collector and Solar Energy System

From information supplied by investigators [1]*, the solar energy collector

was reported to be a flat plate type with overall dimensions of approximately

91 cm (36 in.) wide by 239 cm (94 in.) long by 8.9 cm (3.5 in.) thick.

Glazing consisted of two sheets of tempered glass separated by an air space.

The absorber was pressure-bonded copper with a black chrome selective

surface. The insulation directly behind the absorber plate consisted of

approximately 4.4 cm (1 5/8 in.) of factory-applied spray-in-place urethane

foam. The back of the collector was 0.95 cm (3/8 in.) exterior grade

plywood; the framing was clear heart redwood.

-‘Numbers in brackets pertain to references listed in Section 14

2
The manufacturer of the collector reported that the frame and plywood back

were assembled and then urethane foam was sprayed on the plywood prior to the

absorber and glazing being set in place. This process was said to cause a

natural adhesion between the plywood back and the foam insulation. The

collectors were mounted in direct contact with the roof which had a slope of

6 in. rise per linear foot.

The roof consisted of "90-lb." rolled roofing with a granular surface between

the collector backing and the 1.3 cm (1/2 in.) CDX plywood roof deck.

Insulation for the roof was 15 cm (6 in.) kraft paper faced glass fiber

insulation between 2x8 in. joists which were 61 cm (24 in.) on center. A

layer of gypsum board underneath formed the interior finish of a cathedral

type ceiling. A cross section of the collector and roof is illustrated In

Figures 1 and 2 respectively.

The solar energy system used with the collector was an active fluid type

designed to provide domestic hot water and space heating. The heat transfer

fluid was water, and freeze protection was provided by a drain-down design.

Since the house was unoccupied, it was likely that the collector experienced

non-operating or stagnation conditions during a significant portion of time

between its installation and the fire. Flat plate solar collectors under

stagnation conditions can have absorber plate temperatures over 204°C (400°F)

in the case of double glazed, selective surface collectors. After some period

of time at these temperatures, degradation of plastic foam insulation, wood

and other combustible materials would likely occur within the collectors.

3
3. Cause of the Fire

It was reported that the foam in all the collectors investigated had degraded.

The plywood backing showed signs of blackening or conversion into charcoal

only in areas where the urethane foam layer had partly melted away and become

very thin or when large cracks had opened exposing the plywood. As the

plywood was heated over long periods at moderate temperatures, it is suspected

that charcoal was formed and that it self-heated or smoldered to a glowing

combustion. Porous charcoal containing a large surface area is subject to

rapid oxidation and self-heating. There are a number of articles in the

literature [2-7] which describe the behavior of wood exposed to moderately

elevated temperatures. At low temperatures, wood degrades over a long period

of time. Experiments on ignition of wood after prolonged heating are

difficult to conduct and results vary because of variation in exposure

conditions, wood types and material geometries. Experimental work by fire

researchers indicate that cyclic periods of heating and cooling, the moisture

content of the wood, the oxygen supply, the wood type, and material thickness

may all be factors in determining the length and severity of exposure required

for ignition.

Polyurethane foams have been known to self-heat especially when freshly made

and stocked in large piles [8]. There is very little information available

on the temperature at which rigid urethane foams degrade when heated for

extended periods of time. This is due in part to the fact that their

composition and properties vary widely. Investigators of the Colorado

4
collector fire proposed that the polyurethane did not supply sufficient heat

to start the plywood ignition; however, it may have contributed to the fire

when ignited by glowing plywood. This fire incident clearly suggests that a

need exists to gain an understanding of the relationships between self-heating

and the physical parameters involved.

4. Self-Heating

For the purposes of this report, self -heating will be defined here as that

condition which occurs when the rate of heat generation within a material

exceeds the rate at which heat is lost by radiation, convection, and

conduction to its surroundings.

The problem of self-heating was addressed by Gross and Robertson [9] who used

an adiabatic furnace test apparatus to measure self-heating of materials under

conditions of negligible heat loss. Under these conditions, the specimen

temperature remained uniform throughout its mass and the heat generated within

the body increased its temperature according to the relation:

pc

dT

dt

= A exp (-E/RT) ( 1 )

5
where: p = density

c = specific heat

T = temperature within material

t = time

A = heat generation coefficient

E = activation energy

exp = Naperian base

R = gas constant = 0.0019872

-3

g cm

i - 1 v

" 1 cal g K

K

sec

cal sec "'’cm

kcal mole

^

kcal mole

-1

By transposing terms, taking logarithms and plotting In ^ versus —, the

E dT

resultant line has a slope of - (—) and intercepts the In -j-jr axis at the In —A

. For a test material, the determination of the values of A and E can be

pc

made if the thermal properties p and c are known. General self-heating

problems can then be solved, including the case where size and temperature

become critical and ignition results.

5. Critical Size

The critical condition of ignition in the theory of thermal explosions was

proposed by Frank-Kamenetskii [10]. Chambre [11] presented analytical

solutions in terms of known functions for a sphere, a cylinder, and a

semi -inf inite slab. Analyses by Genensky [12] for a sphere and by Thomas [13]

for all three geometries have assumed that temperature differences within the

material are small in comparison to the absolute temperature. Enig, Shanks,

and Southworth [14] removed this restriction and provided solutions in the

form of convenient tables of the center and surface temperatures for sphere.

6
cylinder, and semi -inf inite slab geometries. They proposed that the critical

size for materials may be determined by the relation.

2 = y^RT

s AE

2 exp (-E/RT) ( 2 )

where: B = radius or half thickness cm

y = experimental parameter

A = thermal conductivity

(assumed uniform and constant)

From their tables, the critical half thickness of a slab B can be determined

The size of material and its corresponding surface temperature are referred

to here as critical size and critical surface temperature because the steady

state condition is critical to ignition. Also, the ambient temperature

corresponding to given critical steady state conditions with given heat

losses is considered the critical ambient or self-ignition temperature. Note

that in this analysis a heat transfer coefficient is not included but would

be required in order to determine the critical size for a given ambient air

temperature. For reasons described in Section 3, critical conditions

developed at the location of the solar collector system's plywood roof deck

polyurethane foam interface where heat energy from the sun collected at this

s given an assumed surface temperature T^.

7
surface faster than it could be dissapated through the well insulated roof

deck. Because the plywood ignited in this particular scenario, it is clear

that material thickness becomes a critical factor for certain levels of

temperature exposure and work needs to be done to determine critical size for

solar collector materials.

6. Test Apparatus

Figure 3 shows a schematic diagram of the adiabatic furnace test apparatus

used to make the self-heating determinations. Briefly the apparatus consists

of a 5 liter capacity Dewar flask which is enclosed within a close fitting

cylindrical shell. The shell consists of two concentric stainless steel

cylinders containing electric heating elements and insulating fill material.

The shell also contains a bottom guard heater; a top heating element is

located in the plug that fits into the top of the Dewar flask. Air is

circulated around the test specimen by a fan whose motor assembly is mounted

at the top of the furnace. A multiple junction thermocouple system consisting

of eight No. 28 gauge chrome 1-alumel thermocouples arranged in series is used

as the sensing element for a servo controller designed to maintain null

temperature differences between the specimen surface and its center. The

controller can sense temperature differences on the order of 0.01°C. Typical

surface/center differentials or sample/air differentials were 0.1. The

rate of drift of furnace temperatures as observed with an inert specimen was

0. 003°C/hr.

8
7. Test Materials

It is unfortunate that the sprayed-in-place urethane foam insulation from the

Colorado solar collector fire incident was not available for this study,

however, a total of 8 polyurethane foam samples gathered from a number of

different solar collector test sites throughout the country were furnished

for test. In the labeling scheme, a letter identifying the collector type

(C,F,G and H) is followed by a number identifying the test site (3,4) which

in turn is followed by another number identifying the test series (2). For

example, C-32 represents collector type C, exposed at test site (3), to a

series (2) exposure condition. For this test series the "32” samples were

exposed at site (3) Palo Alto, California under (2) stagnation conditions.

The "42" samples were exposed at site (4) Gaithersburg, Maryland also under

(2) stagnation conditions. Stagnation here means no fluid flow through the

collector plate. Table 1 lists the color and thickness of the materials and

indicates whether aluminum foil facing was attached to one or both surfaces

of the rigid foam materials. Visual inspection showed that none of the

sample materials were damaged in any way and, except for color, no

differences were noted among the foam materials. Plywood and retardant

treated plywood rounded out the test material listing and these were supplied

by the Center for Fire Research. The retardant treatment used for the

plywood materials is not known.

9
8. Material Conditioning

Test samples were made up of 7.6 cm (3 in.) diameter disks stacked to form a

cylinder 7.6 cm (3 in.) high. Two sample sets were prepared from each of the

plywood test materials. One was conditioned in the laboratory at room

conditions of 20°C (68°F) and 50 percent relative humidity, while the other

was conditioned in an air circulating oven operated at 95°C (203°F). The

length of time a sample remained in the room or the oven before testing,

was recorded as the total number of days conditioned for test samples

listed in Tables 3 and 4. Oven conditioning was used to determine whether

long term aging (up to 2 years) at an elevated temperature would have any

effect on the self-heating characteristics of the plywood materials. The

polyurethane foam samples were not oven conditioned prior to test to avoid

the possibility of ignition of the sample in the oven or the release of

noxious gases into the laboratory.

9. Test Results

Although it may have been possible to start the self-heating tests at lower

ambient air temperatures, a decision was made to initiate testing at

approximately 120°C (248°F). This temperature selection was based in part on

(a) past self-ignition experience with wood materials, (b) a report by the

American Plywood Association [7] that the critical temperature for wood

materials range from 110 to 150°C (230 to 302°F), (c) an attempt to reproduce

those temperatures reported by Boeing Co. in simulated solar collector tests

10
on a built up roof deck, and (d) on a desire to complete the self-heating

tests in a reasonable length of time.

The plywood test samples, randomly selected and tested over a two year

period, self -heated to destruction in times ranging from 40 to 45 hours,

while the eight polyurethane foams required 5 to 10 hours to destruct.

Table 2 lists the physical and kinetic properties for the test materials.

The values of p and c were obtained from handbook sources [15,16]. The

estimated accuracy for the polyurethane foam values is about 3 to 4 percent.

The kinetic constants E and A were determined over the indicated temperature

ranges from a least squares fit of the experimental data assuming first order

reaction (equation 1). Figures 4 and 5 show typical plots of the rate of

temperature change (°C/min) versus the reciprocal of temperature (K) for

plywood and retardant treated plywood samples with zero days conditioning.

Tables 3 to 5 show computed critical half thickness (cm) for slabs of

retardant treated plywood, plywood, and the polyurethane foams respectively,

for projected ambient temperatures ranging from 20 to 120°C (68 to 248°F).

Figures 6 to 11 show plots of these data from zero day ambient conditions

taken from tables 3 to 5.

10. Observations

1. The retardant treated plywood discolored and charred badly as a result of

one week of oven conditioning at 95°C.

11
2. Activation energy values determined for plywood ranged from 21 to 23

kcal/mole. By comparison, Stamm [17] reported values ranging from 23 to

26 kcal/mole for wood, alpha-cellulose and lignin.

3. Room conditioned plywood and retardant treated plywood samples showed

(with one exception) increased critical pile size with the number of days

conditioned before test. Reasons for this phenomenon are not clear. It

may be due to changes in moisture content, glue curing, or loss of

retardant treatment with time.

4. Oven conditioned plywood and retardant treated plywood samples showed

generally lower computed critical size values than did room conditioned

materials for 20°C (68°F). However, at higher temperature levels, 40 to

120°C (104 to 248°F), pile size differences were negligible.

5. Aging of plywood samples at 95°C (203°C) for extended periods of time (up

to 2 years) did not appear to affect the self-heating characteristics of

the plywood and retardant treated plywood materials tested here. MacLean

[4], however, reports charring of wood samples at temperatures as low as

93°C (203°F). He concludes that wood should not be exposed to

temperatures appreciably higher than 66°C (150°F) for long periods.

McGuire [5] suggests that the maximum safe temperature on the surface of a

combustible material adjacent to a constant heat source sould not be more

than 100°C (212°F). Numerous documented fires involving the ignition of

wood members near low pressure steam pipes [6] suggest an upper limit for

12
wood exposed to long-term low-level heating should not be appreciably

higher than 100°C (212°F).

6. Data for polyurethane foams shows that the "42" labeled samples (with the

exception of "G") produced slightly larger half thickness values for slabs

than the "32" labeled foams. Since no information was furnished on the

properties of the urethane foam samples, the significance of slab size

differences between the foam sets could not be established.

7. Considering 20°C (68°F) ambient temperature conditions, slab half

thicknesses for all foams ranged from 11 to 57 cm (4.3 to 22 in.). At

120°C (248°F) slab thicknesses for all foams were similar to those

determined for the plywood samples.

11. Discussion

The Boeing Company (Report TR-2-5612-8000-039) used a solar simulator to

determine the effects of radiant heat transfer on materials used in a full

size solar collector mounted on a simulated roof deck. For tests simulating

non-operating or stagnant condition, Boeing used a solar irradiance ranging

from 0.08 to 0.11 watts/cm (250 to 350 BTU/ft hr), (solar constants 0.566 to

0.792). After seven hours of testing, the peak temperature measured at the

plywood/foam interface was 144°C (292°F) and inside the polyurethane foam

insulation it was 173°C (344°F). Material inspection after the tests showed

degradation of the polyurethane foam and charring on the nlywood collector

base surface. Additional tests showed that for a collector not mounted

13
directly on a simulated roof, the plywood/foam interface peak temperature was

89°C (188°F) and the foam temperature was 136°C (278°F).

Based on Boeing's findings one may conclude that the solar collector not

mounted directly on the roof was a slightly better assembly because the air

space between the bottom of the collector and roof surface allowed for lower

temperatures in the solar collector materials. In effect, the insulation

contribution of the roof to the assembly was negated. Comparison of this

temperature data with the self-heating data reported here, however, indicates

that these polyurethane foam materials would be destroyed in either solar

collector assembly. Plywood/foam interface temperatures in Boeing's work were

173°C and 89°C (344 and 188°F) for the direct mount and air mount assemblies.

Based on the CFR data, the 173°C (344°F) temperature would be unacceptable and

the 89°C (188°F) temperature would only allow for a few cm thickness for foam

and plywood material to be used in the collector.

12. Conclusions and Recommendations

Analysis of self-heating to ignition tests were made under the assumption that

self-heating follows a first order reaction law. Experimental determinations

of kinetic constants of this reaction were made for the solar collector

materials furnished for test and for plywood and retardant treated plywood.

These constants were used to compute critical ambient temperatures as a

function of sample size. The critical half thicknesses for slabs predicted

from these measurements were only a few centimeters for temperatures in the

vicinity of 120°C. This suggests that self-heating ignition may be expected,

14
and that proper fire-safe designs of collectors should ensure that (1)

permissable operational and stagnation temperatures be lowered, or (2)

material thicknesses decreased, or (3) less ignition prone materials be

employed in regions of expected high temperatures. If plywood and

polyurethane materials, similar to those tested, are retained, steps should be

taken to ensure that the allowable ambient temperature limit is reduced

considerably. From the critical size data developed in this test series

it appears that 60°C (140°F) would be about the maximum allowable temperature

for plywood layers but thicknesses of approximately 8 cm (3.2 in.) would be

the largest acceptable for the polyurethane materials.

13. Acknowledgements

This study was sponsored by the U.S. Department of Energy (DOE). The support

of DOE and Mr. Robert Dikkers, of the Center for Building Technology at the

National Bureau of Standards (NBS), is gratefully acknowledged. Thanks are

also due to Dr. Vytenis Babrauskas, Dr. John Rockett, Mr. Emil Braun and Mr.

Daniel Gross for providing input in the analysis and interpretation of test

results.

15
14. References

1. Walton, W. D. , Solar Collector fire Incident Investigation, Nat. Bur.

Stds. (U.S.), NBSIR 81-2326 (1981).

2. Schaffer, E. L. , Smoldering Ignition in Cellulosics Under Prolonged

Low-Level Heating, Fire Technology, Vol. 16, No. 1, 22-28 (Feb 1980).

3. Forest Products Laboratory, Ignition and Charring of Wood, Forest Products

Laboratory Report 1464 (1958, reviewed and reaffirmed in 1963). Madison.

4. MacLean, J. D. , Rate of Disintegration of Wood Under Different Heating

Conditions, American Wood-Preservers Association, Vol. 47, 155-169,

(1951).

5. McGuire, J. H. , Limiting Safe Surface Temperature of Combustible

Materials, Fire Technology, Vol. 5, No. 3, 237-241, (Aug. 1969).

6. Matson, A. F., Dufour, R. E., and Breen, J. F. , Part II Survey of

Available Information on Ignition of Wood Exposed to Moderately Elevated

Temperatures. Performance of Type B Gas Vents for Gas Fired Appliances,

Bulletin of Research No. 51, Underwriters Laboratories, Inc., 269-300,

(May 1959).

7. O'Halloran, M. R. , Plywood in Hostile Environments; Physical Properties

and Applications, American Plywood Association Report 132, 8-9,

(December 1975).

8. Materials Subject to Spontaneous Heating, Fire Protection Handbook,

Fourteenth Edition, 3-206-3-209, NFPA, Boston, MA (Jan. 1976).

9. D. Gross and A. F. Robertson, Self-Ignition Temperatures of Materials from

Kinetic-Reaction Data, Research Paper 2909, J. Research NBS Vol. 61, No.

5, (Nov. 1958).

10. D. A. Frank -Kamenetskii , Acta Physicochim, U.R.S.S. 10, 365 (1939).

11. P. L. Chambre, On the Solution of the Poisson-Boltzman Equation with

Application to the Theory of Thermal Explosions, J. Chem, Phys. 20, 1795

(1952).

12. S. M. Genensky, A Problem in Self-Heating of a Spherical Body, J. Research

NBS 59, Report 2773 (1957).

13. P. H. Thomas, On the Thermal Conduction Equation for Self-Heating

Materials With Surface Cooling, Trans. Faraday Soc. 54, 60 (1958).

14. J. W. Enig, D. Shanks and R. W. Southworth, The Numerical Solution of the

Heat Conduction Equation Occurring in the Theory of Thermal Explosions,

NAVORD Report 4377 (1956) NTIS No. AD 116 873.

16
15. ASHRAE Handbook 1981, Sec. 15, 23, (1981).

16. Perry's Chem. Eng. Handbook, 3-224, 4th Ed., (1963).

17. Stamm, A. J. , Thermal Degradation of Wood and Cellulose, Ind. and Eng.

Chem, 48, 413-417, (1956).

17
TABLE 1 SOLAR COLLECTOR MATERIALS

Material Color Thickness

mm

C-32 urethane foam pink 76

C-42 urethane foam cream 76

F-32 urethane foam tan 25

F-42 urethane foam tan 25

G-32 urethane foam tan 25

G-42 urethane foam tan 25

H-32 urethane foam pink 25

H-42 urethane foam pink 25

plywood natural 12

retardant treated plywood natural 12

* one surface painted

Al. Facing

none

none

2 surfaces

2 surfaces

1 surface

1 surface

2 surfaces*

2 surfaces*

none

none

18
TABLE

2.

PHYSICAL

AND

KINETIC

PROPERTIES

w

Pd o w 5 O NO z o H W 00 00

1—

< <5

O r—1 rH

H pd Z O 1 1 Pd s w ON CM < p Pd ° rH CM H E § rH rH

cn

w '— '—

H

/—

N /^. /—

/^N /^S /~S VO 00 CO CM m vx) o CTn oo rH CM m

r—1 rH rH i—CM CM 1 i rHi i—

1 1 r- oo O v1 1 o o i o 1 o

rH i—CM rH CM i—l CM CM

rH i—rH rH rH rH i—rH

'w' Nw' V-X Sw' 'w' 'w' 'w'

CO1 CM CO CO NO in CO *d- ON o

i-H e + + + + + + + + + + o O o o o o o o o o o rH 1—1 rH 1—

1—1 1—1 rH 1—

i—l 1—rH

CJ

X

X

XXXX

XX

X

X < a>w 00 CM ON NO >d- rH r^» o NO

V UO ON 1—

ON CO ON ON o 1—1

M i—l H to r*^ iH

no CO CM NO i-H i-H

wzM

fcd

CO CO CO t—l NO CM NO o o

t—

CM CM n- NO CM i-H CO

i—

rH rH rH rH rH rH rH CM CM ) Pd m m m m m m m m <1- o o o o o o o o o o £ rH i—l i—

rH rH i—l rH rH i-H rH o X X X X X X X X X X

CJ i-H 1—1 rH i-H rH rH rH rH NO NO

0)

m

m

mmmm

m inm

m

Cfl • • • • m m m m m m m m CM CM

w

rH 00 CO CO 00 00 00 00 00

m

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U 1 CO CO CO CO CO co CO CO

mm

00 o o o o o o o o o o rM

50

CJ

co| o 'd- CM ON ON ^d- NO

Q. £ CM co CO CO CM CM co CO ON

CJ o o o o o o o o m • • • • 00 QWH

PdH

h4cM

PdwH

HZ Q

Q <2 o CM CM CM CM CM CM CM CM o Q O CO co <r co <r CO <f o Pd 3 1 i 1 l 1 1 1 1 < >- u CJ pH PL e> o PC PC >1 H J

_1W Pl, P- Pd

19
TABLE 3. RETARDANT TREATED PLYWOOD

Slabs - Computed Critical Half Thicknesses (cm) at Various Temperatures

Conditioning Temperature (C)

@ Ambient 20°C @ 95°C 20 40 60 80 100 120

Days Days Half Thickness (cm)

0 180 53 18 7 3 1.5

420 214 60 20 7 3 1.4

540 180 52 18 7 3 1.4

730 263 72 23 9 4 1.5

14 51 20 9 4 2 1.3

69 202 59 20 8 3 1.3

100 103 36 15 7 3 1.3

171 114 37 14 6 3 1.4

266 131 42 15 6 3 1.4

630 257 70 23 8 4 1.5

730 78 28 11 5 3 1.5

20
TABLE 4. PLYWOOD

Slabs - Computed Criticl Half Thickness (cm) at Various Temperatures

Conditioning Temperature (C)

Ambient 20°C <3 95°C: 20 40 60 80 100 120

Days Days Half Thickness (cm)

0 96 33 13 5 3 1.3

420 198 56 19 7 3 1.4

540 302 79 25 9 4 1.3

690 401 103 31 11 4 1.8

21 55 20 8 4 2 1.4

85 103 35 14 5 3 1.5

177 62 23 10 5 3 1.3

248 206 60 20 8 3 1.6

540 74 26 11 5 3 1.3

600 112 37 14 6 3 1.4

680 122 40 15 7 3 1.5

21
TABLE 5. POLYURETHANE FOAMS

Slabs - Computed Critical Half Thickness (cm) at Various Temperatures

Foam Material Temperature (C)

20 40 60 80 100 120

Half Thickness (cm)

C-32 16 10 6 4 3 1.9

C-42 23 13 8 5 3 2.2

F-32 13 7 5 3 2 1.2

F-42 22 11 6 4 2 1.4

G-32 57 23 10 5 3 1.6

G-42 52 22 11 6 3 1.9

H-32 11 6 4 3 2 1.1

H-42 24 11 6 4 2 1.3

22
Tempered glass

Air space

Tempered glass

Air space

Absorber plate

Urethane foam

Plywood back

Figure 1. Collector Section

Solar collector

Rolled roofing

Sheathing

Glass fiber insulation

Gypsum board

2” x 8” joist

Not to scale

Figure 2. Roof Section

23
24
05

c\i

(UjOJ/Oo) 3DNVH0 3aniVa3dlAI31 dO 3iva

Figure 4. Plywood: Zero days Conditioning

at Room Conditions. Rate of

Temperature Change Versus

Reciprocal Temperature K

25

2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

TEMPERATURE

(1/T x

10

3

K)
cvi o

cvi

o>

(U!UJ/o 0 ) 39NVH0 3an±Vd3dlAI31 30 31Vd

Figure 5. Retardant Treated Plywood: Zero

Days Conditioning at Room

Conditions. Rate of Temperature

Change Versus Reciprocal

Temperature K

26

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

TEMPERATURE

(1/T x

10

3

K)
(luo) avis v jo

SS3NH0IH1 d"IVH IVOIlldO

27

20

40

60

80

100

120

140

CRITICAL

SURFACE

TEMPERATURE

(°C)

Figure

6.

Plywood

-

Computed

Critical

Half

Thickness

(cm)

of

a

Slab

Versus
(mo) avis v do

SS3N>IOIHl dlVH nvomao

28

20

40

60

80

100

120

140

CRITICAL

SURFACE

TEMPERATURE

(°C)

Figure

7.

Retardant

Treated

Plywood

-

Computed

Critical

Half

Thickness

(cm)

of

a
O C\J o 0 0^*0

(ujo) avis v do

SS3N>I0IH± d"IVH IVOIlldO

29

20

40

60

80

100

120

140

CRITICAL

SURFACE

TEMPERATURE

(°C)

Figure

8.

Computed

Critical

Half

Thickness
co cvj o

o o ° o

(wo) avis v jo

SS3N*0IH± J"1VH nvoiiiao

30

20

40

60

80

100

120

140

CRITICAL

SURFACE

TEMPERATURE

(°C)

Figure

9.

Computed

Critical

Half

Thickness

(cm)

of

a

Slab

for

Urethane

Foam
(iuo) avis v do

SS3N>IOIHl dlVH IVOIlldO

31

CRITICAL

SURFACE

TEMPERATURE

(°C)

Figure

10.

Computed

Critical

Half

Thickness
o

CO CVJ *“ O

O O o o

(wo) avns v do

SS3NN0IH1 dHVH nvoiimo

32

CRITICAL

SURFACE

TEMPERATURE

(°C)

Figure

11.

Computed

Critical

Half

Thickness

(cm)

of

a

Slab

for

Urethane

Foam
NBS-1 1 4 A REV. 2-60

U.s. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 85-3122

2. Performing Organ. Report No. 3. Publication Date

March 1985

4. TITLE AND SUBTITLE

SELF-HEATING TO IGNITION MEASUREMENTS AND COMPUTATION OF

CRITICAL SIZE FOR SOLAR ENERGY COLLECTOR MATERIALS

5. AUTHOR(S)

Joseph J. Loftus

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State, ZIP) U. S. Department of Energy

Office of Solar Heat Technologies

Active Heating and Cooling Division

Washington, DC 20585

10. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Kinetic constants of the self-heating reaction were determined for plywood,

a retardant treated plywood, and eight samples of polyurethane foam representing

possibly two different kinds of foam materials. Under the assumption that

self-heating follows a first order reaction, these constants were used to

calculate the critical half thickness of slabs of these materials for surface

temperatures likely to be experienced during long term use in solar energy

collectors. Based on these calculations, estimates are provided on the

self-heating or ignition hazards associated with the size and use of these

materials in solar energy systems.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate <e, *ords i t

-> <•- Critical temperature; ignition hazards; polyurethane foams; plywood; react i n rate; self-heating; solar collectors; wood.

13. AVAILABILITY 14. NO. OF

| ^Unlimited

| | For Official Distribution. Do Not Release to NTIS

PRINTED PAGES

38

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