NATIONAL BUREAU OF STANDARDS REPORT

2795

INDUSTRIAL RADIANT HEATER CORPORATION

ELECTRIC MAP HEATER

Uy

0* N« McDorman

P. R* Achenbach

Report to

ARM! MAP SERVICE

CORPS OF ENGINEERS

DEPAKEMENT OF THE ARM!

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

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

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1000- 30-4830 September 15, 1953 2795

INDUSTRIAL RADIANT HEATER CORPORATION

ELECTRIC MAP HEATER

ijy

Oo N. McDorman

P» R* Achenbach

Heating and Air Conditioning Section

Building Technology Division

to

Army Map Service

Corps of Engineers

Department of the Army

The publication, n

unless permission

25, 0, C. Such pe

rally prepared If I

. , „ ... , 1 ,1 in pait, Is prohibited Approved for public release by the f J

tandards, Washington Director of the National Institute of been specift- Standards and Technology (NIST) .eport for its own use, on October 9, 2015.
'• 4 J” ,--v

•• - •,•/'* "" '-
INDUSTRIAL RADIANT HEATER CORPORATION

ELECTRIC MAP HEATER

0* N. McDorman & P« R, Achenbach

ABSTRACT

Tests were made of an electric map=heater submitted by the Army Map Service, Corps of Engineers,

U. S. Army to determine uniformity of heater temperature, the portion of the energy transmitted

to the map by radiation, and the wave length of

the energy emitted by the heated surface. The

results showed that the heater, when operating

on a 220-volt source, had a surface temperature

of 720 “F ± 12®. The measurements showed that the

percentage of heat transmitted to the map by

radiation would exceed 90 percent of the total

heat transfer. Nearly all of the energy emitted

from the heater was determined to have a wave

length between 1.2y4</to 6/c/. The radiant energy

would not be appreciably refracted or bent in

travelling through the short path of air en- countered in normal use of the map heater.

I, INTRODUCTION

In accordance with a request dated June 9# 1953 of ‘ttis Army Map Service, Corps of Engineers, U. S. Army, measurements

were made of the operating temperatures and radiation characteristics of an electric map-heater submitted by Mr. Gurtowski

of the Army Map Service.

The following five operating characteristics of the heater

were investigated, as agreed upon with the representative of

the Army Map Service,

1. Heater surface temperature when operated on a 220-volt source.

2. Uniformity of temperature over the full heater

surface.

3. The percentage of the heat absorbed by the map

that was transmitted from the heater by radiation.

h. The wave length of the radiant energy.

5 . The possibility of the radiation bending.

II, TEST SPECIMEN

The outside dimensions of the map-heater were approximately 43 5/8“inches long x 3I l/2-inches wide x 5 3/^“inches

= 2-

high. An area of about 36-inches x 24-inches of the heater

surface was heated electrically by elements underneath the

surface material, A nameplate attached to the heater con- tained the following informations

Industrial Radiant Heat Corp.

Gladstone, New Jersey

220 AC Volts 6820 Watts 3I Amps

Model 2k X 38 Serial No. CF ^k6

Fig, 1 is a side view of the specimen tested and Fig, 2

is a view of the under side of the heater showing the woven

glass cloth which comprised the heat-emitting surface,

III. TEST APPARATUS AND PROCEDURE

A test apparatus was constructed which simulated the

horizontal track arrangement employed at the Army Map Service

in making maps with such heaters. Calorimetric methods were

used to measure the heat emission of the heater and to evaluate

the significance of radiation in the transfer of energy, A

heat receiver was constructed of an aluminum plate 36 inches x

2k inches x 7/8 inch. In the transfer of energy tests the

heater was placed horizontally with the glass cloth 2 inches

above the receiver surface simulating conditions of use by the

Army Map Service., Fig, 1 shows the heater and receiver in

position.

Two separate series of tests were made to determine the

amount of heat which reached the receiver and the portion of

this heat which was radiant in nature. For the first condition

the aluminum surface of the heat absorber was cleaned. For the

second condition the surface of the aluminum plate which was adjacent to the heater surface was painted with two coats of

semi-gloss ^ite enamel to materially change its emissivity

without appreciably changing conduction and convection heat

transfer. The heater was energized and allowed to reach a steady temperature before placing it over the aluminum

receiver. At a chosen time the heater was quickly moved over

the receiver on the horizontal track, allowed to remain over

the receiver for exactly 10 minutes, then disconnected from

the power source and moved away from the receiver. Heater

surface and plate temperatures were observed at one-minute

intervals during the 10-minute heating period and at regular

intervals for a period of 90 minutes

,

The temperatures on the heater surface were measured by

means of 6 butt-welded chromel-alumel thermocouples, one junction being located in each of the 6 square feet of heated

area.

Fig, 2 shows the thermocouple placement. An exploratory

thermocouple capable of being placed at any surface point was

“ 3

“

also constructed to measure temperatures in "bet-ween the permanent= ly attached thermocouples « The rise in reeelTer plate temperature

vas ascertained hy the temperatures observed -with 8 surface-type

copper-constantan shielded thermocouples^ four being secured to

the upper surface of the receiver and four on the lo-wer surface*

When the receiver ms used in its initial state the thermocouple

junctions on the upper surface were covered with a reflective

tape* This tape ms painted the same color as the plate for the

second series of tests*

Emisslvlty measurements were made at the temperature of

boiling water on selected areas of different colors of a typical

map and of the white paint used to cover the aluminum receiver*

IV0 TEST RESULTS

Heater Surface Temperature and Uniformity

The permanently-attached and the exploratory thermocouples

showed that the temperature of the glass cloth surface of the map

heater ranged from 708®F to 732®F at an applied voltage of 220*

The heater current ms 27*2 amperes at this line voltage and the

watthour meter readings indicated a unity power factor* The

observed current and voltage reading® corresponded to a power

consumption of 59^0 mtts *

Emisslvlty of Map and Fainted Surface

Emissivity measurements made of the white paint used to

cover the alurndnum receiver and of a typical map specimen

furnished by the Army Map Service showed almost equal values*

The average of fi^v® observations of the emissl^vity of the white

paint ms 0*920 Pereas the average ©f four readings taken on

different parts of the map was 0 * 926 * These results which -were obtained at boiling temperature of water are summarised in

Table 1* The literature Indicates that there would be only

small changes in emissl^vity of these materials for the temperature rang® from 100®F to 212®F*

Table 1

EMISSIVITISS OF WHITES FABW AM) OF ELASTIC MAP

Test White Paint Map Color ©f

Map Area

1 0*910

2 0*933 0*935 gray

3 0*91% Oo916 gray

k 0*919 0*929 green

5 0o92ii 0*926 white border

Avgo 0*920 0.926

Wave Length and Beniing

•Rie map heater was snoiaiited in front of an Infrared spectrometer

and operated on 208 volts o After the current had heen on for 30

minutes the spectral energy distribution of the emitted energy "was measxiredo The energy spectrum was found to be continuous from

lo^to Gjaj o The energy emitted outside this spectral region

was very small in amount » The energy increased gradually from 1*2 w

to y«/ where it remained constant for a small region of the

spectrum and then decreased gradually to 6^* A globar source

was measured on the same instrument -with similar conditions and

was found to have a spectral energy distribution approxiamtely

that of the map heater when the globar source was at about 700®Fo

This indicates that the map heater has a spectral energy distribu^

tlon which is similar to that of many heated surfaces '5>jhose

emissivity is O08 or greataro

The infrared radiation travelling through short paths of air

is not appreciably refracted ©r bento The index of refraction of

air is nearly unity (I0OOO27) and the coefficient of temperature

change of the refractive index is about 0»000001 per degree

centigrade o The change In direction ©f the radiation when passing

through air at different temperatures would fee very soiallo There

would be a considerable change In direction of transmitted

radiation within the plastic map radiation fell on the surface

at an angle other than perpendicular to the surface.

The transmission of the plastic map material was measiired.

It was found to fee opaque except from 3 ® 5^to Iam* The percent

transmittance was low in this region and 'it reached its greatest

value at 5«2 microns Aere It was 9 percent.

Radiant Heat Transfer

If the heat transmitted from the heater surface to the heat

absorber were transmitted solely fey conduction and convection,

then covering the aluminum surface of the heat receiver with

paint would not alter significantly the amount of heat received

and retained by it with identical heater surface teEsperatures

and the same ambient temperature conditions. If a portion of

this transmitted heat were radiant, altering the receiver surface

emissl^vity would influence the amount of heat received and

retained by the plate. Furthermore, if the emissivity factor of

the receiver and the heater surface is Imown, the percentage of

radiant heat can fee computed by the Stefan=Boltzmann equation,

A fundamental concept of heat transfer by convection is the

movement of a fluid across the heated surface to convey the heat

from one place to another and a further essential concept of

convective heat transfer is that the heat flow is in the same direction as the fluid flo^w. The air flow pattern between the

aluminum absorber plate and the heater surface was observed while

the heater was energized and located over the absorber to reveal

= 5

=

the direction of air flow and heat flow.

Fig 6 3 shows the air flow pattern which was ohserwed when

soioke was introduced "between the heater and receiver surfaces o Air

entered from the outside edge and passed toward the center over the

absorber surface^ rose toward the heater surface^ and then reversed

its direction and flowed over the heater surface toward the edge#

This pattern Indicated that there was probably no heat transferred

by convection from the heater to the absorber*

The heat transferred from the heater surface to the absorber

plate by conduction for a still air condition would be expressed

by the equations

"ifeere

KA Z^Tt

d

K is the thermal conductivity of air ^ approx* 0*20 Btu/hr(sq ft)(®F/in)

He

A is the area of the absorber^ sq ft

^ T is the average tes^erature difference between heater and plate^ ®F

t Is the length of the heating period^ hr

d is the distance between the heater and absorber^ in*

For a separation of 2 inches between the heater and absorber and

a heating period of 10 minutes the heat transmitted by conduction would

be 0*1 times the average temperature difference between heater and

absorber provided the air was still* However^ the air flow was observed to have a eounterflow relationship to the direction of heat

conduction* Hence the air between the heater and absorber was con= stantly being replaced by cool ambient air so the heat conduction

would be consitSerably' less than the values computed for still air

conditions *

Three measurements were made of the heat transferred to the bare

aluminum plate for a range of applied voltage from I38 to I98 eorres«= ponding to averageiae^isr surface temperatures ranging from 637®F to

870®F* Two measurement® were made ©f the heat transferred to the

painted aluminum plate for applied voltages of I98 and 227 corresponding to average heater surface temperatures of 638®F and 695

respectively* The results of these tests are summarized in Table 2*

Since the plate was heated above ambient air temperature during each

test5 there was some loss of heat from the plate by convection to

the aoibient air and by radiation downward during each test* These

corrections are shown in Table 2 in determining the total heat

absorbed by the aluminum plate during each test*

The results in Table 2 for tests 3 and 5 show that with

identical heater surface temperatures of 637 o 5“F ± 0*5®F the

»6-

alundniaa plate retained 7o7 times as much heat -^en coated with

paint as -Mhen hare. The heater surface temperatures during these

tests were adjusted hy regulating the supply voltage

»

Several methods "were used to compute 'what percentage of the

total heat absorbed by the receiver was transferred by radiation.

One approach was to use the data in Table 2 together -with the

measured emissivity of the paint and the map and the results

described under the 'wave length measurements in the StefanBoltzmann equation of transfer of energy by radiation. This

analysis resulted in a value of 89.9 percent heat transfer by

radiation as being the minimum possible. Other methods in=

dicated that the more probable percentage of heat transfer by

radiation was above 97 percent. These computations are sho'^

in the appendix.
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APPENDIX

Computations to determine the percentage of the heat absorbed by

the receiver that was transferred by radiationo

Heat stored in the absorber plate during any test^ Hs,

Hg - Speco heat x weight x temp* rise

Weight of aluminum absorber^ 7^ lb

Specific heat of aluminum^ 0o226 Btu/lb (®F)

Hg s 74 X 0«226 x^ T s 16o 72 Btu/®F tempo rise

Consider Test 5 in Table 2

Rise in absorber tempo ~ 100®F

Therefore Hg ^ l6o72 x 100 ^ I672 Btu

Heat loss of plate to asabient air by convection during testo

Using the formula for free convection from a flat plate

in the ASHVE Guide (See page 9^^ 1953 Guide)

0.25 P

= 0,U78(JL) (^T

=) At for upper surface

1

' L

AT 0,25

Hc2 = 0.239(-k) I^ L

") At lower surface

P ^T 0,25

He = o.7i7(ir) ( °) AT for both surfaces

where is the ratio of the air pressure during the test

to atmospheric pressure

^ T is the average temperature difference between the

plate and ambient air^ ®F

L is the average width of the plate^ ft

For Test 5 in Table 2

Hg - 0,717 X 47 - 70o2 Btu/hr (sq ft)

Hg - 70 Btu during a 10 min test for the entire plate.

The actual convection loss was probably less than that

computed by the above equation because it applies to a plate to

which the air has free access above and below. These conditions

did not exist during the test.

Heat radiated from the bottom of the plate to the supporting

table during the test can be approximated using the Stefan-=Boltzmann

equation with a combined emisslvity factor of 0,1

= 7 Btu for Test 5 in Table 2

Total heat transmitted to the painted absorber plate during

Test 5, Qp^

Qp s Hs + He + ^ 1672 + 70 + 7 = 17^9 Btu

Using the same procedure for Test 3 5 made -with the absorber

plate unpainted^

Qa - 225 Btu

Considering the three methods of heat transfer from the

heater to the absorber plate the total transfer can be

expressed as foUo¥ss

^ 4

Op s c + Ep(^ (Tj^ “ '1^2 ) PaiJited plate

4 4

= c 4- ^7* (Tj^ “ *^2 ^ bare plate

where c is the total heat transfer from the heater to absorber

by convection and conductiono c -would be equal for the painted

and unpainted plate for practical purposes

»

Ep is the combined emissiYlty factor of heater surface

and painted plate

Ej^ is the combined emissl-^rity factor of heater surface

and unpainted plate

Tj_ is the average absolute temperature of the heater

T2 is the average absolute temperature of the absorber

Using the observed temperatures in Tests 3 and 5

( 1)

Qp - c + 2298 Ep

- c ^ 2358 E^

( 2)

(3)

Combining equation® (l)^ (2) and (3)

2298 Ep - 7«78 (e + 2358 E^)

6o78c ^ 2298 Ep = 18345 E;^

c - 339 Ep » 2707 E^ (M

e +

Using the formula for conibined emissivity of two large parallel

surfaces

E »

«1 «2

(V *2^- *1*2

and e_ = emissivity of painted absorber = 0.92 (from experimental

data)

ejj^ emissivity of heater surface^ > 0,8 (from experimental

data)

e^^ ~ emissivity of aluminum plate

The maximum possible value of E s 0«92 taMng e|^ s 1,0

» t The minimum possible value of s 0,0^9^ taMng e^^ ^ 0,05 and ej^ s 0

Then, the maximum possible value of heat transfer by convection and

conduction would be from equation (4)

c = 339 X 0,92 - 2707 x .0^9^ ^ 178 Btu

But the total heat transferred was determined to be 17^9 Btu

Minimum transfer by radiation ^ ^'^^1749^°^^ ~

More probable values of e^^ and taken from the literature are O0O7 and 0,90, respectively. Based on these values the

minimum transfer by radiation would be 94,5^ of the total transfer,

A Second Method for Determining Radiant Heat Transfer

ejj 0,8 (experimental data)

Cp - 0,92 (experimental data)

Ep > 0olh8

The radiant heat transfer for Test 5 by the Stefan=Boltzmann

equation would be

h 4

Hr ^ Ep cr (Ti “ T2 )

ii. 4 ^ Hr > 0,7^^ X 0,173 X 10 X (1,098 = ,595 )

Hr y 1720 Btu

Percent Radiation ^
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A Third Analysis of Radiant Heat Transfer

From the convection currents observed -with smoke around the

absorber and heater, it is probable that the convection heat

transfer from heater to absorber was negligible*

From the analysis in the text the conduction heat transfer,

^ 0*1 X Avg Temp Dlff*

For Test 5

Hj^<0*l (638 - 135)

Hjg< 50.3 Btu

Therefore Percent Radiation ^

n n ^

It is considered probable that the percent radiant heat

transfer to the painted absorber (and therefore to a map of

equal emissivity) was nearer to the values obtained by the

second and third methods of analysis shown above than to the

minimum value computed by the first method*

YfkS = 50

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