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Publisher’s version / Version de l'éditeur:

ASHRAE Transactions, 65, pp. 541-550, 1960-07-01

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Large-scale wall heat-flow measuring apparatus

Solvason, K. R.

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Ser

TH1

N2lr2

no. 101

I

c.

2

I

BLDG

NATIONAL

RESEARCH

COUNCIL

C A N A D A

DIVISION O F BUILDING RESEARCH

LARGE

-

SCALE WALL HEAT

-

FLOW MEASURING APPARATUS

BY K. R. SOLVASON R E P R I N T E D FROM A M E R I C A N S O C I E T Y O F H E A T I N G . R E F R I G E R A T I N G A N D AIR - C O N D I T I O N I N G E N G I N E E R S T R A N S A C T I O N S . V O L . 65. 1959. P. 541

-

550

B ~ i L D M G

RESEARCH

(Xi-Z-T')

RESEARCH PAPER NO. 1 0 1

OF T H E

.!'JL

5

1sg@

DIVISION O F BUILDING RESEA

1

PRICE 25 C E N T S

OTTAWA J U L Y 1960

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It

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Reprinted from

ASHRAE TRANSACTIONS,

Vo1. 65, 1959

AMERICAN SOCIETY OF HEATING, REFRIGERATING AND AIR-CONDITIONING ENGINEERS, INC.

Copyright 1960

Printed in U . S . A .

A N A L Y Z E D

No. 1684

LARGE-SCALE WALL HEAT-FLOW MEASURING

APPARATUS

"r

Division of Building Research of Canada's National Research Council has

THE

recently completed construction of a n apparatus t o measure steady-state ant1 periodic heat flow through 8-ft square wall sections a t its Building Research Centre in Ottawa. The apparatus (Figs. 1 and 2) consists of two 8- by 8- by 4-ft boxes open on one side, betxveen which the test ~vall is placed. One box is maintained a t the desired constant cold side temperature from -35 F t o +50 F for stead)-state tests or varied according to some predetermincd cycle for periodic heat flow tests. The other box is electrically heated t o maintain a constant warm side temperature of from 65 F to 75 F. The heat transmissicn coefficient for the wall specimen is calculatccl from the measured electrical input and temperatures.

In the design of the apparatus a n attempt was made t o overcome the limitations of the follo~ving methods for dcternmining hcat translnission coefficients: ( a ) calcula- tions using predcterininecl thermal conductivities or cond~lctances of t h e ~ a r i o u s components ancl air filins; (b) the ASTRI guarclccl hot box method; and (c) t h e use of hcat flow n~cters.

Many wall combinations are difficult to assess by means of calculation for the follo\+~ing reasons :

1. The only reliable therinal conductivities available for most materials are those obtained by hot plate tests on dry materials which tnay be considerably different for actual operating conditions.

2. Surface conductances are not necessarily constant over the whole of a wall surface and the average conductance will depend on the temperature difference between the \\all surface and air, and that between the wall surface and its surroundings.

t This paper is a contribution from t h e Division of Building Research a n d is published with t h e approval

of the Director.

*

.Associate Research Officer. Division of Boilding Research. National Research Council, Canada.

Presented a t the Annual Mceting of tlle A h ~ ~ n r c a w S o c l ~ r v or: HEATING, REFRIC;ERATING ,\XD AIR-

CONDITIONING ENGINEERS, Lake PIacid. N. Y., June 1959.

541

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ASHRAE TRANSACTIONS

3. Most wall con~binations contain heat flow paths of differing conductance, the effect of which is difficult to assess without resorting to a rigorous mathematical treatment or t o analogue techniques, because of lateral heat flow in some of the components.

The ASTM guarded hot box1 method consists essentially of placing a n electrically heated metering box over thc center portion of the warm side of the t e s t wall. Sur- rounding the metering bos is a larger guard bos. T h e temperatures in the 2 boxes are adjusted so t h a t there is no temperature difference (and hence n o heat flow) across the walls of the metering box. The conductance is calculated from the elec- trical input t o the metering box a n d the temperature difference across the ~vall.

The main disadvantages of this method are: (a) T h e metering box interferes with the convection over the test wall, so t h a t forced convection must be resorted t o and this may give film coefficients different from those occurring in practice. I t is diffi- cult t o produce equal coefficients for the metering area and the guard area, so t h a t lateral heat transfer may occur from the measuring area to the guard area. (b) T h e metering bos placed over the central portion of the test wall measures only the heat flow into t h a t portion but i t has been shown by G. 0. I-Iandegord and

N.

B. Hutcheon2 t h a t this is not necessarily the average heat flow for t h e whole test

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~vall, particularly for walls containing vertical air spaces; blocl~ing of air spaces in the test area m a y change the conductance substantially. (c) Radiation exchange is indefinite and i t is difficult t o produce the same effect on both the metering area and the guard area of the test wall. Differences in t h e radiant exchange from the inner and outer surfaces of the metering box may require t h a t different air tempera- tures be provided in the test box and the guard box in order t o maintain zero heat flow across the test box walls and this m a y lead t o lateral heat transfer i n the test wall. (d) In many cases the metering box will not cover a representative complete module of the test mall.

T h e use of heat flow meters of the multiple differential thermocouple type is considered inadequate for the measurement of heat transmission coefficients for the following reasons:

1. The heat flow must be integrated over the whole wall surface and a n accurate weighted average is clifficult to obtain on walls where large variations in heat flow occur. 2. The meter, if placed on the wall surface, probably interferes with the convective transfer.

3. The thermal resistance of the meter itself reduces the heat flow in the area covered by the meter. Although corrections for this effect can be calculated, for some cases, they are probal~ly not reliable when applied to walls that have surfaces of high conduc- tivity material, since heat transfer in the plane of the wall surface permits part of the heat to bypass the meter.

4. Good contact between the ineter and wall surface is often difficult to achieve and air spaces between the meter and wall surface will increase the thermal resistance through the meter-xvall con~bination; heat flows in the plane of the meter to the points of contact are possible so that, if the thermopile element is localized in, say the center of the meter, the heat flow might bypass the element or the heat flow through it might be exaggerated, depending on where contact occurs.

With these considerations in mind the heat flow apparatus was constructed to meet the following requirements. First, i t was designed to ineter the heat flow into the whole of representative sections of building walls, and second, t o expose the

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ASHRAE TI<.\NWCTIONS

I PLYWOOD r 1 4-

L---l

I/; PLYWOOD

test wall t o controllecl temperature surroundings a s well as to air a t controllrcl tem- perature. Third, it was built to operate without forced air circulation over t h e s

warm side of the test wall in orcler t o produce warm-side film coefficients approach- ing those occurring in practice. Fosirtl~, i t operates with the cold-side temperature either constant or varying according to some predetermined cycle.

The apparatus consists essentially oi a cold box a n d a guarded hot box combina- '

tion, with an 8-ft square measuring arca. This size was chosen in order to accom- modate walls with air spaces 8-it high and with modules up to 4-ft wide. I t was decided to limit the depth of the box to 4 ft for practical reasons. This was con-

,

sidered large enough to produce natural convection effects, approaching those occurring in practice.

T h e warm box was constructed with a liner of aluminunl panels (Fig. 3) similar to those used t o line tlie ASHAE environmental rooms. These panels, which were developed for use in radiant heating and cooling applications, consist of aluminum extrusions with holders for copper tubing, through which liquid is circulated. T h e inside of the box was painted t o increase the enlissil-ity of the panels to about 0.9. A second set of panels mas installed outsicle of the liner and separated from it by

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thermal insulation. \Yhen thc outer panel is maintained a t the same temperature as the inner panel, i t forms a guard t o prevent a n y heat transfer across the malls of the box. Insulation (&in. mineral wool batts) and a plywood covcring were applied t o the outside of the guard panel.

A water reservoir, pump, and pump motor for circulating water through the tubes of the inner panel are located inside the box. The pump is belt driven b y a series- nound d-c motor. T h e voltage-current characteristics of this type of motor permit the energy input to be varied from 25 w a t t (85 B t ~ i per hr) to 500 watt (1700 Rtu per hr) by regulating the voltage from 15 to 90 volt. Direct-current power was con- sidered easier to control and to measure accurately than a-c. The electrical input to the pump motor is normally sufficient to provide all of the heat requirements; thus the energy imparted to the water by the pump serves to heat the panel and the energy loss from the motor and drive serves to heat the air directly. Air heaters and water heaters are also provided so t h a t the ratio of air heat to panel heat may be l~aried if desired.

Twelve parallel water circuits (each circuit supplying 4 by 4 f t of panel) are used t o supply the panels in order to permit the use of a large quantity of water without excessive pump head a n d pump power. At the masimum input of 1700 R t u per hr, about 20 gpm of water are circulated. Since only about 60 percent or less of the total power input will be supplied t o the water, the d r o p in temperature passing through the panel is only about 0.085 F. The ratio of water quantity to input in- creases a s the total input decreases, so t h a t the temperature drop through t h e panel is always less than 0.085 F. I t can also be shown that t h e panel t o water tempera- ture difference and also the temperature variation over the panel surface is quite small. not more than 0.1 F.

~ h k water reservoir is incorporated into a shield in order t o prevent the test wall or the box walls from seeing the pump motor, which will operate a t some tempera- ture higher than the panel.

A second reservoir and pump located outside of the warm box supplies water t o the outer guard panel. The reservoir contains a cooling coil and an electric heater to control the water temperature.

The cold box is simply a well insulated box with a n aluminum panel liner and a sheet metal esterior vapor barrier covering. The panel liner serves a three-fold purpose: (a) t o provide controlled temperature surroundings to which the test wall is exposed; (b) t o provide some convective heat eschange area; and (c) t o intercept heat gains t o the box itself and thus reduce the heat exchange area required inside the bos. Additional heat exchange area (312 sq ft surface area of finned tubing) is provided to bring the air temperature a s close a s possible t o the panel temperature. Liquid ( % water,

56

ethylene glycol a n d methyl alcohol) a t the desired tempera- ture is circulated through the tubes of the panel liner a n d the finned tubing. This liquid is cooled in a shell and tube heat exchanger by a low temperature liquid

(-45 F methyl alcohol) from a central low temperature chiller serving other labora- tories in the Centre. T h e heat exchanger contains eight %-in. outside d i a ~ n copper coils 20 f t long. T h e control valves a r e arranged so t h a t the cold liquid can be cir- culated through 2, 4, 6, or all 8 coils depending on t h e temperature difference desired.

The control and measuring instruments for the apparatus were selected in .order to limit any specific error to a s low a value a s practicable, or to a maximum of one

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546 ASHRAE TRANSACTIONS

percent a t the minimum heat flow and temperature difference. A heat flo~v of 200 Btu per hr and a n overall temperature difference of 30 F were selected as a design basis. Errors in the determination of an overall heat transmission coefficient may arise from errors in nleasurement, in control of tenlperatures and ponrer input, and from heat leakage.

AJeasz~~emeizt of Tenzpcrutzlres: Temperature measurements are made with 30-gauge copper-constantan thermocouples and an electronic self-balancing temperature indicator. Checks of the indicator and tl~ern~ocouple wire indicate t h a t an accuracy

of & 0.05 F can be obtained. T h e temperature difference measurements are thus

accurate t o about & 0.1 F deg. T h e percentage error is then about 0.33 percent a t an overall difference of 30 F.

lVleas7rrenzent a d Control of the n e a t Inpzit, and W a r n ? B o x Tenzperatz~res: Warn1 box control system is required t o adjust the input voltage t o the warm box to a stable value such t h a t the heat input equals the wall transmission within 2 Btu per hr or one percent, and t o maintain the guard panel a t a temperature such as t o prevent a heat transfer of more than 2 Btu per hr across the box walls. The panel liner of the warm box and the water have a large thermal capacity, about 330 Btu

per F deg. The maximum permissible temperature change is, therefore, only 0.006

F deg. per hr in order to limit heat storage t o 2 Btu per hr.

The insulation between the inner and guard panels provides a calculated conduct- ance of approximately 0.125 Btu per (hr) (sq f t ) (F deg) or a total heat flow of 24 Btu per (hr) ( F deg). Thus for precision of one percent, the average temperature difference must be controlled t o about & 0.08 F deg. A moderate ripple in t h e outer panel temperature can be tolerated, provided the average temperature is Ivithin the & 0.08 F deg, since the insulation between the inner and outer panel \vill provide sufficient damping to prevent any appreciable heat flow a t t h e inner panel. The control system is shown in block form in Fig. 1. A thermocouple actuated electronic recorder with a 3-action (proportional

+

reset

+

rate) controller is used to control the guard panel temperature. This controller regulates a solenoid cool- ing valve and electric heater with manual switches, so that heating a n d cooling can be supplied continuously on, continuously off, or pulsed on-off by the controller. The d-c input to the inner panel pump is supplied by a d-c PO\\-er supply3, the output voltage of which is regulated from 0 to 70 volt b y the amplified unbalance from a differential therinocouple to maintain equal inner a n d guard panel temperatures. T h e differential thermocouple signal is first converted to a square mave by a mechanical chopper, amplified by a high gain low level d-c amplifier and rectified by the chopper. The higher voltage d-c signal is then used t o regulate the o u t p u t voltage of the power supply.

This system has been used with only moderate success. Slight fluctuations in the guard panel temperature produce large fluctuations in the inner panel power. The effect of rapid fluctuations has been eliminated successfully b y applying ther- mal damping t o the guard panel thermocouple junction. Lower frequency tem- perature swings produce a similar swing in inside power. This effect, which is not serious for steady-state tests b u t unsuitable for periodic tests, could be eliminated by converting the present thermocouple recorder-controller to a shorter span a n d more accurate resistance-element-type.

Some additional difficulty has, however, been experienced with the chopper-am- plifier system. The chopper point adjustment is difficult t o maintain over long periods and this, together with stray inductive picliup, produces fluctuations in the power and errors difficult to assess. The present power supply, in addition, is n o t

(10)

of sufficient capacity for the maximum inputs required. The control a n d po~ver system is therefore being replaced with ecluipment nhich has recently become avail- able. 11 magnetic amplifier of 500-watt maxinlun~ capacity is being procured for a power supply. The magnetic amplifier regulated by the present recorder (converted to a resistance type) and a current adjusting controller will be used t o control the inner panel temperature. The recorder sensitivity will be approximately 0.001 F deg and the combination is expected to control to 0.002 to 0.003 F deg. The error due t o the capacity effect of the box is thus espected to be no more than 0.5 percent a t the n ~ i n i m u m heat flow.

The guard panel teinperature \\rill be controlled by a n on-off electronic tempera- ture controller which has a sensitivity of 0.001 F deg a n d is expected t o control teillperatures t o well within f 0.03 t o 0.04 F degof the inner panel. The unbalance error is then espected not to exceed about 0.5 percent.

The input voltage a n d current are measured by a millivolt recorder connected across appropriate resistors. T h e maximum recorder error, with calibration, can be limited to less than f 0.25 percent of full scale so t h a t by sizing the resistors to give larger than half-scale deflections this error should always be less than 0.5 percent. T h e error in voltage tinles current is less than 1.0 percent.

Control of Cold B o x Tenzperntzires: No special problen~s were encountered in the control of the cold box temperature. Here a thernlocouple-actuated electronic recorder with a 3-action temperature controller is used t o control 1, 2, or 3 solenoid valves nrhich regulate the flow of cold liquid through t h e heat eschanger. The number of valves used is selected manually, depending o n the teinperature, t o illakc

K,

x,

or all of the heat exchanger area effective. With this combination, the

cold-side temperature can be held constant to about f 0.1 F deg or about 0.33 per- cent of the temperature difference.

T h e controller is now equipped so t h a t an approximate sine wave variation in temperature can be produced with anlplitudes of 5 to 15 F deg and periods of 6, 12 or 24 hr. A program controller may be added later with ~ v l ~ i c h any desired maxe form can be produced.

If all the measurement and control errors were additive, the total error is expected to be less than 2.0 percent. Operatioil t o date indicates that heat transillissioi~ coefficient values can be reproduced within 1.0 perccnt. Tests to assess measure- nlent errors experimentally nrill be conducted in the near future.

Heat leakage a t the edge of the test wall is another source of error. T h e appara- tus is located in a laboratory n-here the temperature is maintained close t o the usual warin-bos operating temperature. H e a t leakage mill, therefore, tend t o occur through the edge of the test \\-all into the cold box. T h e edges of the test xvall will usually be surrounded with low conductivity inaterial (k = 0.25) slightly thicker than the test wall and some 5% in. wide (the width of t h e bos edges) a s sholvn in Fig. 4. Aluminum angles attached to the cold box panel and extending 2 in. over the box edge are installed t o extend the low temperature plane outside of the test wall area. A similar angle is attached to the guard bos pancl so t h a t the heat loss for this portion u~ill be supplied by t h e guard panel rather than the inner panel.

T h e additional thickness of insulation (x in Fig. 4) should be such that i t s thermal resistance approximately equals the air-film resistance, so that the teinperature in the plane of the wall surface will equal wall surface temperature. In many cases, the arrangement of the edges of the test walls may be such that good edge guarding

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ASI-IRAE TRANSACTIONS

is very difficult. I t may therefore be necessary t o use test sections smaller than 8 f t

square and use a filler of known thermal properties. I

I t would be quite difficult t o calculate the error introduced by edge leakage, ex- cept perhaps by one of the relaxation or analogue methods. Tests conducted t o date indicate no significant temperature gradients on the test wall surfaces near the edges, so t h a t this error can be assumed to be rather small. Further tests to assess edge effects \\rill be carried out in the near future.

The total heat transfer a t the \varmsitle surfacc of the test wall will be the s u m of the net radiant exchange from the box panels t o the test wall plus thc convective

eschangc from thc air to the test \\.all." This can bc exprcsscrl as: "3

qt = q.

+

$ = F.F. X 0.173

[(g;)'

-

($)I

+

x ( t n - t , " ) ~

. .

( I ) T l ~ c radiant ant1 con\ecti\c con~ponents are usuall). accountetl for b) cocfhcicnts +

/lr ant1 11, respectively, such t h a t

qt = lzr(lp - t,,)

+

lz,(I,

-

LV)

. .

.

.

.

. .

( 2 ) The usual surface conductance coefficient, f ,, is the sum of h , and lz,, hence,

qt = f , ( t , - t,")

. . . .

( 3 ) If the average air ten1l)craturc (t,) in thc apl,aratus is not equal t o the panel tern-

,

perature,

t,,

a n cqui\alc~it tenlpcratl~rc t i , ran be calculate~l from Equations 1, 2 and 3 xvhich xvould protlucc thc salnc heat flour a s the actual I, and t,.

Thus.

The form factor li, for the test wall and box can be shown to be equal to 1 .O, a n d the emissivity factor F,, to e q ~ ~ a l e,e, where c, and e , arc the ernissivities of the box walls and the test wall surface respectixely.

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If c, and c, are knolvn, the factors for corivecti\-c heat flo~v .Y and y in Equation 1

can be evaluated by measurements of panel temperature, t,, average test wall temperature, t,., and average air temperature, la, a t several values of heat transmis-

sion, pt. For the range of conditions espected, these factors are not espected t o vary so t h a t they need be evaluated for one \\-all only. In a n y later tests, if the aIferage wall surface te~nperature can l)e measured, it n-ill be possible to calculate tlie \\-all s ~ ~ r f a c e enlissivit);. For those \\-alls wl~ere surface temperature variations make the assessment of the a\,erage tliflficult, it \\ill be possible to calculate the average te1i~perat~11-e if the e~nissivit! t . , is kno\vn. The cold-side heat exchange can be treater1 similarly.

T h u s the surlace concluctance can be accul-ately establishecl, tlie average surface temperatures defined, ancl the contluctance of the \vall itself (\vithout surface con-

N O ~ ~ H N C I . A T U K H

gt = total heat flow rate, R ~ L I 1 , = box panel temperature, per (sq f t ) (lir). Fahrenliei t.

q , = net radiant eschange, tw = average wall surface tem-

Btu per (sq ft) (hr). perature, Fahrenheit.

q , = convective escliange, ti = temperature, Fahren- B ~ L I per (sq ft) (hr). Iieit, equivalent to tn and

Fo = the form factor for L.

radiant exchange be- x = constant. tween the box panels and y = constant.

the test wall. k , = surface coefficient for ra-

F, = emissivity factor. diant heat transfer Btu

T , = box panel temperature, per (hr) (sq ft) (I; deg). Fahrenheit absolute. Ir, = surface coefficient for con-

T , = average test wall tem- vective heat transfer Btu

perature, Fahrenheit per (hr) (sq ft) (F deg). absol~~te. f ; = combined surface coeffi-

1, = average air temperature,

Fahrenheit. cient Btu per (hr) (sq f t ) (F deg). ductances) can be calculated. Overall conductances lor a n y other surlace conduc- tances can then be calculated.

T h e warm side surlace conductance, fi, resulting in the apparatus is espected to be quite close to the still air conductances t h a t result in practice, if a wall is exposed to air and surroundings a t nearly equal temperatures. T h i s coefhcient is not a con- stant b u t rather a function of the teillperature level, the temperature difference, and tlie wall emissivity.

T h e cold-side surface conductance coefficients in the apparatus are expected to be much lower than will be desired in most cases. Forced circulation either by fans or induced by jets \\till be provided to increase the conductance a s required.

This apparatus is expected to provide a n accurate and realistic means for obtain- ing steady-state heat flows or conductances through built-up wall sections, \\~indo\\~s, and doors. The sample size used in the apparatus is considered large enough so t h a t the results, particularly for inside surface conductances, will be representative of much present-day construction.

The test wall is esposed to controlled temperature surroundings a s well as t o controlled temperature air. The convective and radiant components of the surface conductances can thus be evaluated.

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550 ASHRAE TRANS~CTIONS

The apparatus also pro\-ides a means for assessing the transient response of wall sections. In this respect the effect of moisture movement induced by cycling temperature can be investigated.

The apparatus can also be adapted to general calorimetry tests such as the lag characteristics and maximum air-conditioning loads from appliances. Tests for water vapor and air transfer can also be carried o u t xvithout a n y major additions o r alterations.

The author is indebted t o N. B. Hutcheon, assistant director of t h e Division of Building Research, and to A. G. GVilson, head of t h e Building Services Section, for their many helpful suggestions in the design of the apparatus a n d in t h e preparation of this paper, a n d to I-I. L. Egan, laboratory assistant, who performed most of t h e construction work.

REFERENCES

A S T M Designatioit C236-54T: Tentative Method of Test for Thermal Condzicta?zce and Traltswzittance of B z J t - U p Sectio7ts B y Means of T h e Guarded Hot B o x (America~~ Society for Testing Materials, 1954 Supplement to Book of ASTM Standards, Part 3, p. 204).

G. 0. Handegord and N. B. Hutcheon: Thermal performanceof franle walls (ASIIVE TRANSACTIONS, Vo1. 58, 1952, p. 171).

T. M. Dauphinee and S. B. Woods: Low-level thermocouple amplifier and a tempera- ture regulation system ( T h e Review of Scieutific Instrz~ments, July 1955, p. 693).

W. H. McAdams: Heat Tra?zs~izission (McGraw-Hill Boolc Co., Inc., New York, New York, 1942, 2nd ed., p. 459).

DISCUSSION

HARRY BUCHBERG, LOS Angeles, Calif.: The author has presented an excellent paper on the design and development of a very well conceived and useful piece of apparatus. As pointed out, many different studies can be made with its use. Of particular interest would be the determination of transfer functions in order to check the validity of calculations that are based on a knowledge of the properties of the materials that go into the structure.

As another comment the impression was gained t h a t the author does not have very much confidence in the use of heat meters. His attention might be drawn to the work of Professors Gier and Dunkle a t the University of California in the determination of wall conductances using heat meters, in which they solved at least many of the difficulties mentioned by the author.

J . G. MACORZIACK, New York, N. Y.: Does the equipment permit orientation of direction of heat-flow? Is it possible to run the equipment in a horizontal position and get the heat-flow downward?

AUTHOR'S CLOSURE: Gier and Dunkle's paper is familiar and use has been made of quite a number of their meters and this is one of the reasons for the remarks made in the paper.

With minor modification the equipment can be turned over and used for floor tests or it can be turned over the other way and used for roof and ceiling heat-flow. This would require some modification in the circulation system, but it is just a matter of getting the pump sitting in the right location.

(14)

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Counc

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O t t a v r ~ , Canada.

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