INDUSTRIAL A N D ENGINEERING CHEMISTRY
May, 1924
pressure outside of the tubes, the following determinations seem justifiable: 1-The instantaneous coefficient of heat transfer on the water side at an elementary section of the heating surface may be calculated from the equation 138 U
z(z)
o.8
?)
It, = (1 + as suggested by ILlcAdams and Frost.'j 2-A constant coefficient of heat transfer on the steam side of 2100-2200 may be assumed. 3-The over-all coefficient of heat transmission throughout the heater varies approximately according to a straight line function of the water temperature-i. e., h = n (m T ) . 4-The amount of heating surface may be determined by the formula
+
Loss of Heat from Furnace Walls' By Robert Calvert and Lyle Caldwell CELITECo., LOMPOC, CALIF.
S Aresult of the survey of data regarding the percentage of heat lost in industry in the United States, it is esti-
A
mated that our waste of industrial heat approximates one billion dollars a year. Obviously, the monetary loss due to radiation, conduction, and convection represents no small figure. I n this article the general, more important, well-known principles of heat transmission will be emphasized. Even the incompleteness of data on some points will, it is hoped, serve a useful purpose in leading others to the further study of these extremely urgent problems.
FACTORS AFFECTINGTHE
NOMENCLATURE
QUANTITY
OF
HEATLOST
It should be clear a t the outset that heat tends to flow from a region of high temperature to one OIlow temperature and thus becomes unavailable for use, just as electricity at high potential dissipates itself unless suitable insulation is used, tubes, in square feet I n the loss of heat from a furnace wall there is involved, = inside diameter of tube, in inches first, the transmission through the wall to the outer surface; = heat transferred through an elementary section of a tube in B. t. u.per hour per square foot of inside surface and second, the giving out of that heat from this outer surface to the air, which carries it away. No more heat can be = over-all rate of heat transfer from steam to water in B. t. u. per hour per degree Fahrenheit per square foot transmitted through the wall, a t equilibrium, than is emitted of inside surface by the outer surface of that wall. = rate of heat transfer through steam side film in B. t. u. Considering the variables separately, the quantity of heat per hour per degree Fahrenheit per square foot of transmitted through a wall varies directly with the area of inside surface. It, = 2500 = rate of heat transfer through metal of tube in B. t. u. the wall, the temperature difference between the hot and cool per hour per degree Fahrenheit per square foot of in- surfaces of the wall, the thermal conductivity of the material side surface. For a 3/r-inch 0.d., 18 B. W. G. brass of which the wall is composed, and inversely with the thicktube, ht = 14,500 ness of the wall. = rate of heat transfer through the water side film in B. t. u. per hour per degree Fahrenheit per square foot The rate of emission of heat from the cooler face of the of inside tube surface wall depends upon various factors-mainly, upon the tem= combined rate of heat transfer through steam side film and metal of tube in B. t. u. per hour per degree Fah- perature, the temperature difference between the wall and the adjacent air, the rate of circulation of air, and, in lesser renheit per square foot of inside tube surface = coefficient of heat transfer through metal of tube = 55 degree, upon the nature of the surface of the wall, its color, B. t. u.per degree Fahrenheit per square foot area per degree of smoothness, and material of construction. foot of thickness, for the heater tested The operating temperature and size of the furnace are = thickness of wall of an elementary section of tube determined usually by necessities other than the most efficient = a constant = a constant utilization of heat. The thickness of wall used in the con1 struction is limited by cost considerations. Moreover, the = total resistance t o heat flow from steam to water, = h temperature of the outside atmosphere is not easily ad1 justable. On the other hand, the thermal conductance of == resistance t o heat flow through steam side film, = ha most furnace walk may be varied readily. The means of = resistance to heat flow through the metal of the tube, SO changing the thermal conductance of a wall as to conserve 1 = heat will therefore be discussed in detail. ht
For convenience of reference the various definitions of symbols used are assembled: A = total amount of heating surface on the water side of
D H h ha ht
h, hi
k
L m n
R
R, Rt
R, = resistance to heat 'I
flow through the water side film, =
THERMAL CONDUCTIVITY UNITS
I
K U
ratio of over-all length of tube t q i t s inside diameter mean temperature of water at an elementary cross section of tube in degrees Fahrenheit temperature of water entering heater in degrees Fahrenheit temperature of water leaving heater in degrees Fahrenheit steam temperature in degrees Fahrenheit temperature of steam side wall of tube in degrees Fahrenheit temperature of water side wall of tube in degrees Fahrenheit T mean temperature of water side film, = Ta 2 mean velocity of water through tube, in feet per second pounds of water heated per hour in heater viscosity of water referred to that of water a t 68' F. in centipoises
+ -
Thermal conductivity is the quantity of heat that passes by internal radiation, conduction, and convection through unit cross-sectional area, of unit length or thickness, a t unit thermal potential difference, in unit time. I n the c. g. s. system, specific conductivity is the number of calories transmitted per square centimeter of material, 1 cm. thick, in 1 second, for a temperature difference of 1" C. In the common engineering units, the conductivity is expressed as the number of British thermal units transmitted per square foot of material, 1 inch thick, in 1 hour, for a temperature difference of 1' F. To convert values for conductivities in the c. g. s. system to B. t. u., it is only necessary to multiply by the factor 2903. The B. t. u. system will be used in this paper since it is the one generally used in technology. 1
Received January 29, 1924.
,
INDUSTRIAL A N D ENGINEERING CHEMISTRY
484
MEASUREMENT OF THERMAL CONDUCTIVITY-BOX METHOD When the experimental data are available, the specific heat conductivity, K , may be calculated from the following formula:
in which Q is the quantity of heat, in B. t. u., transmitted in unit time; t the thickness of material, in inches, through which the heat travels; A the cross-sectional area, in square feet, measured perpendicularly to the direction of heat flow; and ( t i - t 2 ) the temperature difference, in degrees Fahrenheit, between the hot and cold surfaces. The measurement of some of these quantities involves considerable effort. In the conductivity measurements in this paper electrical heat has been used and the B. t. u. equivalent calculated from the wattage. It is necessary, then, to make certain that all this heat passes through the material under test. I n the simpler of the two laboratory methods,],* used by the writers, the heat is generated inside a hollow cube, the six faces of which are composed of the test material in slabs of uniform thickness and joined together snugly a t the corners. The quantity of heat transmitted, the inside surface temperature, and the outside surface temperature are read. At equilibrium all three readings become constant. As much heat is then being put into the box as is being lost from the box; from these data is calculated the conductivity of the wall material. Unfortunately, the corners of the box introduce a complication which must be cared for by the use of a shape factur such as that proposed by Langmuir, Adams, and Meikle.z In the case of a furnace in which all three inside dimensions are greater than one-fifth of the wall thickness S =
A
T+
0.548.L
+ 1.2t
(2)
where S is the shape factor, A the area of the heated surfaces inside the cube, ZL the summation of the lengths of the edges of the heated surfaces inside the cube, and t the thickness of wall. For convenience, all the terms are expressed in feet. Substituting for t / A in Equation 1 the value of S from Equation 2 , we have
* Numbers in t e x t refer to bibliography at end of article.
Vol. 16, No. 5
Q
K = (tl
- tz)
(4 +
0.5481;
+ 1.2t
(3)
A cubical box, 4 inches to a side in the interior and 9 inches to a side a t the exterior surface, composed of insulating blocks, has given typical test data as follows:
... . .. 772O F. .. . . 122" F. . ... , 650' F. . . . , , 61.06 watts
ti, temperature of interior surface In, temperature of exterior surface.. t z ) , temperature difference., . (h Q, heat input . . . . ... . . .. .
-
..
. ..
.
208.4 A =
:/3
X
'/s
or B.t.u. per hour
X 6 = 0 666 square foot
t = 0.208 root
Z L = 4 leet
s-
O.fi66 o,208
4- 0.54(4) 4- 1.2
X 0.208 = 5.61
Substituting in ( 3 ) , we have 208'4 = 0.0572 B. t. u. per hour per square foot K = 650 5.61 per foot of thickness per degree Fahrenheit or 0 0572 X 12 = 0.685 conductivity in B. t. u. per hour per square foot per inch thickness per degree Fahrenheit
Although this box method is not a t present employed in this laboratory for testing conductivities, the results obtained agree very closely with those of the Bureau of Standards and of the Kational Physical Laboratory of England. There are, however, numerous difficulties that present themselves and make the method inconvenient and liable to inaccuracies: ( a ) Because six test pieces, cut t o exact size, must be fitted to form each cube, this apparatus is impracticable where any great amount of data is desired promptly. ( b ) Dificulty arises in maintaining equal temperatures over all the surfaces on the interior of the box, because of the nature of the radiation from the heating coil and of the unequal cooling due to the extra insulation afforded by the corners of the box. (c) I t is necessary t o maintain constant the temperature of the room in which the test is being conducted and to avoid unusual air currents around the test box.
MEASUREMENT OF THERMAL CONDUCTIVITY-METHOD OF DISKWITH GUARDRING An apparatus overcoming most of the foregoing objections has been designed and built by M. S. Van DusenP3of the United States Bureau of Standards, and an equipment du-
ISDVSTRIAL AND ENGINBERING CHE.VIIsTRY
May, 1924
plicating this has heco used to accumulate the experimental data on heat. conductivity which are given in this paper. The equipment is shown in Figs. 1 and 2. The apparatus consists essentially of a circular hot plate. 1 inch thick and 8 inches in diameter, mounted horizontally, midway brtwcen, and parallcl to, two water-cooled copper plates of the same diameter. Two specimcns of t h e material t o be tested 611 the intervening spaces between the heater and the two coolers. The heating plate is constructed of two disks of Alberene stone approximately 0.5 inch thick, one o f these being slightly thicker than the other, so that, when the disks are cemented together, two heating elements imbedded in one of the surfaces are located in a plane equidistant from its outside, flat surfaces.
485
Actual data of test on a thermal insulator with this apparatus
follon:
B9dlibrlum bar been maintained for levem1 hours. 11. temperature oi hot stirfaces of material under
testaverage .......................... 357.2'F. cold surfaces of mateiial
I*,temperature of
under tent average ..................... 5 9 . 4 ' F. (a - I d , temperature difference.. ........... 297.8' F. Q,heat sumlied t o center are%.............. 10.0 watts ~~
~
,
.................................. ...............................
a
1, thickness
or 34.15 B. t. n. per holu n.ns73 foot 1 inch
As there are two identical disks of the test material, the area would he doubled, so Equation 1 becomes
K =
34.15 X I
0.0873
= 0.667 B. t. u. per hour per square
foot per inch per degree Fahrenheit difference in temperature
297.8
RESULTSOF THERMAL CONDUCTIVITX MEASUREMENTS Fig. 3 gims the results of thermal conductivity measurements for some commercial insulators. Thermal conductivities are plotted against the temperature of the hot surface of the insulator; in every case, the cold surface temperature was 65" to 70' F. Similarly, Fig. 4 gives data, from the literature,'~6~efor various structural materials for comparison with a brick commonly used for the insulation of furnaces and boilers. The metals (Table I) are much better conductors than brick. Frc.
?--C"ND"C?I"IrY C n L L P I R T L Y SBT UP hlI.IX ~ O O O C N K E T A I N l l i D ITALL, UFP.Cn COULiNO I'LATS *ND SIL-O-CI:L P O I V O L X 1s PLACE
The healing clemcnts are made from No. 24 (R Pr S ) page Calido, a resistant alloy similar to Nichrome. The center element is spirally wound and extends over the center 6.5 inches of the plate. Similarly, an outer, concentric or erlge~hcatiiig unit covers the remaindcr of the plate. Embedded io the surface of each face of the hot plate are two differential theimocouulcs comrmsed 01 t w a fine wires of eoldpalladium alloy and &e of pl&inum-rhodium alloy. T h c t w o hot-junctions of each couple are so located 3s t o detect any diiferericr of temmratnre of the d a t e betwren a mint near thrrentrr ......... arid a point ;car the edge. The heating currcnts may bc so adjusted that there is no temperature diKerence between these two poiiits. Then the heat Irom the center heater must flow aut through the test material exclusively and cannot escape through the edge of the hot plate. Whet> the apparatus is set up with the tcst pieces in place, a 4-inch thickness of insulating powder is packed around the edge as a n added precaution agzainst heat loss in that direction. Test piece are made in the shape of disks S inclies in diametcr and usually 1 i?ich in thickness. Each sample for test has B platinurn-plntimum rhodium thermocouple embedded in its surlace, in contact with the hat plate and, similarly, a coppcrconstantan couple on the other surface in contact with the cooling plate. In the case of determinations oil powders or granular materials these couples are insulated and ccmented with alundum cement directly to thc hot plate and to the cooling platps, respcctivcly, the nowdei heina held in *lace bv a '/,-inch rine of solid ins;iating.material fitciug around thc outside edge of-the pawtier and hrtwcen the edges of the hot and cold plates. Measurements are made of temneratnre the w9ttar.P innlit ... a n d of ........... ~. within a circular area 4 inches iil diameter at the. center of the apparatus. The method of test is to hrins the hot plate to the desired temperature, then t o reduce and adjust the current in each heating unit until that temperature is maintained and the differential thermocouples read zero. Water is circulated through the two copper plates in large excess so that no difference of temperature may be noted between the iidet and outlet water. A standard potentiometer, with cold-junction cell, mirror galvanometer, and other accurate current control equipment are used in resistwing the results.
.
1~~~
---F-.
Thermal Conductivity 8 . t. u./Sq. Ft./Hr./'F./In.
83 1 818 418
2686
Temperature of MeaSWement 0
n.
129 64
64 212
INDUSTRIAL A N D ENGINEERING CHEMISTRY
486
FACTORS AFFECTING THERMAL CONDUCTIVITY
It is well known that the temperature of a material has an important effect on its conductivity. The extent of this effect is evident from the graphs in Figs. 3 and 4. Magnesia brick is unique in showing a decrease in conductivity coefficient with rising temperature ;refractories and insulators in general show a marked increase. 54
I
\
eral, porosity due to fine pores is more effective in insulation a t elevated temperatures than the same volume of larger pores. The size of the pores in a well-known high-temperature insulator is evident from the photomicrograph. (Fig. 7) In some insulating bricks porosity is induced by incorporation, during manufacture, of organic matter, such as brokenup peach pits, the'granules of which burn out to leave large pore spaces in the finished product. For an understanding of the relative merits of such visible pores and of the extremely fine microscopic pores that occur naturally in each particle of certain other insulators, reference is made to the interesting work of Mel10r.~ For a given size of pore space, states Mellor, there is a particular temperature a t which the quantity of heat traveling across the pore space by radiation equals that which would be carried by conduction by a solid in the same space, Mellor's calculated temperatures, a t which radiation across the pore would equal conduction through the solid, are these: for pore spaces 0.5 cm. across, 730" C.; 0.1 cm. across, 1400" C.; 0.01 cm. across, 3000" C. Such quantitative information is hardly necessary to confirm the fallacy of attempted insulation in furnace walls by the use of so-called "dead air space." Besides the intense radiation through air a t high temperatures, there is also rapid convection. Further, the average velocity of the molecules in air, a t 1100" F., is approximately 60 miles a minute (this calculation can be made from information to be found in many standard texts on physical chemistry); i t might be well, therefore, to use the term "live air space" instead of "dead air space." Finally, there is the thorough work of Ray and Kreisinger,l* of the Bureau of Mines, to show how readily radiant heat leaps across an air space a t the temperature of furnace walls. With an air space between the fire-brick lining of the furnace and the outer common brick wall, the bureau found that "the resistance to heat passage of the air space is very low compared with that of either brick wall, only about onefourth as much." Another factor, elasticity, has been related to the thermal conductivity of solid insulators in a recent article by Thorn-
1 R€fRACTUR/ES AN0 S/L-0-CEL BR/CK I
t8 4 2
Vol. 16, No. 5
1
1
I
I
1
FA%
W L - 0 - CEL E R C X 440
BOO
/ZOO
/so0
2 00
J m
v
O f P#Rf SPAC€S ON CUNDVCTMTY O f THEFHAL-
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
May, 1924
ton." In the belief that the transmission of heat, like the velocity of sound, should depend upon the elasticity and density of the medium, he has made a rather striking tabulation of data, which agree for the most part with his theory. Thornton's work has been chalIenged by Clarke,'% who agrees, however, that a good insulator should he both light and inelastic. However this may he, it seem certain that the thermal conductivity of certain insulators and refractories depends to some extent upon the firmness of the bonding.
l h ,892
T€HPFRATURE @RAG/LMT THRU WALL UF 3ULAT€G B@/CKKLN
,d
N G . 61
487
second brick, the linear shrinkage at 2000° F. is 26.5 per cent, which corresponds to a volume shrinkage of no less than 60 per cent.
--
TABLE I I - S s n r a n ~ a ~os ~ F m n i c h ~ e oINS"L&TINC BBICXS Per cent Linear Shrinkage at Temperature of--------. Btick 1000" F. l200* F. 1400" F. 18dO'F. 1S0Oo F. 2O0Oo F. 0.2
0.2
1 2 3
0.1
0.1
0.0
0.0
0.2 0.6 0.0
0.2 0.2 0.0
3.8 10.0 0.1
6.6 26.5
1.0
The manufacturers of insulators are keenly aware of the effect of shrinkage in opening up cracks through which heat may escape, and do not recommend insulators except for use a t temperatures where the particular product is known to be substantially free from shrinkage. TEMPERATURE GRADIENTS OP COXPOSITE WALLS
Knowing the thermal conductivities, one may calculate the temperature gradients of composite walls by using the potential co~icept.'~The drop in potential, or temperature, is proportional to the thermal resistance per unit area, or to the length of material through which the heat passes, divided by the conductivity. On this basis, the temperature gradient has been calculated for the wall of a commercial brick kiln of which the temperatures of the inside and the outside wall surfaces were known. rAa1.s
cAr,cuLA.ria~ T ~ N ~ S Z A ~ U RGZ nRxicg K ~ L N
III-D.,~~
~
~
09 ~
slDB ~ D
WALL OP SNSVLITSD
KIND oi. B.ICP
Fire brick 3il-O-Cel Iistvra1 Red brick TM*L
A "hard burned" refractory or insulator is a better conductor of heat than is the softer material prepared either by firimg a t lower temperature or not firing a t all. A porous, siliceous pon-der, packed to the density of 18 pounds per cubic foot and tested a t a temperature of 200' F., shows a conductivity of 0.48 B. t. u.; calcined powder, of the same apparent density, gives 0.61 B. t. u. Those wishing evidence as to the effect of firing temperatures on the conductivity of refractories are referred to the articles by DougiU, Hodsman, and Cobb,6 of tVologdiue,13 of Dougill," or to that most useful book, "Industrial Furnaces," by Professor "rinks.' That the higher conductivity obtained by firing to the higher temperature is not due entirely to the increase in specific gravity, already shown in one case, seems reasonably certain. Occasionally, an insulating brick is found with a lower conductivity than a lighter brick of a different manufacturer. Investigation in a number of such cases has shown that the former is not hard-burned, as evidenced by ita heginning to shrink when raised to a temperature lower than that at which the less dense hut better conducting brick was originally fired. Unfortunately, it is not feasible to choose always that temperature of firing most favorable to the insulating value of the product. Many insulating bricks, except those sawed from natural rock, contain added clay as the hinder. Such brick will show appreciable shrinkage if used at a temperature much above that of firing during manufacture, as illustrated by the shrinksge percentages for a number of fabricated insulating bricks given in Table 11. For the
Thickness Inches 0
2.5 12 23.5
Averspe Thermal Conductivity at Ki!n Wall
Total Thermal Rrsistanre (Thickness
9.5
0.95
TempeXst"re 1.1 0.0
...
i Conductivity
per Inch) 2.27 2.00 5.22
~
T
488
INDUSTRIAL A N D ENGINEERING CHEMISTRY
The kiln was of the circular, down-draft type, 32 feet in diameter, with a capacity of 63,000 brick per setting. During the "soaking" period, when the temperature of the kiln was held as nearly constant as possible, the inside wall temperature was 1895" F., the outside surface 162" F., and the air 67" F. T€/YP€RATUEE GRAD/E"T;S O f COMpOS/TE /-URN%'€ WALL
The results finally obtained are good evidence that the temperature gradient of a wall may be calculated from laboratory data. Further, such calculations based upon very careful determinations of conductivity are more reliable than the gradient actually determined, unless unusual care has been taken in its measurement.
i 1736
8
VoI. 16, No. 5
TEMPERATURE ' GRADCNT
THRU UMNSULATED FURNACE WALL
f
E E
f
I
There is a total temperature drop of 1895 - 162, or 1733" F., for a total thermal resistance of 5.22. This is a t the rate of 1733/5.22, or 332" F., for each unit of thermal resistance. Then the temperature gradient should show a drop of 332 X 0.95, or 315' F., in the 9 inches of fire brick; similarly, 754" F. in the 2.5 inches of insulating brick, and 664" F. in the 12 inches of red brick. The gradient is shown in Fig. 6a. The effect of the insulation in holding the heat in the fire brick is obvious. Realizing that a greater thickness of insulation would have given a more pronounced effect, the owner of this kiln has used three times as much insulation in his later kilns. Fig. 8 shows temperature gradients through a composite wall, the temperatures a t the several points of which were determined experimentally by use of calibrated thermocouples and a high-grade potentiometer. The furnace was gasolinefired and well insulated. The test section of the wall, 27 inches square, was thoroughly insulated, on the bottom with several courses of insulating brick and on the sides and top with more than 12 inches of insulating powder. The fire brick were laid with fire clay, the insulating brick with an insulating mortar, and the red brick with lime and sand. Several months' tests were discarded as being necessary to develop technic sufficient to give even approximate results.
Fig. 8 shows temperature gradients through a wall consisting of 9 inches of fire brick, 2.5 inches of insulating brick, and 4 inches of red brick, for three different furnace temperatures. The breaks in the graphs represent the changes in temperature where the heat crosses the mortar joints separating the course of fire brick from the insulating brick, and the latter from the red brick. Many preliminary tests gave graphs which were convex upward for the temperatures through the fire brick, this being due to the greater conductivity of the inner, hotter portion of the fire brick, and, therefore, to a smaller drop in temperature through the first 4.5 inches than through the second 4.5 inches of the brick. Fig. 9 shows similar gradients through a wall consisting of the same fire brick and red brick, but without the insulating layer. I n this latter test the thermocouples located in the center of the fire-brick layer, half way from the hot to the cold surfaces, became disarranged and could not, therefore, be read. I n both tests, with the insu ted and the uninsulated wall, the couple on the inner face o the fire brick also was lost. There remained good two couples in the furnace proper, one couple of Chromel-Alumel, the other of platinium-platinum rhodium. The latter projected through the fire brick 0.25 inch into the fire box, and was covered with two cup-shaped shields, one fitting loosely around the
f
INDUSTRIAL A N D ENGINEERING CHEMISTRY
May, 1924
489
TABLEIV-CALCULATIONS OF HEATLOSSESFROM TEMPERATURE GRADIENTS OF WALLS
TEST 1 2 3 5 6
Insulation Used 2.5 inches of Sil-0-Cel brick 2.5 inches of Sil-0-Cel brick 2.5 inches of Sil-0-Cel brick None None None
-Wall Inside
Temperatures, Outside
F.h-fa
-Inches Fire Ll
of Brick in WallSil-0-Cel Red Ln La
-ConductivitiesFire Sil-0-Cel Red Ki Ka K8
Heat Loss
B.t. u./Sq. Ft./Hr.
Q
1817
I88
1629
9
2.5
4
9.2
0.93
6
376
1510
164
1346
9
2.5
4
8.6
0.88
6
296
980
134 283 225 202
846 1453 1202 771
Q
2.5
4
7.4
0.80
6
169
9
0 0 0
4
8.6 8.0 7.3
7 6.5 6
902 695 408
1736 1427 973
Q 9
4
4
... ... ...
* other, and each so perforated that the hot gases, but not the direct radiation from the flame, had access to the couple. Many preliminary tests showed that this couple read from 20" to 39' F. higher than the temperature of the actual face of the f i e brick, and this difference was allowed for in selecting the figure for the temperature of the face of the fire brick.
HEAT LOSSESFROM INSULATED AND UNINSULATED WALLS The heat losses can be calculated from the temperature gradients and the thermal conductivity of the wall materials. Again the potential concept is used-namely, that the quantity of heat transmitted varies directly with the drop in potential and inversely with the sum of the resistances in
series. The formula for flat walls, which is similar to Ohm's law, follows: f1- Q Q = L1 La L*
%+E+%
in which
Q = the total quantity of heat lost in B. t . u. per square foot of wall per hour h = the inside wall temperature, in O F. ta = the outside wall temperature, in
L1
~7 4-
La
' F.
+ 2 = thematerial thickness, in inches, of each course of in the furnace wall divided by the conductivity in B. t. u. per inch of thickness of that course
The application of the formula will be evident from Table
IV. ECONOMICS OF HEATINSULATION The insulated wall loses only 40 per cent as much heat as the uninsulated wall. At a temperature of 1800" F., for example, the losses are, respectively, 370,000 and 940,000 B. t. u. for each thousand square feet of wall surface. The saving, 570,000 B. t. u. an hour, represents a monetary saving of $2500 yearly. To insulate this 1000 square feet of wall and save this $2500 a year would have required 3560 insulating brick of the kind used. The saving due to the use of insulation varies with the temperature of the furnace. The variation with temperature may be seen from Fig. 10, or from Table V. Again the cost of heat has been taken as 50 cents per million B. t. u. TABLEv-RELATION Furnace Temperature O F. 900 1400 I900 2400
OF FURNACE TEMPERATURE TO NEED OF INSULATION
Heat Losses B. t. u./Sq. Ft./Hr. Insulated Uninsulated Diff. Wall Wall 204 153 357 408 272 680 606 396 1002 808 L520 1328
Saved by Insulation for Every 281 Sq. Ft. of Wall (or Area Covered by 1000 Insulating Brick Laid 2.5 In. Thick) Millions of Dollars B. t. u. a Year a Year 502 251 1005 502 1492 746 1990 9135
SUMMARY I-Industrial heat losses are very large, probably equivalent, for the United States, to an amount of fuel costing a billion dollars a year. 2-The loss of heat through the walls of heated equipment may be decreased by a number of expedients, the most feasible of which is the use of thermal insulators. 3-Data for various insulators and refractories shorn that a layer 0.5 inch to 1.5 inches thick, of the various insulators, has an insulating value equivalent approximately to 9 inches of fire brick or silica brick. 4-The thermal conductivity of insulators varies with the temperature of use, the density, unevenness of distribution of the pore spaces, the temperature to which the material has been subjected during manufacture, and, possibly, with the elasticity. 5-Temperature gradients through walls follow roughly the slopes calculated from the thermal conductivities.
490
INDUSTRIAL A N D ENGINEERING CHEMISTRY
6-The surface emissivity of a wall, as affected by its color, texture, or composition, is probably a small factor only in heat losses from high temperature furnaces. The adjustments of temperature undergone by the outer face of the wall, in order to emit to the air the varying quantities of heat transmitted to its surface, are so small as not to affect appreciably the temperature gradient between the inside and outside faces of the furnace wall; and it is this gradient that determines the heat loss. Thus, an increase in furnace temperature from 980” to 1817” F. requires an upward adjustment of but 54” F. in the outside wall temperature, a change which is only 3.3 per cent of the total temperature difference between the interior and outside temperatures of the wall. 7-The results for heat losses from an insulated and from an uninsulated wall show remarkable economy from the use of insulation. 8-The need of insulation for the efficient operation of a furnace increases with an increase in temperature of the furnace. 9-Much remains to be done in this rich and profitable field of heat economy. A list of a few of the many valuable contributions already made to the subject follows. BIBLIOGRAPHY 1-Fitzgerald, Trans. A m . Elecfrochem. SOC.,21, 535 (1912). a-Langmuir, Adams. and Meikle, Trans. A m . Elecfrochem. SOC., 24, 53 (1913); C. A . , 7 , 3713 (1913). 3-Van Dusen, J. A m . SOC.Heating Ventilating Eng., 26, 385 (1920), describes a somewhat similar apparatus designed for testing thermal conductivities of low temperature insulators. 4-Trinks. “Industrial Furnaces,” p. 67, from the average curve of results of numerous investigators. 5-Dougill, Hodsman, and Cobb, J. SOC.Chem. Ind., 34, 469 (1915). 6-Norton, Proc. Nat. Assoc. Cemen; Users, 7 , 78 (1911). T h e thermal conductivities of concrete are plotted against the hot surface temperature, the other conductivities of Fig. 4 against the mean temperature of t h e hot and cold surfaces. 7-Kent’s Mechanical Engineers’ Handbook, 10th ed., p. 602. &Smithsonian Physical Tables, 7th ed., 1921, p. 213. g-Mellor, J. Soc. Chcm. Ind., 38, 140R (1919). 10-Ray and Kreisinger, Bur. Mines, Bull. 8, 12 (1922). 11-Thornton, Phil. Mag., 38, 705 (1919). 12--Clarke, Ibid., 40, 502 (1920). 13-Wologdine, Bull. soc. encour., 3, 879 (1909); J. SOC.Chem. Ind., 28, 709 (1909). 14-Dougill, Gas World, 68, 269 (1918). 15-Walker, Lewis, and McAdams, “Principles of Chemical Engineering,” McGraw-Hill Book Co., Inc., New York, 1923.
Convention to Discuss World Metric Standards Adoption of metric units of weights and measures in merchandising will be a topic of discussion before the convention of the Chamber of Commerce of the United States, to be held a t Cleveland this month. On May 5 the national council will be called upon to advise whether the pending metric referendum shall be submitted to nation-wide vote of business organizations. A year of study and conference was devoted to world standardization by the Metric Committee of the Chamber of Commerce of the United States, and the report of this group will be the basis of the vote. Already the national council is on record in favor of sympathetic consideration of the metric advance, and i t is believed that the referendum will be called forthwith. Need for expanding the world markets for American products is cited as a vital reason for considering world standardization a t this time. Japan and Russia in 1921 adopted metric units for commercial use, and China is also gradually standardizing on the metric measures. The World Metric Standardization Council states that all the civilized world is now on the metric basis, except the United States and the British Commonwealths. The Congress of Chambers of Commerce of the British Commonwealths voted overwhelmingly for adoption of the metric units, and American business men are expected to do likewise. The Britten-Ladd Metric Standards Bill is before Congress, and the vote of the Chamber of Commerce of the United States will aid the decision of the national legislators.
Vol. 16, No. 5
A Heat Transmission Meter’ By P. Nicholls R E S ~ A R CLABORATORY, B AiU8RICAN S O C I 8 T Y OF HSATINGAND VBNTILaTI N G ENGINEERS, u.s. B U R ~ AOBU MINBS,PITTSBURGH, PA.
HE heat transmission referred to in this paper is chiefly that in or out of flat surfaces, such as of building, boiler, or tunnel walls, of floors, and of roofs. There is only one method available for measuring such a heat flow which is applicable to all conditions-vie., by determining the drop in temperature produced through a known thermal resistance. As this is merely the reverse of the process of obtaining the thermal conductivity of a material, there is nothing new in the idea. The earliest recorded practical application of it that the author knows of is the attempt made in 1914 by Professor Henky,2 of Munich, to measure the heat flow into the floor of a brewery cellar by covering it with a 4-inch thickness of cork board. As the primary use of such a heat transmission meter should be to measure the flow that occurs under natural conditions, and since it will be an added thermal resistance in series with the natural ones, it is desirable that this addition should be as small as consistent with the use of not too delicate temperature measuring instruments. It was also evident that the meter plates must be calibrated by passing known heat currents through them, as, even if the conductivity of the material were known, the uncertainty in surface temperature measurements involves a large possible error. The problem, therefore, included the following factors:
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1-To develop a method of construction that would not change with time, and would be fairly rugged. 2-To develop a method of calibration and to so conduct the tests that the order of accuracy of the meters would he determined. 3-To find the design that would give the smallest additional thermal resistance. CONSTRUCTION AND CALIBRATION OF METERPLATES Since the meter plates and the calibration apparatus must be interlinked in design, and since the plate values are dependent on the accuracy of the calibration apparatus, the problem involved an investigation of both. The general plans for the work were therefore laid out to include as many checks as possible through analytical comparisons. Two feet square was adopted for the plate size, and as the ring-guarded test hot plate was given a 15 X 15-inch center plate, only this area of the meter plate would be taken as the flow area to be measured. For measuring the surface temperatures the electrical resistance method and thermocouples were available. The former would be cheaper, but the multiplication possibilities in the latter make them better adapted for use by untrained observers, and, moreover, the calibration is less liable to be changed by strain when used with flexible boards. Thermocouples give a convenient means of measuring the difference in temperature of the surfaces by having alternate junctions on the two sides. This could be done by bringing the wire around the edges or by taking them through holes in the plate. The former makes a very complicated wiring system and greatly increases the electrical resistance of the couple system. In addition to measuring the temperature difference, one surface temperature is needed to be able to connect the differential electromotive force with some known temperature, and also because of possible variation of thermal conductivity with temperature. Since the plates were to be calibrated by test and not by 1 Received
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February 4, 1924.
Kdlte-Ind., August. 1915.
