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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). The thermal conductivities of concrete are plotted against the hot surface temperature, the other conductivities of Fig. 4 against the mean temperature of the 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
a Z . ges.
February 4, 1924. Kdlte-Ind., August. 1915.
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INDUSTRIAL A N D ENGlNEERING CHEMISTRY
the known conductivity of the material, it is not necessary to deal with actual temperatures, but only with the readmgs given by the thermocouple system. This in itself takes care of variations in the closeness of the vires to the surface, of the electromotive force given by different junctions, and of the plate material, provided a given rate of heat flow at a given plate temperature always gives the same couple reading.
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compared with the plate thickness; also, although the surfaces of the plates are comparatively smooth, it is believed that there would be an improvement in their action if the coverings were applied with the aid of steam presses so as to make them smoother and of a more uniform thickness than can be done by hand.
APPLICATION In applying such plates to a surface the added thermal resistance will include the necessary small $1. gap, so that halving the thickness of the meter plate will not halve the total thermal resistance introduced. Tests of plates on a plaster finish that was not free from waves gave the following values:
Fie. I-FINISIIBDMGIERPLATE
The differential couples were made in strings. All junction beads were squeezed flat so that they were little thicker than the wire diameter. They were threaded through the board, cemented in place, and given several coats of spar varnish. The cork plates were covered with a '/&nch layer of corksheeting, and theotberswith "4 pound"ashestospaper. Full details are given elsewhere2 regarding the development, construction, and calibrating of tho meter plates. Fifteen plates a t various materials and thicknesses m r e made, and a number were calibrated a t the same time by passing the same heat through them, thus making it possible to compare tbcir accuracy and also to distinguish departures from consistent readings as due to the plates themselves and to test errors. A finished plate made from materia,l '/ls-ineh thick is show= in Fig. 1. A diagram of the testiig scheme is shoa-n in Fig. 2. By using two hot plates with oneof themeterplates between them the temperature condition can be kept so that all the heat supplied to each hot plate flow in one direction. The meter plates n'ere tested as shown in Fig. 2, both with close contact and also with an air gap between them. The same general scheme was used, except that the last plate had its outer surface exposed to air maintained at a constant temperature, thus simulating its application to a wail in order to measure the heat flowing into or out of it. As a brief summary of the calibration resu1t.s it can br stated: 1-The plates as constructed, after 9 months' use, have shown no tendency to change their readings for a given flow due to changes in the plates themselves. 2-The thin plates have a slightly lower calibration value for one side exposed to the air than they have with close contact on both sides. 3-The readings given by a plate fell within 0.5 per cent of a mean curve, and can safely be taken as a true measure of the heat flow to within plus or minus 1 per cent. &The variations of the readings irom consistent values, as caused by the test method or manipulation, can, with care, be kept within 1 per cent.
The reason for the lower readings with air exposure is undoubtedly the appreciable size of the thermocouple wircs as J . I-.
sac.nIatinl vlntiiaiingE = ~ .80, , 33 (1924).
For the */,&nch plate it will be noted that the thermal resistance added to the wall is 4.7 times as large as the resistance across which the temperature drop is measured to obtain the flow. The air-gap resistance is 63 per cent of the total, R. By carefnlly leveling the wall surface and getting closer contact, the resistance of the air gap could be reduced. In trial applications of the plates to various surfaces with air exposure, it was found that the rate of heat flow varied more rapidly with time than was expected. The flow through a very thin surfacelayer will theoretically fluctuate from positive to negative if the surface temperature decreases by a very small amount, but since the thickness used is appreciable and a stationary air a m is supposed to exist, it might be expected that the change would be slow with long wave lengths. Typical results are shown in Figs. 3,4, and 5. Curves are shown of readings a t '/*-minute intervals. The base lme is shown for one, but the side scales fix the range of the variations relative to the total flow. Fig. 3 is for a plate on a vertiealZ4-inch brick wall. Because of room conditions there would be more air motion and temperature variation than is usually found. Figs. 4 and 5 are for plates on a concrete floor, solid with the foundation and with steady room conditions. I t d l be noted that the */,,inch plate registers larger fluctuations than does the '/&ch plate.
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To illustrate how rapidly the plate records new conditions, a 60-watt lamp was suspended above each of the floor plates. The immediate action of the radiation is very marked. The plates are thus very sensitive in their record, hut this
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same sensitivity increases the difficulty of taking readings, which have to be averaged. It would thus be of decided advantage to bury the plate a small distance in the wall. Because of their small mass and heat capacity, such plates
will give a true instantaneous value of the heat flow for any rate of variations that occurs in practical applications, and will be particularly useful where it is desirable to follow the changing values, such as in the heating up of any apparatus.
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I N D USTRIAL AND ENGINEERING CHEMISTRY
As a general rule, however, the integral of the heat transfer over a time period is the more practical value, and such an automatic integration of the electromotive force is needed. The possibility of doing this with accuracy and with not too expensive apparatus does not seem very hopeful at present. The primary use of such meter plates would seem to be the measurement of the flow that is actually occurring, without consideration of the causes. As far as it is desired to use them to determine the conductivity coefficients under variable natural conditions, the observations must include all necessary temperature measurements as well as the flow over a cycle of changes, or long enough to make the error due to change in stored heat negligible. When the conductivity coefficient of an ordinary building wall is required, the time can be shortened and the accuracy improved by measuring the flow a t each surface, although the closing of the surfaces by the plates will shut out the effects of air infiltration and possibly some of the change due to the influence of moisture. The ability to measure the heat flow thus has possibilities in connection with laboratory investigation. As applied to refractories it will avoid the need of measuring the input and will only require the maintenance of a constant temperature on the hot side. Such an application, of course, assumes
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a practical thickness of wall so that its cold side temperature will not be above that which the plates can stand. With the plates as made and as applied with possible different degrees of closeness of contact, the order of accuracy will be safely within 2 per cent. Several applications have been made, and have presented no difficulty. The floor was tested, ns there were pipe tunnels around it, the 3.0 B. t. u. flow occurring near the tunnel edge and the 1.1 a t the greatest distance from them. (Partial results shown in Figs. 4 and 5.) A test on a 24-inch concrete basement wall under natural outside weather co ditions gave a conductivity coefficient of about 12 B. t. u. pe square foot per inch of thickness per degree Fahrenheit per hour. This compares with the usually accepted value of 8.7 as obtained in laboratory tests. Such a thickness of concrete will be denser than small samples, and moisture is more likely to be present. It is believed that the conductivity of commercial walls will usually be found to be larger than that indicated by laboratory determinations. Such an additional tool as these heat transmission meters should prove of use, and though only comparatively small areas of the whole structure can be covered, yet there should be more confidence in such measurements of actual flow than in those based entirely on small samples.
f
A Laboratory Stirrer' By C. E. Waters BUREAUOF STANDARDS, WASHINGTON, D. C.
HIS note describes a stirrer run by compressed air, which has been in use for several years in cases where it wm not convenient to use an electric motor. The fan, F, is made from a circular piece of sheet brass 9 cm. in diameter and 0.3 mm. thick. Twelve radial cuts running nearly to the center are made, and the sectors are twisted so that their outer edges are nearly vertical. The short brass bushing, B, soldered to the center of the fan, is drilled to fit the steel shaft and is provided with a set screw. The steel shaft, 8, 3 mm. in diameter and 10 cm. long, rotates in babbitt bearings. The bearing metal is cast in a glass tube and short pieces of it are turned to fit in the ends of a brass tube. The holes allow the steel rod to rotate easily without lateral play. The brass tube is held by a clamp. The curved glass tube a t the left has four air outlets that lie on a circle of 3 or 4 mm. greater diameter than the fan. It is clamped horizontally so that the clearance between the tips and the vanes is 2 mm. or less. If the air pressure is equsl to 7 or 8 cm. of mercury, and a little typewriter oil is used on the bearings, motor oils and other equally viscous liquids crtn be stirred a t a rate of several hundred revolutions per minute. For convenience in adjusting the stirrer a t any desired height, the clamps that hold it and the air tube are fastened to an 18-cm. length of brass tubing that just slides easily over the vertical rod of an ordinary laboratory stand. Near the lower. end of the tube is a 4-mm. hole. Through this passes one of the screws of a clamp fastener or holder, which thus serves to hold the tube and all it carries a t any height. Before placing the tube on the rod of the stand, one or more perforated corks are placed on the rod, so that the lower end of the stirrer cannot be broken against the base of the stand. The shaft proper of the stirrer is a straight piece of glass tubing that just slips easily over the steel rod, so that when
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1 Received March 31, 1924. Published by permission of the Director. U. S. Bureau of Standards, Department of Commerce.
the two are cemented together by means of melted shellac, they will be coaxial. The shelIac is most readily applied as varnish, which is heated to drive off the alcohol and melt the gum. One of the simplest forms of stirrer is a T-tube with short, closed arms. A variant of it has little paddles made of glass rod flattened at the end. If two immiscible liquids are to be stirred together, the paddles can be set a t an angle so as to exert an upward or a downward thrust as desired. The stirrer shown attached to the steel shaft was made for hastening the oxidation of an oil in chromic acid mixture. The oil was drawn in through the two lower holes in the shaft and thrown out of the open arms in thousands of tiny drops. The upper hole was to admit air and prevent back suction when the fan was stopped. The stirrer in the lower corner will pick up coarse sand or even mercury and sling it out of the arms. The vertical shaft is closed by fusion just above the junction of the arms. For some kinds of work, one of the special forms of stirrer is nearly or quite as efficient as a shaking machine, and is less cumbersome. For work with volatile, inflammable liquids, the advantage of avoiding the use of an electric motor is obvious.
