ANALYTICAL EDITION
396
Vol. 3, No. 4
Studies in Adiabatic Calorimetry' S. W. Parr and W. D. Staley BURGESS-PARR COMPANY, MOLINB,ILL.
I
N THE Power Test Codes of the American Society of
Mechanical Engineers for instruments and apparatus now approaching their final form, Part 9 for the Heat of Combustion says, in paragraph 14: An effective method of minimizing cooling errors consists in accurately controlling the temperature of the calorimeter environment. This is done by providing a complete water jacket, the temperature of which is kept constant to two or three hundredths of a degree by means of a sensitive thermostat,* * * * * * but no commercial instrument of this type is at present available.
The directions then proceed on the theory that a waterjacketedeinstrument is available and directions are elaborated for applying a correction factor for radiation. This matter of deriving the radiation correction in single-unit instruments
TMEINMINUTES Figure 1-Radiation
Loss Due t o High Heat Head
is cumbersome, requires the close attention of the operator, and at best is susceptible of errors that it would be highly desirable to avoid. It is useless to hold up the ideal of a water-jacketed instrument thermostatically controlled and having such a constant radiation rate that the correction may be read from a chart if, as seems to be the case, only two such instruments are in existence. The Code acknowledges two, one a t the Bureau of Standards, and one in use by the Bureau of Mines. With a water-jacket mass
T O E IN flINUTEJ
Figure 2-Radiation
Gain Due to High Heat Head
of something like a thousand gallons, it is obvious that these conditions can easily be maintained. It is obvious also that the production of a unit for individual use by a manufacturer 1 Received April 11, 1931. Presented before the Division of Gas and Fuel Chemistry at the Slst Meeting of the American Chemical Society, Indianapolis, Ind., March 30 to April 3, 1931.
observing these conditions is beset with difficulties, and the Code says no such instruments are available. The attempt to obviate these difficulties is the most natural thing to expect and it is believed that further progress is indicated by the experiments recorded in this paper. The most natural thing to undertake would be the avoidance of radiation factors by securing adiabatic conditions, and in this direction of effort, the most natural line of procedure would be to utilize the insulating properties of a vacuum jacket. F~~ reason, instruments of this typehave not met with very hearty endorsement* The statement usually accompanying 8 discussion of this feature is that the idea is good but the practice bad. White (2) states that ''the Sources of error and the inconvenience which characterizes the vacuum-walled calorimeter are clearly serious only in work of high precision, hence its most unqualified success has been in work of low precision." It is obvious that a manufacturer of calorimeters would not like to advertise his ware as adapted to work of low precision. Other experimenters have found this adiabatic type of insulation erratic and uncertain and have abandoned the attempt at its utilization. Stansfield and Sutherland (1) list three reasons for discontinuing the use of a vacuum cup calorimeter: (1) The vacuum cups were subject to collapse; (2) the water equivalent of the calorimeter apparently changed; and (3) there was a change in the rate of cooling per minute per degree rise in calorimeter temperature. The exact reasons for the results were not definitely determined. This s i t u a t i o n seemed to present an opportunity to v e r i f y the &advantages existing i n t h e use of a v a u u m w a 11 jacket in calorimeters for &termining heat of cornbustion or to de- E termine a means of o v e r c o m i n g these difficulties. The first series of e x p e r i m e n t s i n v o l v e d the use Figure 3-Type of Insulation of a vacuum vessel of 136 mm. inside diameter, 165 mm. outside diameter, and with an inside depth of 292 mm. This was mounted in a square Bakelite housing with outside dimensions of 305 mm. by 305 mm. by 375 mm. The jacket cover was constructed with a circular piece of felt on the underneath side and then a smaller disk of Bakelite which just fitted within the top of the vacuum vessel. The temperature of the testing laboratory of the BurgessParr Company was allowed to drop overnight to between 50" and 60" F. (10" and 15.56' C.). Upon starting the tests in the morning, with the room at 71" F. (21.67' C.) and the calorimeter water a t 71" or 72" F. (21.67" or 22.22" ca), there was marked heat leakage Outward' It was pected that with the vacuum wall, 2 or 3 inches (5.08 or
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INDUSTRIAL AND ENGINEERING CHEMISTRY
October 15, 1931
7.62 em.) of good Balsam wool insulation, and a retaining jacket of Bakelite, there was too much or too good insulation, and a high heat head existed in the calorimeter, which accounted for the leakage outward. The Balsam wool was removed and a thermometer inserted in the air-jacketing space. Upon repeating the experiments it was found that the temperature of the air-jacketing space was low, thus
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2
5
4
5
6
7
8
&ADYEN7 BETWEEN AIR JACKET ANDcALORlMETEe U / r t i VACUUM WALL ZNT€iCVENING. L'EBPEEJ E
Figure 4-EfIect
of Low Gradient between Air Jacket and Calorimeter w i t h Vacuum Wall Intervening
demonstrating the presence of a high heat head in the calorimeter, with a gradient of from 5' to 8'F. (2.77'to 4.44OC.), which accounted for the transmission of heat .outwardly. . These conditions are shown by the curve in Figure 1. These results suggested the next series of experiments in which the temperature of the testing laboratory was maintained for half a day a t 90' F. (32.22' C.). The next series of experiments was conducted under the same conditions as before by bringing the working temperature of the laboratory to 72' or 73' F. (22.22" or 22.78" C.), and using the same temperature in the calorimeter water. Here the heat leakage was equally pronounced but in the other direction, being inward. The results obtained are shown by the curve in Figure 2. This seemed to furnish an explanation for the erratic behavior referred to by other operators, and the tests were continued by installing an air-circulating system whereby the air-jacketing spaces could be made to bring the jacketing material more nearly to an equilibrium with the surrounding atmosphere. The type of insulation is shown in Figure 3. The Illium bomb, D, is suspended in the vacuum jacket, C, which rests on a cork ring, I . The air space, B, separates the vacuum-wall container from the Bakelite jacket, A. The vacuum flask rests on a cork ring, I , and the top of the flask is sealed off from the air space, B, by means of a felt pad, H . A Bakelite ring, F , holds the vacuum flask erect. The Illium bomb, D, is suspended in p holder, K , which is raised and lowered by movement of telescoping tubes, J, on either side of the Illium bomb ho€der. Stirring of the water is accomplished by the stirrer, E, operated by the pulley, G. A heating coil was located just outside the jacket with a small fan whereby, upon turning the switch, the circulating air could be heated, thus more quickly bringing the container and jacketing spaces up from a low temperature. Similarly, without the heat and by circulating the air of the room, an equalization of temperatures could be readily obtained. By establishing a condition of equilibrium and then slowly raising the jacket temperature (approximately 4" F., or 2.22' C., per hour), a gradient of over 3" F. (1.66' C.) was required to obtain an indication of heat leakage, as shown by the curve in Figure 4. Another test, in which the conditions approximate those present for an actual B. t. u. determination, was made by circulating air from the room until equilibrium conditions
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were reached. The temperature of the water surrounding the bomb was the same as the jacket and constant. The temperature of the jacket was quickly raised (within 1 minute) 5.5" F. (3.05" C.) and maintained at that temperature, a gradient which would be more than that encountered in the usual determination. The calorimeter water remained constant for 31 minutes and then began to change a t the rate of 0.001" F. (0.0005" C.) per minute during the next 30 minutes. To test the comparative stability of temperatures that might be secured by use of a water jacket instead of an air jacket, an equipment was assembled of the type illustrated in Figure 5, in which the temperature of the jacket was controlled by means of quick accessibility to either hot or cold water. The Bakelite container, A , was the same as in Figure 3. The inner container, B, was circular and constructed of brass with a capacity of about 8 liters. An air jacket, D, surrounded the water container, E, in which the Illium bomb, C, was located. A stirring of the water in the jacket and circulation through the cover was maintained by the pump, I . The water in the cover portion of the jacket returned through to opening H . The two pulleys, G, operate the pump, I , and the stirrer, F.
Figure 5-Apparatus for Testing Comparative Stability of Temperatures
Under these conditions, it is possible to obtain a quick and immediate response to any desired changes in temperature, and to easily maintain a gradient of less than 0.05" F. (0.027" C,). The method of maintaining adiabatic conditions, illustrated in Figure 5, has been in general use for a number of years. However, the instrument indicated in Figure 5 includes a number of simplifications in the means of stirring and circulation, the type of heater for supplying hot water, and the general construction, which permit a greater ease of operation without sacrificing any of the accuracy in the previous instrument of this type. It should be noted that with either one of these devices the mechanical construction is such that the calorimeter container holding the water and bomb can be sealed off from the surrounding space between the calorimeter and the outer jacket, thus practically eliminating the error of heat of vaporization. It is believed that with these devices a marked advance has been made in the accuracy and convenience of manipulation, and that this is due to the attaining of true adiabatic conditions. Literature Cited (1) Stansfield and Sutherland, Can. J . Res., 8, 464 (1930). (2) White, "The Modern Calorimeter," p. 155, Monograph Series 42, Chernicnl Catalog.
