CCCXXIII.

A SIMPLE CALORIMETER.

BY C. H. PEABODY, BOSTON, MASS.

(Member of the Society.)

A CALORIMETER for determining the quality of moist steam, has been devised by the writer, which depends on the property that dry steam is superheated by wire drawing, and which appears to have valuable features.

The first calorimeter of this type, made and used in the laboratory of the Massachusetts Institute of Technology, was made as follows: A piece of pipe, six inches in diameter and ten inches long, was capped at each end. Into the upper end was fitted a half-inch pipe bringing the steam to be tested, a thermometer cup, and a steam gauge. From the lower cap an inch pipe led away the exhaust steam. The supply pipe brought steam from the main steam pipe nearly overhead. Near the calorimeter was a T which formed a pocket, with a drip at the lower opening, and a branch from the side opening leading to an angle valve in the upper cap of the condenser. The pipe further was well wrapped with hair felt, and it was assumed that the steam had the same quality as in the main pipe. The calorimeter itself was wrapped in asbestos board and hair felt, and covered with Russia iron.

Two other calorimeters have been made, which differ from the first only in size. One is made of a piece of two-inch pipe, eight inches long, and the other of a piece of four-inch pipe of the same length. The only difference in the action of these three calorimeters appears to be that the smaller ones are more sensitive, i. e., they respond more quickly to any change of condition.

To make an experiment, the valve in the supply pipe is opened a slight amount, about of a turn, and a valve in the exhaust pipe is regulated to give a suitable pressure in the calorimeter. After the gauge and thermometer attached become steady, their readings are taken, together with the reading of the boiler gauge. If p is the boiler pressure, then r is the heat of vaporization, and q the heat of the liquid corresponding, and x may represent the

dry steam in one pound of the mixture drawn from the main steam pipe, so that 1 is the water or priming. The heat in one pound of the mixture is

Let Pi be the pressure in the calorimeter, and A1 the total heat, and the temperature corresponding. Let t, be the temperature of the superheated steam by the thermometer. Then the heat in one pound of steam in the calorimeter is

in which is the specific heat of the superheated steam at constant pressure (0.4805).

Assuming that no heat is lost

The following experiments were made on the first 6" calorimeter. The boilers were forced during the test to supply an unusual draught of steam for heating and other purposes, and the pressure was less than the usual pressure in the main steam pipe, and fluctuated during the test.

After the smaller calorimeters were completed, all three were set

up and compared. In Table II. the several groups of two and three experiments were made simultaneously.

A comparison of the several groups shows that all of the calorimeters give substantially the same results.

A little consideration shows that this type of calorimeter can be used only when the priming is not excessive, otherwise the wiredrawing will fail to superheat the steam, and in such case nothing can be told about the condition of the steam, either before or after wire-drawing. To find this limit for any pressure t, may be made equal to 4 in equation (2); that is, we may assume that the steam is just dry and saturated at that limit in the calorimeter. Ordinarily the lowest convenient pressure in the calorimeter is the pressure of the atmosphere or 14.7 pounds to the square inch. Table III. has been calculated for several pressures in the manner indicated. It shows that the limit is higher for higher pressures,

but that the calorimeter can be applied only where the priming is moderate.

When this calorimeter is used to test steam supplied to a condensing engine, the limit may be extended by connecting the exhaust to the condenser. For example, the limit at 100 pounds absolute, with 3 pounds absolute in the calorimeter, is 0.064 instead of 0.046 with atmospheric pressure in the calorimeter.

In case the calorimeter is used near its limit, that is when the superheating is a few degrees only, it is essential that the thermometer should be entirely reliable, otherwise it might happen that the thermometer would show superheating when the steam in the calorimeter was saturated or moist. In any other case a considerable error in the temperature would produce an inconsiderable effect on the result. Thus, at 100 pounds absolute with atmospheric pressure in the calorimeter, 10° F. of superheating indicates 0.035 priming, and 15° F. indicates 0.0 2 priming. So also a slight error in the gauge-reading has little effect. Suppose the reading to be apparently 100.5 pounds absolute instead of 100, then with 10° of superheating the priming appears to be 0.033 instead of 0.032. It is, however, to be remarked that no gauge is to be trusted for such work unless it has been compared with a correct mercury column.

It is of interest to compare this calorimeter with the Barrus superheated steain calorimeter* more especially as that calorimeter can be most advantageously applied with steam of moderate or low at which the new calorimeter has a narrow limit. It is

pressure,

* Trans. A. S. M. E., Vol. VII., p. 178, and Vol. VIII., p. 235.

scarcely necessary to recall the fact that in the Barrus calorimeter the steam to be tested is dried and 'superheated in an instrument resembling a surface condenser, by a stream of highly superheated steam. To show the difference between the two types of calorimeter, the following table has been calculated on the assumption that the superheated steam has an initial temperature of 500° and a final temperature of 10° above the temperature of saturated steam of the given pressure, while the moist steam is supposed to be dried and superheated 5°. It will be seen that the limit under these conditions is widest for lowest pressures and that it is narrower at high pressures than that of the new type. While the limit is determined by arbitrarily assumed conditions, it is believed that it will be found narrower rather than wider in practice.

Prof. Jas. E. Denton.-I will make one remark about the calorimeter. I expect I will have my hands fuil to-morrow when I read a paper on the subject. The thermo-dynamic formula there is open to two queries: You do not know the specific heat of steam at any but atmospheric pressure. The common figure .480 was determined by Regnault for atmospheric pressure only. It has never been determined experimentally for any other pressure. There is a calculation, I think, by Zeuner, surmising what it would be for other pressures, and I think there can be a question raised there as to whether that he at which is originally in the steam,