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3 X 885.4
96.8 per cent. dry steam in steam under test.
FIG. 53. Side view of oil fired Stirling boilers showing the steam piping, soot blower, piping and explosion doors, station A, Pacific Gas and Electric Company, San Francisco.
Surface Condenser Tank Calorimeter. This method varies from the one just set forth in that the condensed steam does not mingle with the water in the barrel. To accomplish this the steam is passed through a coil of piping which is inserted in the tank. As the steam comes in contact with the cooling surface of this pipe, it is condensed into water and of course the heat thus liberated or given out is absorbed by the water in the tank and its temperature correspondingly raised. Hence in this instance, it is necessary to weigh the water in the tank and the
condensed steam discharged through the coil. It is also necessary to take the pressure of the steam under observation and to note the temperature of the tank of water before and after application as well as the temperature of the water discharged from the coils.
Proceeding by similar reasoning as set forth in the former instance, the heat lost by each pound of steam is sure to be (ho + XoLo hз), wherein the subscript 3 is to denote the condition of the steam condensed into water as it emerges from the coil. The heat gained by each pound of water in the tank is also seen to be (hhi) heat units. Hence if Wo lb. of condensed steam are discharged and W1 lb. of water are found in the tank, since the heat lost by the steam is equal to that gained by the water, neglecting radiation and other minor losses, we have
To illustrate, let us assume that one pound of steam at a pressure of 100 lb. per sq. in. absolute is passed through coils immersed in a tank containing ten pounds of water at an initial temperature of 100°F. At the conclusion of the condensation the water in the tank is found to be at a temperature of 204.5°F., while that emerging from the coils is 210°F. The quality of the steam is at once found by substitution in the formula as follows:
Since the quality of steam is greater than unity, it is evident that the steam in this instance is superheated.
The principle upon which the more accurate steam calorimeters operate is in general accomplished along similar lines. We shall, however, reserve further discussion on the subject until the next chapter wherein we shall deal at length with calorimeters.
THE STEAM CALORIMETER AND ITS USE
We come now to a consideration of the methods used in steam engineering practice to accurately determine the moisture content of saturated steam. In the preceding chapter certain approximate methods were set forth, but in the following discussion it will be seen that by care and patience the moisture content of saturated steam may be ascertained with a wonderful degree of accuracy.
The Chemical Calorimeter.-The Chemist has a method of determining the moisture content which finds little application in the steam engineering laboratory, but in the chemist's laboratory it is performed with a remarkable degree of accuracy. Certain salts absorb moisture held in a vapor. Hence by passing wet saturated steam over such salts, the moisture content is taken from the steam and by weighing the moisture so absorbed the degree of moisture held in suspension is ascertained.
The Throttling Calorimeter.-By reference to the steam tables it is seen that when saturated steam exists at say 200 lb. pressure per sq. in., each pound of steam represents a storage of heat equal to 1197.6 B.t.u. For it is seen from the steam tables that it took 354.4 B.t.u. to bring the original pound of water from 32°F. to its boiling point and then an additional 843.2 B.t.u. to evaporate this water into dry saturated steam.
Let us suppose for a minute this steam at 200 lb. per sq. in. were allowed to flow through an orifice and expand into a chamber which was at but 14.7 lb. per sq. in. From the steam tables it is seen that saturated steam existing under such a pressure holds in storage but 1150.4 B.t.u. What then becomes of the difference between 1197.6 B.t.u. and 1150 B.t.u. represented by the heat held in storage in the two instances? Evidently if the main at the lower temperature be well hooded so that no heat escapes, the heat given out must go toward superheating the steam at the lower pressure. Since the specific heat of superheated steam at the lower pressure is about 0.47, the 47.2 B.t.u. that are liberated
would evidently superheat the steam about 100°. The actual measurement, then, of this superheat gives us at once a most accurate method of determining the quantity of moisture present in the steam at the original pressure. For if we find that the steam is superheated only 25°F., instead of 100°F., evidently some of the mixture must have been water, for otherwise its existence at the higher temperature as steam would aid in superheating still further the lower temperature.
FIG. 54.—The throttling calorimeter and the sampling nozzle.
In the typical throttling calorimeter, steam is drawn from a vertical main through the sampling nipple, then passed around the first thermometer cup, then through a one-eighth inch orifice in a disk between two flanges, and lastly around the second thermometer cup and to the atmosphere. Thermometers are inserted in the wells, which should be filled with mercury or heavy cylinder oil. Due to the fact that the heat content in the steam under the expanded condition with which it reaches the second thermometer, is much less, the heat thus liberated superheats the steam at this point and thus a means is given for ascertaining the moisture originally in the steam sample.
A throttling calorimeter, then, is simply a contrivance by which we allow steam to pass from its high pressure through a small opening where its temperature and pressure are taken before it passes out into the atmosphere. Prior to its passage through the small opening, the temperature and pressure of the steam is noted. Let us denote by "s" subscripts the conditions of superheated steam in the low pressure chamber, “o” subscripts the steam in the steam main, and "3" subscripts saturated steam at the pressure of the low pressure chamber.
Each pound of wet steam in the steam main has X. parts by weight existing as dry steam. Hence the total heat represented in each pound of this steam is evidently (X.L. + ho) heat units
as seen from close inspection. In the same manner cach pound of steam in the lower pressure chamber holds in storage [H3 + Cpm(ts- ts)] heat units as seen from previous reasoning. Since no heat is allowed to escape, evidently these
expressions are equal one to the other, or
Cpm for the low pressures has a value of 0.47, hence, we have
in which H. is the total heat of superheated steam in the low pressure chamber. Its numerical value may be taken directly from the steam tables when the pressure and degree of superheat are known.
As an illustration, let us assume that the pressure in the steam main is 153.6 lb. per sq. in. abs. and that its temperature is found to be 362.9°F., thus indicating at once that the steam is saturated and not superheated. After it has expanded into the low pressure chamber it is found to have a temperature of 261.3°F. and a pressure of 14.8 lb. per sq. in. absolute. From the steam tables we find L.
Therefore the steam is evidently 97.58 per cent. dry.
Normal Reading of the Calorimeter.-For accurate work it is necessary to make a correction for the radiation loss from a calorimeter. This may be done by taking readings on the instrument when absolutely dry saturated steam is passing through it. This reading is called the Normal Reading of the calorimeter. A boiler that is standing by without a fire under it but with pressure up will deliver practically dry steam. It is, therefore, possible to secure the normal reading of the calorimeter right in place, by shutting off the oil burners and allowing the circulation within the boiler to come to rest. A more accurate method of obtaining the normal reading is shown in the illustration on page 89. In this arrangement the calorimeter is supplied from near the top of a horizontal pipe containing quiescent steam, a drain being provided to remove the moisture from the bottom.