per sq. in. will be produced before the water boils or the formation of steam begins at 231°F. This pressure, by the way, is in absolute units and would not be the pressure read on the steam gage of a boiler room. Since the steam gage indicates Table 1: Temperatures pressures above FIG. 37.-A typical page from the steam tables. the atmosphere, one must subtract from this reading in the steam. tables the atmospheric pressure of the day in order to find the proper gage pressure. Thus, in this instance, if the atmospheric FIG. 38.-Marks & Davis method of collating data for specific heat of water from three noted investigators. pressure of the day be 14.7 lb. per sq. in., a steam gage in a boiler room would read 6.46 lb. per sq. in., when the water in the boiler is 231°F. This precaution is most important and the student should carefully reread the former chapter on pressures if he does not thoroughly understand the conversion of gage pressures, inches of vacuum, inches of mercury, etc., into standard absolute pressure units. Pressures in Atmospheres. In many engineering computations pressures are given as so many atmospheres instead of pounds per square inch. The pressure of the standard atmosphere is usually taken as 14.7 lb. per sq. in. but for very exact work it is more accurately 14.696 lb. per sq. in. Hence this column is computed by dividing each item in the preceding column by 14.696, which in this instance is found to be 1.440 atmospheres. When, however, the reading is below that of ordinary atmospheric pressure, such values are often desired in inches of mercury since vacuum pressures for the condenser are given in such units. This particular column is therefore found by dividing the corresponding line in the preceding pressure column by the number of inches of mercury equivalent to one pound pressure per square inch. It is to be remembered that this does not even yet give the reading in inches of vacuum. Pressures in absolute inches of mercury and inches of vacuum cause seemingly endless confusion. A complete discussion of this feature was taken up under the chapter on pressures and its careful review is emphatically recommended if any unsettled question still exists in the mind of the reader. Specific Volume.-The cubic feet occupied by one pound of dry saturated steam at a given temperature and pressure is known as the specific volume of the steam for that temperature and pressure. This is a factor often necessary in steam engineering computations. Yet no known means has ever been invented whereby this factor can be accurately ascertained by experiment. The task is indeed one that involves such difficulties as to make its determination by experiment practically impossible. The science of higher mathematics has come to the rescue and here is indeed an instance where purely theoretical deductions have brought about a practical solution of an otherwise unsolvable problem in steam engineering. This relationship involves the latent heat of evaporation L; the absolute temperature T at which the saturated steam is formed; the ratio of the increase in pressure Ap to the increase in temperature At of boiling points taken immediately below the temperature under consideration and immediately above it; the specific volume of the steam v that is found, which of course, is the unknown value we are desirous of computing; and the specific volume of a space occupied by one pound of water vi immediately before its conversion into steam. Algebraically the From the steam tables we will take our values for Ap and At immediately below corresponding to 230°F. and immediately above corresponding to 232°F. Hence The value in the table is 19.05 which is seen to be about onethird of one per cent. in error. This difference is probably due to the fact that decimals neglected in computation were made use of by the comp ler of the steam tables, and then too the small pressure and temperature variations were probably taken nearer together than is possible in the data actually set forth in the steam tables. Specific Density. The weight in fractions of a pound of one cubic foot of dry saturated steam is known as its specific density. It is evident that if one pound of steam occupies 19.05 cu. ft. as taken from the previous column, then 1 cu. ft. of steam would weight 1/19.05 of a pound which is 0.0525 lb. Hence this column is computed in each case by taking the reciprocal of the data given in the preceding column. The Heat of Liquid. This is one of the most important columns necessary in steam engineering practice. Since the heat of liquid technically means the quantity of heat necessary to raise one pound of water from 32°F. to the temperature under consideration, it is evident that by experimental data as given in this column it has been found that to raise one pound of water from 32°F. to 231°F., 199.1 B.t.u. are necessary to be applied from an outside source. FIG. 39.-Determination of the specific heat of superheated steam from investigations of Knoblauch. The Latent Heat of Evaporation.-Data for the latent heat of evaporation has been determined by careful experimental means. It is by definition the quantity of heat necessary to convert one pound of water at the temperature and pressure indicated into dry saturated steam at the same temperature and pressure. In this instance it is seen that to convert one pound of water at 231°F. into dry saturated steam at 231°F., 958.1 B.t.u. are necessary to be applied from an outside source. Total Heat of Dry Saturated Steam.-The total quantity of heat required to raise the temperature of one pound of water at 32°F. to the temperature at which dry saturated steam may exist under the pressure exerted in the particular instance, added to the quantity of heat then necessary to convert this water completely into dry saturated steam is known as the total heat of dry saturated steam. Numerically speaking, it is seen that this column is at once obtained by adding the heat of liquid and the latent heat of evaporation. In a word, this column is the sum of the two preceding columns. Thus H231 h231 +L 231 ..H231 = = 199.1958.1 = 1157.2. (2) Internal and External Work.-One wonders where the heat disappears when it is being continually applied to water at the boiling point and yet the temperature of the water or steam does not increase. Upon careful investigation it is found that it disappears first in an internal absorption due to intermolecular rearrangement as water passes into steam which thereby stores up a considerable quantity of energy to be given out again when the steam is condensed back into water. The energy that disappears in this manner is known as energy necessary to perform internal work. On the other hand in the generation of steam from water the volume is vastly increased. The pushing back against external pressure to make room for such an increased volume performs external work. So that the energy applied in steam generation which goes toward latent heat of evaporation may be divided into two classifications, known as external and internal work. No one has as yet found a method of directly measuring internal work. We may, however, measure external work or even compute it and then by subtraction from total energy absorbed arrive at a value for internal work. In a former chapter on gases it was shown that the external work accomplished by a gas expanding under constant temperature and pressure is computed universally by subtracting the initial volume from the final volume and then multiplying this result by the pressure. Thus External Work (3) |