92 STEAM 



5. The specific volume, V, which is the volume, in cubic ft., 

 of 1 Ib. of steam at the given pressure. 



6. The density, w, which is the weight, in pounds, of 1 cu. ft. 

 of steam at the given pressure. It is the reciprocal of the 

 specific volume. 



The pressures, p, given in the first column of the Steam 

 Table, are absolute pressures. The pressure registered by the 

 gauge on the boiler is the gauge pressure, or the pressure of the 

 steam above that of the atmosphere. The pressure of the 

 atmosphere at sea level, with the barometer at about 30 in., 

 is approximately 14.7 Ib. per sq. in. Therefore, the abso- 

 lute pressure at sea level is equal to the gauge pressure 

 plus 14.7. In using the Steam Table, the atmospheric pres- 

 sure, 14.7 Ib. per sq. in., must always be added to the gauge 

 pressure. 



Use of Steam Table. For any absolute pressure p given in 

 the first column of the Steam Table, the corresponding temper- 

 ature /, total heat H, or other property is found in the same 

 horizontal line, under the proper column heading; but if the 

 pressure lies between two of the values given in the first column, 

 the corresponding temperature, total heat, etc. must be found 

 by the process known as interpolation, as illustrated in the 

 following examples: 



EXAMPLE 1. Find the temperature corresponding to a 

 pressure of 147 Ib. per sq. in., absolute. 



SOLUTION. Referring to the Steam Table, 



for/>=1501b., / = 358.26 



and for p = 145 Ib., t - 355.59 



Difference, 5 Ib., 2.67 



2.67 



Difference for 1 Ib. difference of pressure is = .534. 



5 



147 lb.^145 lb. = 2 Ib., the given difference of pressure; and 

 for this, the difference in temperature is 2 X. 534= 1.068 or 

 1.07, taking two decimal places. Hence, the increase of 2 Ib. 

 from 145 Ib. to 147 Ib. is accompanied by an increase in temper- 

 ature of 1.07. Therefore, adding the increase 1.07 to the 

 temperature 355.59 corresponding to 145 Ib., the temperature 

 for 147 Ib. is 355.59 +1.07 = 356.66. 



