When steam is generated under ordinary conditions it is termed "steam of one atmosphere" (15 lbs. per square inch). One cu. in. of water will produce approximately 1 cu. ft. of steam (1728 cu. in.). If the pressure is increased the volume is diminished; i.e., the pressure varies inversely as the volume. Thus with a pressure of 30 lbs. the volume is only one-half of what it would be under normal pressure. One cu. in. of water produces 864 cu. in. of steam under 30 lbs. pressure.
P : P' = V' : V 15 : 30 = V : 1728 30 V ' = 15 X 1728
V' = 15 X 1728 / 30
V' = 864 cu. in.