Consider, for example, the vaporization of 1 lb of water into steam at 212 F under a pressure of 1 atmosphere. The volume of 1 lb of water at 212 F is 0.01672 ft3 /lb, whereas the volume of 1 lb steam at 212 F is 26.80 ft3/lb. Consequently, during the vaporization process the fluid expands at constant pressure from a volume of 0.01672 ft3/lb to a volume of 26.80 ft3/lb, thereby doing work in expanding against the pressure of the atmosphere.