The effect on water vapour pressure of superimposed air pressure

In a compressed gas saturated with water vapour the vapour pressure of the water exceeds the normal value by an amount which is greater than can be predicted by Poynting's thermodynamic relationship. A quantitative knowledge of this behaviour is important in the design of equipment for drying compressed gases, particularly for drying by refrigeration, which is commonly used in the low‐temperature separation of air. The equilibrium moisture‐content of compressed air at pressures from atmospheric to 200 atm, has been experimentally determined at temperatures between 15° c. and −35° c. It has been found that the vapour pressure of water and ice increases progressively as the total pressure on the system is increased and that the relative increase in vapour pressure at a particular pressure becomes grester as the temperature of the system is lowered. At a total pressure of 200 atm. and a temperature of −35° c. the water content of saturated air is more than three times the calculated value assuming the normal vapour pressure and 2.6 times that derived from Poynting's relationship.