Properties of Steam

Saturated Steam

The thermodynamic properties of saturated steam are shown in the table to the right. Saturated steam is pure steam in direct contact with the liquid water from which it was generated and at the same temperature and pressure as the Water. For example, saturated steam at 50 psig has a temperature of 2980E

Steam pressure is commonly expressed as psia or psig. psia is pounds per square inch absolute with reference to a perfect vacuum. Psig is pounds per square inch gauge with reference to atmospheric pressure of 14.7 psi psia. = psig + 14.7 psi (1 atmosphere).

The heat content of liquid is the heat energy in Btu/lb required to heat the 4uid to the condition indicated starting with water at 32°F

Latent heat is the beat energy in Btu/lb absorbed when a pound of boiling water is converted to a pound of steam at the same temperature. The same amount of heat is released when the steam condenses back to water at the same temperature. Latent heat varies with temperature.

Boiler Feed Water Temperature

The temperature of boiler feed water directly affects the steam output of a boiler. The following table can be used to determine the kilowatt rating of a boiler when the steam load, gauge pressure and boiler feed water temperature are known.

Example - A process requires 450 lbs of steam per hour at 75 psig. The available feed water temperature is 500F. From the chart read the kW/lb required for 50OF water an~ a gauge pressure of 75 psig. Multiply the factor by the pounds of steam: 0,3417 x 450 lbs 1518 ft.