Enthalpy, for practical purposes, is the total amount of heat in one pound of a substance calculated from an accepted temperature base. The temperature of 32 F. is the accepted base for water and water vapor calculations. For refrigerant calculations the accepted base is -40 F.
Example: What is the enthalpy of 1 lb. of water at 212 F. assuming 0 enthalpy at 32 F.?
Solution:
Enthalpy at 32 F.= 0.
Heat to raise the temperature of 1 lb.of water from 32 F. to 212 F.= 212- 32 . 180 Btu.
This is one Btu per pound of water per degree F.
Total heat at 212 F.= 180 Btu.
Example: What is the enthalpy of 10 lbs. of steam at 212 F.?
Solution:
Enthalpy at 32 F.= 0.
Enthalpy of water at 212 F. = 180 Btu
Enthalpy of steam at 212 F. = 970 Btu
Total heat of 1 lb. of steam at 212 F. = 1150 Btu Total heat of 10 lbs. of steam at 212 F. will be 1150x 10= 11,500 Btu.
The latent heat of vaporization of water at atmospheric pressure is 970 Btu/lb., as shown in Par. 1-27.
Tags:
Basics of Refrigeration