19.1 Introduction

Psychrometrics deals with the thermodynamic properties of moist air, which is the final heat transport medium in most air conditioning processes. The use of psychrometric tables and charts allows the designer to make a rational and graphic analysis of the desired air conditioning processes.

The general use of psychrometric data and charts began with the publications of Dr. Willis Carrier in the 1920s. In the 1940s, a research project conducted at the University of Pennsylvania by Goff and Gratch [sponsored by American Society of Heating and Ventilating Engineers (ASHVE)] resulted in new, more accurate data, which remained definitive until the results of further research were published in the 1980s.

This chapter deals with the subject rather briefly and simply, but in sufficient depth to provide an adequate background for HVAC design. For further study see Ref. 1.

19.2 Thermodynamic Properties of Moist Air

Moist air is a mixture of atmospheric air and water vapor. Dry air contains no water vapor. Saturated air contains all the water it can hold at a specified temperature and pressure. The properties of moist air can be evaluated by the perfect gas laws with only a small degree of error, which is not significant in most HVAC processes. The properties of interest in this discussion are the dry-bulb (db), wet-bulb (wb), and dew point temperatures; humidity ratio; degree of saturation; relative humidity (RH); and enthalpy and density.