Overview

Psychrometrics and psychrometric processes are used in the conversion of the cooling and heating loads into equipment loads for system design and optimization. The use of psychrometrics enables the designer to determine air quantities and air conditions that can be used for equipment selection and duct and piping design. This section deals with the fundamental properties of moist air and the basic processes used in HVAC systems. More detailed information is available in the ASHRAE Handbook, particularly Fundamentals and Systems.

There are two methods used to determine the properties of moist air. The calculations can be made using the perfect gas relations or the various properties can be determined from a psychrometric chart, which is based on a somewhat more rigorous analysis. For all practical purposes, properties computed from the perfect gas relations agree very well with those read from a psychrometric chart. Many computer programs are available to compute the properties of moist air, and these programs may display a chart as well.

The psychrometric chart is a valuable visual aid in displaying the processes in the HVAC system. Figure D1 is the abridged ASHRAE Psychrometric Chart for the normal temperatures of air-conditioning applications and for a standard barometric pressure of 29.921 in. of mercury. Charts for other temperature ranges and other altitudes are also available. For example, ASHRAE Psychrometric Chart 4 is for an elevation of 5000 ft. The various properties may be identified on the chart where dry-bulb temperature is plotted on the horizontal...