TABLE OF CONTENTS The cold-air distribution system designer must perform a psychrometric analysis of the temperature and humidity conditions throughout the system. This analysis is essential to accurately determine system design requirements, particularly the air condition entering the cooling coil. The commonly used rules of thumb that often take the place of such an analysis are not applicable to cold-air distribution designs, where coil latent cooling capacity requirements must be carefully evaluated.
The calculations presented here are based on volume airflow rates expressed in terms of standard density. As discussed in section 4.1, actual airflow rates should be converted to standard flow rates for accurate calculations.
Psychrometric properties most pertinent to this analysis include dry-bulb and wet-bulb temperatures, relative humidity, and humidity ratio. Other properties are enthalpy, dew-point temperature, and specific volume. In general, if any two properties are known, the others can be calculated by any of several methods. Chapter 6 of the 1993 ASHRAE Handbook Fundamentals discusses methods for calculating psychrometric properties. A number of computer programs are on the market that perform psychrometric calculations. A psychrometric chart can be used to graphically determine properties, although the results are generally not as precise as with the numerical methods.
The analysis of heat transfer and psychrometric processes in an air distribution system generally proceeds as follows:
Determine the air condition leaving the cooling coil.
Determine the final supply air temperature to the space.
Calculate the required supply airflow.
Determine the return air condition entering the air-handling unit.
