This calculator is used to compute the thermodynamic properties of moist air. These properties are used by mechanical engineers to design Heating Ventilating & Air Conditioning (HVAC) systems and processes.

The program uses equations which are derived from the perfect gas relations and is suitable for most HVAC engineering applications - within the range of -40 to 150 degrees fahrenheit. Specifically, the program is based on the procedure outlined in the 1997 ASHRAE Fundamentals Handbook, Chapter 6 - Psychrometrics, section: Numerical Calculation of Moist Air Properties. The program also uses data (absolute pressures vs temperature for water at saturation) from Table 3 (Thermodynamic Properties of Water at Saturation), also from chapter 6 of the 1997 ASHRAE Fundamentals Handbook.

The following terminology will be used in the discussion of this program:

• TDB is the drybulb temperature in degrees F or C
• TWB is the wet bulb temperature in degrees F or C
• TD is the dew point temperature in degrees F or C
• RH is the relative humidity in percent
• W is the humidity ratio
• WSTDB is the humidity ratio at saturation using the dry bulb temperature
• WSTWB is the humidity ratio at saturation using the wet bulb temperature
• H is the enthalpy in BTUs per pound of dry air or KJ/Kg
• V is the volume in cubic feet per pound of dry air or M^3/Kg
• U is the degree of saturation in percent
• PWSTDB is the saturation pressure for water at the drybulb temperature in PSI or MPa
• PWSTWB is the saturation pressure for water at the wetbulb temperature in PSI or MPa
• PWSTD is the saturation pressure for water at the dew point temperature in PSI or MPa

The ASHRAE procedure outlines three situations which are commonly encountered.

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Situation 1

Given: Dry Bulb Temperature (TDB) and Wet Bulb Temperature (TWB)
Calculate: PWSTWB, WSTWB, W, PWSTDB, WSTDB, U, RH, V, H, PWSTD, AND TD.

Situation 2

Given: Dry Bulb Temperature (TDB) and Dew Point Temperature (TD)
Calculate: PWSTD, W, PWSTDB, WSTDB, U, RH, V, H, and TD.

Situation 3

Given: Dry Bulb Temperature (TDB) and Relative Humidity (RH)
Calculate: PWSTDB, PWSTD, W, WSTDB, U, V, H, TD, and TWB.