Psychrometric Calculator

by Michael J. Rocchetti PE, Mechanical Engineer
in cooperation with the
University of Connecticut Architectural & Engineering Services

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:

bulletTDB is the drybulb temperature in degrees F or C
bulletTWB is the wet bulb temperature in degrees F or C
bulletTD is the dew point temperature in degrees F or C
bulletRH is the relative humidity in percent
bulletW is the humidity ratio
bulletWSTDB is the humidity ratio at saturation using the dry bulb temperature
bulletWSTWB is the humidity ratio at saturation using the wet bulb temperature
bulletH is the enthalpy in BTUs per pound of dry air or KJ/K
bulletV is the volume in cubic feet per pound of dry air or M^3/Kg
bulletU is the degree of saturation in percent
bulletPWSTDB is the saturation pressure for water at the drybulb temperature in PSI or MPa
bulletPWSTWB is the saturation pressure for water at the wetbulb temperature in PSI or MPa
bulletPWSTD 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.

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.

Dry Bulb Temp:

Wet Bulb Temp:

Temp Units: Fahrenheit Celsius

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Situation 2

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

Dry Bulb Temp:

Dew Point Temp:

Temp Units: Fahrenheit Celsius

*

Situation 3

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

Dry Bulb Temp:

Relative Humidity %:

Temp Units: Fahrenheit Celsius

*