I want to create a program that will accurately simulate a condensor. I want to use the data in psychrometric charts. But I cannot and hence want to use equations that show similar data. Any idea where to start?

  • $\begingroup$ This is what an interpolating fit function is for. $\endgroup$
    – Colin K
    Commented Apr 18, 2012 at 3:41

2 Answers 2


Data in the psychrometric charts are essentially experimental. If you want to do approximate calculations, you can do the following:

For saturated vapor pressure (100% humidity line) you can use one of many approximate functions:

$$p = 611 \exp\left( \frac{a \theta}{b + \theta} \right)$$

where $\theta$ is temperature in $^\circ$C, $a = 22.44, b=272.44$ for negative temperatures and $a = 17.08, b=234.18$ for positive temperatures.

For specific enthalpy lines (entalphy over mass of dry air), you can calculate them using the data for specific latent heat for water at $0^\circ$C and heat capacities of vapor and dry air.

$$h = (c_{p,a} + x \; c_{p,w}) \theta + x \; l_0$$

where again $\theta$ is temerature in $^\circ$C, $l_0 = 2500$kJ/kg specific latent heat of water at $0^\circ$C, $c_{p,a} = 1.005$kJ/kgK heat capacity of dry air at constant pressure and $c_{p,w} = 1.926$kJ/kgK heat capacity of wapor at constant pressure. $x$ is absolute humidity.

Of course heat capacities are not temperature-independent constants, but the result will be fairly correct.


There are existing software libraries of psychrometric functions available. These could be suitable for use directly, or at least as points of reference if for some reason you need to create your own. i.e., you can inspect the formulae, coefficient values, etc.

One set of psych equations is the PsychroLib project: https://github.com/psychrometrics/psychrolib

PsychroLib is a library of functions to enable the calculation of psychrometric properties of moist and dry air. Versions in Python, C, Fortran, JavaScript, and Microsoft Excel Visual Basic for Applications (VBA) are available. The library works in both metric (SI) and imperial (IP) systems of units. For a general overview and a list of currently available functions, please see the overview page.

A second source would be EnergyPlus which uses psychrometric functions for various building energy analysis purposes.

EnergyPlus has a full complement of psychrometric functions. All the routines are Fortran functions returning a single precision real value.

The code actually appears to have decent comments in addition to the actual documentation.


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