# Equations instead of psychrometric charts

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?

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This is what an interpolating fit function is for. –  Colin K Apr 18 '12 at 3:41

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.

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