Sidebar 3: How to Calculate Enthalpy

To calculate enthalpy, a simplifying assumption is made, followed by a series of calculations.

By definition, the degree of saturation (µ) of air can be calculated using Equation 12 from Chapter 6 of the ASHRAE Handbook of Fundamentals, 2001, which is outlined below. 

However, the degree of saturation and relative humidity are approximately the same at temperatures below 100°F.  This simplifying assumption is then applied to the previous equation by substituting relative humidity for m, obtaining values for W_s from a psychometrics chart and calculating W.  Rewriting the equation above, we get:

W = %RH x W_s

Where:

W = specific humidity (lb_water / lb_{dry air)}

%RH = relative humidity of the air

W_s = specific humidity at saturation (lb_water / lb_{dry air)}

Finally, enthalpy is calculated using Equation 32 from Chapter 6 of the 2001 ASHRAE Handbook of Fundamentals.

H = (0.24 x T) + [W x (1061 + 0.444 x T)]

Where:

H = enthalpy (Btu/lb)

T = dry-bulb temperature (°F)

W = specific humidity (lb_water / lb_{dry air)}