For superheated gases, enthalpy is a function of both temperature and pressure:

However, enthalpy is more strongly a function of temperature, and thus you will frequently see the approximation as follows:

where the specific heat at constant pressure is used as a proportionality constant. For processes that are not at constant pressure, the above approximation starts to lose accuracy.

AFT Arrow offers three ways to calculate the enthalpy. Below is a description of each.

Reference Enthalpy

The Reference Enthalpy Method uses the following equation to calculate enthalpy

The reference enthalpy and temperature is entered in the fluid library, and should be for a state that is as near as possible to the operating conditions of your model. As seen from the equation, the pressure dependence of enthalpy is ignored. In many cases, this assumption is acceptable.

Generalized Enthalpy

The Generalized Enthalpy Method is an extension of the Reference Enthalpy Method. The Generalized Enthalpy, similar to the Three-Parameter Method, uses critical property data to calculate a Residual Enthalpy. The method is shown below.

where:

B₀ and B₁ are as given previously, and:

Table Lookup Enthalpy

The third method of obtaining enthalpy data is by using AFT Arrow's two-dimensional table. In this method, a sequence of polynomial curve fits as a function of temperature are used. Each step in the sequence is for a different pressure.

With this method, enthalpies would be obtained by interpolating the polynomials between 25 and 50 psia at a given temperature. Up to 10 pressure table entries can be used. If the pressure is outside the table range, the enthalpy is extrapolated.