Enthalpy of Gases

The following have been derived from the best available data, using spectroscopic values. Historically, tables of heat capacities have been used for deriving heat contents, but since these are invariably integrated with respect to temperature, it is more convenient to tabulate the integrals themselves. Formulae for these functions are extremely complex and unweildy (Note 1), and for modelling purposes it is best to use the table to derive simpler equations over the range of interest. For convenience, all values are related to the state at 0°C. Values are in kJ/kg at a pressure of 101.325 kPa. The air and de-oxygenated air compositions are those for air at 50% humidity as defined in the Air Composition page. H₂O, CO₂ and SO₂ are all subject to decomposition at the top end of the temperature range, but because the extent of this is a function of their partial pressure, this is not accounted for here.

Note 1. Of course, we all have computers to do these multi-range fifth-(or sometimes as much as ninth)-order equations. But in most heat exchange modelling processes, it is necessary not only to calculate enthalpy from temperature, but also to calculate temperature from enthalpy. Again, because the equations are mostly differentiable, the computer can do this, using Newton-Raphson iteration. But if speed is an issue, this is a slow process. It is much better to focus on a restricted range of interest, and to fit the data to a simpler (second-order at most) formula that can be solved by explicit algebra.