Enthalpy Lines. For ASHRAE’s chart, the molar enthalpy of moist air is calculated from the formulation recommended by Hyland and Wexler (1983) in their paper “Formulations for the Ther- modynamic Properties of Dry Air from 173.15 K to 473.15 K, and of Saturated Moist Air from
173.15 K to 473.15 K, at Pressures to 5 MPa.” For ASHRAE’s chart, the enthalpy h lines incline at an angle of 25° to the horizontal lines. The scale factor for the enthalpy lines Ch, Btu / lb ft (kJ / kg m), is