A. The Mollier chart

In most cases the drying agent is air. During the drying process, heat is transferred from the air to the product while vapor is transferred from the product to the air. This combined transfer process of heat and mass leads to a change of enthalpy as well as of the vapor concentration of the drying agent. It is convenient to depict those changes in an enthalpy concentration chart for the drying agent, the so-called Mollier chart for humid air as shown in Figure 1 [see also Krischer and Kast (1978)].The enthalpy of the humid air is defined as

\[\label{eq1} h=c_{p,\rm{air}}T+X\left(\Delta h^{0}_v+c_{p,\rm{vapor}}T\right)\tag{1}\]

Figure 1 Mollier chart for humid air at constant total pressure (Krischer and Kast, 1978)

c_p,air (in J/kg dry air K) is the specific enthalpy of the dry air, c_p,vapor is that of the pure vapor. Δh_v⁰, in joules per kilogram, is the standard latent heat of vaporization at 0 °C, X is the vapor content in kilograms vapor/kilogram dry air or the absolute humidity, T is the temperature in degrees celsius, and h is the enthalpy of the humid air in joules per kilogram dry air. 𝜑 in Figure 1 is the relative humidity, which is equal to the ratio of the partial vapor pressure p_v to the saturation vapor pressure at a given temperature p_v^*(T). The absolute humidity can be calculated from the partial vapor pressure p_v and the total pressure p: