where h is a heat transfer coefficient and Text the temperature of the external fluid far from the boundary, the moisture flux is described by the equation

where Mv is the molar mass of water vapor, hm is a moisture transfer coefficient and cv,ext the vapor concentration of the external fluid far from the boundary.

In Ref. 20, the heat and mass boundary layer analogy is used to express the moisture transfer coefficient hm as a function of the heat transfer coefficient h:

where Le is the Lewis number, D is the vapor diffusion coefficient in air, k is the thermal conductivity, ρ is the density, and Cp is the specific heat capacity of the external fluid evaluated at the same conditions as those specified in the heat transfer correlation used to compute h.

This relation is valid for laminar and turbulent external flows, and the value n=1/3 is suggested as a reasonable assumption for most applications in Ref. 20.

Note that the analogy applies only when the density variations due to temperature and vapor concentration have the same trend. In other words, if the hot air is heavier than cold air due to the moisture, the correlations are not suitable. Else, all the correlations described in Heat Transfer Coefficients — External Natural Convection, Heat Transfer Coefficients — Internal Natural Convection, Heat Transfer Coefficients — External Forced Convection, and Heat Transfer Coefficients — Internal Forced Convection may be used to evaluate a moisture transfer coefficient hm to be used in Equation 4-173.