The Lewis relation in air-water vapor psychrometrics states that convective heat transfer coefficient $h_c$ and mass transfer coefficient $h_m$ are related by:
Options:
(A) $\frac{h_c}{h_m c_s} \approx 1$ (where $c_s$ is humid heat)
(B) $h_c \times h_m = c_s$
(C) $\frac{h_c}{h_m} = \rho^2$
(D) $h_c = h_m / c_s^2$
Answer Key:
A
Step-by-Step Solution & Explanation
Because the Lewis number $Le = \frac{Sc}{Pr} = \frac{\alpha}{D_{AB}} \approx 1.0$ for dilute water vapor in air, the convective heat and mass transfer coefficients satisfy the Lewis analogy: $h_c / (h_m c_s) \approx 1.0$, making wet-bulb temperature virtually identical to adiabatic saturation temperature.