The separation performance of air dehumidification is investigated over a wide range of experimental conditions by using an isothermal PSA apparatus. Velocity ratio of purge to feed (gamma), half cycle time, column length (L), and particle size are round to have a great effect on the product concentration, while the independent effect of pressure ratio (beta) seems relatively small with fixed values of the other parameters. A simplified analytical model proposed previously, short cycle time approximation, is extended to the Langmuir isotherm to derive the following new algebraic equation which identifies the combination of the operating and design parameters. In[(1 - beta)(C-A1/C-A0)] - In(1 - 1/gamma) = -(N-0(1) - A)/r in which N-0(1) is a combined number of mass transfer units given by N-0(1) = (K(A)amL)/(1 + K-A/K-D)(1/u(A) -1/u(D)) and the parameter A in the equation is a simple function given by the velocity ratio gamma and Langmuir coefficient r. Good agreement between the calculated results and the experimental data show that the short cycle time approximation is suitable for estimating the separation performance of the purification PSA process such as the present air dehumidification system with a better understanding of the role of each parameter on the separation performance.