An effectiveness model has been developed from the approximate analytical solution of a rotary desiccant wheel. The resulting equations clearly show that the maximum air temperature and humidity differences are two independent driving forces commonly acting in the heat and mass transfer processes. Comparison with the numerical model from the preceeding study revealed that the effectiveness model could make realistic predictions roughly with 10% uncertainty. The model may be useful for the rough design and analysis of desiccant evaporative cooling systems.