Partial recirculation of the dryer outlet air is a common practice in many batch-type drying processes. However, energy consumption and total duration of drying are directly affected by the recycled air ratio (RAR). In order to assess the influence of different ambient air conditions on favorable values of RAR, a new quantity is introduced. The so-called critical drying efficiency determines the most favorable starting point for partial air recirculation. It is shown that by increasing RAR successively in the further course of drying, substantial energy savings can be achieved with only a marginal extension of drying duration. It is also pointed out that optimized control of drying plants should be based not only on dryer outlet air conditions but also on ambient air conditions.