The temperature and relative humidity of the air are the operating conditions that determine the drier performance. since the time involved and the final water content depend directly on them, and in turn determine the operation costs and the product quality. In this work, the relevant variables governing the drying operation are presented as surfaces, such that once some variables are fixed. the remaining are univocally set. In our case we have fixed final water content and drying time, usually the variables of commercial importance. and the air conditions result from the calculation. We have worked with Valenciana-type onions in a rotary drier and the result is a working zone, in terms of humidity and temperature, that yields a final water content of 5% of the original one, for drying times not larger than 8 h. (C) 2002 Elsevier Science Ltd. All rights reserved.