This study investigates the performance of a heat pump assisted mechanical opener drying system. This system was constructed and tested at the Energy Laboratory, Mechanical Engineering Department, Balikesir University, Turkey. Wetted wool, which is a material with fibres; was used as the test material being dried. The air velocities at the inlet of the dryer were varied from 0.65 to 1.25 m/s, while the material loading ratios (material/dryer volume) ranged from 1.39 to 2.78, with an optimal value of 1.85. The bypass air ratio, defined as the air mass flow rate bypassing the evaporator divided by the total air mass flow rate, was in the range of 20-80%. The dryer was shown to be capable of specific moisture extraction rates ranging from 0.65 to 1.75 kg/kW h. The heating coefficient of performance of the dryer was found to be between 2.47 and 3.95, depending upon condenser and evaporator temperatures. The drying curve equation for the wool was given, and the experimental findings were also discussed in the light of available models. (C) 2002 Elsevier Science Ltd. All rights reserved.