A closed-type water source heat pump dryer integrated with a drying programme (DP) was designed and tested. The DP was developed using psychometric conditions of desired drying air and thermodynamic balance of heat pump. A drying algorithm was developed according to the technical specification of the dryer. Drying air temperature, relative humidity and drying air volumetric flow rate were entered from a programmable logic controller (PLC) screen. In this study, the drying of mint leaves was examined. Mint leaves were dried at different temperatures (35 degrees C, 40 degrees C and 45 degrees C) and at different volumetric flow rates 300 m(3)/h for a velocity of 0.75 m/s and 600 m(3)/h for a velocity of 1.5 m/s in the PLC-controlled heat pump dryer. The relative humidity of the drying air entering the dryer chamber was kept between 13.88% and 6.81% and the two air dampers were controlled according to predetermined value. The coefficient of performance (COPhp) values obtained were between 3.81 and 2.29. The specific moisture extraction rate for the whole system (SMERws) was between the 0.034 and 0.044 kg/kWh. The drying air temperatures were controlled with an accuracy of +/- 0.41 degrees C to +/- 0.76 degrees C depending on the mode of operation.