Abstract In this paper, a new computer modeling has been used to study the irreversibility of evaporation in timber dryer assisted heat pump. Also, energy and exergy analysis was carried out to determine the irreversibility of evaporation in this timber dryer. In the model the irreversibility was calculated for ambient temperature of 0 to 30 , relative humidity of 40%, 60% and 80% and recirculation air ratio (RAR) of 0 to 0.9. The results show that the minimum irreversibility was obtained at 30 ambient temperature. Also, increasing relative humidity, decrease in irreversibility slightly. Heat pump dryer (HPD) with recirculation air, had minimum irreversibility compare with HPD without recirculating air especially in lower air temperature. The using of heat pump in timber drying process limited the amount of RAR in the unit but the advantage of this is better controlling the drying air temperature and the humidity of timber drying in process.

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