Abstract In this paper, heat pump timber dryer has been utilized at different ambient air temperature, relative humidity and recirculation air ratio by computer modeling. Indeed, the base model has been experimentally prepared by the others. For this system, energy and exergy analysis was accomplished to determine the best irreversibility of timber dryer. It was obtained at, far from that means, this is the critical temperature for this system, in the other words the minimum and maximum possible ambient air temperatures are , , that occur in relative humidity at 0.6 and 0.8 respectively. Also, recirculation air ratio was changed in range of 0.6 to 0.9.

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