| Abstract: | The loss mechanisms in convective drying systems are examined from a second law viewpoint. The avoidable and unavoidable loss processes in heat pump driers are identified and compared with those in conventional heat and vent driers. In the quasi-static limit the only unavoidable loss in a heat pump system is due to the irreversible diffusion of water vapour into the unsaturated atmosphere of the drier. Thermal losses, due to heat transfer within the drier and venting to the atmosphere are avoidable in principle by the use of a heat pump. In practice the efficiency of a heat pump drier depends on how effectively these processes are managed, whereas in conventionally-heated driers the major loss is due to the heating process. Manipulation of the residual thermodynamic loss can have only a restricted effect on the efficiency in such driers. The influence of drier design on the unavoidable losses in heat pump driers is shown to be important. In the most advantageous case, a multi-pass system with a high recirculation ratio, the unavoidable loss is related only to the relative humidity. Temperature is an important parameter in other systems, especially single-pass driers. The significance of the unavoidable thermodynamic loss is that it is closely related to the actual thermodynamic loss in an effective heat pump drier design because it sets the scale of the actual losses. Thus, the limiting loss is a relevant consideration when selecting the operating conditions for heat pump driers. |
