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Drying is a complex and energetically intensive process accounting for at least 15% of industrial energy usage and even higher in some specific sectors: 30-55% in medicinal plants processing, 50% in textile manufacturing and up to 70% in wood industry.
In general, most heat losses in industrial dryers are mostly due to the discharge of moist air. Heat pumps assisted dryers has emerged as one of the best solutions for this problems due to the potential of saving up to 50% or more of the primary energy used.
The dryer exhaust air serves as the heat source for the evaporator of the heat pump (HP). After cooling and dehumidification, the working air can (partially) be recirculated and heated up to a higher temperature level in the condenser. In case of completely closed air-cycle advantages such as the avoidance of fire hazard, annoying odour or environmental load of contaminants can be exploited. Moreover, other gases than air can be applied in order to avoid the drawbacks of oxygen: explosion risk or product degradation. Another promising possibility is to utilize the condensate, but in some cases additional costs due to the need of cleaning before discharge could be caused.
During the last few decades a lot of academic research was performed on heat pump dryers (HPDs) to reduce energy consumption. However, reliable industrial applications are still exceptional. HPDs are much more complex than each of their components separately. Hence, theoretical and experimental analyses of the overall system must consider the complex interaction between the drying process and the HP cycle.