Design and evaluation of an R290-based freezing system and air distribution in a domestic-scale microwave-heated freeze-dryer prototype

Freeze-drying is a drying process that can result in long-term and cost-effective storage of food products. In the case of domestic applications, a compact freeze-dryer may lead to the production of added-value products in households and small farms. For this reason, this study aimed to determine the design of R290 refrigeration system together with air distribution system components in a one-chamber microwave-heated freeze-dryer prototype. The device is capable of drying both fresh and frozen food products. The freezing time of food products was experimentally analysed concerning various evaporator configurations to fulfill the final device design's compactness and high food load. The study showed that an internal evaporator located at the bottom of the freeze-dryer chamber outperformed the external heat exchangers with mini- and micro-channels. The freezing process was optimised based on the CFD model of airflow distribution in the microwave-heated chamber with a stationary multi-shelf system by proposing the shape and location of air guides and radial fans. The results showed that the solution involving air guides positioned at the ends of the shelves at a 60-degree angle provides uniform air distribution. The combination of experimental and numerical approaches resulted in a complete design of a prototype freeze-dryer chamber in which, according to the calculations performed, the freezing time of food products was 40% faster compared to the reference device.