Multi-Step Design Method for Achieving Heating Uniformity in a Domestic Microwave-Heated Freeze-Dryer with a Stationary Multi-Shelf System

This paper aimed to develop a multi-step design method for achieving heating uniformity in an all-in-one standalone microwave-heated high-load freeze-dryer for domestic applications. In conventional freeze-dryers, the refrigeration unit is located outside the freeze-drying chamber, while heat is provided to the product through conduction by heating mats located above or under the food shelves, respectively. The proposed multi-step method was applied to design the microwave heating system in a compact freeze-dryer with a stationary multi-shelf system and integrated with a crystalliser unit. For this reason, the proposed method consists of four steps. Firstly, a computational model of a standard microwave oven was developed and experimentally validated. The model was then used to perform a parametric analysis of the design parameters of the microwave heating system for the investigated freeze-dryer. As a result, the method allowed for the assessment of the back wall as the best location for magnetrons. Additionally, it was concluded that a rotating mixer blade located between microwave windows greatly improves the heating uniformity for any configuration of magnetrons. Based on the last stage of the multi-step method, a single case with the highest heating uniformity was selected. To achieve this, the key performance parameter was applied in the form of the Heating Uniformity Index in the freeze-dryer analysis. Finally, the multi-step method allowed for the selection of the best configuration with the Heating Uniformity Index equal to 18.59, which is 72% better than the average case considered in the final step of the developed method.