INFLUENCE OF HEAT PROPERTIES OF METALLIC OR DIELECTRIC CONTAINERS ON THERMAL YIELD AND ENERGY EFFICIENCY IN MICROWAVE HEATING APPLICATIONS
Isabelle Polaert, Michel Delmotte, Nassima Benamara, Lionel Estel
Abstract: Microwave heating is often considered as a volumetric and uniform heating and also as the very selective heating for polar products and materials. Vessels, pipes, moulds, conveyor belts, made of lossy materials are required for containing products or carrying materials. For microwave processes design and calculations, the steady state of the physical system is always considered. The total yield of the microwave equipment, equal to the power absorbed by the process divided by the total electrical power consumed, takes into account, in most of the cases, the magnetron output power and the thermal efficiency of insulators only. But when looking at microwave applications such as thermoset polymer curing or flow chemistry under pressure, one can realize that transient phases are crucial for microwave generator and process design.
For curing unique items made of moulded thermoset polymers, the heating has to transform the material, from the heat sensitive polymer up to its solid and definitive shape. Consequently, the microwave equipment runs in thermal transient state. In this study, we observed the predicted variation of the absorbed electromagnetic power all along the thermal curing process. A thermal model was developed correlating the temperature increase with thermal characteristics such as heat capacities, heat diffusivities and thermal conductivities and consequences on the process design were drawn.
The case of a special reactor-applicator for high pressure chemical reaction was also examined, thus led to similar consideration. The heat capacities (beyond 20 kJ.K-1) of the stainless steel container, the applicator and the insulator as well, condition the heating time and required power. Thermal gradients require time for space stabilization. Indeed, we can show that the time constant in all the system is proportional to its whole heat capacity. The role of heat losses, fluid nature and flowrate is examined and typical parameters are discussed.
This paper exhibits the crucial parameters on thermal yield and energy efficiency of microwave processes. In a context of energy saving, and precisely in the domain of microwave applications, these results show how important it is to calculate not only the applied power necessary to maintain a steady state but also the energy consumed all along the preliminary transient state.
L. Douadji , M. Delmotte, J.M.P.E.E., 2012, 46 (3), 174-484.