Low order dynamic model of a vapor compression cycle for process control design

Control of food processes involving vapor compression cycles as actuators is often a difficult task: indeed this particular device is itself a complex process including coupled unit operations as evaporation, compression, condensation and expansion. Nevertheless, an accurate control of heat transfer rate is often essential for global quality of product and stability of flow in exchangers. Moreover a number of vapor compression systems are already equipped with variable speed-compressors and fans. However, due to lack of knowledge about the dynamic behavior of these systems, the industry has not taken full advantage of these variable devices to get substantial control performance improvement. This paper presents a lumped-parameter model for describing the dynamics of vapor compression cycles. Based on moving-boundary approach for the description of two-phase/single phase interface inside the heat exchangers, this low-order model composed of only ordinary differential equations can be highly useful for design of control strategies. This model has been validated on an experimental device and a good agreement between measurements and computed data has been found. Use of this model in food process control design is discussed.