Simulation of the Transient Behavior of Household Refrigerators and Freezers

Due to their contribution to the ozone depletion and the anthropogenic greenhouse warming effect caused by the refrigerant as well as the foam blowing agent household refrigerators and freezers were attracting public attention especially in Western Europe. The conversion to the use of environmentally benign fluids as refrigerants and foam blowing agents requires a large number of extensive, expensive and time consuming performance tests. This number can be reduced significantly by the assistance of a simulation program. A program to simulate the dynamic behavior of household refrigerators and freezers has been developed at the institute. The main object of this program is not to simulate a type of refrigerator very exactly but to calculate the effect of constructive modifications on a refrigerator with regard to energy consumption and compartment temperature. In order to develop a model, only the geometric data, the material properties and the performance graph of the compressor have to be known. The thermodynamic basic principles as well as the application of the simulation model are shown for a simple refrigerator. Using the simulation program, the effect of various parameters on the cycling operation and the energy consumption is examined and a comparison with measured data is made. Provided that the modifications of the specific refrigerator are not too far from the original one, an exact prediction of variations to the characteristics of the refrigerator can be expected. INTRODUCTION Unlike many other refrigeration plants, for the simulation of household refrigerators, the inclusion of the housing is, besides the calculation of the refrigerant circuit, very important. Through the large thermal inertia of the insulation used in the housing, compared to that of the. refrigeration cycle, the transient behavior of the refrigeration cycle is mainly influenced by the housing. Once a model of a refrigerator or freezer is developed parameter studies can be performed very easily with the program. These parameters can be : compressor type thickness, heat conductivity and material of insulation size and material of condenser and evaporator -size of the compartments switching temperatures of control device SIMULATION MODEL Very fast transient changes in the refrigeration cycle occur only during cycling operation for a short period directly after the compressor has started to operate. The prevailing part of the cycling period can assumed as quasi-stationary. In the housing there are on the contrary changes in the temperature during the complete cycle of operation which have to be taken into account for the simu~ati~n. Even in the l!leasurements, the behavior of the whole system of refrigeration cycle and housmg 1s always determmed. A separate development of each of these two parts may not influence the whole system to the same degree.