Unsteady State Analysis Of A Hermetic Reciprocating Compressor: Heat Transfer Inside the Cylinder And Valve Dynamics

In this work, an unsteady state analysis of the compression cycle of a small hermetic reciprocating compressor for domestic refrigeration was carried out. The mass and energy balances were applied to the refrigerant inside the cylinder in order to determine the mass and temperature behaviour through the compression process. A specific onedimensional model of the valves was developed to calculate the mass flow rates that were processed. The compression cycle unsteady state analysis was inserted into a traditional steady state model of the compressor to evaluate the combined effect of the heat transfer inside the cylinder and the valve dynamics on compressor performance and efficiency. The whole simulation code was validated against the experimental measurements carried out on an R134a commercial unit in a wide range of operative conditions. INTRODUCTION The performance of a small hermetic reciprocating compressor is greatly affected by the heat transfer inside the cylinder during the compression cycle and by the valve dynamics. The evaluation of the heat transfer between the refrigerant and the cylinder wall requires an unsteady state analysis of the compression cycle as suggested by Todescat et al. 1992. This analysis may be complicated by the phase lag between the heat transfer and the temperature difference which is due to the simultaneous heat and work transfer as illustrated by Fagotti et al. 1998. The study of valve dynamics involves the evaluation of the mass and stiffness parameters and the analysis of the unsteady state flow through the valves. The most relevant unsteady state effects in refrigerant flow through the valves are those of the inertia of the refrigerant stream, which produces a time delay between the pressure variation and the relative velocity change, and the work transfer between the refrigerant flow and the valve plate. The latter modifies the kinetic energy and, hence, the velocity of the refrigerant flow, as shown by Böswirth 1984 and 1990. In this work, an unsteady state analysis of the compression cycle was inserted into a traditional steady state model of the compressor (Cavallini et al. 1996) in order to evaluate the combined effect of the heat transfer inside the cylinder and the valve dynamics on compressor performance and efficiency. THEORETICAL MODEL AND COMPUTER CODE The basic assumption behind the model was to consider the refrigerant flow through the compressor, except for the valves and inside the cylinder, as a one-dimensional steady state current. In this way it is possible to establish a steady state thermal balance to compute the temperatures and the heat and work flow rates for each component and for the overall system. The compressor was subdivided into six parts (shell, body, suction muffler, suction chamber, discharge chamber, discharge line) (see figure 1) and, for each of them, the mass and energy balances were established. The main irreversibilities inside the compressor (electric energy conversion losses, friction losses) were taken into account by suitable electrical and mechanical efficiencies. The compression cycle, the refrigerant flow through the valves and the valves dynamics were analysed by an unsteady state approach. This allows the direct computation of the heat and work flow rates exchanged together with the mass flow rate processed, as well as also the behaviour of the main characteristic parameters during the compression cycle.