Thermal Analysis of a Hermetic Reciprocating Compressor

The development of more efficient hermetic compressors for refrigeration involves a sound analysis of the heat exchanges inside the machine to evaluate the interaction among compressor structure, flow characteristics, refrigerant properties and operative conditions. This work presents a computational procedure for the steady-state thermal analysis of a hermetic reciprocating compressor. The machine is subdivided into six parts (shell, compressor body, suction muffler, suction chamber, discharge chamber, discharge line). The energy balance based on the first law is established for each component and for the overall system to obtain the temperature distribution inside the machine and the heat flow rates exchanged. The results of the simulation are compared against the experimental measurements carried out on commercial units operating with R600a and R134a. INTRODUCTION Several computational models for the thermal analysis of refrigeration compressors are to be found in the open literature: they range from simple simulations on the effects of heat transfer on compressor performances to more complex procedures concerning the overall system. Among the first type of analyses, the one by Brocket al. 1980 [I] is particularly interesting, considering the heat exchanges from the compression process to the external medium and to the suction gas (internal heat transmission). This analysis is carried out by applying both simple overall thermodynamic relations (isentropic and polytropic equations) and also by a simulation model complemented with heat transfer equation for the cylinder, the suction and discharge lines. This model is limited only to open-type compressors and, therefore, it does not include the heat exchanges relative to electric motor, compressor shell and lubricant oil, which are very important in hermetic units. The most recent and interesting numerical codes for the overall compressor are those by Meyer et al. 1990 [2] and by Todescat et al. 1992 [3] and 1994 [4]. The model developed by Meyer et al. for a small reciprocating hermetic compressor is based on a steady-state energy balance for the different components and for the overall system, where the heat transfer coefficients are derived from available correlations or from experimental measurements. The mass flow rate and the compression process is analysed by assuming experimental values for the volumetric efficiency and the compression efficiency respectively. This program gives the temperatures and the heat flow rates relative to each component and the refrigerant gas outlet conditions, but it requires the value of the electric power input, which is usually something one would like to determine. The model by Todescat et al. is also based on a steady-state energy balance of the different compressor component<; and it also includes a sound analysis of the heat and work transfer during the compression cycle. A companion simulation program is used to evaluate mass flow rates, enthalpies and pressures inside the machine. This approach allows a computation of the temperature distribution inside the compressor, the refrigerant outlet conditions and also the power input. The results obtained agree well with the experimental measurements on a small hermetic compressor. Present work reports a new model for the thermal analysis of a hermetic reciprocating compressor which is compared against the experimental data measured both on a R600a and a R134a commercial units in a wide range of operative conditions.