An Experimental Method for Design and Performance Evaluation of a Hermetic Compressor

Experimental determination of the effects of parameters which can be used as guidelines in design modifications and the evaluation of their effect on the performance and EER of a hermetic compressor is proposed. The Pressure-crank angle record obtained during standard calorimeter test offers the necessary data. Displacement of the valves is not measured explicitly but the opening and closing of the valves are noted as the points of discontinuity on the continuous polytropic processes. Design changes of the valves were conceived and evaluated using this method. An improvement in performance and energy efficiency ratio was obtained with the modifications adopted. INTRODUCTION The pressure crank angle (P-6) relationship along with the valve opening and closing timings have been some of the main parameters for design improvements in the automobile industry. The accessibility of the cylinder allowed the use of simple mechanical indicator diagram equipment and the cam operated valves gave direct timings of valve action. However, a hermetically sealed compressor poses multiple problems such as:: High speed running necessiates instruments with good dynamic response Limited space and accessibility for fixing the transducers Sealing of the shell to be undisturbed by the instrumentation Performance to be undisturbed by the instrumentation 428 Separate monitoring of valve timings. Early researchers, Speich1 , Me Connell & Cohen2 have measured the pressure-volume (P-V) relation of compressors successfully. The criteria limiting the dynamic range of pressure adaptors were suggested qy Elson and Soedel3. However, the effect of adaptors on clearance volume is not discussed. Microprocessor based data acquisition helps in recording multipleparameters such as pressure, temperature, displacements and crank angle. Time averages, variation from cycle to cycle and during the cycle can be automatically calculated, Surfuzi & Nakayamma4, Chang K.Y., Kerr S.V., Maclaren T.F.T.5, Murati and Fuzitani6 have acquired data through a microprocessor based system. Along with the experimental methods, reliable computer oriented numerical models are available to obtain the ~ressure, volume, valve displacements, and mass flow rates. Soedel has compiled an introductory model with a Fortran listingS. The model was enhanced by Hamilton9. Many improved models with additional fac~ taken into consideration are found in literature. The criteria for analysis of a compressor independent of the refrigeration cycle are not standardized. Pandeya & Soedel10 have defined the term efficiency of performance which combines all the factors that affect the compressor efficiency. Shaffer11 has also discussed the mechanical, gas compression, suction gas heating, and overall efficiencies in evaluating compressor performance. He obtained an improvement in performance using larger discharge port areas and reduced suction gas temperature. Jacobs12 took an overall view of the performance losses due to suction gas heating, manifold pressure drop, and pulsation, valve dynamics, piston ring blowby, and clearance volume. Since the capacity increases as shown by bare calorimeter data do not always lead to EER increase, the present study is an attempt to make use of the detailed analysis of the pressurevolume diagram for estimation of the effectiveness of the design changes made to improve both the performance and EER of a compressor. The experimentation is designed following conventional technique in which the transducer output is fed to an oscilloscope and photographed. Displacement of valves were not separately measured but the valve opening and closing timings were read from the pressure-crank angle diagram with reasonable accuracy. The instrumentation required is simplified to avoid the introduction of physical changes which might affect the true performance of the compressor. EXPERIMENT The compressor under study is fixed in a bolted shell in~