Fast Response Temperature Measurements in a Reciprocating Compressor

Mathematical modeling of the dynamic processes in a compressor involves a knowledge of the various forms of energy exchange occurring in the system. Heat transfer to and from the gas is one such. Any detailed modeling of the heat transfer process would require a detailed knowledge of the temperature variations resolved to temporal scales at least as fine as the finest generated by the dynamics of the valves and their interaction with the cylinder and piping. This paper presents measurements of gas temperature inside the cylinder and valve chambers made with a new type of thermocouple sensor which can meet the fast response requirements. The measurements were used to predict the effect of suction gas heating on capacity loss which agreed quite well with direct capacity loss measurements. INTRODUCTION The ever increasing demand for improving energy efficiency and pollution control has fueled lot of research on the understanding of the dynamic processes in machinery including reciprocating compressors and its application to analytical modeling. In reciprocating compressors, until recently , the heat transfer process had assumed a back burner role. This was partly because Of the notion that heat transfer has very little impact on the performance of a reciprocating compressor and partly due to the difficulty in modeling the complex heat transfer processes. Even determination by experiment is also not an easy task, since it is very difficult to isolate its effects. Most of the work on heat transfer effects have been done in the area of refrigeration compressors. Meyer and Thompson ( 1) have studied heat transfer effects on the performance of refrigeration compressors by using a steady state modeling of the complete system. They particularly studied the impact of inlet system design on heat transfer effects. The comparison of the results of their model with experimental data indicated reasonable agreement and suggested that the discrepancies could perhaps be reduced by considering the unsteadiness in the heat transfer process occurring inside the cylinder. Pandeya and Soedel [2) have ·derived a simple relationship for the change in mass flow rate expressed as a function of the magnitude of suction gas heating using thermodynamic principles. Adair, Qvale and Pearson [3) have provided a correlation for the instantaneous heat transfer across the cylinder ba13ed on dimensionless quantities with constants derived from their experimental data. Jacobs [ 4] has reported measurements of the important losses in a compressor. In addition, he observed the benet! t of cooling of the suction gas on volumetric. efficiency increase. He also indicated that a suction gas heating of 10°F would reduce the volumetric efficiency by approximately 2 %. on the other hand, Brok, Touber and Vander Meer [5] provided credence to the conflicting opinion about the extent of influence of heat transfer on volumetric efficiency decrease by suggesting that the impact (was only 2.5 % for the compressor they modeled) is only nominal. Lee and Smith [ 6 j have measured instantaneous temperature inside the cylinder in order to understand the heat transfer