Performance Study of a Variable Speed Compressor with Special Attention to Supercharging Effect

Compressor performance is ioiluenced by many factors. Besides the commonly considered factors such as suction gas heating, leakage, compressed gas re-expansion, gas pulsations, and back flow, supercharging effect on performance is also studied. Since the compressor used in this study operates in a wide speed range, from 1100 rpm up to 9000 rpm, how those factors affect the compressor performance under different speeds is quantitatively studied. However, it must be noted that the compressor studied was a research compressor (not a production compressor) and all simulation data is therefore to be interpreted on a relative scale. Introduction Tremendous improvements of system performance have been achieved in the field of refrigeration and airconditioning during the past few decades. Energy conservation has been a very important research topic and is still the focus of the researchers in this field. Besides the legal requirement established by the United States government, the efficiency of refrigeration products is a very important parameter to the customers. In the area of residential air-conditioners and household refrigerators, ON/OFF control is commonly used as a capacity control system. This technique is cheap and simple to implement. However, its disadvantage is the unavoidable reduction of seasonal energy efficiency ratio (SEER) due to the losses caused by the frequent ON/OFF cycling during actual changing load conditions [ 1). At the emergence of variable frequency inverters, variable speed compressors became a reality. Therefore, the capacity of refrigerators and air -conditioners can also be controlled by the speed of the compressors. This new technique has attracted a growing interest for several reasons [2]. (1) In most applications, a variable speed controlled refrigeration system is more efficient than the system using ON/OFF or stepped capacity control. (2) A variable speed controlled system is capable of controlling its capacity in a wide range, and is able to adjust compressor output closely according to actual changing load conditions. The performance of a variable speed rolling piston rotary compressor, which is shown in Figure 1, is investigated in this paper. Controlled by a frequency converter, this compressor can operate in the speed range from 1100 rpm to 9000 rpm. Compressor performance has been analyzed by many researchers [1, 3-5], but most of them assume that the compressor runs at the constant conventional speed. The intent of this paper is to study the effects of various factors on the compressor under different speeds, especially the speed dependent factor such as supercharging. Thus the performance of the compressor can be improved. Overview of the Compressor Simulation There are a number of complicated phenomena occurring inside the compressor during each cycle of operation: unsteady flow through the suction and discharge passages, thennodynarnic processes inside the compressor