Theoretical comparison of the performance of two-stage reciprocating compressor, a two-stage scroll compressor, and a vapor-injection scroll compressor

Abstract

[EN] Vapor-injection technique comprises the injection of vapor refrigerant into an intermediate stage of compression. This technique is used in refrigeration systems and heat pumps working in extreme conditions. The advantages of vapor-injection are the improvement of capacity and COP in severe climates and the reduction of the compressor discharge temperature. The vapor-injection scroll compressor (SCVI) is one of the most frequently used compressors in heat pump systems with vapor-injection technique. In this case, the refrigerant is injected during the compression process into the scroll chambers. The intermediate pressure and refrigerant flow depend on the compressor design and the injection port size and location. Another common compressor technology with vapor injection is the two-stage reciprocating compressor (TSRC). In this case, the vapor-injection is performed after the first stage of compression. Therefore, the refrigerant discharged for the first stage of compression is mixed with the injection refrigerant before entering the second-stage compressor. The advantage of the scroll technology is the high volumetric efficiency for all compressor-working envelope, and it has a good compressor efficiency for moderate pressure ratios (< 7). On the other hand, the reciprocating compressors have a flatter compressor efficiency curve, but the volumetric efficiency rapidly decreases with the pressure ratio. Another possibility of the vapor-injection compressor is the two-stage scroll compressor (TSSC). Nevertheless, there is not a systematic evaluation of the performance of this compressor technology and a comparison with the previously described technologies. This paper presents a performance comparison of three compressor technologies with vaporinjection, two-stage reciprocating compressor, two-stage scroll compressor, and vapor-injection scroll compressor. The comparison is performed in terms of the compressor efficiencies, discharge temperature, heating capacity, and COP. In order to do that, semi-empirical models of the compressors have been implemented for each compressor technology. The models consider the main sources of losses in the compression process and are able to predict the compressor efficiencies in terms of empirical parameters, which have a direct physical interpretation. Results show that SCVI presents better COP and efficiency with pressure ratios below 5; the TSSC and TSRC perform better with higher pressure ratios (above 7), nevertheless the TSSC present higher volumetric efficiency and heating capacity than the TSRC. The irreversibilities in the compression process of the TSSC decrease the compressor efficiency for low and moderate pressure ratios compared with the SCVI.