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R046, Page 3 It can be observed from the above figures that the specific heat of supercritical carbon dioxide is changing over a much bigger range when compared to the heat sink (air) or the CO2 at the evaporator’s outlet. The difference in specific heats will thus influence the heat transfer performance of the heat exchanger and the shape of the heat exchanger’s temperature profile (in both GC & IHX). Consequently, so called “pinching”, which may limit the performance of the heat exchangers, may also occur in the heat exchanger. Therefore, the specific heat of different heat exchanger working fluids should be carefully examined when evaluating the heat exchangers that operate with supercritical carbon dioxide. 2. BASIC CYCLE ANALYSIS A typical carbon dioxide transcritical refrigeration cycle can be analyzed as follows to show the influence of supercritical carbon dioxide specific heat’s variation on the heat exchanger. The basic carbon dioxide transcritical refrigeration system is composed of five parts, namely: evaporator, compressor, GC, expansion valve and IHX. The schematic system layout is showed in figure 3. Figure 3. Schematic layout of carbon dioxide transcritical refrigeration system The cycle operating conditions are selected according the most commonly used working condition suggested by other researchers and in CO2 automobile A/C prototype testing (Kim et al., 2004). The evaporation temperature is selected to 5°C and the corresponding pressure will be 3.97 Mpa. The compressor’s isentropic efficiency is assumed to 75% according to the research done by Rozhentsev and Wang (2001). The gas cooler is assumed to be cooled by air with 20 oC inlet temperature and 0.5kg/s available mass flow rate. For the heat rejection pressure, Liao et al. (2000) proposed a correlation to predict the optimum heat rejection pressure in terms of evaporation temperature and the GC’s outlet temperature, which is expressed as equation (1). popt =(2.778−0.0157te)tc +(0.381te −9.34) (1) Based on equation (1), the optimum heat rejection pressure for the proposed working condition will be 8.7 Mpa Moreover, a 5 oC superheat after the evaporator is assumed as a fixed value to ensure that there is no moisture at the compressor inlet. The cycle operating conditions are given in table 1 and the corresponding cycle T-S chart and logP-H chart are also plotted in figure 4. Items Evaporator pressure Evaporation temperature Cooling capacity Superheat after Evaporator Gas cooler pressure Gas cooler outlet temperature Compression efficiency Table 1. Basic combined cycle working condition V alue 3.97 5 10 5 (fixed value) 8.705 35 75% Unit Mpa oC KW K Mpa oC - International Refrigeration and Air Conditioning Conference at Purdue, July 17-20, 2006PDF Image | Analysis of Supercritical CO2 Heat Exchangers in Cooling
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