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100 75 50 25 0 -25 -1,75 102 101 b 0,2 100 -0,75 -0,50 -300 Carbon DioxideTranscritical Cycle R046, Page 4 Carbon Di oxide Transcritical Cyc le c 12 MPa 10 MPa 8 MPa 6 MPa 4 MPa d e 0,2 0,4f0,6 0, 2 MPa 45°C 35°Ce d 15°C 25°C 5°C - 5°C 0,4 f 0,6 0,8 -200 -100 a b c 0 100 -1,50 -1,25 8a -1,00 S (kJ/kg-K) (a) h (kJ/kg) (b) Figure 4. (a)T-S chart of carbon dioxide transcritical refrigeration cycle (b) logP-H chart of carbon dioxide transcritical refrigeration cycle (from EES) The model implanted in EES shows that under the pre-described working condition, the air temperature after passing the condenser will be 46.3 °C and the supercritical carbon dioxide’s temperature at the IHX’s inlet will be 36.21 °C. Based on the calculations, a Cp-∆h chart is developed for the integrated total heat exchanger length, which includes both IHX and the GC4, to show the specific heat variations of all the working fluids (i.e. supercritical CO2, evaporator’s outlet CO2 and the GC’s cooling air) along the two heat exchangers (figure 5). In figure 5, the left part of the curves shows the CP variation along the IHX (d-e for supercritical CO2, a-b for evaporator’s outlet CO2) and the right part shows the CP variation along the GC (c-d for supercritical CO2, g-h for gas cooler’s cooling air). The arrows show the direction of the fluid flow. It can be noticed from figure 5 that after being compressed to the supercritical region, the supercritical CO2 enters the GC with a moderate Cp value (c). Inside the GC, the Cp value of supercritical CO2 increases slightly at the beginning and then increases sharply until it reaches its peak value. After this point, the Cp value starts to decrease until the supercritical CO2 reaches the outlet of GC (d). After entering the IHX, the CP value of supercritical CO2 keeps decreasing until it reaches the IHX’s outlet (e). On the other side, the Cp value of evaporator’s outlet CO2 shows a slightly decreasing along the IHX (a-b), while the Cp value of GC’s cooling air shows an almost “constant” value along the GC (g-h). As mentioned before, the difference in the trend of Cp variations for different fluids will influence the shape of the temperature profile in both IHX and GC, which is showed in figure 6. As showed in figure 6, the supercritical CO2’s temperature has a more obvious drop near the inlet of GC (c) due to its relatively moderate Cp increment. After that, the temperature profile becomes relatively flat and then slightly drops again before the supercritical CO2 exits the GC due to its Cp variation. On the other side, the temperature of GC’s cooling air is increasing steadily due to its almost constant Cp value. In the IHX, the temperature of supercritical CO2 decreases while the temperature of evaporator’s outlet carbon dioxide increases respectively. It can be also seen from figure 6 that the supercritical CO2’s Cp variation has its main influence on the shape of temperature profile in the GC, which causes the temperature profile to show a concave shape. Due to this shape, the temperature differences at the heat exchanger ends are much bigger than inside the heat exchanger, thus the so-called “pinching” may occur inside the GC. Meanwhile, the temperature difference, which is the “driving force” for heat transfer to take place, is much smaller inside the GC than at its ends. Therefore, the required heat transfer area for the GC to remove a certain amount of heat will be much larger than the one without such a shape of temperature profile. Further, the logarithmic mean temperature difference, which is calculated by the measured temperature difference at both heat exchanger ends (equation 2), will over predict the real temperature difference of the heat exchanger (GC). Consequently, the UA value of the heat exchanger, which calculated by equation 3, will be under-estimated. 4 All the heat exchangers analyzed in this paper are referring to counter flow heat exchangers. International Refrigeration and Air Conditioning Conference at Purdue, July 17-20, 2006 T (°C) P (MPa) 0,0057 0,01 -1 0,034m3/kg 0,019 0,0017 -0,9 -0,7 -0,6 kJ/kg-KPDF Image | Analysis of Supercritical CO2 Heat Exchangers in Cooling
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