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M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 149 Fig. 40. Effect of internal heat exchanger (IHX) in mobile air-conditioning system at idling speed and 43.3 8C ambient (L is the length of IHX in meter). however, these heat exchangers have much greater primary surface area. At this time there is relatively little published data that would allow detailed comparisons of these heat exchangers with the microchannel type, because their geometry lies outside the range of many correlations developed for stationary applications. 7.2.3. Internal heat exchangers The benefits of an internal heat exchanger for transcritical automotive air-conditioning systems have been documented through extensive experiments in CO2 prototype systems, and subsequent analyses using a validated simulation model. It has been demonstrated [120] that internal heat exchange can increase cycle efficiency up to 25%. Fig. 40 shows experimental results for a base case automotive air-conditioning system with no internal heat exchanger, and two others of lengths 1.0 and 1.5m. The longer internal heat exchanger provides the greatest increase in COP and the greatest decrease in the correspond- ing optimal discharge pressure. In automotive air-condition- ing systems, internal heat exchange provides the greatest capacity enhancement when it is needed most, while idling at high ambient temperatures. Boewe et al. [37] showed how three microchannel tubes could be stacked to provide many parallel ports to control pressure drop in the cold suction gas, while forcing the supercritical fluid through smaller ports to maximize heat transfer coefficients and areas upstream of the expansion device where larger pressure drop can be tolerated. Compared to conventional concentric tube designs, the microchannel configuration shown in Fig. 41 reduced Fig. 41. Illustration of new designs of internal heat exchanger.PDF Image | CO2 Vapor Compression Systems
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