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Rapport 2414173 - October 2007 __________________________________________________________________ Refrigerant-to-liquid plate heat exchangers are not available for pressures higher than about 40 bars, except for very costly, customized, industrial applications. Plate heat exchangers are therefore not used in transcritical cycle because of the pressure constraints (as of 2007). The Ecocute units use finned-tube air-to-refrigerant evaporators that use copper tubes and aluminum fins. Basically, these evaporators are not different from heat exchangers using HFC refrigerants. However, they are equipped with thicker tubes and specific circuitry that takes into account the special R744 properties. Figure 35 shows such an evaporator from Daikin. The "all-aluminum" evaporators are used for mobile air conditioning, but their use is envisaged in stationary equipment for cost considerations. 4.3.Gas coolers The gas cooler is the R744-to-water heat exchanger that heats up water in a heat pump. Most gas cooler designs use a double wall separation between R744 and water. In the event of a leak, R744 will thus never mix with water. Some sample gas coolers are shown in Figures 36 to 39. Plate heat exchangers are not used, due to pressure limitations on their design (as of 2007). The Sanyo gas cooler uses superimposed wounded coils, with alternate circulation of water and R744. Water flows through flattened tubes, while R744 flows through flat tubes with several ports. Stiebel Eltron (Figure 37) use a layout that is similar to that used by Denso (Figure 38). In these designs, R744 flow through small diameter copper tubing that is welded to copper plates though which water flows. The Denso design uses extra fins/baffles between the water plates to increase surface area and turbulence. The Daikin design uses regular water tubing around which is soldered to small helically wounded copper tubing (see Figure 39). __________________________________________________________________________________________ SHERPHA PROJECT - Transcritical R744 Heat Pumps – Technician's Manual 32/63PDF Image | Transcritical R744 (CO2) Heat Pumps 2017
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