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M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 147 Fig. 37. Effect of gas cooler exit temperature on transcritical cycle COP for realistic high-side pressures. is obvious, owing to the fact that the flat tubes are usually less than 2-mm thick. What they have in common is their single slab cross-flow configuration that exposes all tubes to the inlet air. Some of the early theoretical analyses of CO2 system performance assumed that both indoor and outdoor heat exchangers would be of conventional flat-fin/round- tube design [36]. One issue in compact gas cooler design is internal conduction due to large temperature differences across small lengths. As pointed out by Pettersen et al. [76] internal conduction in fins, tubes and manifolds may lead to performance reduction. Solutions to avoid these problems include splitting of fins, use of several heat exchanger sections, and careful design of manifold geometries. In the transcritical CO2 cycle, system performance is very sensitive to gas cooler design. A small change in refrigerant exit temperature can produce a large change in gas cooler exit enthalpy (and evaporator inlet enthalpy) because specific heat becomes infinite at the critical point. Fig. 37 shows how COP can be increased 11% and the COP-optimizing discharge pressure reduced 0.5 MPa by a gas cooler in an automotive air-conditioning system (T0 1⁄4 3:9 8C; IHX effectiveness 0.8) that cools the refrigerant exit an additional 2 8C [109]. This indicates that a CO2 transcritical cycle is so sensitive to the refrigerant exit condition that a counterflow configuration is important for the gas cooler to exploit the large refrigerant-side temperature glide. Moreover, the steep refrigerant tempera- ture glide allows for ideal cycle efficiency to be achieved at finite air flow rate, in contrast to the infinite air flow required to achieve ideal efficiency in the subcritical cycle. Yin et al. [110] validated a gas cooler simulation model using measured inlet data for a diverse set of 48 operating conditions, predicting refrigerant outlet temperature within ^ 0.5 8C for most of the experimental data. They proposed a multislab gas cooler design (Fig. 38(b)), and reported the new design offered better performance than the commonly used multipass design (Fig. 38(a)). For the given heat exchanger volume, they reported that a newly designed cross-counter flow gas cooler could be improved system capacity and COP by 3–4 and 5%, respectively, compared to the old design (Fig. 38(a)). Additional details on the selection of heat transfer and pressure drop correlations may be found in [67,111–113]. The model was used to design the next-generation prototype gas cooler shown in Fig. 38(b), where a multislab overall counterflow configur- ation concentrates the cool air stream on the exiting refrigerant, because the transcritical cycle is so sensitive to this exit condition [31,113,114]. The new gas cooler design achieves approach temperature differences ,2 8C at most operating conditions because air flowing over the first slab undergoes only a small temperature change, and that DT is what places an upper bound on the approach temperature difference [109]. The flat tubes are vertical in this prototype, to facilitate condensate drainage and defrosting in heating mode. Finally, the refrigerant flows in a single pass from the inlet to outlet, with no intermediate headers, to accommodate reversibility and facilitate refrigerant distribution in heating mode. It is clear that flat tubes must be oriented vertically for any air-source heat Fig. 38. Gas cooler design for a CO2 air-conditioning system [110]. (a) One-slab three-pass design, (b) three-slab one-pass design.PDF Image | CO2 Vapor Compression Systems
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