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M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 135 Fig. 24. Voorhees dual-effect compressor circuit (left) and thermodynamic cycle in temperature–entropy diagram (right) [10]. the control disc sealing surfaces. Hesse [51] proposed using a gear machine as expander in CO2 vehicle air conditioning systems. By using helical gears, acceptable efficiency of the expansion process was predicted. 4.3. Two-stage cycle The performance deterioration of the basic single-stage cycle can be largely mitigated by using multistage compressors and with intercooling of liquid and vapor refrigerant. In 1905, Voorhees [10] introduced a dual-effect compressor. The principle was that a supplementary suction orifice opened during compression, which allowed the refrigerant to be taken in at two different pressures. Figs. 24 and 25 show Voorhees dual-effect cycle [10], and Plank cycle [39] using an additional ‘pump’ stage near the expansion valve, respectively. The latter cycle uses two-stage compression, but instead of dividing the pressure rise into two stages, as commonly used, the cycle adds another, higher, pressure level before the compressed refrigerant is cooled. This reduces the enthalpy before throttling, and thus increases the cooling capacity. Due to the high refrigerant density in the second-stage compression, the power requirement is low—almost comparable to a liquid pump. In another publication, Plank [52] found that the intercooling of vapor by evaporation of liquid in a flash intercooler resulted in an increase of COP except for operating conditions near the critical point. Thiessen [53] devised a two-stage system with single- stage compression, using the intermediate pressure accumulator as a buffer for pressure control. Advantages Fig. 25. Plank’s two-stage cycle with high-pressure ‘pump’ [39].PDF Image | CO2 Vapor Compression Systems
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