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Appendix A – CO2 as a Working Fluid in Heat Pumps The low-pressure receiver should be designed to prevent possible liquid droplets from entering the suction line, as well as to provide sufficient volume to avoid excessive pressures if the system is inoperative at high ambient temperatures. It is important to ensure that the total CO2 charge in the system, and with that the initial liquid volume in the receiver, is sufficient to provide adequate pressure control at all operating conditions. If the CO2 charge is too small, the receiver will be emptied and the evapo- rator will be underfed at high operating pressures. This will in turn result in excessive superheating and reduced evaporation temperature, and as a consequence poor system performance. An oil return system can be arranged by bleeding off lubricant and possible CO2 liquid from the bottom of the receiver, as illustrated in Figure A9. However, this requires that the lubricant is fully miscible with the CO2, or that the density of the immiscible lubricant is higher than that of liquid CO2. Depending on the CO2 flow rate in the oil return pipeline, the flow from the evaporator will either be saturated vapour or a mixture of vapour and liquid droplets. A suction gas heat exchanger (internal heat exchanger) may be used to evaporate the liquid droplets in the suction line. The influence of a suction gas heat exchanger on the system perfor- mance for CO2 heat pumps is discussed in Section A2.3 and Section 7.2.4, Possibilities of Efficiency Improvements. EVAPORATOR Mvapour Mevap x LPR Mliquid CO2 mass flow rates Mevap = Mvapour + Mliquid Mliquid≈0→ x≈1 Mliquid>0→ x<1 Figure A9 The CO2 mass balance of the LPR and the oil return system. Lorentzen (1990) and Pettersen and Skaugen (1994) have discussed several alternative concepts for controlling the supercritical pressure including high-side volume control and systems using a medium-pressure receiver. Rieberer et al. (2000) analysed the operating characteristics of CO2 heat pump systems using a thermostatic expansion valve for controlling the CO2 liquid feed to the evaporator. A13PDF Image | Residential CO2 Heat Pump System for Combined
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