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Appendix D: Vapor Compression Cycle Control/Distribution D.1 Low pressure receiver in subcritical / transcritical cycle The objective of the low-pressure receiver, illustrated in Figure D.1, is to separate liquid from vapor sending only vapor to the compressor, and to provide a reservoir of refrigerant to accommodate changing operating conditions and system leakage over time. The thermodynamic cycle for a system with a low-pressure receiver is the same in subcritical and transcritical cycles. From Condenser Exp Valve Figure D.1 Low pressure receiver Evaporator To Comp. In the ideal cycle, only vapor exits the receiver. As a result, if refrigerant entering the receiver has a quality less than one, the liquid content of the refrigerant will remain in the receiver and only the vapor will pass. This will reduce the charge of refrigerant in the system. For the same heat transfer in the evaporator, the lower mass flow will exit with a higher quality until at steady state the quality of the refrigerant entering the receiver (exiting the evaporator) will be one. Similarly, if the refrigerant entering the receiver is superheated, the temperature and pressure of the refrigerant reservoir will rise. This will increase the amo unt of charge in the system, reducing the superheat of the refrigerant until at steady state the entering quality is one. In this way the low-pressure receiver modulates the mass flow of refrigerant through the system so that the quality of refrigerant entering the receiver is one. In a real cycle, a controlled amount of liquid needs to be allowed past the receiver to ensure lubricant return—this is typically done by placing a small hole in the bottom of the “j” on the compressor suction line. In the transcritical cycle, increasing the discharge pressure while maintaining a constant gas cooler outlet temperature until the effect of additional work required by the compressor offsets that of the additional refrigerating effect can increase COP. This is in contrast to the subcritical cycle in which COP is maximized at the lowest compressor discharge pressure. Control of high side pressure in the transcritical cycle with the expansion device was patented by Sinvent (Lorentzen et al., 1993), and, although not required, the “preferred embodiment” in the patent features a low side receiver. With a low-pressure receiver, maximum cycle efficiency can be accomplished by controlling one valve, the expansion valve, which sets the high side pressure. Stating that low side receiver/expansion valve control is effective only in systems from 2-10 kW, Mercedes-Benz patented transcritical system control based on varying the capacity of the compressor (Abersfelder et al., 1997). Varying the compressor capacity allows for the capacity of the cycle to be efficiently maintained over a wider range of system capacities. If maximum efficiency is to be achieved over all conditions, then a controllable expansion device as described by in the Sinvent patent needs to be incorporated into the variable compressor cycle 69 Rec.PDF Image | Comparison of R744 and R410A
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