Vapor compression cooling cycles deviate from the Carnot refrigeration cycle in several ways, such as isenthalpic expansion
of saturated liquid at the condenser outlet and desuperheating of refrigerant vapor at the compressor outlet.
Because most gases will see a reduction in temperature during isenthalpic expansion (this is the Joule-Thompson Effect), it is possible that even a dry gas can develop liquids if it is subject to the pressure drop in a typical fuel supply system (Figure 1).
Negative Joule Thompson coefficients will cause the fuel gas temperature to rise during isenthalpic expansion (for example through a leak), which may result in an explosion if autoignition temperatures are reached.
Additionally, the enthalpy of the refrigerant at the expander outlet is less than it would be if an isenthalpic expansion
device were employed.
Specifically, the isenthalpic expansion
process across the orifice in the MGJT cycle can be characterized by the Joule-Thomson effect temperature change ([DELTA][T.sub.JT]).
With outlet state from inner condenser, and initialized outdoor heat exchanger inlet pressure, the expansion valve gives outdoor heat exchanger inlet state by assuming isenthalpic expansion
. This initialized inlet pressure is modified to match exit quality with compressor suction quality.
The ideal vapor compression cycle consists of four processes: (1) isentropic compression in a compressor, (2) isobaric heat rejection in a condenser, (3) isenthalpic expansion
in an expansion valve, and (4) isobaric heat absorption in an evaporator.
One method for increasing the efficiency of a [CO.sub.2] cycle is to approach isentropic expansion with a work-recovery machine (expander) rather than isenthalpic expansion
with an orifice-type expansion device.
Since the inherent COP of an air-cooled [CO.sub.2] cycle is lower than that of HFCs, the use of an expander as an expansion device is recommended to improve the cycle performance of the [CO.sub.2] cycle by recovering the throttling loss that occurs in an isenthalpic expansion
process (Lorentzen 1995; Robinson and Groll 1998).
When an expander replaces an expansion valve in the [CO.sub.2] transcritical cycle, the isentropic expansion process (state point 3-5) takes place instead of the isenthalpic expansion
process (state point 3-6).
The admission process can be seen as nearly isentropic expansion with ensuing dissipation or as isenthalpic expansion
. The approach chosen for the calculation depends on the objective of the investigation.
The transcritical C[O.sub.2] cycles were drawn based on the following assumptions: gas cooler outlet temperature of 30[degrees]C, evaporation temperature of 0[degrees]C, superheat of 0 K, and isentropic compression and isenthalpic expansion