Refrigerant vapor compression systems are well known in the art and commonly used for conditioning air to be supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility.  Refrigerant vapor compression system are also commonly used in refrigerating air supplied to display cases, merchandisers, freezer cabinets, cold rooms or other perishable/frozen product storage areas in commercial establishments. 
Refrigerant vapor compression systems are also commonly used in transport refrigeration systems for refrigerating air supplied to a temperature controlled cargo space of a truck, trailer, container or the like for transporting perishable/frozen items by truck, rail, ship or intermodal.  Refrigerant vapor compression systems used in connection with transport refrigeration systems are generally subject to more stringent operating conditions due to the wide range of operating load conditions and the wide range of outdoor ambient conditions over which the refrigerant vapor compression system must operate to maintain product within the cargo space at a desired temperature.  The desired temperature at which the cargo needs to be controlled can also vary over a wide range depending on the nature of cargo to be preserved.  The refrigerant vapor compression system must not only have sufficient capacity to rapidly pull down the temperature of product loaded into the cargo space at ambient temperature, but also operate efficiently at low load when maintaining a stable product temperature during transport.  Additionally, transport refrigerant vapor compression systems are subject to vibration and movements not experienced by stationary refrigerant vapor compression systems. 
Traditionally, conventional refrigerant vapor compression systems commonly operate at subcritical refrigerant pressures and typically include a compressor, a condenser, and an evaporator, and expansion device, commonly an expansion valve, disposed upstream, with respect to refrigerant flow, of the evaporator and downstream of the condenser.  These basic refrigerant system components are interconnected by refrigerant lines in a closed refrigerant circuit, arranged in accord with known refrigerant vapor compression cycles, and operated in the subcritical pressure range for the particular refrigerant in use.  Refrigerant vapor compression systems operating in the subcritical range are commonly charged with fluorocarbon refrigerants such as, but not limited to, hydrochlorofluorocarbons (HCFCs), such as R22, and more commonly hydrofluorocarbons (HFCs), such as R134a, R410A, R404A and R407C.
In today's market, greater interest is being shown in “natural” refrigerants, such as carbon dioxide, for use in air conditioning and transport refrigeration systems instead of HFC refrigerants.  However, because carbon dioxide has a low critical temperature, most refrigerant vapor compression systems charged with carbon dioxide as the refrigerant are designed for operation in the transcritical pressure regime.  In refrigerant vapor compression systems operating in a subcritical cycle, both the condenser and the evaporator heat exchangers operate at refrigerant temperatures and pressures below the refrigerant's critical point.  However, in refrigerant vapor compression systems operating in a transcritical cycle, the heat rejection heat exchanger, which is a gas cooler rather than a condenser, operates at a refrigerant temperature and pressure in excess of the refrigerant's critical point, while the evaporator operates at a refrigerant temperature and pressure in the subcritical range.  Thus, for a refrigerant vapor compression system operating in a transcritical cycle, the difference between the refrigerant pressure within the gas cooler and refrigerant pressure within the evaporator is characteristically substantially greater than the difference between the refrigerant pressure within the condenser and the refrigerant pressure within the evaporator for a refrigerant vapor compression system operating in a subcritical cycle. 
It is also common practice to incorporate an economizer into the refrigerant circuit for increasing the capacity of the refrigerant vapor compression system.  For example, in some systems, a refrigerant-to-refrigerant heat exchanger is incorporated into the refrigerant circuit as an economizer.  U.S. Pat.  No.  6,058,729 discloses a subcritical refrigerant vapor compression system for a transport refrigeration unit incorporating a refrigerant-to-refrigerant heat exchanger into the refrigerant circuit as an economizer.  The disclosed system also includes a suction modulation valve (SMV) for throttling refrigerant flow to the suction inlet of the compressor and an intermediate pressure-to-suction pressure unload circuit for compressor capacity control.  U.S. Pat.  No.  7,114,349 discloses a refrigerant vapor compression system with a common economizer and liquid-suction heat exchanger interdisposed in the refrigerant circuit downstream of the condenser with respect to refrigerant flow and upstream of the evaporator with respect to refrigerant flow.  Through various bypass lines and manipulation of various open/closed solenoid valves associated with the bypass lines, the common heat exchanger may be operated either as an economizer heat exchanger or as a liquid-suction heat exchanger. 
U.S. Pat.  No.  6,694,750 discloses a subcritical refrigeration system that includes a first refrigerant-to-refrigerant heat exchanger economizer and a second refrigerant-to-refrigerant heat exchanger economizer disposed in series in the refrigerant circuit between the condenser and the evaporator.  Refrigerant passing through the primary refrigerant circuit from the condenser to the evaporator passes in series through the first pass of the first refrigerant-to-refrigerant heat exchanger and thence through the first pass of the second refrigerant-to-refrigerant heat exchanger before traversing the primary refrigerant circuit's single evaporator expansion valve prior to entering the evaporator.  A second portion of the refrigerant passing from the condenser is diverted from the primary refrigerant circuit and passed through an auxiliary expansion valve and thence through the second pass of the first refrigerant-to-refrigerant heat exchanger prior to being injected into a high pressure stage of the compression process.  A third portion of the refrigerant passing from the condenser is diverted from the primary refrigerant circuit and passed through another auxiliary expansion valve and thence through the second pass of the second refrigerant-to-refrigerant heat exchanger prior to being injected into a low pressure stage of the compression process. 
In some systems, a flash tank economizer is incorporated into the refrigerant circuit between the condenser and the evaporator.  In such case, the refrigerant leaving the condenser is expanded through an expansion device, such as a thermostatic expansion valve or an electronic expansion valve, prior to entering the flash tank wherein the expanded refrigerant separates into a liquid refrigerant component and a vapor refrigerant component.  The vapor component of the refrigerant is thence directed from the flash tank into an intermediate pressure stage of the compression process.  The liquid component of the refrigerant is directed from the flash tank through the system's main expansion valve prior to entering the evaporator.  U.S. Pat.  No.  5,174,123 discloses a subcritical vapor compression system incorporating a flash tank economizer in the refrigerant circuit between the condenser and the evaporator.  U.S. Pat.  No.  6,385,980 discloses a transcritical refrigerant vapor compression system incorporating a flash tank economizer in the refrigerant circuit between the gas cooler and the evaporator.