2017 Tech Report Nov-Dec

Figure 2 Diagramed

Subcritical System and Transcritical System. Courtesy of Sporlan Valve Company a Division of Parker Hannifin Corp.

drop, heat is transferred from the surroundings to the liquid CO2 refrigerant, and it boils. The vapor is drawn off by the compressor, compressed to a higher pressure—and temperature—and pushed out to the discharge line. As the gas exits the compressor it is now above the critical point and the dynamics change. The refrigerant is now in a state that does not strictly behave as a gas or a liquid, and is called a supercritical fluid . The fluid cannot be condensed while in this state. A conventional condenser is not used at this point, instead a gas cooler is used. Although

a gas cooler may be similar in appearance and mechanical function to a condenser, the heat transfer done in a gas cooler is sensible—at a constant pressure—rather than latent. As the name indicates, the gas temperature is reduced. During low ambient conditions, air temperatures below 80ºF, the gas cooler may operate as a condenser. This is because the temperatures are below the critical point of the CO2. When the supercritical fluid exits the gas cooler it is expanded— the pressure is dropped—at a constant temperature and enthalpy to below the critical point into an expansion tank.

The CO2 will now be a mixture of gas and liquid at normal state conditions. The liquid can now be used to supply the evaporator, and the gas is drawn off by the compressor. The gas cooler and the expansion tank are maintained at a constant pressure using two pressure control valves. The compressor can be a single stage compression system with one or multiple compressors. The single stage system has one suction pressure and one load temperature. Other load temperatures can be achieved by using an evaporator pressure regulator. The single stage system is graphed on

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