release as much of it as possible to the outside air, and this is also why under these conditions the condenser is referred to as a gas cooler since it is cooling the gas as opposed to fully condensing it.
Enthalpy – P-H Diagram
In the booster system, low-temperature subcritical compressors operate at around 200 psig, well below the critical point for CO2 receiving suction gas from the low-temperature display case and freezer evaporators. The low-temperature compressors then discharge that gas at about 425 psig combining it with the medium-temperature suction gas from the medium-temperature display cases and walk-in cooler evaporators before entering the medium-temperature transcritical compressors. Medium-temperature discharge gas leaves the compressors, depending on ambient conditions, anywhere from 560 psig to as much as 1450 psig, which is above the critical point of CO2 at 1070 psig (at 88°F). Under warmer conditions in which the pressure rises above 1055 psig, the system operates in the transcritical range.
Under any conditions, however, the discharge gas from the medium-temperature compressors feeds to the gas cooler, where the heat it carries is rejected to the outside environment. This heat rejection process is standard in any refrigeration system.
The CO2 leaving the condenser feeds to a high-pressure control valve that controls the pressure in the gas cooler, similar to a traditional hold back valve that allows a pressure drop to assure some liquid CO2 refrigerant flows into an intermediate pressure receiver called a flash tank (a high-pressure tank similar to a traditional receiver). The gas enters the valve from 560 to 1450 psig, depending on ambient conditions, and regardless of those conditions exits the valve at 540 psig. The valve is designed to work somewhat like a holdback valve in order to maintain optimum pressure through the condenser when it is working as gas cooler for the most efficient operational performance of the system.
The flash tank works the same way, in principle, as a component used on most types of refrigeration systems that’s called a receiver. This is usually a vessel that contains all of the refrigerant charge of the system when needed such as during maintenance or if the system ever has to shut down. The pressure in the tank is held at a constant level that is sufficient to maintain differential pressure throughout the system.
From the flash tank, where both liquid and gas can be present, the liquid refrigerant is supplied to the medium- and low-temperature evaporators controlled by conventional electronic expansion valves. Vapor from the flash tank is fed through a flash gas bypass valve (another specialized component) back to the medium-temperature compressors. The flash gas bypass valve maintains a constant pressure in the flash tank and removes the gas buildup in the flash tank.
Apart from some of the special components just described, the system works similarly to other types of DX systems. The main differences are related to the two-stage design of the system and the fact that all evaporators in the system are supplied with liquid from the same source. For most experienced technicians, the system will not seem overly complicated.