CO2 as a Refrigerant – Introduction to Retail Transcritical Systems

This is post number nine of a series.

Introduction to Retail Transcritical Systems

The diagram in Figure 1 is a simple single-stage transcritical system. The refrigerant discharged from the compressor flows into the gas cooler where heat is removed. If ambient is low the gas cooler will condense the vapor, however if ambient is high the refrigerant will not condense in this part of the system because it is operating above the critical point.

The high pressure refrigerant vapor passes through the expansion device and condenses when its pressure drops below the critical point. The liquid in the bottom of the flash tank (receiver) feeds the MT and LT cases. Electronic valves control superheat at the outlet of all cases and vapor is drawn back to the compressors.

Figure 1: Simple transcritical system for medium temperature

In this simple system:

• • The temperature of the refrigerant at the gas cooler exit depends on the size of the cooler
  • The pressure of the refrigerant in the gas cooler depends on the quantity of refrigerant in the system and the ambient temperature

The capacity and efficiency of this type of system vary significantly with ambient temperature and the quantity of refrigerant in the system.

Three example systems are shown on the pressure enthalpy chart in Figure 2. Each have identical evaporating conditions.

Figure 2: Pressure enthalpy chart showing transcritical operation

In a subcritical system the refrigerant would de-superheat and then condense, rejecting heat at a constant temperature. In transcritical operation the R744 does not condense; it rejects heat as a supercritical fluid, cooling during this process. (Even with a wide temperature-glide HFC such as R407A, the temperature change through the condenser is small compared to that of a gas cooler in a transcritical system.)

In each example above, the R744 exits the gas cooler at a temperature of 104 °F (40 °C). This exit temperature is a function of the size of the gas cooler and the ambient temperature, in the same way as condensing temperature is a function of the size of the condenser and the ambient temperature.

The cooling capacity of each system varies significantly. When operating in supercritical mode the cooling capacity increases (at constant temperature) with an increase in pressure. This is the opposite of what happens in a subcritical operation, where cooling capacity is increases at lower discharge pressures.

The compressor power input of each system also varies. The lower the pressure the lower the power input, as in subcritical systems.

Variation in power input is not proportional to the variation in cooling capacity. For example, increasing the head pressure from condition 1 to condition 2 provides a significant increase in cooling capacity with a very low increase in compressor power input. Increasing the pressure from condition 2 to condition 3 increases cooling capacity less than the increase in compressor power input.

Unlike subcritical systems, the maximum coefficient of performance (COP) while in supercritical operation does not occur at minimum condensing pressure. Optimum COP depends on evaporating conditions and gas cooler exit temperature, but is typically 90–100 bar (1310–1450 psi). In general the pressure for optimum capacity is greater than that for optimum COP.

In a retail transcritical system, gas cooler pressure is controlled to provide optimum capacity or optimum efficiency while maintaining the pressure below the maximum allowed at all times. Figure 3 shows how this pressure is controlled in a typical retail system with single-stage compression.

Figure 3: Gas Cooler High pressure control

Two additional valves in this system control the gas cooler and intermediate pressures:

• • The gas cooler pressure valve 1 (also called the high-pressure regulating valve) controls the pressure in the gas cooler. It is a pressure-reducing valve controlled from the R744 pressure in the gas cooler and its exit temperature.
  • The receiver pressure valve 2 (also called the medium-pressure regulating valve or the flash gas valve) controls the pressure of the refrigerant in the receiver and associated liquid distribution pipe work. It is controlled from the pressure in the receiver. This receiver is also called a flash tank.

Begin Callout
Gas cooler pressure is usually selected for optimum COP unless greater capacity is needed, in which case a higher pressure would be selected.
End Callout

Subcritical Operation

The ambient temperature profile determines the proportion of time a system runs in transcritical mode. For many regions a proportion of the operation will be subcritical, typically when the ambient temperature is below 68 °F to 77 °F (20 °C to 25 °C). In this case the gas cooler pressure valve usually controls the refrigerant in the condenser such that it exits the condenser with a specified degree of sub cooling.

In the next article of this series we’ll take a closer look at retail booster systems.