CO2 as a Refrigerant — Introduction to Transcritical Operation

This is post number 4 of a series.

Many R744 systems operate above the critical point some or all of the time. This is not a problem; the system merely works differently and is designed with these needs in mind.

• • R744 systems work subcritical when the condensing temperature is below 31 °C (88 °F).
  • R744 systems work transcritical when the gas cooler exit temperature is above 31 °C (88 °F).
  • HFC systems always work subcritical because the condensing temperature never exceeds the critical temperature (e.g., 101 °C / 214 °F in the case of R134a).

The pressure enthalpy chart in Figure 1 shows an example of a simple R744 system operating subcritically at a low ambient temperature and transcritically at a higher ambient temperature. The chart shows that the cooling capacity at the evaporator is significantly less for transcritical operation.

Figure 1: R744 pressure enthalpy chart showing subcritical and transcritical systems

An efficiency drop also occurs with HFC systems when the ambient temperature increases, but the change is not as great as it is with R744 when the change is from sub- to transcritical.

It is important that appropriate control of the high side (gas cooler) pressure is used to optimize the cooling capacity and efficiency when transcritical. For example, increasing the high side pressure will increase the cooling capacity when operating above the critical point

Behavior in the Reference Cycle

Simple comparisons between R744 and other refrigerants can be misleading because its low critical temperature either leads to differences in system design, such as the use of cascade systems, or to transcritical operation. As a result, like-for-like comparisons are not easy to make.

Theoretical comparisons between R744 and common HFC refrigerants are outlined in the list below.

• • R744 compares reasonably well with HFC systems when subcritical and at low condensing temperatures. But the comparison is less favorable at higher condensing temperatures and when transcritical.
  • The high suction pressure and high gas density of R744 results in very good evaporator performance. In like-for-like systems the evaporator temperature of an R744 system would, in reality, be higher than for an HFC equivalent.
  • The index of compression is very high for R744, so the discharge temperature is higher than for the HFCs. This can improve heat reclamation potential in retail systems, although the requirement for heat in the summer when the system is transcritical is limited.
  • The density of R744 results in very high volumetric capacity. This reduces the required compressor displacement, but not the motor size, which would be similar to that required for HFC refrigerants.
  • The required suction pipe cross-section area is in proportion to the volumetric capacity. For R744 the diameter of the suction line is approximately half that required for R404A.
  • The compression ratio for R744 is less than for HFCs. This can result in higher isentropic efficiency.

Upcoming CO2 as a Refrigerant series topics will cover the potential hazards of R744, compare it to other refrigerants (both traditional and new), and weigh its advantages and disadvantages as a refrigerant.

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