CO2 as a Refrigerant — Introduction to R744 Systems

Transcritical, cascade, and secondary CO₂ systems

This series continues with the introduction of transcritical, cascade and secondary systems; it explains how each system works; and then compares their advantages and disadvantages.

Overview of Application Considerations for R744

The properties of R744 affect how the refrigerant is applied (see post 3 of this series for more details):

• • The high density of R744 compared to hydrofluorocarbons (HFCs) results in the requirement for less compressor displacement, typically 1/5th of that needed for R404A. However the motor size is similar since the work done is approximately the same. Smaller pipe diameters are another result, especially on the suction side of the system.
  • Because of the higher pressures of R744, all components require a higher maximum pressure rating.
  • The high discharge temperatures of R744 (because of the high index of compression) result in the need for two-stage compression for LT systems that reject heat to ambient air.
  • The low critical temperature of R744 results in differences in system design and control. In the retail sector this results in R744 being used mainly in the following types of systems:

• • Transcritical systems: Systems are called transcritical when they “transition” from subcritical to supercritical operation. In supercritical operation the heat rejection takes place above the critical point of the refrigerant (for CO₂ ambient temperatures from 68 °F to 77 °F (20 °C to 25 °C) (See Figure 1)
  • Booster systems: Systems with two temperature levels, e.g., -31 °F and -4 °F (-35 °C and -20 °C) evaporating temperature and with low-stage and medium stage compressors (see Figure 4).
  • Cascade systems: R744 is the low-stage refrigerant in a cascade system in which the R744 is always subcritical. Heat rejected by condensing R744 is absorbed by the evaporating high-stage refrigerant. The high-stage system is usually a conventional system using HFC or hydrocarbon (HC) refrigerants, known as hybrid cascade.
  • Secondary systems: R744 is used as a secondary volatile fluid pumped through the heat exchangers (cooling load). The CO₂ is not completely evaporated; the gas is condensated by a chiller.

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