Skip to content

CO2 as a Refrigerant — Introduction to R744 Systems

This is post number eight of a series.

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.

Andre Patenaude
Director – CO2 Business Development, Emerson Climate Technologies

Visit our website for additional information on CO2 Solutions from Emerson. 
Excerpt from original document; Commercial CO2 Refrigeration Systems, Guide for Subcritical and Transcritical CO2 Applications.

To read all posts in our series on CO2 as a Refrigerant, click on the links below:

  1. Series Introduction
  2. Criteria for Choosing Refrigerants
  3. Properties of R744
  4. Introduction to Trancritical Operation
  5. Five Potential Hazards of R744
  6. Comparison of R744 with Other Refrigerants
  7. R744 Advantages / Disadvantages
  8. Introduction to R744 Systems
  9. Introduction to Retail Transcritical Systems
  10. Retail Booster Systems
  11. Introduction to Retail Cascade Systems
  12. Introduction to Secondary Systems
  13. Selecting the Best System



One Comment Post a comment
  1. Yves Moore #

    Bonjour André! il est prévu que nous ayons une formation avec vous le 4 avril 2016 chez AubinPélissier à Trois-Rivières.
    Ma question est la suivante:
    j’ai vu dans le fundamental handbook de 1981 de l’ASHRAE que le CO2 avait un COP inférieur au R-12 et c’est une raison pourquoi les CFC ont pris le marché dans les années après guerre. j’apporterai le tableau de comparaison des expériences faites en transcritique et sous-critique. J’aimerais qu’on puisse commenter ces analyses.

    March 25, 2016

Leave a Reply

%d bloggers like this: