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CO2 as a Refrigerant – Introduction to Secondary Systems

This is post number 12 of a series.

Secondary Systems

Figure 6 shows a simple secondary system. The high-stage system cools the liquid R744 in the secondary circuit. The R744 is pumped around the load. It is volatile, so unlike a conventional secondary fluid such as glycol it does not remain as a liquid. Instead it partially evaporates, providing a significantly greater cooling capacity. This reduces the pump power required and the temperature difference needed at the heat exchanger.

Figure 6

Figure 6: Simple cascade system

R744 would typically be cooled to 26.6 °F (-3 °C ) for the MT load, and to -13 °F (-25 °C) for the LT load.
The high-stage system is a simple chiller-type system, typically running on an HFC, HC or Ammonia refrigerant.

In the next article of this series we’ll review the advantages and disadvantages of booster, cascade, and secondary systems. We’ll also review some case studies to determine how to select the correct system for our application.

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


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