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CO2 as a Refrigerant — Properties of R744

This is post number 3 of a series.

Carbon dioxide is a naturally occurring substance; the atmosphere is comprised of approximately 0.04 percent CO2 (370 ppm). It is produced during respiration by most living organisms and is absorbed by plants. It is also produced during many industrial processes, in particular when fossil fuels such as coal, gas or oil are burned to generate power or drive vehicles.

The triple point of carbon dioxide is high and the critical point is low compared to other refrigerants. The chart in Figure 1 shows the triple point and the critical point on a phase diagram.

Figure 1: R744 / CO2 phase diagram

Figure 1: R744/CO2 phase diagram

The triple point occurs at 4.2 bar (60.9 psi) and -56.6 °C (-69.8 °F). Below this point there is no liquid phase. At atmospheric pressure (0 bar/0 psi), solid R744 sublimes directly to a gas. (Dry ice produces 845 times its volume in gas at 59 °F and 1 atm. Example: 1 oz. of dry ice will produce 845 oz. of CO2 vapor as it sublimes.) Solid R744 (also known as dry ice) has a surface temperature of -78.5 °C (-109.3 °F). If R744 is at a pressure higher than the triple point and the pressure is reduced to below the triple point (e.g., to atmospheric pressure), it will deposit directly to solid. For example, this can occur when charging an evacuated refrigeration system with liquid R744.

The critical point occurs at 31 °C (88 °F), which is below typical system condensing temperatures for part or all of the year, depending on the climate. Above the critical point the refrigerant is a transcritical fluid. There is no phase change when heat is removed from a transcritical fluid while it is above the critical pressure and temperature. In a refrigeration system transcritical R744 will not condense until the pressure has dropped below the critical pressure.

CalloutNo other commonly used refrigerant has such a low critical temperature. As a result, other refrigerants always condense as heat is removed on the high side of the system.

The boundaries of the transcritical fluid region are:

  • The critical temperature (31 °C / 87.8 °F) to the sub-cooled liquid region
  • The critical pressure (72.8 barg / 1,055.9 psig) to the superheated gas region

Table 1 compares the basic properties of R744 with other refrigerants commonly used in the retail sector.

Table 1: Basic properties of R744 compared with other refrigerants

Table 1: Basic properties of R744 compared with other refrigerants. Footnotes: 1. The GWP values are from the Intergovernmental Panel on Climate Change, 4th assessment report: Climate Change 2007; 2. GWP for R407A from EN388; 3. GWP for R407F from supplier’s data.

The pressure enthalpy chart in Figure 2 shows the critical point and the extent of the transcritical fluid region.

Figure 2: Pressure enthalpy chart for R744

Figure 2: Pressure enthalpy chart for R744

A significant challenge with the application of CO2 as a refrigerant is the higher operating pressures compared to other commercial refrigerants. The chart in Figure 3 compares the pressure of R744 with R404A and R134a.

Figure 3: Pressure-temperature relationship comparison

Figure 3: Pressure-temperature relationship comparison

The saturation curve for R744 does not extend beyond 31 °C (88 °F) because this is the critical point. Above this condition there is no distinction between liquid and gas. Operation above this pressure is current practice in transcritical systems, which we will discuss in the next post.

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

 

CO2 as a Refrigerant — Criteria for Choosing Refrigerants

This is post number 2 of a series.

How R744 meets different conditions and criteria

Table 1: How R744 meets different conditions and criteria

Multiple factors need to be carefully considered when selecting any refrigerant, including its cooling capacity, safety, environmental impact, ease of use, cost, and availability of components and expertise. Table 1 summarizes these and other key criteria, and shows how well R744 meets them.

In general, the table shows that R744 offers a more superior cooling capacity than conventional refrigerants while meeting the demand for a natural refrigerant with low global warming impact, but presents challenges in both its application and handling.

In the next post, we’ll look at the properties of R744 as a naturally occurring substance.

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

 

CO2 as a Refrigerant — Series Introduction

CO2 offers refrigeration with low global warming impact, but with new application and handling considerations

Weighing up natural refrigerant alternatives

This is post number 1 of a series.

Commercial refrigeration has been in the environmental spotlight for more than a decade, especially as leakage studies have revealed the true effects of hydrofluorocarbon (HFC) emissions. Considerable reductions in emissions are possible, but implementing them is having a major impact on the refrigeration industry. In response, many new refrigerant options and system architectures have appeared — both on paper and in practice — creating tricky choices for decision makers in commercial refrigeration.

The significant environmental advantages of R744 (CO2) have guaranteed its position as a leading option for future refrigeration systems. It has demonstrated favorable results in different system configurations over many years, particularly in Europe, Australia and Canada. Initially high investment costs are now on a downward trend, while innovations in component technology and application methods continue to reveal potential performance gains. These results have ensured that CO2 will be a long-term option in the foreseeable future.

This post is the first in a series that will introduce CO2 as a refrigerant. The blog series will summarize the properties of R744 and examine how well it meets traditional and emerging needs for refrigerants. The series will also covers some of the reasons why CO2 refrigeration systems differ from conventional systems, notably the design considerations created by the need for transcritical operation under certain conditions. Other topics concerning R744 will also be examined, including the general aspects of R744 systems; more detailed information about the design of R744 cascade, transcritical booster and secondary systems; and key points about their commissioning, operation and service.

In the next post I will discuss the basics and considerations in criteria for choosing refrigerants. Multiple factors need to be carefully considered and we’ll take a look at how well R744 meets key criteria.

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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