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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

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.

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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Modulation Technologies: Designing Great Commercial Atmospheres

Getting Comfortable with Designer Air, a new webinar series on how modulation technologies are enabling enhanced comfort and improved efficiency in homes and businesses around the U.S.  This webinar series is designed specifically for air conditioning contractors and facility managers to become more informed on the latest information about compressor modulation and the role it plays in enabling ‘Designer Air’.

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In the next installment of Getting Comfortable with Designer Air webinar series, Bart Powelson, Director of Commercial Air Conditioning Marketing, and Jacob Groshek, Director Global Commercial Scroll Engineering discuss “Modulation Technologies: Designing Great Commercial Atmospheres.”

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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

 

EEVs Make Low Condensing Refrigeration a Reality

217-P-EX6_M21_ART6It’s a problem all operators face: if I lower the head pressures in my refrigeration systems, I can lower operating costs. On top of that, I can meet the Department of Energy’s 2017 energy reduction targets. But, how do I do that with existing technologies?

Some in the industry have tried using thermal expansion valves (TXV) to lower head pressures, but TXVs can’t maintain capacity and digest flash gas that forms when pressures float lower. So operators are still forced to keep head pressures near 105 °F.

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