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CO2 as a Refrigerant — R-744 Advantages/Disadvantages

This is post number seven of a series.

Weighing the Advantages and Disadvantages of R744

Table 1 outlines the advantages and disadvantages of R744 as a refrigerant. The list of disadvantages appears smaller than the advantages list, but these issues should not be overlooked, as they have a significant impact on the safety and reliability of R744 systems.

Table 1: Advantages and disadvantages of R744 as a refrigerant

Table 1: Advantages and disadvantages of R744 as a refrigerant

Future articles in this series will cover additional topics concerning R744 in more detail, including the general aspects of R744 systems; more specific information about the design of R744 cascade, transcritical booster and secondary systems; and key points about their commissioning, operation and service.

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 — Comparison of R744 with Other Refrigerants

This is post number six of a series, and compares CO2 to both traditional and new refrigerants.

R744 vs. Other Refrigerants

Table 1 shows a simple comparison of R744 with other types of refrigerants, including those that are currently commonly used and those that are currently being evaluated for future use. It uses a simple “traffic light” system and employs the common HFCs, such as R404A and R134a as a baseline.

This provides a simple introduction to the options — the situation varies globally, especially in the availability of refrigerants, components and expertise.

For retail applications a well-designed and installed R407A/F system generally has better efficiency than R744 systems. However, the overall environmental performance of R744 systems is better, primarily because of the low GWP in the event of leakage.

Table 1: Comparison of R744 with other refrigerants

Table 1: Comparison of R744 with other refrigerants

Blog post seven in this series will weigh the advantages and disadvantages of R744.

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 — Five Potential Hazards of R744

This is post number five of a series, and continues our overview of CO2 as a refrigerant by touching on the potential hazards associated with handling systems where R744 is present.

R744 Hazards

R744 is not flammable, but its high pressures, toxicity at high concentration, and potential for dry ice formation must be taken into account when applying and handling. This post explains some of the hazards and provides very general guidance on reducing them.

1. Asphyxiation

R744 is odorless, heavier than air, and is an asphyxiant. The practical limit[1] of R744 is lower than hydrofluorocarbons (HFCs) because of its potential for high toxicity (HFCs are non-toxic):

  • Practical limit of R744: 0.1 kg/m3 (56,000 ppm);
  • Practical limit of R404A: 0.48 kg/m3 (120,000 ppm)

Note: The practical limit is defined in EN378 but may vary in regional regulations.

CO2 TLV Threshold Limit Value is 5000PPM (0.5%), and Ammonia TLV is 25 PPM (0.0025%), which is the highest concentration for an eight-hour limit.

Table 1 summarizes the effect of CO2 at various concentrations in air.

Table 1: Effects of CO2 at various concentrations in air

Table 1: Effects of CO2 at various concentrations in air

If a leak of R744 could result in a concentration exceeding the practical limit in an enclosed occupied space such as a cold room, precautions must be taken to prevent asphyxiation. These include the use of permanent leak detection, which activates an alarm in the event of a leak.

2. High Pressures

R744 systems operate at significantly higher pressures compared to conventional systems, especially when ambient temperatures cause the system to operate above the critical point. As a result, system components, pipe work, tools and equipment must be rated to safely operate at these higher pressures (see Table 2 for more details). It should be noted that the standstill pressure on some systems (e.g., cascade systems) is higher than the maximum rated suction pressure PS (hence, the pressure-relief valve setting). The pressure-relief valve will discharge in the event of a fault such as a power failure.

Table 2: R744 standstill and typical system operating pressures

Table 2: R744 standstill and typical system operating pressures

To ensure the pressure does not rise to the relief pressure in the event of a power failure or sudden system shutdown, these systems can be fitted with a small auxiliary cooling system. This typically runs on an auxiliary (uninterruptable) power supply and will switch on when the pressure rises above a set point (this is lower than maximum allowable suction pressure PS, but higher than the normal operating pressure). The auxiliary cooling system is sized to remove sufficient heat to keep the standstill pressure below the safe low side limit when there is no load on the system (apart from heat absorbed from the ambient).

Care must be taken when charging R744 systems. The maximum operating pressure of some systems (such as cascade systems and parts of transcritical systems) is normally below the R744 cylinder pressure. These systems must be charged slowly and carefully to prevent pressure relief valves discharging. This topic will be covered in greater detail in a future article.

3. Trapped Liquid

The coefficient of expansion for R744 is significantly higher than for other refrigerants. The practical impact of this on liquid R744 trapped between closed valves is shown in the graph in Figure 1.

Figure 1: Relationship between temperature and pressure of trapped liquid R744.  Source: Danish Technological Institute

Figure 1: Relationship between temperature and pressure of trapped liquid R744.
Source: Danish Technological Institute

The example shows the effect of a 20 °C (36 °F) temperature rise on liquid that is trapped at an initial temperature of -10 °C (14 °F). The pressure will increase from 44 bar (638 psi) to approximately 240 bar (3,480 psi). This condition could potentially occur in a liquid line of a cascade system, and similar situations can arise in other parts of the system and in other R744 systems. As a rule of thumb, trapped R744 liquid will increase in pressure by 10 bar (145 psi) for every 1 °C (1.8 °F) temperature increase.

The pressure of trapped liquid refrigerant always increases, but the pressure increase of R744 is much greater than for other refrigerants. This is exacerbated by the potential to trap R744 at low temperatures and hence for the liquid temperature to rise more than for other refrigerants.

Systems should be fitted with pressure-relief protection wherever liquid could be trapped, either during operation or service. Methods of providing this protection will be covered in upcoming articles on the design of R744 systems.

4. Dry Ice

Dry ice (solid R744) is formed when R744 pressure and temperature are reduced to below the triple point (4.2 bar/60.9 psi, -56 °C/-68.8 °F). This will not occur within a properly working refrigeration system, but can occur when:

  • A pressure-relief valve discharges if it is venting vapor R744
  • Venting R744 during service (component change or replacement, for example)
  • Charging a system which is below 4.2 bar/60.9 psi (e.g., an evacuated system)

Dry ice does not expand when it is formed, but dry ice will become gas as it absorbs heat (e.g., from ambient). If the dry ice is trapped within the system, it will absorb heat from the surroundings and turn into gas. This will result in a significant pressure increase.

Dry ice can block vent lines, so care must be taken to ensure that this cannot occur:

  • Appropriate pressure-relief valves should be used — see the upcoming articles on system design for more information about these and how safety valves should be applied;
  • When R744 is vented from a system during service it should be vented as a liquid, and the pressure in the system monitored. R744 should always be vented outside a building.

5. Freeze Burns

Contact with solid or liquid R744 will cause freeze burns and should be avoided. Suitable gloves and goggles should always be worn when working with R744. (The surface temperature of dry ice is -78.5 °C [-109.3 °F]).

My next blog post will compare R744 to both traditional and new refrigerants.

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.

[1] EN378 Refrigerating systems and heat pumps — Safety and environmental requirements
ISO 5149 mechanical refrigerating systems used for cooling and heating — Safety requirements.


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 — 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

 

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

 

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