Commercial refrigeration has been in the environmental. Refrigerant. as a. Basics Considerations PART 1:

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1 PART 1: CO 2 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 R-744 (CO 2 ) 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 CO 2 will be a long-term option in the foreseeable future. This article is the first of two introducing CO 2 as a refrigerant. It will summarize the properties of R-744 and examine how it meets traditional and emerging needs for refrigerants. 26 RSES Journal JANUARY 2015 as a Refrigerant Basics Considerations CO 2 offers refrigeration with low globalwarming impact, but with new application and handling considerations. BY ANDRE PATENAUDE Images courtesy of Emerson Climate Technologies, Canada. Criteria Cooling capacity Efficiency Operating conditions Environmental impact Availability of refrigerant Availability of system components Availability of competent service technicians Cost Safety Ease of use Availability of appropriate standards Composition Suitability as a retrofit refrigerant How well does R-744 meet the criteria? Significantly higher volumetric capacity than conventional refrigerants. Varies, depending on system type and ambient temperature. Operating and standstill s significantly higher than for all other common refrigerants. Global-warming Potential (GWP) = 1, significantly lower than for commonly used HFCs. Varies globally, but generally available. Many components differ from those used in HFC retail systems, but all are now generally available Varies globally, but generally low; service technicians must have a good understanding of refrigeration best practices and will require training for R-744. Refrigerant cost significantly lower than for HFCs, but system costs are generally higher. Low toxicity and nonflammable; high s and associated hazards present challenges. High and low critical point drive the need or more complex systems. Europe standards: EN 378; ISO U.S. standards: ASME B31.5; ASHRAE 15; UL 1995/CSA 22.2 No ; UL ; UL ; Single molecule, no temperature glide in subcritical operations. 1 EN 378: Refrigerating systems and heat pumps Safety and environmental requirements ISO 5149: Refrigerating systems and heat pumps Safety and environmental requirements ASME B31.5: Refrigeration piping and heat transfer components ASHRAE 15: Safety standard for refrigeration systems and designation and classification of refrigerants UL 1995/CSA22.2 No : Heating and cooling equipment UL : Safety of household and similar electrical appliances General requirements UL : Particular requirements for electrical heat pumps, air conditioners and dehumidifiers UL : Particular requirements for motor-compressors Table 1 How R-744 meets different conditions and criteria.

2 Figure 1 R-744/CO 2 phase diagram. It also covers some of the reasons why CO 2 refrigeration systems differ from conventional systems, notably the design considerations created by the need for transcritical operation under certain conditions. Following these two introductory articles, other specific topics concerning R-744 will also be examined, including the general aspects of R-744 systems; more detailed information about the design of R-744 cascade, transcritical booster and secondary systems; and key points about their commissioning, operation and service. Criteria for choosing refrigerants 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 R-744 meets them. In general, the table shows that R-744 offers 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. Properties of R-744 Carbon dioxide is a naturally occurring substance; the atmosphere is comprised of approximately 0.04% CO 2 (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. Circle Reader Service No. 72 JANUARY 2015 RSES Journal 27

3 Figure 2 Pressure enthalpy chart for R-744. The critical point is the condition at which the liquid and gas densities are the same. Above this point distinct liquid and gas phases do not exist. The triple point is the condition at which solid, liquid and gas coexist. Refrigerant R-744 R-404A R-134a R-407A R-407F Temperature at atmospheric Critical temperature Critical Triple point Pressure at a saturated temperature of 20 C (68 F) F (-78.5 C) Temp. of dry ice 87.8 F (31 C) 1,056 psig ( psig (4.2 bar 815 psig ( F (-46 C) (Saturation F (72 C) 503 psig ( psig (0.03 bar 144 psig (9.9 Table 2 Compares the basic properties of R-744 with other refrigerants commonly used in the retail sector. 28 RSES Journal JANUARY F (-26 C) (Saturation F (101 C) 590 psig ( psig (0.005 bar 68 psig ( F (-41 C ) (Mid-point saturation F (82 C) 641 psig ( psig (0.013 bar 133 psig ( F (-43 C ) (Mid-point saturation F (83 C) 674 psig ( psig (9.6 Globalwarming 1 3,922 1,430 1, ,824 3 potential 1 1 The GWP values are from the Intergovernmental Panel on Climate Change, 4 th assessment report: Climate Change GWP for R-407A from EN GWP for R-407F from supplier s data. The triple point occurs at 4.2 bar (60.9 psi) and C (-69.8 F). Below this point there is no liquid phase. At atmospheric (0 bar/0 psi) solid R-744 (also known as dry ice) sublimes directly to a gas. This produces 845 times its volume in gas at 59 F and 1 atm. Example: 1 oz. of dry ice will produce 845 oz of CO 2 vapor as it sublimes. Solid R-744 has a surface temperature of C ( F). If R-744 is at a higher than the triple point and the is reduced to below the triple point (e.g., to atmospheric ), it will deposit directly to solid. For example, this can occur when charging an evacuated refrigeration system with liquid R-744. 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 and temperature. In a refrigeration system transcritical R-744 will not condense until the has dropped below the critical. No 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 enthalpy chart in Figure 2 shows the critical point and the extent of the transcritical fluid region. The boundaries of the transcritical fluid region are: g The critical temperature (31 C/87.8 F) to the subcooled liquid region; and g The critical (72.8 barg/1,055.9 psig) to the superheated gas region. A significant challenge with the application of CO 2 as a refrigerant is the higher operating s compared to

4 other commercial refrigerants. The chart in Figure 3 compares the of R-744 with R-404A and R-134a. The saturation curve for R-744 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 is current practice in transcritical systems. transcritical. For example, increasing the high side will increase the cooling capacity when operating above the critical point. An introduction to transcritical operation Many R-744 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: g R-744 systems work subcritical when the condensing temperature is below 31 C (88 F); g R-744 systems work transcritical when the gas cooler exit temperature is above 31 C (88 F); and g HFC systems always work subcritical because the condensing temperature never exceeds the critical temperature (e.g., 101 C/214 F in the case of R-134a). The enthalpy chart in Figure 4 shows an example of a simple R-744 system operating subcritically at a low ambient temperature and transcritically at a higher ambient temperature. The chart shows that the cooling capacity at the evaporatoris significantly less for transcritical operation. An efficiency drop also occurs with HFC systems when the ambient temperature increases, but the change is not as great as it is with R-744 when the change is from sub- to transcritical. It is important that appropriate control of the high side (gas cooler) is used to optimize the cooling capacity and efficiency when Circle Reader Service No. 73 JANUARY 2015 RSES Journal 29

5 Behavior in the reference cycle Simple comparisons between R-744 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, likefor-like comparisons are not easy to make. Theoretical comparisons between R-744 and common HFC refrigerants are outlined in the folowing list: gr-744 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; gthe high suction and high gas density of R-744 results in very good evaporator performance. In like-for-like systems the evaporator temperature of an R-744 system would, in reality, be higher than for an HFC equivalent; gthe index of compression is very high for R-744, 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; gthe density of R-744 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; gthe required suction pipe cross-section area is in proportion to the volumetric capacity. For R-744 the diameter of the suction line is approximately half that required for R404A; and gthe compression ratio for R-744 is less than for HFCs. This can result in higher isentropic efficiency. Part 2 of this article, scheduled for March 2015, will cover the potential hazards of R-744, compare it to other refrigerants (both traditional and new), and weigh its advantages and disadvantages as a refrigerant. Figure 3 Pressure-temperature relationship comparison. Andre Patenaude is the Director of Marketing with Emerson Climate Technologies, Canada. He leads market strategy, planning and implementation of programs for Emerson Canada s refrigeration and A/C business. Patenaude is a certified Mechanical Engineering Technologist and has worked for Emerson since 1984 in a variety of technical and marketing positions, allowing him to gain a deep understanding of customers needs. For more information, andre.patenaude@emerson.com or visit Figure 4 R-744 enthalpy chart showing subcritical and transcritical systems. 30 RSES Journal JANUARY 2015