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
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.
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 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. 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
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
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.
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