The comparison between CO2 and other commons refrigerants can be misleading because the unique properties of the CO2 such as low critical temperature. The CO2 can be compare reasonably with R-404A only when the condensing temperature is below 20°C and the pressure of 56.3 bar (Pearson, 2014). However, is really difficult to achieve these conditions all the time.
The low critical temperature usually leads to design the CO2 systems in different way comparing the other refrigerants. Therefore, like-for-like comparisons between CO2 and other common refrigerants are not easy to predict.
Table 2.3 represents the performance comparison between CO2 and selected refrigerants under the same operating conditions for medium (MT) and low temperature (LT) refrigeration applications. The comparison operating conditions are shown in Table 2.2.
The refrigerants which are selected for comparisons are: CO2, R-134A, R404A, R-290, R-1270 and R-717. The refrigeration capacity for the comparison is lower in order to be served from HC refrigerants as well.
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Table 2.2 Operating conditions
MT LT
Refrigeration capacity (kW) 5 5
Degree of superheat (K) 5 5
Sub-cooling (K) 0 0
Evaporating Temperature (°C) -8 -32
Condensing Temperature (°C) 28 28
The refrigeration effect of CO2 is higher comparing with R-404A but lower than the other natural refrigerants. The mass flow rate required to be circulated in the refrigeration system is a proportional of the refrigeration effect. CO2 required 30%
lower refrigerant charge comparing with R-404A and 6% than the R-134A for the LT applications under the same conditions. On the other hand, CO2 refrigeration systems required more refrigerant charge compare with the other natural refrigerants.
Table 2.3 shows that CO2 has really low pressure ratio than the selected refrigerants.
The lower pressure ratio leads to the higher isentropic and volumetric efficiencies of the system. As shown in table, the pressure ratio of CO2 is 20 to 64% for LT applications and 8 to 33% lower for MT refrigeration system.
Table 2.3 Performance comparison between CO2 and selected refrigerants
LT application
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Continue from the Table 2.3 the suction gas specific volume and latent heat of evaporation for the given refrigerants are shown in the Table 2.4.
Table 2.4 Suction gas specific volume and Latent hat of evaporation
Another important advantage of the CO2 operation in both LT and MT applications is the very small suction gas specific volume comparing with HFCs and other natural refrigerants. The suction gas specific volume for CO2 is 0.030 m3/kg for LT applications when the values for R-717 and R-404A are 1.082 and 0.102 m3/kg respectively. It is mean by reducing the suction gas specific volume we can reduce the diameter of the pipework in the system and the compressor size. For the same operating condition the ammonia compressor is much bigger comparing with the CO2. This directly influence on the capital cost of the system, maintenance cost, safety and direct and indirect emissions from the system to environment. The table also shows CO2 has higher latent of evaporation values comparing with HFCs refrigerants but lower comparing with other natural refrigerants.
The main advantage associated with the high working pressure of the CO2 is the low vapour pressure. This allows us again to design the system by using smaller compressor size and distribution pipework.
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The comparison between CO2 and selected refrigerants can be related by applying the same operating conditions, geometry and load with Table 2.3 to the following pressure drop equations (Moreno and Thome, 2007a; Moreno and Thome, 2007b).
𝛥𝑃 = 4 𝑓
𝐿𝐷 𝐺2
2𝜌
2.2
The friction factor is calculated by Blasius equation for laminar flow conditions (Cheng et al, 2008):
𝑓 =
0.079𝑅𝑒0.25 2.3
Assuming the refrigerant enters the evaporator as saturated liquid and evaporates completely. For the same capacity, pipe length and diameter the above equations used to compare selected refrigerants under different saturations temperatures. The data were derived by using EES.
Figure 2.11 Saturated vapour pressure drop ratio
Figure 2.11 illustrates the saturated vapour pressure ratios for selected refrigerants. The values for CO2 vapour pressure drop is much lower comparing with other refrigerants
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apart from ammonia. For the saturation temperature below 0 °C the gap between CO2
and other selected refrigerant is getting bigger. The saturated vapour pressure ratios for CO2 and ammonia is nearly the same for different for saturated temperatures between -10 and -32 °C.
In Fig. 2.12 the comparison between CO2 and selected refrigerants based on the saturated liquid pressure drop are presented. Eq. 2.2 and 2.3 used to define the liquid pressure drop under different saturation temperatures and assuming that the quality entering is 100% liquid.
It can been seen from the graph, that the liquid pressure drop of CO2 is lower comparing with R-404A for saturation temperatures below 5 °C. The pressure drop values for CO2
and R-134A related with the saturation temperature. As is can been seen for saturation temperature below -24°C, CO2 shows higher pressure drop. Pressure drop ratios of R-290 and R-1270 shows much lower pressure drop ratios comparing with CO2. Ammonia gives a pretty constant liquid pressure drop characteristics for varying evaporating temperature.
Figure 2.12 Saturated liquid pressure drop ratio
The acceptable liquid pressure drop of the applications where the CO2 used is higher comparing with other refrigerants as a proportional of the unique thermos-physical properties. Based on that, CO2 and R-717 can performed better compared with the other
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selected refrigerants. Due to the low saturated pressure drop CO2 is suitable to use as a secondary fluid in centralized refrigeration systems.