*3.1. R32–R41–R1234ze(E)*

The first of the mixtures contains the now popular R32 refrigerant, which is treated as a substitute for the R410A refrigerant and has very good thermodynamic properties, as well as a relatively low GWP; nevertheless, it is in the A2L flammability class. The second HFC component is R41, which has a very low global warming potential. The representative from the HFO group is the well-known R1234ze(E), which is freely available on the market. Analyzing the composition of the discussed mixture, it is most likely to belong to the A2L flammability class.

The paper presents the value gradients determined for the R32–R41–R1234ze(E) mixture. This started from the basic values: the GWP and the normal boiling point, presented in Figure 3.

**Figure 3.** Summary of the basic properties for the R32–R41–R1234ze(E) mixture: (**a**) GWP and (**b**) normal boiling point.

The mixture, regardless of the shares of individual components, has a GWP below 750, as none of the components exceed this threshold. However, in order to meet the criterion for low-temperature cycles, it is necessary to limit the share of R32 to a maximum of 20%. Thus, only the left side of the triangle shown in Figure 3a is potentially usable. Attention should also be paid to the lower left corner of the normal boiling point diagram (Figure 3b). The boiling points at normal pressure in this area are higher than −30 ◦C, which means that the system will work under pressure. In the case of failure and leakage, it may lead to the appearance of air inside the system, which in the case of flammable refrigerants may pose a real risk of ignition of the installation due to the possible exceeding of the lower flammability limit of the mixture. Moreover, analyzing Figure 4b, it can be seen that the area at the left edge of the triangle is almost entirely covered by high temperature glides, reaching even over 25 K. In practice, only the mixtures located in the upper corner and in the right corner should be considered as suitable for low-temperature cycles.

For air conditioning cycles, the GWP limit according to [1] is 750, which is not exceeded at any point in the graph. The only limit here is the temperature glide, the value of which should not exceed the assumed threshold of 10 K (compare Figure 4a). Figure 5a,b show that it is impossible to select the mixture in such a way as to ensure both the possible high COP and the high volumetric cooling capacity of the mixture. Optimizing the mixture for high volumetric cooling capacity will provide benefits in terms of reducing the compressor (piston) displacement, which will translate into the number of pistons, compressor weight, dimensions, and price. Optimization for the possible high COP will reduce operating costs due to the minimization of the energy needed to compress the refrigerant. Regardless of the chosen direction of optimization, only a few mixtures are possible to use. Taking into account that GWP should be limited to 150, only two ternary mixture are available with mass fractions 0.1/0.8/0.1 and 0.1/0.7/0.2. Unfortunately, both of these mixtures contain a high proportion of R41, a refrigerant that is currently hardly available on the market due to the lack of widespread use in refrigeration. At the same time, it can be seen that there is no other R41-free mixture that meets these criteria. Extending the analysis to the threshold of 750 adopted by Regulation (EU) 517/2014, it is possible to select two binary mixtures R32–R1234ze(E) with the weight shares of 0.9/0.1 and 0.8/0.2. Of the two mixtures, the first is the more promising, as it is characterized by an increase in volumetric cooling capacity by 8%, with an almost identical COP. It should be remembered that the GWP threshold of 750 applies only to single split air-conditioning systems containing less than 3 kg of F-gases and only in this group of devices can the mixture proposed above be used.

**Figure 4.** Temperature glide at the evaporation pressure for cooling cycles with R32–R41–R1234ze(E) mixture as refrigerant: (**a**) AC system (*t*e/*t*c = 0/30 ◦C); (**b**) low-temperature system (*t*e/*t*c = −30/30 ◦C).

**Figure 5.** Volumetric cooling capacity (**a**) and (**b**) the COP for air-conditioning system (*t*e/*t*c = 0/30 ◦C) with R32–R41– R1234ze(E) mixture as refrigerant.

In the case of low-temperature cycles, additional problems in finding the proper mixture are the areas of high pressure ratio and high discharge temperature (Figure 6a,b). The use of a mixture with a mass fraction of R1234ze(E) greater than or equal to 0.4 leads to exceeding the assumed permissible level of the compression ratio for a single-stage cycle. In addition, a fraction greater than 0.8 largely leads to an evaporation pressure below 1 bar. For freezing circuits, it is not possible to select a mixture that does not contain R41, as the normal boiling point and GWP limitations effectively exclude the use of the R32–R1234ze(E) binary mixture. It is necessary to introduce a third component that will lower both these parameters at the same time. Similarly to the case of air-conditioning cycles, for the considered ternary mixture, there is only one composition that meets all the assumptions—0.1/0.8/0.1. However, further analysis of Figure 6a–d shows that by abandoning the R1234ze(E) component, all the operating parameters of the circuit are improved. The use of the binary mixture R32–R41 (0.1/0.9) allow the increase of both the COP and the volumetric cooling capacity, by 3% and 20.6%, respectively. A 6 K reduction in discharge temperature is also achieved, while the pressure ratio value drops below 5.5.

**Figure 6.** Low-temperature system parameters (*t*e/*t*c = −30/30 ◦C) of the R32–R41–R1234ze(E) mixture: (**a**) pressure ratio; (**b**) temperature of the refrigerant vapour at the compressor discharge; (**c**) volumetric cooling capacity; and (**d**) COP.
