*3.4. R1243zf–R152a–RE170*

The last analyzed mixture is similar in composition to the third mixture, because it combines the two previously used refrigerants, RE170 and R152a. The HFO group refrigerant has been changed to a less popular R1243zf, as ye<sup>t</sup> unused in any of the encountered mixtures. The compositions discussed, like those in the previous point, will most likely belong to the higher flammability group due to the presence of dimethyl ether. As with the previous combination of fluids, the GWP does not exceed that of R152a, and due to the normal boiling point, the mixture is considered only in high-temperature circuits (Figure 14).

**Figure 14.** Summary of the basic properties for the R1243zf–R152a–RE170 mixture: (**a**) GWP and (**b**) normal boiling point.

The mixture R1243zf–R152a–RE170, regardless the composition, fulfils the requirements for the <sup>Δ</sup>*t*glide and π limits. For this mixture, an interesting parameter is the normal boiling point (Figure 14b), which reaches its minimum in the center of the triangle, not at the corners or sides. The lowest temperature achieved was −27.4 ◦C for the composition of 0.4/0.4/0.2. This phenomenon did not occur in the previously tested mixtures. Although all the points reached a negative normal boiling point, it is not low enough to use this mixture safely in freezing equipment. The temperature glide presented in Figure 15 is definitely below 1 K and has the lowest values in the center of the graph.

**Figure 15.** Temperature glide at the evaporation pressure for air-conditioning system (*t*e/*t*c = 0/30 ◦C) with R1243zf–R152a–RE170 mixture as refrigerant.

Moreover, a significant number of mixtures with a weight fraction of R152a in the range of 40–60% show a temperature glide significantly below 0.1 K, which makes them azeotropic. Considering the volumetric cooling capacity and the COP (Figure 16), as well as the previously mentioned normal boiling point and temperature glide, the best composition turns out to be 0.2/0.5/0.3. This is a compromise between a moderately high COP and a nearly zero temperature glide. Furthermore, it is a ternary mixture, and the RE170 belonging to the A3 flammability class does not make up the largest share. The second optimal substance, with a slightly higher COP, but also <sup>Δ</sup>*t*glide, is the mixture with a weight composition of 0.1/0.5/0.4. At the same time, it is a composition that achieves almost the highest volumetric cooling capacity of 2284 kJ/m3.

**Figure 16.** Volumetric cooling capacity (**a**) and (**b**) the COP for air-conditioning system (*t*e/*t*c = 0/30 ◦C) with R1243zf– R152a–RE170 mixture as refrigerant.
