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Refrigerant blends Fig. 11 Evaporating and condensing behavior of zeotropic blends Fig. 12 Pressure level of R404A in comparison to R502 With regard to a uniform and easily compre- hensible definition of the rated compressor capacity, the revised standards EN 12900 and AHRI540 are applied. Evaporating and condensing temperatures refer to saturated conditions (dew points). the pure gas and liquid phases is mainly non-critical. Leaks in the phase change areas, e.g. after the expansion valve, within the evaporator and condenser/receiver are considered more significant. It is therefore recommended that soldered or welded joints should be used in these sections. Extended investigations have shown in the meantime that leakage leads to less serious changes in concentration than initially thought. In any case it is certain that the fol- lowing substances of safety group A1 (see page 42) which are dealt with here cannot develop any flammable mixtures, either inside or outside the circuit. Essentially sim- ilar operating conditions and temperatures as before can be obtained by supplemen- tary charging with the original refrigerant in the case of a small temperature glide. Further conditions/recommendations con- cerning the practical handling of blends must also be considered: o The plant always has to be charged with liquid refrigerant. When vapour is taken from the charging cylinder, concentration shifts may occur. o Since all blends contain at least one flammable component, the entry of air into the system must be avoided. If the proportion of air is too high, a critical shift of the ignition point can occur under high pressure and while evacuating. o The use of blends with a significant tem- perature glide is not recommended for plants with flooded evaporators. A large concentration shift is to be expected in this type of evaporator, and as a result also in the circulating refrigerant mass flow. * In the new edition (2020) of the standard for condens- ing units EN13215, the declaration of performance data based on the mean evaporating temperature has also been included. This enables a direct com- parison with performance data for single-component refrigerants. o Evaporating temperature according to point A (Fig. 11) o Condensing temperature according to point B (Fig. 11) In this case the assessment of the effective superheat and subcooling temperatures will be simplified. It must however be considered that the actual refrigerating capacity* of the system can be higher than the rated compressor capacity. This is partly due to an effectively lower temperature at the evaporator inlet. A further characteristic of zeotropic refriger- ants is the potential concentration shift when leakage occurs. Refrigerant loss in 14 Dew linePDF Image | REFRIGERANT REPORT 21
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