General characteristics of zeotropic blends

As opposed to azeotropic blends (e.g. R502, R507A), which behave as single substance refrigerants with regard to evaporation and condensing processes, the phase change with zeotropic fluids occurs in a "gliding" form over a certain range of temperature.

This "temperature glide" can be more or less pronounced, it depends mainly on the boiling points and the percentage proportions of the individual components. Certain supplementary definitions are also used, depending on the effective values, such as "near-azeotrope" or "semi-azeotrope" for less than 1 K glide.

Essentially, this results in a small temperature increase already in the evaporation phase and a reduction during condensing. In other words: At a certain pressure level, the resulting saturation temperatures differ in the liquid and vapour phases (Evaporating and condensing behavior).

To enable a comparison with single substance refrigerants, the evaporating and condensing temperatures have been often defined as mean values. As a consequence, the measured subcooling and superheating conditions (based on mean values) are unrealistic. The effective difference – based on dew and bubble temperature – is less in each case. These factors are very important when assessing the minimum superheat at the compressor inlet (usually 5 to 7 K) and the quality of the refrigerant after the liquid receiver (vapour bubbles).

With regard to a uniform and easily comprehensible 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).

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. (In the 2020 edition of the standard for condensing units EN13215, the declaration of performance data based on the mean evaporating temperature has also been included. This enables a direct comparison with performance data for single-component refrigerants.)

A further characteristic of zeotropic refrigerants is the potential concentration shift when leakage occurs. Refrigerant loss in 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 following substances of safety group A1 (Refrigerant data) which are dealt with here cannot develop any flammable mixtures, either inside or outside the circuit. Simply by recharging with the original refrigerant, largely similar operating conditions and temperatures as before can be achieved for refrigerants with low temperature glide.

Further conditions/ recommendations concerning the practical handling of blends must also be considered:

Evaporating and condensing behavior of zeotropic blends
Evaporating and condensing behavior of zeotropic blends
Pressure level of R404A in comparison to R502
Pressure level of R404A in comparison to R502