Manual for Refrigeration Servicing Technicians - UNEP - Division of ...

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Refrigerants
Using Refrigerant Blends – issues
and concepts
The following graphic allows you to consider different concepts and issues
relating to the use of refrigerant blends.
Temperature glide 4
Composition 4
Flammability 4
When considering these factors combined there are pros and cons for blends.
Pros and cons for blends 4
Temperature glide
The characteristic called “temperature glide” refers to the
temperature range over which components in a blended refrigerant
boil or condense at a given pressure. A pure substance (like water)
at a constant pressure will go through a complete phase change at
a constant temperature. Conversely, a zeotropic blended refrigerant
must proceed through a temperature range in order to complete the
phase change process. The name temperature glide refers to this
range of temperature. Similarly, if a zeotropic refrigerant was left
to boil off within a bucket, the temperature at which it would start
to boil would be the bubble-point temperature, and eventually the
last few drops would be boiling off at the dew-point temperature.
However boiling and condensation in “flooded” type evaporators
and condensers will not normally exhibit any temperature glide
because they are constantly being replenished with new refrigerant.
Thus the refrigerant is seen to change phase at a single temperature
- the bubble-point in an evaporator and dew-point in a condenser.
Composition
The specific characteristic of zeotropic blends, i.e., the liquid and
vapour composition, is different at most given temperature and
pressure. This causes concerns relating to composition changes in
the refrigerant supply chain, including liquid removal from containers
for multi-component refrigerant mixtures in the manufacturer plant,
and refrigerant transfers to smaller containers by dealers. A study
conducted by the Air Conditioning and Refrigeration Institute (ARI) in
USA indicates “refrigerant mixtures can have composition changes
during the handling procedures that lead to out-of-specification
composition”. The refrigerant transfer and equipment charging by
technicians, and refrigerant equipment leakage could also change
the composition. The change of the composition will affect the
performance to some extent. Accordingly, ARI and Deutsches Institute
für Normung (DIN) sets composition tolerances for specific blends, for
example, ARI composition tolerances for R410A (R32/R125) are +0.5,
-1.5% for R32, and +1.5, -0.5% for R125.
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