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

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CHAPITRE The 2 next illustration shows action of a zeotropic mixture of
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refrigerant components, fluid A and fluid B, as it flows through a
heat exchanger tube. In the case of a pure fluid, the temperature
of the refrigerant remains the same as the liquid vaporises, or
the vapour condenses. However, with a zeotropic blend, as the
refrigerant vaporises, the saturation temperature rises, or as the
vapour condenses, the saturation temperature falls. The refrigerant
is at the bubble temperature when it is just a pure liquid (e.g. when
it is just evaporating) and is at the dew temperature when it is just a
pure gas (e.g. when it is just condensing).
Refrigerants
The range of temperature between the dew point and bubble point is called
the temperature glide. This is illustrated here:
TEMPERATURE
EVAPORATION
LIQUID VAPOUR
CONDENSATION
PROPORTION VAPORISED/CONDENSED
Behaviour of a zeotropic blend with changing temperature
PURE FLUID A
BLEND OF FLUID
A AND FLUID B
PURE FLUID B
DONE
TEMPERATURE
ZEOTROPIC BLEND
PRESSURE - TEMPERATURE
CHART
LENGTH OF HEAT EXCHANGER
GLIDE
BUBBLE POINT
(LIQUID WITH A
«BUBBLE» IN IT)
SUBCOOLED
BOILING
DEW POINT
(VAPOR WITH A
«DEW DROP» IN IT)
Condensation and evaporation of a zeotropic blend
Prior to the Montreal Protocol, zeotropic mixtures were
rarely used, perhaps the only example being R400 (a
mixture of R12 and R114). In the search for alternatives
for CFCs, and latterly, for alternatives for HCFCs, a large
number of zeotropic mixtures have been formulated. Some
of the HFC and HC based zeotropic blends to replace
CFCs and HCFCs are: R404A, R407C, R410A and R436B.
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