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

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Refrigerants
Azeotropic Blends
An azeotropic blend is a mixture of usually two substances, which
behaves as if it were a pure fluid. When heat is added to or removed
from an azeotropic refrigerant blend, the composition (mole
fraction) of the vapour and the liquid remain essentially unchanged
throughout the complete process. In other words, in a blend of
50% of fluid A and 50% of fluid B, for every molecule of fluid A that
vaporises or condenses, a molecule of fluid B does the same.
An azeotropic blend behaves like a single refrigerant when condensing or
evaporating, i.e., the temperature remains constant at a given pressure, as
shown in this figure:
TEMPERATURE
SINGLE COMPONENT
PRESSURE - TEMPERATURE CHART
LENGTH OF HEAT EXCHANGER
SUBCOOLED
SUPERHEATED
BOILING
EVAPORATING
EVAPORATING
(CONSTANT PRESSURE)
Condensation
and evaporation of
an azeotropic blend
Historically, the commonly used ozone depleting substance (ODS)
blends were R500 (mixture of R12 and R152a) and R502 (mixture of
R22 and R115). More recently, an azeotropic blend using only HFCs
has been found, R507A (mixture of R125 and R143a).
Zeotropic blends
A zeotropic blend is a mixture of refrigerants whose different
volatilities are seen when observing the performance of a
refrigeration cycle. For example, a change in the molar composition
and/or a change in saturation temperature during boiling or
condensation; in this way, it does not behave like a single refrigerant
when condensing or evaporating.
Two different situations arise, depending upon the type of system.
Direct expansion system:
• Flow-boiling occurs in the evaporator and flow-condensation in
the condenser. Fluid A boils or condenses at a different rate than
fluid B, and because the liquid composition of fluid A and fluid
B changes, whilst the pressure remains the same, the saturation
temperature along the heat exchanger varies.
• Flooded system: Pool-boiling occurs in the evaporator and filmcondensation
in the condenser. Fluid A boils or condenses at a
different rate from fluid B, which results in an accumulation of
the most volatile fluid in the condenser and the less volatile fluid
in the evaporator. Whilst the phase-change process occurs at
a constant temperature, there is essentially a short-circuiting of
the refrigerant, which results in a loss in performance.
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