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

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Refrigerants
Chemical properties and stability
The stability of a refrigerant is linked to the way it behaves in the
presence of other substances, particularly within the refrigerating
system. It is important that the refrigerant will not react with, or act
as a solvent with, any of the materials within the system. These
include metals used for pipes and other components, compressor
oils and associated additives, plastic motor materials, elastomers
in valves and fittings, and desiccants within filter dryers. This
should also be considered with respect to the small quantities of
contaminants such as moisture and air.
In general CFCs, HCFCs, hydrofluorocarbons (HFCs) and HCs
are compatible with most materials (since most components are
designed for these refrigerants). However, many components are
designed using proprietary mixtures and additives, so there is always
a possibility of incompatibility with certain materials if an unspecified
refrigerant is used. Carbon dioxide has some compatibility problems
with certain elastomers, which is why only dedicated components for
R744 should be used with this refrigerant.
Ammonia is not compatible with many materials, such as copper,
copper alloys and many electrical wiring insulation materials.
Therefore construction metals inside ammonia systems are normally
limited to carbon steel and stainless steel.
In all cases, component manufacturers should be consulted to check
that their materials are compatible with a non-standard refrigerant.
Operating pressures
It is important to consider the likely operating pressures in both the
suction and discharge sides of the system. Ideally, a refrigerant is
chosen that will have an evaporating pressure above atmospheric
pressure under normal operating conditions, so as to avoid air and
moisture being drawn into the system in the event of a leak. Thus, a
refrigerant should be chosen with a normal boiling point (NBP) that
is lower than the anticipated evaporating temperature. A selected
refrigerant should also have a condensing pressure that does not
exceed the pressure that the system components are designed for,
as this can have safety implications.
Thermodynamic and transport properties
The most important performance criteria for a refrigerating system
are cooling (or heating in the case of heat pumps) capacity and
efficiency, or coefficient of performance (COP). These performance
characteristics are influenced by a number of properties, including:
• saturation pressure-temperature characteristics
• critical temperature
• latent heat
• density
• viscosity
• thermal conductivity
• specific heat capacity
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