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

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Refrigerants
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.
The capacity and COP are mainly dictated by the design and
control of the system itself (compressor, heat exchangers, piping,
etc), although the properties of the refrigerant play a part in this.
The COP can be affected by the compression ratio (which is
dictated by the saturation pressure-temperature characteristic), heat
exchanger performance and pressure losses around the system,
which are all influenced by latent heat, density, viscosity, thermal
conductivity, specific heat.
For a given evaporating and condensing temperature, the cooling
(or heating) capacity of a system is strongly influenced by the
latent heat and density of the gas entering the compressor. For
conventional systems, a fairly high critical temperature is preferred
(at least 20K above the condensing temperature), unless the
system is specially designed for operation near or above the critical
temperature, such as with R744 systems.
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