Handbook of air conditioning and refrigeration / Shan K

9.6 CHAPTER NINE
Evaporating and Condensing Pressures
Oil Miscibility
Inertness
Thermal Conductivity
Refrigeration Capacity
Physical Properties
Operating Characteristics
It is best to use a refrigerant whose evaporating pressure is higher than that of the atmosphere so
that air and other noncondensable gases will not leak into the system and increase the condensing
pressure. The condensing pressure should be low because high condensing pressure necessitates
heavier construction of the compressor, piping, condenser, and other components. In addition, a
high-speed centrifugal compressor may be required to produce a high condensing pressure.
When a small amount of oil is mixed with refrigerant, the mixture helps to lubricate the moving
parts of a compressor. Oil should be returned to the compressor from the condenser, evaporator, accessories,
and piping, in order to provide continuous lubrication. On the other hand, refrigerant can
dilute oil, weakening its lubricating effect; and when the oil adheres to the tubes in the evaporator
or condenser, it forms film that reduces the rate of heat transfer.
An inert refrigerant does not react chemically with other materials, thus avoiding corrosion, erosion,
or damage to the components in the refrigerant circuit.
The thermal conductivity of a refrigerant is closely related to the efficiency of heat transfer in the
evaporator and condenser of a refrigeration system. Refrigerant always has a lower thermal conductivity
in its vapor state than in its liquid state. High thermal conductivity results in higher heat transfer
in heat exchangers.
The cubic feet per minute (cfm) suction vapor of refrigerant required to produce 1 ton of refrigeration
(liters per second to produce 1 kW of refrigeration) depends mainly on the latent heat of vaporization
of the refrigerant and the specific volume at the suction pressure. It directly affects the size
and compactness of the compressor and is one of the criteria for refrigerant selection.
Discharge Temperature. A discharge temperature lower than 212°F (100°C) is preferable because
temperatures higher than 300°F (150°C) may carbonize lubricating oil or damage some of the
components.
Dielectric Properties. Dielectric properties are important for those refrigerants that will be in direct
contact with the windings of the motor (such as refrigerants used to cool the motor windings in
a hermetically sealed compressor and motor assembly).
Leakage Detection. Refrigerant leakage should be easily detected. If it is not, gradual capacity reduction
and eventual failure to provide the required cooling will result. Most of the currently used
refrigerants are colorless and odorless. Leakage of refrigerant from the refrigeration system is often
detected by the following methods: