Chapter 4 Compressor and Package Unit Operation
UD-series Screw Compressor
4.1 Lubricating Oil (Refrigerant Oil)
4-1
Chapter 4 Compressor and Package Unit Operation
4.1
Lubricating Oil (Refrigerant Oil)
Selecting and managing lubricating oil (refrigerant oil) is very significant to keep the compressor in a
good operating condition.
Take the following notes when selecting and managing lubricating oil.
4.1.1 Precautions for Selecting the Lubricating Oil
Selection of the lubricating oil should depend on the type of the refrigerant, the type of the
evaporator used with the compressor, and the conditions under which the compressor is
operated. Also to be considered when selecting lubricating oil are the properties of the oil that
include not only the viscosity but also such characteristics as solubility in refrigerant, separability
from refrigerant, low temperature fluidity, high temperature thermal stability, etc. We therefore
recommend contacting our sales offices or service centers for choice of a specified brand for
your system.
Lubricating oil used for compressors must have a viscosity appropriate for lubricating the
bearings and other components in the compressors. The viscosity to be considered in this case
should be the viscosity the oil shows at the oil inlet of the compressor. The viscosity of the
lubricating oil significantly changes depending on the type of the refrigerant used in combination
with the oil. If the refrigerant dissolves in the oil (or the oil and refrigerant are compatible), the
viscosity of the oil drops to a level remarkably below the level required for operation of the
compressor under some operating conditions. On the contrary, if the refrigerant does not
dissolve in the oil (or the oil and refrigerant are incompatible), the viscosity may become too high
when the supply oil temperature is low. For this reason, the lubricating oil must be selected such
that it is supplied to the compressor with an appropriate viscosity (kinematic viscosity of 13 - 40
mm
2
/s) in the operating state.
In a refrigeration system using a screw compressor, the lubricating oil supplied to compressor is
discharged together with the compressed refrigerant gas and separated from the refrigerant by
an oil separator. However, the oil cannot be separated completely in the oil separator, so very
small part of the oil enters the condenser and can remain there. Part of the oil can also enter the
evaporator. For this reason, the lubricating oil must be thermally stable under high temperatures,
be separable from the refrigerant gas, and maintain adequate fluidity under low temperatures.
Note that some lubricating oils are incompatible with a certain type of refrigerant.
The caution below is an example case that is required especially attention.
Be careful since polyolester synthetic oil (POE) cannot be used with ammonia
refrigerant.
