Support
INCORRECTLY ADJUSTED EXPANSION DEVICE
The next possible cause can be an expansion device that has been set for
too low a superheat or is not operating correctly.
Many systems are controlled by expansion valves. The basis of
control of an expansion valve is suction superheat. In a normal operating
system, there should be roughly 6K of suction superheat when
operating at full load.
At an average of 6K suction superheat, there should be no more
than occasional traces of liquid in the suction line; if at all. However, if
the valve superheat setting is too low, this can result in unacceptable
amounts of liquid in the suction line moving toward the compressor.
An oversized value also causes the same problem. If the expansion
valve is oversized, it will tend to hunt and this in turn may allow liquid
refrigerant into the suction line.
If a refrigeration system is being forced to work at too low a load,
this can effectively bring about a situation where the valve is oversized
for the operation at that time. Again, liquid will intermittently be passed
into the suction line, and possibly into the compressor.
BLOCKED OIL STRAINER
Blocked oil strainers will gradually cut off the oil supply to the oil pump
and in turn this will cause a reduction of oil pressure.
Strainers should be checked, and renewable filter cartridges should
be renewed at least once a year. If a problem with contaminated oil
has arisen, the checks, cleaning functions and cartridge renewals must
be much more frequent. Keep a plant room logbook and record the
relevant pressures as part of your servicing procedures.
INCORRECT OIL TYPE OR VISCOSITY
It is very important that the correct oil is used. Should the oil be too thin,
the oil pump will not be able to supply the oil at the correct pressure as
the oil will leak back through the pump. In addition, the oil clearance
gaps between components will be too great and allow the oil to move
through too quickly, further lowering the gallery pressure.
It is therefore very important to follow the manufacturer’s
recommendations regarding the oil type and viscosity.
WORN OIL PUMP
A worn oil pump will produce low oil pressure as the pump will not
be able to supply sufficient oil to keep the compressor lubricated. Oil
pumps are not normally problematic but do fail and can be replaced on
larger systems.
Many of the smaller compressors are built with an oil pump which
includes the pump end bearing as part of its construction. These pumps
are not built to be repaired. If such a compressor is overhauled, its oil
pump is automatically replaced at the same time.
OIL PRESSURE RELIEF VALVE
Oil pressure relief valves are usually factory set. Refrigerant oil at
pressure is fed to the relief valve, while the pressure of the feed is slowly
increased. If the valve starts to open before the prescribed oil pressure
for the system is reached, you will again observe insufficient oil pressure.
In some cases this can be caused by a weakened spring in the valve.
WORN BEARINGS
Worn bearings allow excessive amounts of oil to be passed through the
bearings, dropping system oil pressure. Once it has been confirmed
that refrigerant in the oil is not the cause of low oil pressure, and the oil
temperature is normal, worn bearings will become the next most likely
reason for weak oil pressure.
HIGH OIL TEMPERATURE
Overheated oil will thin out creating oil pressure issues as previously
stated.
Normal operating temperature for crankcase oil is 55°C to 65°C. If
it is an open drive compressor, the shaft seal housing will be somewhat
hotter. It could operate in the range of 70°C to 75°C.
Note: The compressor should not be operated with the shaft seal
housing being above 75°C, as this will be harmful to the seal.
Operation temperature of the oil tends to rise with high discharge
temperatures which may be due to bad condensing conditions, or due
to a high compression ratio in low temperature applications.
When condensing conditions are at fault, the problem should be
rectified. If high oil temperature is due to a high compression ratio, the
machine should have been fitted with an oil cooler.
OIL PUMPED OUT INTO THE REFRIGERATION SYSTEM
Foaming of the oil at startup has already been given as a potential
reason for the sudden pump-out of oil into the system by the
compressor. All compressors pump oil out continuously with the hot
gas. This could amount to about half a litre per hour for every kW of
refrigeration. Under normal circumstances, this oil returns steadily to
the compressor. When oil return problems do manifest themselves,
they are usually at times of low load when refrigerant gas velocities
www.hvacronline.co.za RACA Journal I October 2020 39