Support
GRANT LAIDLAW
Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy (ACRA) in Edenvale.
He holds a Bachelor of Business Administration and an associate degree in educational administration. He
has a National Technical Diploma and completed an apprenticeship with Transnet. He has dual-trades status:
refrigeration and electrical. He has been involved with SAIRAC for 17 years and has been a Johannesburg
committee member for the past eight (chairman in 2011 and 2012). Currently he is the treasurer (Johannesburg
council) as well as president (national council).
COMPRESSORS AND
OIL PRESSURE FAILURES
By Grant Laidlaw
Even if the oil level is correct, the pressure may not be adequate to
ensure lubrication of all the components in the compressor.
38
Jackson asks: Hello Grant, we do work on servicing and
sometimes see compressor failure. I noticed that somehow
there seems to be a lubrication issue. I was looking at the
article on lack of maintenance causing compressors to fail.
But why would the oil pressure be too low if the oil level is
correct? Thanks.
Hi Jackson, with regard to oil pressure failure, let us examine
the individual causes:
• Liquid refrigerant in the oil
• Poor practice
• Incorrectly adjusted expansion device
• Blocked strainers
• Incorrect oil viscosity
• Worn oil pump
• Weakened spring on oil pressure relief valve
• Worn bearings
• Excessively high temperature
• Oil pumped out into refrigeration system
• Leaking unloader mechanisms
LIQUID REFRIGERANT IN THE OIL
Liquid refrigerant entering into the oil has the effect of diluting
the oil. This will thin the oil and have a negative impact on oil
pressure and may allow bearings to run metal on metal instead of
running (floating) on a layer of oil.
In addition, the lubricating qualities of the oil will be reduced,
increasing friction. When liquid refrigerant has migrated into the
compressor, on start up the refrigerant will rapidly boil off causing
the oil to foam at the oil pump inlet, affecting pump operation
and may cause slugging with resultant damage.
RACA Journal I October 2020
This problem can come about in various ways. For example:
Crankcase can be too cold during shut down periods (no
crankcase heater or crankcase heater not working).
A crankcase heater is incorporated to keep the oil
warm, not to keep the oil thin but to discourage refrigerant
migration.
The process of refrigerant migration is brought about by the
natural tendency of refrigerant to evaporate from warm parts
of a standing system, and to condense in cold parts. During the
plant off-period, the compressor, which often is located outside,
may well represent the colder part of the system (for example: at
night).
Cold oil in the crankcase results in a situation of refrigerant
migrating into the crankcase. It is a dangerous situation and can
result in:
• Compressor damage
• Oil being lost from the sump into the system
• The oil pressure switch tripping
POOR PRACTICE
Moving on, we may find a technician charging the system
with liquid refrigerant, directly into the compressor. As with
migration, the liquid refrigerant entering into the oil will have
the effect of diluting the oil. And again this will thin the oil and
have a negative impact on oil pressure and may allow bearings
to run metal on metal instead of running (floating) on a layer
of oil.
When liquid refrigerant has been added to the oil in this way,
again on start-up, the refrigerant will rapidly boil off, causing the
oil to foam at the oil pump inlet. This will affect pump operation
and may causing slugging with resultant damage.
www.hvacronline.co.za