Proper lubrication is essential for compressor reliability and longevity. Oil must
reduce friction, remove heat, seal clearances, and protect surfaces — and it must
be compatible with the refrigerant in the system. This lesson covers the functions of
compressor oil, how it is delivered, which oil types apply to which refrigerants,
and how oil is managed in the field.
The oil charge refers to the quantity and type of lubricating oil contained within a
compressor. Proper oil level and type are critical for compressor longevity. The oil
must be compatible with the refrigerant used in the system and must maintain its
lubricating properties across the entire operating temperature and pressure range the
compressor will experience.
Oil Types Used in Refrigeration Compressors
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Mineral Oil
Petroleum-derived oil used historically with CFC and HCFC refrigerants (R-12,
R-22). Mineral oil is not miscible with HFC refrigerants (R-410A,
R-404A, R-134a) and cannot be used in systems operating with these refrigerants.
Still found in older R-22 equipment still in service; must not be mixed with POE
or alkylbenzene oil without a thorough flush.
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Alkylbenzene (AB) Oil
Synthetic oil compatible with HCFC refrigerants (R-22). More resistant to
oxidation and moisture than mineral oil. Often used as a transitional oil during
R-22 system retrofits. Not fully miscible with HFC refrigerants at low temperatures;
not the preferred choice for new HFC systems.
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Polyolester (POE) Oil
The standard lubricant for HFC and HFO refrigerants (R-410A, R-32, R-134a,
R-454B). POE is hygroscopic — it absorbs moisture rapidly
from the atmosphere; keep containers sealed and minimize exposure to air. Moisture
in POE oil reacts with HFC refrigerants to form acids that destroy motor windings
and bearings. Contaminated POE must be replaced, not dried.
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Oil Incompatibility
Mixing incompatible oil types creates deposits, sludge, and poor lubrication.
Never add mineral oil to an HFC system, and never add POE to a system that has
run on mineral oil without first removing the old oil. Always verify the compressor
manufacturer’s approved oil specification before adding or replacing oil.
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Oil Level and Oil Return
Correct oil level is visible in the compressor sight glass (where fitted); normal level is typically between 1/4 and 3/4 of the sight glass height when the compressor is running
A small amount of oil always circulates with the refrigerant through the system; system design and pipe sizing must ensure oil returns to the compressor at the same rate it leaves — inadequate oil return leads to progressive oil loss and eventual bearing failure
Low oil level symptoms: compressor runs hot, bearing noise, reduced capacity; high oil level symptoms: liquid slugging risk (oil can carry liquid refrigerant into the cylinders during rapid load changes)
After any major repair involving refrigerant recovery and recharge, verify oil level and condition; contaminants from a burnout or moisture ingress require oil replacement before restarting
1.2.2 — Functions of Compressor Oil
Lubricating oil in a refrigeration compressor performs five essential functions. A
lubricant that fails at any one of them puts the compressor at risk.
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Friction Reduction
The primary function. Oil forms a thin film between moving surfaces, preventing
metal-to-metal contact. This reduces wear, heat generation, and the power consumed
overcoming friction at bearings, pistons, and other sliding surfaces.
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Cooling
Oil carries heat away from bearings, pistons, cylinders, and other components. In
screw compressors, injected oil is the primary cooling medium for the
compression process. Oil coolers may be required to maintain oil within its rated
temperature range during continuous operation.
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Sealing
In screw and rotary compressors, oil fills the clearances between rotors and
housing, reducing internal leakage and improving volumetric efficiency. In
reciprocating compressors, oil on piston rings helps seal the cylinder, preventing
high-pressure gas from bypassing the piston into the crankcase.
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Cleaning
Oil carries wear particles, contaminants, and decomposition products away from
working surfaces, transporting them to filters or settling areas where they can be
removed from circulation. This keeps internal surfaces clean and prevents abrasive
wear from recycled particles.
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Corrosion Protection
Oil coats metal surfaces, protecting them from moisture and corrosive contaminants
that would otherwise cause rust, pitting, and surface degradation. This function is
especially important during compressor off-cycles when surfaces are exposed to
refrigerant vapour and any moisture that has entered the system.
1.2.3 — Lubrication Methods
Oil is delivered to compressor components by one of two methods, or a combination
of both. The method used depends on compressor size, design, and the bearing loads
involved.
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Splash Lubrication
Moving parts dip into the oil sump and splash oil throughout the crankcase.
Simple and reliable, this method is common in small hermetic compressors. The
crankshaft may include oil slingers or paddles to improve distribution. Splash
lubrication requires a fixed compressor orientation and consistent oil level
to be effective.
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Pressure (Forced) Lubrication
A positive-displacement pump delivers oil under pressure directly to bearings,
pistons, and other critical surfaces. This method ensures adequate lubrication
regardless of oil level variations and is required for larger compressors with
higher bearing loads. Typical oil pressure is 15–60 psid
(103–414 kPa) above crankcase pressure.
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Combined Systems
Many compressors use pressure lubrication for main bearings and critical
components, with splash lubrication for cylinder walls and secondary surfaces.
This provides coverage redundancy and ensures all lubricated surfaces receive
adequate oil even if one delivery path is partially restricted.
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Oil Pressure Safety Controls
Compressors using pressure lubrication include an oil pressure differential switch
that monitors the difference between oil pump discharge pressure and crankcase
pressure. If this differential falls below the safe minimum, the control shuts the
compressor down on a time-delay to allow the oil pressure to stabilize at start-up
before tripping. A tripped oil pressure control requires investigation — low
oil pressure is not self-correcting.
1.2.4 — Types of Lubricating Oils
Oil selection is determined by refrigerant compatibility. Using an incompatible oil
causes poor miscibility, oil trapping in the evaporator, bearing starvation, and
possible acid formation. Always verify the compressor manufacturer’s approved
oil specification before adding or replacing oil.
Oil Type
Compatible Refrigerants
Key Characteristics
Mineral Oil
CFC, HCFC (R-12, R-22)
Petroleum-derived; miscible with CFC/HCFC; not miscible with HFC; naphthenic and paraffinic types
Alkylbenzene (AB)
HCFC, some HFC
Synthetic; better thermal stability than mineral oil; used as a transitional oil during R-22 retrofits; not fully miscible with HFC at low temperatures
Synthetic; excellent lubricity; not compatible with all system materials; extremely hygroscopic; primarily for automotive applications
Polyvinyl Ether (PVE)
HFC (alternative to POE)
Synthetic; good miscibility with HFC refrigerants; less hygroscopic than POE; used where moisture contamination risk is a concern
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POE Oil — Moisture Contamination Is Irreversible
POE oil absorbs moisture from the atmosphere within minutes of exposure to open
air. Moisture in POE reacts with HFC refrigerants to form hydrofluoric and
hydrochloric acids that attack motor windings, bearings, and copper plating.
Contaminated POE cannot be dried and must be replaced. Keep containers sealed,
minimize pour time, and use only a clean, dry system when servicing HFC equipment
that uses POE oil.
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Never Mix Incompatible Oil Types
Mixing mineral oil with POE or PAG creates sludge, deposits, and inadequate
lubrication. When converting a system from R-22 (mineral or AB oil) to an HFC
refrigerant (POE oil required), the system must be flushed to remove residual
mineral oil — typically by running the system with the new refrigerant
and POE and replacing the oil charge multiple times until residual mineral oil
falls below 5%.
1.2.5 — Oil Management Considerations
Oil circulates continuously through the refrigeration system along with the refrigerant.
Managing oil level, temperature, return, and contamination is an ongoing field
responsibility.
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Oil Return
Oil that leaves the compressor with discharge gas must return from the evaporator
and all downstream components. Adequate refrigerant velocity in suction and discharge
piping carries oil back. Oil traps, oil separators, and proper pipe sizing and slope
are essential — inadequate oil return leads to progressive oil loss and eventual
bearing failure.
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Oil Level Monitoring
Sight glasses allow visual inspection during operation. Normal oil level is
typically between ¼ and ¾ of the sight glass. Automatic oil level
controls can add oil or shut down the compressor if level is too low. Too little
oil starves bearings; too much raises liquid slugging risk and reduces efficiency.
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Oil Temperature
Oil viscosity drops as temperature rises. Cold oil is too viscous to flow to
critical areas; hot oil is too thin for adequate film strength. Crankcase
heaters prevent refrigerant migration into the oil during off-cycles
(which dilutes and thins the oil). Oil coolers prevent viscosity
from falling too low during continuous high-load operation.
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Oil Contamination
Contaminants include moisture, acid, wear particles, and decomposition products.
Oil filters remove particles; filter-driers (molecular
sieve) remove moisture. Acid in the oil indicates a system problem requiring
corrective action. Regular oil analysis detects contamination trends before they
cause compressor failure.