Compressor Oil Contamination Part II - Prevention
In
Part I we noted two ways compressor lubrication failure can occur: Oil
contamination and oil breakdown. Either of these can result in compressor
failure even though the Net Oil Pressure of the system appears good. Here in
Part II we will list steps you can take to help avoid these problems.
Foreign Particles
Due to handling.
Foreign material, solids, liquids, or gasses can be introduced
when adding lubricant to the compressor or charging the refrigerant loop.
Refrigerants are excellent degreasers and as such any contaminant in the
refrigerant will eventually migrate to the compressor oil.
- Fill the compressor crankcase
from a known-good source of lubricant. Always use an oil specified by
your service manual.
- Use clean rags and fill from
clean containers when adding lubricants.
- Always use an oil pump to
replace the oil, using good techniques such as making sure there is a
solid hose of oil (no bubbles) from the pump.
- Charge your refrigerant loop
from a known-good source of gas.
- If a refrigerant or
lubricating loop must be opened, be sure it is protected from
contamination and is clean when re-sealed.
Due to operation.
No matter how well lubricated, it is a fact of life that moving
parts wear and wear generates foreign particles. Periodic service, oil and
filter changes, insure that the contaminants are removed before they can build
up to a point of causing damage.
Common causes of contamination:
- Chips from broken reed valves
- Dirt from the casting process
used for the compressor body, heads, and other cast iron components.
- Moisture from the atmosphere.
- Acid from chemicals in the
atmosphere.
- Air- Oxygen (oxidizer) and
Nitrogen (non-condensable).
Due to sludge.
Sludge is caused by chemical breakdown of the oil. Frequently
this breakdown is due to operating temperatures which are higher than the oil
rating, usually above 300 degrees F internal.
High temperature can be caused by:
- High discharge temperatures
- High compression ratio
- Damaged reeds on the valve
plate
- Suction leaks
- Reduced lubrication
- Clearances which are less than
the manufacturer's recommended tolerances.
Another source of sludge is moisture in the oil. Moisture can be
from three sources: Contaminated oil or contamination during handling, suction
leaks introducing air, or cooling loop leaks into either the refrigerant loop
or the compressor crankcase.
To limit the creation of sludge:
- Always use clean oil and fill
carefully.
- Continuously monitor for
suction leaks.
- Periodically sample your oil
and check for moisture.
Chemical Breakdown
Caused by acids.
Acid formation is a significant cause of lubrication failure.
There are two strong acids which may form when there is a refrigerant leak:
Hydrochloric acid or hydrofluoric acid, depending upon the composition of the
refrigerant. Suction leaks introduce air and the oxygen in the air reacts with
the refrigerant under high temperature and high heat to form the acid. The acid
attacks the copper piping of the refrigerant system causing corrosion and
producing particulates of copper salts.
The copper particulates plate out onto the connecting rod
bearing surfaces and also form sludge in the oil channels of the crank shaft.
The sludge restricts the flow of oil while the copper plating on the bearing
surfaces decreases clearances, and operating temperatures rise. Once this cycle
begins it is an ever-accelerating breakdown of the oil due to high temperature.
Prevent this by continuously monitoring for refrigerant leaks.
Caused by moisture.
As noted above, moisture has three primary sources and can be
prevented by careful handling when filling the compressor crankcase, prevention
of suction leaks, and maintaining a leak-free refrigerant cooling loop.
Caused by excessive
temperatures.
Too high an operating
temperature can cause oil breakdown as well as sludge. The prevention is the
same as noted above: Maintain correct clearances and always operate within the
manufacturer's stated parameters.
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Tech Tips Home
These technical tips are for use by qualified maintenance
personnel who are familiar with their specific freeze drying equipment. These
tips are intended only as general guidelines. Lyophilization equipment is
frequently custom configured and some tips may not be appropriate for all
freeze dryers. Always read and follow the directions of your equipment's
maintenance manual. If you would like to discuss one of our tech tips, please
contact Dave Clayton at 215-672-7800 ext-1376.