By Mike Horst, Division Manager, Stellar
Refrigeration screw compressors use axial thrust load bearings — and they have a life expectancy that will
begin to fail over time.
At one time or another, all food processors and beverage manufacturers will be in
the market for a refrigeration compressor
rebuild or replacement. Because the compressor and its bearings will deteriorate over
time, staying on top of its performance is
crucial to avoiding a catastrophic failure.
Here are three methods that can help
gauge your compressor’s fitness:
1. Oil analysis
Take an oil sample from your compressor
(downstream of the oil filters), and have it
analyzed for impurities and wear properties,
per original equipment manufacturer (OEM)
guidelines. This is an easy thing to do and
the sample can be tested at an independent
laboratory that is knowledgeable on the
specific parameters for the product or by
the OEM.
Typical red flags:
• High iron count (unacceptable range
levels vary by product and are typically
between 15-20 parts per million)
• Abnormal particle counts (specific to your
particular brand of oil)
• High water content (per the Karl Fischer
method) (unacceptable range levels vary
by product and are typically between
100-150 parts per million)
2. Vibration analysis
For this analysis, an authorized professional collects the data with a certified/
calibrated vibration meter and evaluates the
compressor’s spectrums.
Typical red flags:
• Bearing defect frequencies
• High acceleration/ velocity values (each
OEM has specific values and guidelines)
3. Bearing end play measurement
This is when a service professional
measures the amount of wear movement
within the bearing. This is measured in thou-
sandths of an inch with a certified/calibrated
dial indicator. Take these measurements
when the machine is hot, or at operating
temperature. Different types of bearings
require different test procedures—follow
OEM guidelines for their compressors.
Typical red flags:
• Wear movement as low as one thou-
sandths or as high as five thousandths,
based on your bearing manufacturer
Be sure to incorporate the above mea-
sures into your predictive maintenance
program. Each of the methods should be
performed on a routine, scheduled basis, at
least twice a year for vibration and oil analy-
sis and once a year for bearing end play.
This will establish trends to help analyze
your compressor’s fitness and performance.
While an oil analysis is a great, low-cost
way to determine wear properties affecting
your compressor’s’ fitness, give more weight
to your vibration analysis and bearing end
play measurements. These are stronger indicators of compressor fitness. For example, if
you had an oil analysis that showed a high
iron count but the vibration analysis and
bearing end play measurement were normal, you should not replace or rebuild your
compressor based solely on the oil analysis
results. A better and more prudent approach
would be to monitor the machine more frequently and to take another oil sample.
Compressor rebuild vs.
replacement
After performing the above three tests, it
may become clear it’s time to either rebuild
or replace your refrigeration compressor.
How do you know which option to go with?
It’s up to you.
Rebuild pros and cons
A compressor rebuild involves a complete
disassembly and precision measurement of
all critical areas. The assembly also encom-passes setting all critical dimensions and
levels to OEM specifications, tolerances and
procedures. A rebuild is a lower cost alternative to a replacement; however, it requires
more down time. It depends, but the rebuild
process usually takes about a week.
Replacement pros and cons
If you can’t afford to have your compres-
sor offline for several
days, a replacement
may be a better
option, though it is
more expensive. Replacements are either
a remanufactured or new compressor.
Remanufactured compressors may be
supplied by qualified and knowledgeable
Rebuilders or by the OEM. When replacing a
compressor, you must ensure the replace-
ment will be a suitable match to the existing
compressor—pay close attention to detail
during this process. There are many vari-
ables within the same model line that can
adversely affect the performance and opera-
tion when applied incorrectly, such as:
• Internal Vi’s
• Liquid injection angles
• Pressure-specific connections
The same strict adherence to OEM rebuilding
procedures and guidelines must be followed
in the remanufacturing process, as well.
Best practices for both
If you do opt for a compressor rebuild or
remanufactured replacement, ensure that
your provider is OEM qualified. Your provider
should have the capabilities and knowledge
to follow all OEM guidelines/tolerances and
have the required “special” tooling for your
particular brand of compressor to install the
precision bearings for proper life expectancy
and performance and to be able to balance
the rotors.
Additionally, the use of a clean climate
control facility enables the compressor to be
precisely reset to the OEM clearances and
protects the precision internal components.
Some organizations may be equipped to
solely change out parts of the compressors—not remanufacture the entire piece to
a “like new” condition in a controlled environment exclusively for the rebuilding and
remanufacturing of compressors.
Stellar is a fully integrated firm focused
on planning, design, pre-construction, construction, refrigeration, mechanical & utility,
building envelope, and total operations &
maintenance services worldwide. Visit the
company's blog at www.stellarfoodfor-thought.net
