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Introduction
Copeland Scroll refrigeration compressors have severalinherent
design differences from Copeland Discussemi-hermetic compressors.
These differences requiredifferent considerations for oil
management when usingCopeland Scroll refrigeration compressors in
parallelrack applications. Emerson Climate Technologieshas worked
closely with several supermarket rackmanufacturers to develop
acceptable system design
parameters. This bulletin will describe the designdifferences,
explain how they affect oil management andprovide guidelines to
ensure proper oil management.
Oil Volume
The typical horsepower range for Copeland Scrollrefrigeration
compressors in these applications is7.5 to 15 HP. Although Copeland
Scroll refrigerationcompressors have more oil capacity (140 oz.)
thancomparably sized Copeland 3D Discus compressors(125 oz.), the
amount of useful oil is much smaller. Thisis a result of the type
of oil protection that is requiredfor the different
compressors.
Oil Protection
Copeland Discus compressors utilize the Sentronic oilprotection
system which monitors oil pressure from theoil pump to determine if
there is satisfactory oilflow goingto the bearings. The oil level
can go down to the top ofthe oil pick up screen before there will
be insufficient oilavailable to maintain oil pressure. This means
that thecompressor has approximately 100 oz. of useable oil.
Copeland Scroll refrigeration compressors do not havean external
oil pump that readily allows the measurementof oil pressure.
Therefore, an external monitor of oil levelis required. The Alco
OMB, is required and combines the
functions of oil level control and time compressor shutoff.
This device is mounted on the sight glass and monitorsthe level
between and the bottom of the sight glass.Therefore, the amount of
useful oil prior to a potentialcontrol trip is only about 15 oz.
This becomes verycritical during start up commissioning.
Note: Due to issues that have been experienced withthe Trax Oil
level control, Emerson recommendsthat OEMs not use the Trax Oil on
future new scrollcompressor rack applications. We may
considerdenial of warranty for lubrication related failureswhen the
Trax Oil control is used on new systemsin the future.
Oil Return Pressure
Both low pressure and high pressure oil return systemshave been
successfully applied. However, they havedifferent issues. The high
pressure system requires anadditional reservoir. The low pressure
system alreadyemploys a separate reservoir. The main concern in
thelow pressure scheme is that the reservoir may dropbelow the
required 20 PSIG differential to feed oil duringlow load
conditions. The reservoir pressure dependson the oil separator
feeding to keep adequate pressureabove the crankcase pressure. The
oil separator is sizedfor worst case conditions and in the event
that only acouple of compressors are operating, the separator
isineffective. Therefore, the reservoir may be depleted ifmultiple
compressors are calling for oil.
Start Up Commissioning
During initial start up, oil will be lost from the compressoras
it coats the various system surfaces. During this time,the OMB will
cause the oil reservoir level to fluctuateuntil equilibrium is
reached. Since there is less usefuloil available, it is necessary
to monitor the oil levelsclosely to prevent unnecessary trips.
Service techniciansfrequently relate trips to the need to add oil,
when indeedthis may not be the case and results in a system withtoo
much oil.
Oil Reservoir
To prevent nuisance oil level trips, it is imperative that oilbe
available to the OMB when they are required to fill.This means that
a larger reservoir is needed than mightbe used on comparable Discus
parallel applications.
Compressor oil circulation rates vary by mass flow.Therefore, a
larger increase in reservoir volume is needed
Oil Management for Copeland ScrollCompressorsin Parallel
Applications
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Application Engineering
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Application Engineering
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for medium temperature racks than low temperatureracks. Our
studies indicate that per compressor, increasein reservoir capacity
should be:
ZS MEDIUM TEMPERATURE = 50 OZ. PERCOMPRESSOR
ZF LOW TE MPERATURE = 20 OZ. PER
COMPRESSOR
On systems having a common discharge header forthe medium and
low temperature racks, the increase inreservoir capacity should be
the sum of the number oflow temperature and medium temperature
compressorsfeeding the header. For example, a rack with 4 ZS
mediumtemperature compressors and 3 ZF low temperaturecompressors
would require,
For example: (450) + (320) = 260 additional oz. of oil.
Systems utilizing an integral oil separator/oil
reservoir,supplying 8 10 compressors may not have sufficientoil
capacity available to satisfy the OMB control.
Consequently, the hot foaming oil being suppliedcauses the oil
float to fluctuate resulting in erraticcontrol performance. This
condition also elevates the oiltemperature, bottom shell
temperature and generatesnumerous oil level trips. The additional
reservoireliminates this problem.
Oil Filters
The OMB, oil level controls, utilize a Hall Effect magnetto
actuate the flow valve. Therefore, they are somewhatsensitive to
any metallic particles and wear debris thatmay be in the oil.
Therefore, we strongly recommendthat a replaceable core oil filter
be used.
Incorporating the above recommendations will providevaluable
trouble free parallel rack applications withCopeland Scroll
refrigeration compressors. Should you
have any questions or need any additional information,contact
your Emerson Climate Technologies ApplicationEngineer.
