Emerson Copeland Scroll Introduction Manual page 38

3. Active oil control
Individual oil control for each compressor can be attained when each compressor has Flow Controls OMB oil level regulator fitted
using an adaptor at the sight glass location. The regulator is fed from an oil reservoir which is itself fed from an oil separator. Filters
can be installed in the oil feeding line. The reservoir allows variations in oil quantity in the system to be accommodated. Oil carried
over from the compressor to the system is replaced when insufficient oil is being returned. There is a sight glass on each oil level
regulator where the oil level can be visually checked approximately 10 seconds after switching off the compressor. This method is
recommended by Emerson Climate Technologies provided that certain design points are observed. Because the scroll compressor
has no positive displacement oil pump and therefore no oil pressure switch, it is recommended that the oil control system
incorporates protection. It will be necessary to add oil to the system and the Emerson Climate Technologies approved lubricants are
Emkarate RL 32-3MAF and Mobil EAL Arctic 22 CC. The oil level regulating system for applications should be installed on site. Well
tested oil level regulating systems are available from refrigeration wholesalers.
3.1 Low pressure oil reservoir
The oil reservoir is maintained at a pressure slightly above the compressor sump pressure and this limits the amount of refrigerant
dissolved in the oil in the reservoir. The pressure drop is low when the oil enters the compressor and the amount of flash gas formed
in the sump is small.
Float switches or other devices can be used as regulators. It is recommended that the regulator has an electrical output which
can be wired into the control circuit and stop the compressor if the oil level falls below the minimum level and remains below the
minimum level for a period of time (maximum 2 minutes). This protects against failure of the oil supply to the individual compressor.
A level switch in the reservoir will only protect against insufficient oil in the receiver, but not against loss of oil supply to an individual
compressor or against a fault on an individual regulator.
Regulators currently on the market which meet these requirements include Flow Controls OMB oil level regulator. The OMB requires
the oil reservoir to be pressurized to 3.5 bar above suction pressure for reliable operation.
When using a regulator care must be taken to set the oil level in the upper half of the sight glass. If an adapter is used to connect the
regulator this may have a smaller internal diameter than the glass and this could give rise to a false oil level reading.
When commissioning these systems it is important to allow sufficient time for equilibrium running conditions to be attained.
Because the oil carry over rate from the scroll compressors is low it may take some time for stable oil quantities to build up in the
coolers. Until this has happened the total oil requirement of the system cannot be determined.
3.1 High pressure oil reservoir
The need for a separate oil receiver may be avoided if a combined separator/receiver is used, but in this case the oil will be stored at
discharge pressure. It will therefore cause much more disturbance and foaming when it enters the compressor sump. For this reason
it is advisable to limit the quantity of oil entering the sump when the valve opens. Flow Controls OMB oil level regulator is suitable for
this type of application and has been proven to operate satisfactorily with a high-pressure oil supply.
4. Passive oil control
4.1 Equalisation lines with passive oil control
The need for a separate oil receiver may be avoided if a combined separator/receiver is used, but in this case the oil will be stored at
discharge pressure. It will therefore cause much more disturbance and foaming when it enters the compressor sump. For this reason
it is advisable to limit the quantity of oil entering the sump when the valve opens. Flow Controls OMB oil level regulator is suitable for
this type of application and has been proven to operate satisfactorily with a high-pressure oil supply.
Simple systems which link the compressor sumps via tubes with no control are obviously attractive. They are quite common
on air conditioning applications, but in refrigeration additional considerations apply. They can of course only be considered for
compressors working at a common suction condition.
If there are large variations in oil quantity in the system due to changing conditions or defrost, this may result in too much or too
little oil in the compressors. Usually the only means of checking oil level is via the sight glass connection. Once the oil level is above
the sight glass it is not possible to determine if the maximum oil quantity is reached. Likewise if no oil is visible in the sight glass there
is a danger of operation below the minimum quantity.
A tube connecting the oil level adjustment valves on the compressors is not adequate because when a compressor is stopped, the
pressure in the shell rises and oil will transfer to the running compressor(s). The adjustment valve is located below sight glass level
and therefore the stationary compressor will show no oil level. Even if all compressors run together at all times, there will be small
pressure differences which will cause the same effect.
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