1
CP1
CP2
CP3
2
1
2
1
2
CC-000048
1
2
CP1
CP2
CP3
On
Off
Compressor 1
Compressor 2
Compressor 3
Manage off cycle migration
Off-cycle migration
Off -cycle refrigerant migration is likely to occur when the compressor is located at the coldest part of the
installation, when the system uses a bleed-type expansion device, or if liquid is allowed to migrate from the
evaporator into the compressor sump by gravity. If too much liquid refrigerant accumulates in the sump it will
saturate the oil and lead to a flooded start: when the compressor starts running again, the refrigerant evaporates
abruptly under the sudden decrease of the bottom shell pressure, causing the oil to foam. In extreme situations, this
might result in liquid slugging (liquid entering the scroll elements), which must be avoided as it causes irreversible
damage to the compressor.
Danfoss CH scroll compressors can tolerate occasional flooded starts as long as the total system charge does not
exceed the maximum compressor refrigerant charge. A suitable test to evaluate the risk of off –cycle migration is the
following:
• Stabilize the non-running system at 5°C ambient temperature,
• Raise the ambient temperature to 20°C and keep it for 10 minutes,
• Start the compressor and monitor sump temperature, sight glass indication and sound level.
The presence of liquid in the crankcase can be easily detected by checking the sump level through the oil sight
glass. Foam in the oil sump indicates a flooded start.
A noisy start, oil loss from the sump and sump cool down are indications for migration. Depending on the amount
of migration graduate measures shall be taken:
•
Sump heater
•
Liquid line solenoid valve
•
Pump down cycle
Sump heater
The surface sump heaters are designed to protect the compressor against off-cycle migration of refrigerant.
When the compressor is idle, the oil temperature in the sump of the compressor must be maintained at no lower
than 10 K above the saturation temperature of the refrigerant on the low-pressure side. This requirement ensures
that the liquid refrigerant is not accumulating in the sump. A sump heater is only effective if capable of sustaining
this level of temperature difference. Tests must be conducted to ensure that the appropriate oil temperature is
maintained under all ambient conditions (temperature and wind). Note that below -5°C ambient temperature and a
wind speed of above 5m/second, we recommend that the heaters be thermally insulated in order to limit the
surrounding energy losses.
Since the total system charge may be undefined, a sump heater is recommended on all stand-alone compressors
and split systems. In addition, any system containing a refrigerant charge in excess of the maximum recommended
system charge for compressors requires a sump heater. A sump heater is also required on all reversible cycle
applications.
At initial start-up or after power shortage, it is recommended to energize surface sump heater to remove refrigerant
6 hours in advance. A quicker start-up is possible by “jogging” the compressor to evacuate refrigerant in the
compressor.
The heater must be energized whenever the compressor is off to avoid liquid refrigerant entering the compressor.
© Danfoss | Climate Solutions | 2022.09
AB236986442743en-000501 | 31
Scroll compressors, CH290 and CH485 | Application