System design recommendations
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 WSH 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:
Stabilise 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
More detailed information can be found in the paragraphs hereafter.
Please contact Danfoss Technical Support for any deviation from these guidelines.
Recommended
Required
No test or additional safeties required
Note: for special conditions such as low ambient temperature, low refrigerant load or brazed plate heat exchangers please refer to
corresponding sections.
REQ
REC
BELOW charge limit
ABOVE charge limit
Cooling only systems,
Packaged units
No test or additional safeties required
Refrigerant migration and flood back test
Sump heater
Cooling only systems
with remote condenser
and split system units
Refrigerant migration and flood back test
Sump heater
Refrigerant migration and flood back test
Sump heater
Liquid receiver (in association with LLSV and
pump down)
Reversible heat pump system
Specific tests for repetitive flood back
Sump heater
Defrost test
For more details, refer to section "Reversible heat pump system"
REC
REC
REQ
REQ
REQ
REQ
REQ
REQ
REQ
REC
23
FRCC.PC.028.A5.02
Application Guidelines
