System design recommendations
Refrigerant charge limit
MLZ/MLM scroll compressors can tolerate liquid
refrigerant up to a certain extend without major
problems. However, excessive liquid refrigerant in
the compressor is always unfavourable for service
life. Besides, the installation cooling capacity may
be reduced because of the evaporation taking
place in the compressor and/or the suction line
instead of the evaporator. System design must be
such that the amount of liquid refrigerant in the
compressor is limited. In this respect, follow the
guidelines given in the section: “essential piping
design recommendations” in priority.
Use the tables below to quickly evaluate the
required compressor protection in relation
with the system charge and the application.
More detailed information can be found in the
paragraphs hereafter. Please contact Danfoss for
any deviation from these guidelines.
Model
Refrigerant charge limit (kg)
MLZ015-026
3.6
MLZ030-048
5.4
MLZ058-076
7.2
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 could
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, 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 too much oil
leaving the compressor, which must be avoided
as it causes irreversible damages due to possible
lack of lubrication.
MLZ/MLM scroll compressors can tolerate
occasional flooded starts as long as the system
has been evaluated.
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:
• Crankcase heater
• Liquid line solenoid valve
• Pump down cycle
Note: for special conditions such as low ambient temperature, low load operation or brazed plate heat exchangers please refer to corresponding sections
Recommended
Required
No test or additional safeties required
REQ
REC
BELOW charge limit
ABOVE charge limit
Packaged units
No test or additional safeties required
Off cycle migration test
Liquid flood back test
System with remote heat exchanger
Off cycle migration test
Off cycle migration test
Liquid flood back test
REC
REQ
REQ
REQ
REQ
Crankcase heater:
when the compressor is
idle, the oil temperature in the sump 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 crankcase 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). Below –5°C ambient
temperature and a wind speed of above 5m/
sec, it's recommended to thermally insulated the
heaters in order to limit the surrounding energy
losses.
Depending on test results, crankcase heaters,
Liquid Line Solenoid Valve, pump down or
suction accumulator must be applied see below.
35
FRCC.PC.015.A8.02
Application Guidelines
