Refrigerant Charge Limit; Off -Cycle Migration - Danfoss MLM Series Application Manuallines

A G
PPLICATION UIDELINES

Refrigerant charge limit

Packaged units
System with remote heat exchanger
Recommended
REC REQ
Note: for special conditions such as low ambient temperature, low load operation or brazed plate heat exchangers please refer to corresponding sections
Off -cycle migration
28
S
YSTEM DESIGN RECOMMENDATIONS
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
Model
MLZ015-026
MLZ030-048
MLZ058-076
Depending on test results, crankcase heaters, Liq-
uid Line Solenoid Valve, pump down or suction
accumulator must be applied see below.
BELOW charge limit

No test or additional safeties required
Off cycle migration test
REC

Required No test or additional safeties required
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
fl ooded start: when the compressor starts, the
refrigerant evaporates abruptly under the sudden
A suitable test to evaluate the risk of off -cycle mi-
gration is the following:
• Stabilize the non running system at 5°C ambi-
ent temperature.
• Raise the ambient temperature to 20°C and
keep it for 10 minutes.
• Start the compressor and monitor sump tem-
perature, sight glass indication and sound level.
Crankcase heater: when the compressor is idle,
the oil temperature in the sump must be main-
tained at no lower than 10 K above the satura-
tion 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 eff ective if capable of
sustaining this level of temperature diff erence.
FRCC.PC.015.A4.02
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 re-
quired compressor protection in relation with the
system charge and the application. More detailed
information can be found in the paragraphs here-
after. Please contact Danfoss for any deviation
from these guidelines.
Refrigerant charge limit (kg)
3.6
5.4
7.2
ABOVE charge limit
Off cycle migration test
REQ
REQ Liquid fl ood back test
REQ Off cycle migration test
Liquid fl ood back test
REQ
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 occa-
sional fl ooded starts as long as the system has
been evaluated.
The presence of liquid in the crankcase can be eas-
ily detected by checking the sump level through
the oil sight glass. Foam in the oil sump indicates
a fl ooded start.
A noisy start, oil loss from the sump and sump
cool down are indications for migration. Depend-
ing on the amount of migration graduate meas-
ures shall be taken:
• Crankcase heater
• Liquid line solenoid valve
• Pump down cycle
Tests must be conducted to ensure that the ap-
propriate oil temperature is maintained under all
ambient conditions (temperature and wind). Be-
low –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 sur-
rounding energy losses.