A
PPLICATION
GUIDELINES
S
YSTEM
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 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.
Model
Refrigerant charge limit (lb)
MLZ015-026
8
MLZ030-048
12
MLZ058-076
16
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 oc-
casional flooded starts as long as the system has
been evaluated.
A suitable test to evaluate the risk of off-cycle mi-
gration is the following:
Stabilize the non running system at 40°F ambi-
•
ent temperature.
Raise the ambient temperature to 70°F and
•
keep it for 10 minutes.
Start the compressor and monitor sump tem-
•
perature, sight glass indication and sound level.
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 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
•
REQ
REC
9
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
Recommended
Required
No test or additional safeties required
Note: for special conditions such as low ambient temperature, low load operation or brazed plate heat exchangers please refer to corresponding sections
REC
REQ
REQ
REQ
REQ
9
Crankcase heater:
when the compressor is idle,
the oil temperature in the sump must be main-
tained at no lower than 20°F above the saturation
temperature of the refrigerant on the low-pres-
sure side. This requirement ensures that the liquid
refrigerant is not accumulating in the sump. A
crankcase heater is only effective if capable of sus-
taining 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 25°F
ambient temperature and a wind speed of above
16ft/s, it's recommended to thermally insulated
the heaters in order to limit the surrounding en-
ergy losses.
Depending on test results, crankcase heaters, Liquid Line Solenoid Valve, pump down or suction accu-
mulator must be applied see below.
26
FRCC.PC.016.A1.22
