Danfoss MLM116 Application Manuallines page 25

Application Guidelines System design recommendations
In addition, any system containing a refrigerant
charge in excess of the maximum recommended
system charge for compressors requires a sump
heater.
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. Start the compressor for 1 second,
then wait for 1 to 2 minutes. This operation must
be repeated for each compressor individually:
on
Compressor 1
off
on
Compressor 2
off
on
Compressor 3
off
Liquid line solenoid valve (LLSV): This feature is
very convenient and can be used on all types of
applications.
An LLSV is used to isolate the liquid charge in the
high pressure side, thereby preventing against
Pump-down cycle: Once the system has reached
its set point and is about to shut off, the LLSV
on the liquid line closes. The compressor then
pumps the majority of the refrigerant charge into
the high pressure side before the system stops on
the low pressure pump-down switch. This step
reduces the amount of charge on the low side in
order to prevent off-cycle migration.
A pump-down cycle represents one of the
most effective ways to protect against the
off-cycle migration of refrigerant; however it is
only convenient to apply on application with
thermostatic control.
For MLM/MLZ116, pump down cycle is
mandatory to use.
Rack application with pressostatic control can use
timer delay to empty the evaporators before the
stop. Time should be carefully set to not interfere
with the low safety pressure switch.
For low pressure pump-down switch settings,
refer to section "High and low pressure
protection". For suggested wiring diagrams,
please see section "Electrical data".
Single compressors
After 3 or 4 jogs the compressor can be started.
The heater must be energized whenever the
compressor is off to avoid liquid refrigerant
entering the compressor.
Provide separate electrical supply for the heaters
so that they remain energized even when
the machine is out of service (e.g. seasonal
shutdown).
Surface sump heater accessories are available
from Danfoss (see section "Accessories").
charge transfer or excessive migration to the
compressor during off-cycles. The quantity of
refrigerant remaining in the low-pressure side
of the system can be further reduced by using a
pump-down cycle in association with the LLSV.
Under certain conditions, the internal valve may
not completely seal, and due to the refrigerant
back flow the compressor might restart during
pump-down applications. Repeated short cycling
can result in a compressor breakdown.
Tests for pump down cycle approval:
• As the pump-down switch setting is inside the
application envelope, tests should be carried
out to check unexpected cut-out during
transient conditions (i.e. defrost – cold starting).
When unwanted cut-outs occur, the low
pressure pump-down switch can be delayed. In
this case a low pressure safety switch without
any delay timer is mandatory.
• While the thermostat is off, the number of
pressure switch resets should be limited to
avoid short cycling of the compressor. Use
dedicated wiring and an additional relay which
allows for one shot pump-down.
The pump-down allows stroage of all the
refrigerant in the high pressure side circuit. On
unitary or close-coupled systems, where the
system refrigerant charge is expected to be both
correct and definable the entire system charge
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FRCC.PC.035.A5.02