Danfoss MLM Series Application Manuallines page 36

Application Guidelines
36
FRCC.PC.015.A8.02
System design recommendations
Since the total system charge may be undefined,
a crankcase heater is recommended on all
systems with remote heat exchangers. In
addition, any system containing a refrigerant
charge in excess of the maximum recommended
system charge for compressors requires a
crankcase heater.
Belt-type crankcase heater accessories are
available from Danfoss (see section "Spare parts &
Accessories").
The heater must be energized whenever the
compressor is off.
!
Provide separate electrical supply for the
heaters so that they remain energized even when
the machine is out of service (eg. Seasonal
shutdown).
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.
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".
Models MLZ/MLM015-048 incorporate an internal
low leak check valve that is appropriate for
pump-down operations. This valve prevents the
back flow of refrigerant from the high pressure
to the low pressure side through the compressor
so pump down conditions can be achieved and
maintained.
It is recommended that the heater be turned on
for a minimum of 8 hours prior to starting the
compressor.
Optimum
location area
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.
Models MLZ/MLM058-076 are not equipped
with this low leak check valve. 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. It is recommended
to install an external magnetic check valve
(such as Danfoss Part No. 120Z5046) close to
the compressor's discharge connector so the
discharge volume is minimized.
A magnetic check valve is recommended for this
as it offers the best solution regarding minimal
required and maximal pressure drop over the
wide application envelope of the MLZ/MLM
compressors. If a Danfoss NRV check valve is
applied it has to be carefully selected for the
specific operation conditions of the individual
system.
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 (ie. 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.