Danfoss MLM Series Application Manuallines page 29

A G
PPLICATION UIDELINES
S
YSTEM DESIGN RECOMMENDATIONS
Due to the Danfoss scroll compressors inherent
ability to handle liquid refrigerant, crankcase
heaters are not required when the system charge
does not exceed the recommended maximum
charge.
Since the total system charge may be undefi ned, 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 avail-
able from Danfoss (see section "Spare parts &
Accessories").
The heater must be energized whenever the com-
pressor is 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
eff ective ways to protect against the off -cycle
migration of refrigerant; however it is only con-
venient 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 page 24. For suggested wiring diagrams,
please see page 17.
Models MLZ/MLM015-048 incorporate an inter-
nal low leak check valve that is appropriate for
pump-down operations. This valve prevents the
back fl ow of refrigerant from the high pressure
to the low pressure side through the compressor
so pump down conditions can be achieved and
maintained.
FRCC.PC.015.A4.02
!
Provide separate electrical supply for the
heaters so that they remain energized even when
the machine is out of service (eg. Seasonal
shutdown).
It is recommended that the heater be turned on
for a minimum of 12 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 condi-
tions, the internal valve may not completely seal,
and due to the refrigerant back fl ow the com-
pressor might restart during pump-down ap-
plications. 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 compres-
sor's discharge connector so the discharge vol-
ume is minimized.
A magnetic check valve is recommended for this
as it off ers the best solution regarding minimal re-
quired and maximal pressure drop over the wide
application envelope of the MLZ/MLM compres-
sors. If a Danfoss NRV check valve is applied it has
to be carefully selected for the specifi c 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 tran-
sient conditions (ie. defrost – cold starting).
When unwanted cut-outs occur, the low pres-
sure pump-down switch can be delayed. In this
case a low pressure safety switch without any
delay timer is mandatory.
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