A
PPLICATION
GUIDELINES
S
YSTEM
DESIGN
RECOMMENDATIONS
Due to the Danfoss scroll compressors inherent
ability to handle liquid refrigerant, crankcase heat-
ers are not required when the system charge does
not exceed the recommended maximum charge.
Since the total system charge may be undefined, a
crankcase heater is recommended on all systems
with remote heat exchangers. In addition, any sys-
tem 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 page 39).
The heater must be energized whenever the com-
pressor is off.
!
Provide separate electrical supply for the heat-
ers so that they remain energized even when the
machine is out of service (eg. Seasonal shut-
down).
It is recommended that the heater be turned on
for a minimum of 12 hours prior to starting the
compressor.
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 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 flow of refrigerant from the high pressure
to the low pressure side through the compressor
so pump down conditions can be achieved and
maintained.
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 flow the compres-
sor might restart during pump-down applications.
Repeated short cycling can result in a compressor
breakdown. It is recommended to install an exter-
nal 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 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 specific operation
conditions of the individual system.
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
charge transfer or excessive migration to the com-
pressor during off-cycles. The quantity of refrig-
erant remaining in the low-pressure side of the
system can be further reduced by using a pump-
down cycle in association with the LLSV.
27
FRCC.PC.016.A1.22
Optimum
location area
