System 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 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).
It is recommended that the heater be turned on
for a minimum of 8 hours prior to starting the
compressor.
Optimum
location area
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 to section “Electrical connections”.
Models MLZ015-048 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.
Models MLZ058-076 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
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.
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
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.
AB243286444663en-001003
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Application Guidelines
Summary of Contents for MLZ Evolution A
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