© Danfoss | Climate Solutions | 2022.03
System design recommendations
Suction accumulator:
a suction accumulator
offers protection against refrigerant flood back
at start-up, during operations or defrosting by
trapping the liquid refrigerant upstream from
the compressor. The suction accumulator also
protects against off-cycle migration by providing
additional internal free volume to the low side
of the system. For MLZ/MLM116 application,
suction accumulator is mandatory to use.
A suction accumulator must be carefully
dimensioned, taking into account the refrigerant
Liquid flood back
During normal operation, refrigerant enters
the compressor as a superheated vapor. Liquid
flood back occurs when a part of the refrigerant
entering the compressor is still in liquid state.
A continuous liquid flood back will cause oil
dilution and, in extreme situations lead to lack
of lubrication and high rate of oil leaving the
compressor.
Liquid flood back test
- Repetitive liquid flood
back testing must be carried out under TXV
threshold operating conditions: a high pressure
ratio and minimum evaporator load, along with
the measurement of suction superheat, oil sump
temperature and discharge gas temperature.
During operations, liquid flood back may be
detected by measuring either the oil sump
temperature or the discharge gas temperature.
If at any time during operations, the oil sump
temperature drops to within 10 K or less above
the saturated suction temperature, or should
the discharge gas temperature be less than 35 K
above the saturated discharge temperature, this
indicates liquid flood back.
Continuous liquid flood back can occur with
a wrong dimensioning, a wrong setting or
malfunction of the expansion device or in case of
evaporator fan failure or blocked air filters.
A suction accumulator providing additional
protection as explained hereunder can be used
to solve light continuous liquid flood back.
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/MLZ, 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".
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
may be stored in the condenser during pump-
down if all components have been properly sized.
Other application needs a liquid receiver to store
the refrigerant.
Receiver dimensioning requires special attention.
The receiver shall be large enough to contain
part of the system refrigerant charge but it shall
not be dimensioned too large. A large receiver
easily leads to refrigerant overcharging during
maintenance operation.
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AB243586442698en-000601
Application Guidelines
Single compressors
