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.
A suction accumulator must be carefully
dimensioned, taking into account the refrigerant
charge as well as the gas velocity in the suction
line. Depending on the operating conditions it
may happen that the recommended connections
of the accumulator are one size smaller than the
suction line.
Liquid flood back
During normal operation, refrigerant enters the
compressor as a superheated vapour. 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 10K (18°F) or less
above the saturated suction temperature, or
should the discharge gas temperature be less
than 35K (65°F) 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.
• 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 to store 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.
AB243286444663en-001003
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Application Guidelines
Summary of Contents for MLZ Evolution A
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