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
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.
Selection of a suction line accumulator should
be made on the basis of the following three
capabilities:
1. The accumulator should have an adequate
liquid-holding capacity that can vary with the
system. Normally this should not be less than
50% of the system charge. If possible, this value
should be checked based on actual tests.
2. The accumulator should perform without
adding excessive pressure drop to the system.
3. An accumulator should have the capability of
returning oil at the proper rate and under a range
of load conditions.
Guideline of suction accumulator needs to be
respected in making a selection.
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.
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.
26
FRCC.PC.035.A5.02
Application Guidelines
Single compressors