APPLICATION GUIDELINES
32
FRCC.PC.007.B5.02
SYSTEM DESIGN RECOMMENDATIONS
Liquid fl ood back
Suction accumulator: a suction accumulator
off ers protection against refrigerant fl ood 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 dimen-
sioned, taking into account the refrigerant charge
as well as the gas velocity in the suction line.
The accumulator should not be sized for less than
50 % of the total system charge. Tests must be
conducted to determine the actual refrigerant
holding capacity needed for the application
.
Depending on the operating conditions it may
happen that the recommended connections of
the accumulator are one size smaller than the
suction line.
During normal operation, refrigerant enters the
compressor as a superheated vapour. Liquid fl ood
back occurs when a part of the refrigerant enter-
ing the compressor is still in liquid state.
Performer SH scroll compressors can tolerate oc-
casional liquid fl ood back. However system de-
sign must be such that repeated and excessive
fl ood back is not possible.
A continuous liquid fl ood back will cause oil di-
lution and, in extreme situations lead to lack
of lubrication and high rate of oil leaving the
compressor.
Liquid fl ood back test - Repetitive liquid fl ood
back testing must be carried out under expan-
sion valve threshold operating conditions: a high
pressure ratio and minimum evaporator load,
along with the measurement of suction super-
heat, oil sump temperature and discharge gas
temperature.
During operations, liquid fl ood back may be de-
tected by measuring either the oil sump tempera-
ture or the discharge gas temperature. If at any
time during operations, the oil sump temperature
drops to within 10K or less above the saturated
suction temperature, or should the discharge gas
temperature be less than 35K above the saturated
discharge temperature, this indicates liquid fl ood
back.
Continuous liquid fl ood back can occur with a
wrong dimensioning, a wrong setting or malfunc-
tion of the expansion device or in case of evapora-
tor fan failure or blocked air fi lters.
A suction accumulator providing additional pro-
tection as explained hereunder can be used to
solve light continuous liquid fl ood back.
While the thermostat is off , the number of pres-•
sure 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 refriger-
ant in the high pressure side circuit. On unitary or
close-coupled systems, where the system refrig-
erant charge is expected to be both correct and
defi nable the entire system charge may be stored
in the condenser during pump-down if all com-
ponents 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 mainte-
nance operation.
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