System design recommendations
Liquid flood back
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.
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
flood back occurs when a part of the refrigerant
entering the compressor is still in liquid state.
Danfoss PSH scroll compressors can tolerate
occasional liquid flood back. However system
design must be such that repeated and excessive
flood back is not possible.
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
expansion valve 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 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 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.
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FRCC.PC.022.A4.02
Application Guidelines
