Defrost Cycle; Sump Dilution Control - Danfoss MLM116 Application Manuallines

Application Guidelines

Defrost cycle

Sump dilution control

Operating conditions
In refrigeration system applications, there are
different defrost methods, such as electric
heating defrost, hot gas bypass defrost,
reversible defrost etc. For the system which use
hot gas bypass or reversible defrost method,
suction accumulator is necessary as a result
of the possibility of a substantial quantity of
liquid refrigerant remaining in the evaporator,
this liquid refrigerant can then return to the
compressor, either flooding the sump or as
To avoid refrigerant migration to idle compressor
or oil dilution in oil sump, sump superheat must
be maintained at above10 K. Depending on the
amount of migration below, measures shall be
taken:
Manifold compressors
a dynamic liquid slug when the cycle switch
back to normal cooling operations. Sustained
and repeated liquid slugging and flooding can
seriously impair the oil's ability to lubricate the
compressor bearings. In such cases a suction
accumulator is a must. For system using
reversible defrost, it is highly recommended that
both compressor should keep running or both
stop during 4-way valve switch.
- add a sump heater, recommendations on sump
heater are the same as single compressor guide
under section "off cycle migration. "
- keep enough suction superheat, minimum
suction superheat should be above 5 K.
- the idle compressor should keep running time
to time.
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FRCC.PC.035.A4.02