Liquid Refrigerant Control And Charge Limits - Danfoss Maneurop NTZ048 Selection And Application Manuallines

SYSTEM DESIGN RECOMMENDATIONS
Liquid refrigerant control
and charge limits
Off-cycle migration
Liquid floodback during
operation
16
des the thermostat, mounting bracket
and insulation. The thermostat must
be attached to the discharge line as
Discharge line
Bracket
Refrigeration compressors are basi-
cally designed as gas compressors.
Depending on the compressor de-
sign and operating conditions, most
compressors can also handle a limited
amount of liquid refrigerant. Maneu-
rop® NTZ compressors have a large in-
ternal volume and can therefore handle
relatively large amounts of liquid refri-
gerant without major problems.
However even when a compressor can
handle liquid refrigerant, this will not
be favourable to its service life.
During system standstill and after
pressure equalisation, refrigerant will
condensate in the coldest part of the
system which may be the compressor
when it is placed in a cold environ-
ment. Ultimately, the full system refri-
gerant charge can condensate in the
compressor crankcase. A large amount
will dissolve in the compressor oil un-
til the oil is completely saturated with
During normal and stable system ope-
ration, refrigerant will leave the eva-
porator in a superheated condition
and enter the compressor as a supe-
rheated vapour. Normal superheat
values at compressor suction are 5 to
30 K. However the refrigerant leaving
the evaporator can contain an amount
of liquid refrigerant due to different
reasons:
wrong dimensioning, wrong setting
●
or malfunction of expansion device
RECIPROCATING COMPRESSORS
indicated below at no more than
150 mm from the discharge connec-
tion.
Thermostat
Insulation
Liquid refrigerant will dilute the oil,
wash out the bearings causing wear
and eventually seizure. Furthermore,
high oil carry over will cause lack of oil
in the sump.
Good system design can limit the
amount of liquid refrigerant in the
compressor, which will have a positive
effect on the compressor service life.
Liquid refrigerant can enter a com-
pressor in various ways, with different
effects on the compressor as descri-
bed in the following points.
refrigerant. When the compressor is
started, the pressure in the crankcase
decreases rapidly and refrigerant will
violently evaporate, causing the oil
to foam (boiling). Both dilution and
foaming reduce the lubrication pro-
perties of the oil. In extreme situations
liquid could enter the compressor cy-
linders with immediate compressor
break-down as a result.
evaporator fan failure or frosted-up
●
evaporator coils.
In these situations, liquid refrigerant
will continuously enter the compres-
sor. The negative effects from conti-
nuous liquid floodback are:
permanent oil dilution
●
in extreme situations with high
●
system refrigerant charge and large
amounts of floodback, liquid slugging
could occur.