20
SYSTEM DESIGN RECOMMENDATIONS
Liquid refrigerant control
and charge limits
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
internal volume and can therefore han-
dle relatively large amounts of liquid
refrigerant without major problems.
However even when a compressor can
handle liquid refrigerant, this will not
be favourable to its service life.
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
eff ect on the compressor service life.
Liquid refrigerant can enter a compres-
sor in various ways, with diff erent ef-
fects on the compressor as described
in the following points.
Thermostat
Insulation
Bracket
Discharge line
Off -cycle migration
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 re-
frigerant charge can condensate in the
compressor crankcase. A large amount
will dissolve in the compressor oil un-
til the oil is completely saturated with
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 prop-
erties of the oil. In extreme situations
liquid could enter the compressor
cylinders with immediate compressor
break-down as a result.
Liquid fl oodback during
operation
During normal and stable system
operation, refrigerant will leave the
evaporator in a superheated condition
and enter the compressor as a super-
heated vapour. Normal superheat val-
ues at compressor suction are 5 to 30
K. However the refrigerant leaving the
evaporator can contain an amount of
liquid refrigerant due to diff erent rea-
sons:
z
wrong dimensioning, wrong setting
or malfunction of expansion device
z
evaporator fan failure or frosted-up
evaporator coils.
In these situations, liquid refrigerant
will continuously enter the compres-
sor. The negative eff ects from continu-
ous liquid fl oodback are:
z
permanent oil dilution
z
in extreme situations with high sys-
tem refrigerant charge and large
amounts of fl oodback, liquid slug-
ging could occur.
the thermostat, mounting bracket
and insulation. The thermostat must
be attached to the discharge line as
indicated below at no more than
150 mm from the discharge connec-
tion.
