18
SYSTEM DESIGN RECOMMENDATIONS
Liquid refrigerant control
and charge limits
Off -cycle migration
Liquid fl oodback during
operation
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. Danfoss
MPZ compressors have a large internal
volume and can therefore handle rela-
tively large amounts of liquid refriger-
ant without major problems. However
even when a compressor can handle
liquid refrigerant, this will not be fa-
vourable to its service life. Liquid re-
frigerant can dilute the oil, wash oil out
of bearings and result in high oil carry
over, resulting in loss of oil from 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 com-
pressor in diff erent ways, with diff er-
ent eff ects on the compressor.
During system standstill and after
pressure equalisation, refrigerant will
condense in the coldest part of the
system. The compressor can easily
be the coldest spot, for example
when it is placed outside in low
ambient temperatures. After a while,
the full system refrigerant charge
can condense in the compressor
crankcase. If the entire system is at a
uniform temperature and there are
no obstructions to vapor fl ow during
the off cycle the refrigerant charge
will slowly migrate to the compressor
because of the solubility of refrigerant
in the oil. If other system components
are located at a higher level, this
process can be even faster because
gravity will assist the liquid refrigerant
to fl ow back to the compressor. When
the compressor is started, the pressure
in the crankcase decreases rapidly.
At lower pressures the oil holds less
refrigerant, and as a result part of the
refrigerant will violently evaporate
from the oil, causing the oil to foam.
This process is often called “boiling”.
The negative eff ects of migration on
the compressor are:
• oil dilution by liquid refrigerant
• oil foam, transported by refrigerant
gas and discharged into the system,
causing loss of oil and in extreme
situations risk for oil slugging
• in extreme situations with high
system refrigerant charge, liquid
slugging could occur (liquid entering
the compressor cylinder).
During normal and stable system
operation, refrigerant will leave the
evaporator in a superheated condition
and enter the compressor as a
superheated 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 diff erent
reasons:
• wrong dimensioning, wrong setting
or malfunction of expansion device
• evaporator fan failure or blocked air
fi lters.
In these situations, liquid refrigerant
will continuously enter the
compressor.
The negative eff ects from continuous
liquid fl oodback are:
• permanent oil dilution
• in extreme situations with high
system refrigerant charge and large
amounts of fl oodback, liquid slugging
could occur.