SYSTEM DESIGN RECOMMENDATIONS
Liquid refrigerant control
and charge limits
Off-cycle migration
Liquid floodback during
operation
Liquid floodback at
changeover cycles in
reversible heat pumps
Liquid floodback
and zeotropic refrigerants
Refrigeration compressors are basically
designed as gas compressors. Depen-
ding on the compressor design and
operating conditions, most compres-
sors can also handle a limited amount
of liquid refrigerant. Maneurop® MT
and MTZ 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, it is not favo-
rable to a long service life. Liquid refri-
gerant 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, and have a positive effect
on the compressor service life.
Liquid refrigerant can enter a com-
pressor in different ways, with different
effects on the compressor.
During system standstill and after
pressure equalization, 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 tem-
peratures. After a while, the full system
refrigerant charge can condense in the
compressor crankcase. A large amount
will dissolve in the compressor oil until
the oil is completely saturated with re-
frigerant. If other system components
are located at a higher level, this pro-
cess can be even faster because gravity
will speed the flow of liquid refrigerant
to flow back to the compressor. When
the compressor is started, the pressure
in the crankcase decreases rapidly.
At lower pressures the oil holds less re-
frigerant, and as a result part of the refri-
gerant will violently evaporate from the
oil, causing the oil to foam. This process
is often called “boiling”.
The negative effects on the compressor
from migration 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 of oil slugging
• in extreme situations with high
system refrigerant charge, liquid
slugging could occur (liquid entering
the compressor cylinders).
During normal and stable system ope-
ration, refrigerant will leave the evapo-
rator in a superheated condition and
enter the compressor as a superheated
vapor.
Normal superheat values at compressor
suction are 9
o
F to 54
o
F. The refrigerant
leaving the evaporator, however, can
contain an amount of liquid refrigerant
for various reasons:
• wrong dimensioning, wrong setting
or malfunction of expansion device
• evaporator fan failure or blocked air
filters.
In these situations, liquid refrigerant will
continuously enter the compressor.
The negative effects from continuous
liquid floodback are:
• permanent oil dilution
• in extreme situations with high
system refrigerant charge and large
amounts of floodback, liquid
slugging could occur.
In heat pumps, changeover from coo-
ling to heating cycles, defrost, and low
load short cycles may lead to liquid re-
frigerant floodback or saturated refrige-
rant return conditions.
The negative effects are:
• oil dilution
• in extreme situations with high
system refrigerant charge and large
amounts of floodback, liquid
slugging could occur.
Liquid floodback in systems working
with a zeotropic refrigerant such as
R-407C introduces additional negative
effects. A part of the refrigerant leaves
the evaporator in liquid phase and this
liquid has a different composition than
the vapor.
This new refrigerant composition may
result in different compressor operating
pressures and temperatures.