Application guidelines
28
FRCC.PC.004.A9.22
System design recommendations
A crankcase heater protects against the off-cycle
migration of refrigerant and proves effective if
oil temperature is maintained 18
o
F above the
saturated LP temperature of the refrigerant. Tests
must thereby be conducted to ensure that the
appropriate oil temperature is maintained under
all ambient conditions. A PTC crankcase heater
is recommended on all stand-alone compressors
and split systems. PTC crankcase heaters are
self-regulating.
Under extreme conditions, such as very low
ambient temperature, a belt type crankcase
heater could be used in addition to the PTC
heater; although this is not a preferred solution
for 1 and 2 cylinder compressors. The belt
crankcase heater must be positioned on the
compressor shell as close as possible to the oil
sump to ensure good heat transfer to the oil.
Belt crankcase heaters are not self-regulating.
Control must be applied to energize the belt
heater once the compressor has been stopped
and then to de-energize it while the compressor
is running. The belt heater must be energized 12
hours before restarting the compressor following
an extended down period.
If the crankcase heater is not able to maintain the
oil temperature at 18
o
F above the saturated LP
temperature of the refrigerant during off cycles
or if repetitive floodback is present a the Liquid
Line Solenoid Valve (LLSV) + pump-down cycle is
required, eventually in conjunction with a suction
accumulator.
Crankcase heater
Liquid line solenoid valve &
pump-down
Suction accumulator
Liquid floodback at change over
cycles in reversible heat pumps
Liquid floodback and zeotropic
refrigerants
In refrigeration applications, the Liquid Line
Solenoid Valve (LLSV) is highly recommended.
During the off-cycle, the LLSV isolates the liquid
charge in the condenser side, thus preventing
against refrigerant transfer or excessive migration
of refrigerant into the compressor. Furthermore,
when using a LLSV in conjunction with a pump-
down cycle, the quantity of refrigerant in the low-
pressure side of the system will be reduced.
A pump-down cycle design is required when
evaporators are fitted with electric defrost
heaters.
A suction accumulator offers considerable
protection against refrigerant floodback at
start-up, during operation or after the defrost
operation. This device also helps to protect
against off-cycle migration by means of
providing additional internal free volume to the
low pressure side of the system.
The suction accumulator must be selected in
accordance with the accumulator manufacturer
recommendations. As a general rule, Danfoss
recommends to size the accumulator for at least
50% of the total system charge. Tests however
must be conducted to determine the optimal
size.
A suction accumulator shall not be used in
systems with zeotropic refrigerant mixtures.
In heat pumps, change over from cooling to
heating cycles, defrost and low load short cycles
may lead to liquid refrigerant floodback or
saturated refrigerant return conditions.
The negative effects are:
• oil dilution
• in extreme situations with high system
refrigerant charge and large amounts of
floodback, liquid slugging could appear.
Liquid floodback in systems working with a
zeotropic refrigerant such as R407C 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.
• 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.