SYSTEM DESIGN RECOMMENDATIONS
Crankcase heater
Liquid line solenoid valve
& pump-down
Suction accumulator
A crankcase heater protects against
the off-cycle migration of refrigerant
and proves effective if oil temperature
is maintained 18
o
F above the satura-
ted LP temperature of the refrigerant.
Tests must be conducted, therefore, to
ensure that the appropriate oil tempe-
rature is maintained under all ambient
conditions. A PTC crankcase heater is
recommended on all stand-alone com-
pressors and split systems. PTC crank-
case heaters are self-regulating.
Under extreme conditions such as very
low ambient temperature a belt type
crankcase heater could be used in addi-
tion to the PTC heater, although this is
not a preferred solution for 1 and 2 cy-
linder compressors. The belt crankcase
heater must be positioned on the com-
pressor shell as close as possible to the
oil sump to ensure good heat transfer to
the oil.
Belt crankcase heaters are not self-re-
gulating. Control must be applied to
energize the belt heater once the com-
pressor 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 18
o
F
above the saturated LP temperature of
the refrigerant during off cycles or if re-
peated floodback is occuring, a pump-
down cycle using an LLSV is required.
In such cases, a suction accumulator is
recommended.
In refrigeration applications, a Liquid
Line Solenoid Valve (LLSV) is highly re-
commended. During the off-cycle, the
LLSV isolates the liquid charge in the
condenser side, thus preventing refri-
gerant transfer or excessive migration
of refrigerant into the compressor. Fur-
thermore, when using an LLSV in with
a pumpdown cycle, the quantity of re-
frigerant in the low-pressure side of the
system will be reduced.
A pump-down cycle design is also re-
quired when evaporators are fitted with
electric defrost heaters.
A suction accumulator offers considera-
ble protection against refrigerant floo-
dback at start-up, during operation or
after the defrost operation. This device
also helps to protect against off-cycle
migration by means of providing addi-
tional internal free volume to the low
pressure side of the system.
The suction accumulator must be selec-
ted in accordance with the accumulator
manufacturer's recommendations. As a
general rule, Danfoss recommends si-
zing the accumulator for at least 50% of
the total system charge. Tests, however,
must be conducted to determine the
optimal size.
A suction accumulator must not be
used in systems with zeotropic refrige-
rant mixtures.