17
sYsteM design RecoMMendations
Refrigerant charge
limit and compressor
protection
crankcase heater
liquid line solenoid valve
(llsV)
pump-down cycle
suction accumulator
A suction accumulator offers protec-
tion against refrigerant floodback at
start-up, during operation, or after
heat pump defrosting. Sustained and
repeated liquid slugging and flood-
back can seriously impair the oil’s
ability to lubricate. The suction line
accumulator also protects against
off-cycle migration by providing addi-
tional internal free volume on the low
side of the system. The accumulator
should be sized for not less than 50%
of the total system charge. Tests must
be conducted to determine the actual
refrigerant-holding capacity needed
for the application.
Performer® scroll compressors incor-
porate internal safety devices such
as motor temperature protection,
reverse rotation protection, and a
discharge gas check valve. Additional
safety devices or system components,
however, may be required to en-
sure adequate protection of the
compressor. Depending on the system
configuration and application, one
or several of the following methods
should be considered.
When the compressor is idle, the oil
temperature in the sump must be
at least 18
o
F above the saturation
temperature of the refrigerant on the
low pressure side. This ensures that
liquid refrigerant does not accumulate
in the sump. A crank case heater is
only effective if capable of sustaining
a temperature difference of 18°F
or more. Tests must be conducted
to ensure that the appropriate oil
temperature is maintained under all
ambient conditions (temperature and
wind).
Below 23°F ambient temperature with
wind speed above 11 mph, however,
we recommend thermal insulation
of crankcase heaters in order to limit
the energy losses to the surrounding
air. Since the total system charge may
be undefined, a crankcase heater is
recommended on all stand-alone
compressors and split systems. In
addition, any system containing a
refrigerant charge in excess of the
maximum recommended system
charge for compressors requires a
crankcase heater. A crankcase heater
is also required on all reversible cycle
applications.
notes
: Belt-type crankcase heater
accessories
are
available
from
Danfoss (see page 34). The heater
must be energized for a minimum of
12 hours before initial start-up (with
compressor service valves open), and
must remain energized whenever the
compressor is off. Provide separate
electrical supply for the heaters so that
they remain energized even when the
machine is out of service (eg. seasonal
shutdown).
An LLSV may be used to isolate the
liquid charge on the condenser side,
thereby preventing charge transfer or
excessive migration to the compressor
during off-cycles.
The quantity of refrigerant on the
low pressure side of the system can
be further reduced by using a pump-
down cycle in association with the
LLSV.
A pump-down cycle is one of the
most effective ways to protect against
the off-cycle migration of liquid
refrigerant. Once the low pressure
cut off is activated, a solenoid valve
closes on the condenser outlet.
The compressor then pumps the
majority of the system charge into the
condenser, and the system stops on
the low pressure switch. This reduces
the amount of charge on the low side
and prevents off-cycle migration.
Recommended settings for the low
pressure switch can be found in the
table on p. 22. For suggested wiring
diagrams, please see p. 14-15.
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