System design recommendations
Sump heater
The surface sump heaters are designed to
protect the compressor against off cycle
migration of refrigerant. When the compressor
is idle, the oil temperature in the sump of the
compressor must be maintained at no lower
than 18°F above the saturation temperature of
the refrigerant on the low-pressure side. This
requirement ensures that the liquid refrigerant
is not accumulating in the sump. A sump heater
is only effective if capable of sustaining this
level of temperature difference. Tests must be
conducted to ensure that the appropriate oil
temperature is maintained under all ambient
conditions (temperature and wind). However,
below 23°F ambient temperature and a wind
speed of above 16 ft/sec, we recommend that
the heaters be thermally insulated in order to
limit the surrounding energy losses.
Since the total system charge may be undefined,
a sump 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.
The heater must be energized for a minimum
of 6 hours before initial start-up (compressor
service valves opened) 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).
Sump heater accessories are available from
Danfoss (see section "Accessories").
Liquid line solenoid valve
(LLSV)
Pump-down cycle
An LLSV may be used to isolate the liquid charge
on the condenser side, thereby preventing
against 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 represents one of the most
effective ways to protect against the off-cycle
migration of liquid refrigerant. Once the controls
has been satisfied, a solenoid valve closes on
the condenser outlet. The compressor then
pumps the majority of the system charge into
the condenser and receiver before the system
stops on the low pressure pump-down switch.
This step reduces the amount of charge on the
low side in order to prevent off-cycle migration.
Recommended settings of the low-pressure
pump-down switch can be found in the table
section "High and low pressure protection". For
suggested wiring diagrams, please see section
"Suggested wiring diagram logic".
In certain conditions, the discharge valve may
not completely seal and result in compressor
restarts during pump down applications. An
external, non-bleeding check valve may need to
be installed.
Tests for pump down cycle approval
:
• As the pump-down switch setting is inside the
application envelope, tests should be carried
out to check unexpected cut-out during
transient conditions (ie. defrost – cold starting).
When unwanted cut-outs occur, the low
pressure pump-down switch can be delayed. In
this case a low pressure safety switch without
any delay timer is mandatory.
• While the thermostat is off, the number of
pressure switch resets should be limited to
avoid short cycling of the compressor. Use
dedicated wiring and an additional relay which
allows for one shot pump-down.
The pump-down allows to store all the
refrigerant in the high pressure side circuit. On
unitary or close-coupled systems, where the
system refrigerant charge is expected to be both
correct and definable the entire system charge
Application guidelines
34
FRCC.PC.003.A5.22
