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 10 K 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 -5°C ambient temperature and
a wind speed of above 5 m/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 may be
stored in the condenser during pump-down if all
components have been properly sized.
Other application needs a liquid receiver to store
the refrigerant.
Receiver dimensioning requires special attention.
The receiver shall be large enough to contain
part of the system refrigerant charge but it shall
not be dimensioned too large. A large receiver
easily leads to refrigerant overcharging during
maintenance operation.
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AB237986441643en-010801
Application Guidelines
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