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AB243586442172en-001401
System design recommendations
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A 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.
Liquid line solenoid valve
(LLSV)
Pump-down cycle
A pump-down cycle represents one of the most
effective ways to protect against the off-cycle
migration of liquid refrigerant. Once the system
has reached its set point and is about to shut
off, the LLSV on the condenser outlet closes.
The compressor then pumps the majority of
the refrigerant 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. The recommended
low-pressure pump-down switch setting is 1.5
bar below the nominal evaporating pressure.
It shall not be set lower than 1.7 bar(g). For
suggested wiring diagrams, please see section
”Suggested wiring diagrams logic“.
In certain conditions, the discharge valve in the
SH090 - 105 - 120 - 140 - 161 - 184 compressor
may not completely seal and result in compressor
restarts during pump down applications. An
external, non-bleeding check valve may need to
be installed.
SH180
to
SH485
SH90 to SH184
The selection of surface sump heater could refer
to below principle:
Compressor Surrounding
Ambient
Surface Sump Heater
Unit has enclosure, no wind
48W SSH
Unit has no enclosure, with
wind
80W SSH
Unit has no enclosure,
wind >5m/s & ambient
temperature <-5°C
80W SSH + additional
SSH/thermal insulation
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 sump heater.
A sump heater is also required on all reversible
cycle applications.
At initial start-up or after power shortage, it is
recommended to energize surface sump heater
to remove refrigerant 6 hours in advance. A
quicker start-up is possible by “jogging” the
compressor to evacuate refrigerant in the
compressor. Start the compressor for 1 second,
then wait for 1 to 2 minutes. After 3 or 4 jogs the
compressor can be started. This operation must
be repeated for each compressor individually:
Compressor 1
Compressor 2
Compressor 3
The heater must be energized whenever the
compressor is off to avoid liquid refrigerant
entering the compressor.
Provide separate electrical supply for the heaters
so that they remain energized even when
the machine is out of service (e.g. seasonal
shutdown).
Surface sump heater accessories are available
from Danfoss (see section “Accessories”).
Application Guidelines
Summary of Contents for SH485
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