System design recommendations
A LLSV must 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".
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.
Sump heater
The surface sump heaters or other equivalent
heaters are required to protect the PSH
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).
Note that below –5°C ambient temperature and a
wind speed of above 5m/second, we recommend
that the heaters be thermally insulated in order
to limit the surrounding energy losses.
Initial start-up: due to light commercial platform
compact design, it is recommended to energize
surface sump heater in advance to remove
refrigerant at initial start-up only 6 hours in
advance.
Then 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 (eg. Seasonal
shutdown).
Surface sump heater accessories are available
from Danfoss (see section "Accessories").
23
FRCC.PC.022.A4.02
Application Guidelines
