System design recommendations
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
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 2.15 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.
25
FRCC.PC.026.A4.02
Application Guidelines
