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 and Spare parts).
Figure 23: Sump heater
Liquid line solenoid valve (LLSV)
A Liquid line solenoid valve (LLSV) is used to isolate the liquid charge on the condenser side, thereby preventing
against charge transfer 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.
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.
Scroll compressors, CH290 and CH485 | Application
© Danfoss | Climate Solutions | 2022.09
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