Dual Pump Package Sequence of Operation
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Dual Pump Package Sequence of Operation
On a call for cooling, the compressor contactor and/or the R5 Econ-O-Coil relay in the Liebert unit is
energized. The relay and contactor are in the Liebert indoor evaporator section. Each compressor con-
tactor has a side switch wired in parallel with the R5 relay and is responsible for closure of the
low-voltage pump-control circuit.
This low voltage circuit has a series of contactors, relays, stats, selector switch, and a flow switch.
This circuit controls the start of the pumps and the energizing of the drycoolers. Once the circuit is
closed, 24 volts is passed to the selector switch and the auxiliary relays are energized. The auxiliary
relay(s) closes a set of contacts that energizes the drycoolers. The switch will be in position 1-2 for #1
pump primary and #2 as standby or 2-1 for #2 pump primary and #1 as standby.
For this example the switch will be set 1-2. Voltage from the switch then passes through the normally
closed contacts of the R2 relay (standby pump relay), through the current overloads and to the #1
pump contactor. At this point, the #1 pump and appropriate drycoolers are running. When the pump
establishes flow it opens the system flow switch. The pump has approximately 10 seconds to establish
full flow. If it does, the system will run in this state until the call for cooling is satisfied and the circuit
drops out. If this pump cannot establish flow or it has been running and fails, the flow switch will
close and energize an adjustable relay, typically set for 10 seconds.
Once this relay times out, it energizes the R2 switch over relay. This relay will drop out the voltage to
the #1 pump contactor and energize the #2 pump contactor. Along with the R2 relay the AL relay
(alarm relay) will energize. This will provide a set of closed contacts for remote indication of the
switch over situation.
Once the problem with the lead pump is repaired, the controls will need to be reset. To reset the con-
trol box, turn off the main power to the control box and then restore the main power to the control box.
