Chiller Staging Conditions; Load Balancing - Carrier ChillerVu 23XRV Series Application Manual

Chiller staging conditions

After a successful system start, the following staging is initiated to satisfy the system load:
•
When enabled, each chiller modulates its individual capacity using its internal controls to maintain its leaving
chilled water setpoint, sent by the application settings, which default to 45°F (configurable).
•
The control program monitors the downstream chiller leaving chilled water temperature sensor. If the
downstream chiller leaving chilled water temperature sensor exceeds the deviation from setpoint for the 10-
minute default (configurable), the lag chiller start sequence initiates.
•
Once the lag start sequence initiates, the lead chiller's operating capacity is calculated. If the capacity has
exceeded the efficiency optimization threshold, plus the deadband for the 10-minute default (configurable),
then the lag chiller is commanded on.
•
Both chillers run until the efficiency optimization threshold, minus the deadband, has been met for the 10-
minute default (configurable), and then the lag chiller is commanded off.
•
The lag chiller is enabled or disabled, as needed, to maintain the leaving chilled water setpoint.
•
If the lead chiller fails during operation, an alarm signal is issued over BACnet, converts the lag chiller to the
lead chiller, and assigns the appropriate leaving chilled water setpoint to the lead chiller.
•
If the lag chiller fails during operation, an alarm signal is issued over BACnet.
Pull Down
During pull down, there is an alarm after the 10-minute default, or the time you set, if high chilled water
temperature conditions exist.
There is an alarm if entering or return chilled water temperature readings are out of range for more than the
configured time.
Power Loss
In the event of power loss to a chiller, and if the power loss relay is closed (optional), the application attempts to
start the remaining chiller in the branch. If the remaining chiller is either already operating or unable to start, the
application attempts to restart the inoperative chiller 30 seconds after its power is restored.
You can monitor the power loss binary input and disable the start command to prevent undesired restarts after
power failure.

Load balancing

When both chillers must operate, the application calculates the appropriate setpoint to maintain equal load
balance between the upstream and downstream chillers. The return water temperature and the leaving water
temperature are monitored to determine the optimum leaving water temperature setpoint of the lead machine to
balance the load between the 2 chillers. The system then sends the calculated setpoint to the upstream chiller
and sends the configured leaving water temperature setpoint to the downstream chiller to ensure optimal
efficiency and load balance. See the sequencing preconditions (page 9).
If both chillers are operating, and the leaving water setpoint of the downstream chiller is not met after a
configured time delay, the leaving water setpoint is adjusted and sent to the upstream chiller to automatically
compensate until the required downstream setpoint is achieved.
Carrier® ChillerVu™
23XRV Series Counterflow Application Guide 10
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