The Waterside Economizer Sequencer module is designed to automatically switch the central plant between chiller (mechanical) cooling and waterside economizer cooling with minimal disturbance to flows and to the chilled water supply temperature. The module uses a finite state machine to accomplish transitions between nine possible states listed in the following table.
|
State |
Description |
|---|---|
|
Off |
The system is disabled. All devices are off. |
|
Chiller Control |
Chillers are controlled to provide chilled water to the building. |
|
Stop Chillers |
Chillers are disabled prior to initiating waterside economizer cooling with heat exchangers. |
|
Start Hx |
Waterside economizer cooling is initiated. Cooling towers are run at maximum speed to bring the condenser water temperature down to meet the Economizer CWS-T Setpoint as quickly as possible. |
|
Hx Control |
Heat exchangers are controlled to provide chilled water to the building. Cooling towers are modulated to control the CWS-T to the Economizer CWS-T Setpoint and the Heat Exchanger Selector stages heat exchangers to meet the building load. |
|
CHWS-T Timing |
Waiting state. Heat Exchangers are in control, but the CHWS-T has risen above the Economizer CHW-Tsp plus Differential, which indicates a transition back to chiller control may be possible due to an increase in building load. |
|
Warm CW-T |
Precursor state to return to chiller control. Flows are directed through the heat exchangers, but cooling towers are turned off and fully bypassed to allow the CWS-T to warm up to an acceptable chiller startup temperature as quickly as possible. |
|
Start Chillers |
Chillers are started. Time in this state is brief because this state is exited as soon as chiller status is True. |
|
Stop Hx |
Heat exchangers are disabled. Heat exchanger isolation valves are closed. Time in this state is brief because this state is exited as soon as Hx status is False. |
For information on how the FSM determines the state, see FSM (Waterside Economizer Sequencer).