The Open Loop Tower Control with Low Limit module provides a cooling tower control signal using the Near Optimal Open Loop Tower Control block. Normally, the percent command output of the module is the output of the Near Optimal Open Loop Tower Control block only. However, if the condenser water temperature drops below the low limit setpoint, the module starts low limit control. In this condition, the module output becomes the minimum of the block output and the output of a PID controlling the condenser water temperature to the low limit setpoint. When conditions are favorable for exiting low limit control [that is, CW-T > (LL Spt + Differential) and (OA Wet Bulb Temp + Tower Approach) > LL Spt], the module output is handed back to the block output only. This module is implemented as a standard Control Activity.
In addition, this module has a high limit feature using a High Control Setpoint input. If the CW-Temp is above the High Control Setpoint, the fan spins at 100%. This module also allows for a limited max mode of operation.
For equations and more information on the Near Optimal Open Loop Tower Control logic block used by this module, see Near Optimal Open Loop Cooling Tower Control in the Control category of the Logic section.
This module also incorporates tracking the overridden value of the actual output object (AO, PAO). The module resets the PIDs to default tuning parameters if the Reset PID Tuning input is not Normal.
This module provides EWMA diagnostic values calculated by the PID. Controllers have the ability to collect data about their inputs, outputs, and internally calculated variables. Inside the controller, the EWMA minimizes memory requirements, reduces communication traffic, and provides an easy way to analyze collected data. See EWMA Diagnostics in the Commissioning a System section for more information.
You can view and modify the modules in this group’s logic.