Condenser Water System (CPO) - Johnson Controls - Metasys - LIT-12011147 - Software Application - Controller Configuration Tool - 13.1

Bypass Valve Only; River or Lake Water Cooling

Use this option if the plant can use a natural cold water source to remove the waste heat from the chillers. This method uses a bypass valve to maintain the condenser water temperature setpoint until the valve fully closes (to the bypass). At this point, the water source temperature is at or above the condenser water temperature setpoint.

Cooling Towers Only

This choice is appropriate for hot and humid climates, where the ambient wet-bulb temperature never (or very seldom) drops below the condenser water temperature setpoint. For these applications, the condenser water is expected to run through the cooling towers any time the chillers are operational.

Cooling Towers with Bypass Valve

This choice is appropriate in moderate to cold climates, where the returning water from the cooling towers may be below the condenser water temperature setpoint. Due to the chiller’s manufacturer requirements, you may need to maintain the condenser water temperature above a minimum value. The bypass valve ensures that the minimum temperature setpoint is maintained.

Towers with Single Speed Fans

During normal operation, the CPO 10 application first modulates the bypass valve to maintain the condenser water temperature setpoint. The Cooling Tower Selector module commands towers on and off based on the CW Commanded Flow. Typically, this is one tower for each condenser water pump. The controlling PID stages tower fans as needed.

Towers with Multiple Speed Fans

During normal operation, the CPO 10 application first modulates the bypass valve to maintain the condenser water temperature setpoint. The Cooling Tower Selector module commands towers on and off based on the CW Commanded Flow. Typically, this is one tower for each condenser water pump. The controlling PID stages tower fans as needed. As the condenser water temperature increases (requiring more cooling from the towers), the operating towers are commanded one at a time, to the next higher speed until all the towers reach this second speed. If more cooling is required, the application continues to command the operating towers to successively higher speeds, until all the towers are running at the highest speed. The following figure explains how eight three-speed towers are staged. This strategy is the most energy efficient, since the fan power consumption increases with the cube of its speed.

Towers with Variable Speed Fans

During normal operation, the CPO 10 application first modulates the bypass valve to maintain the condenser water temperature setpoint. The Cooling Tower Selector module commands towers on and off based on the CW Commanded Flow. Typically, this is one tower for each condenser water pump. The controlling PID stages tower fans as needed. As the condenser water temperature increases (more cooling is required from the towers), all towers are modulated by a single PID to maintain the cooling tower temperature setpoint. This strategy is the most energy efficient, since the fan power consumption increases with the cube of its speed.

Two different strategies are available for controlling cooling towers with variable speed fans: Closed Loop (PID) Control or Open Loop Control.

Closed Loop (PID) Control

During normal operation, the CPO 10 application first modulates the bypass valve to maintain the condenser water temperature setpoint. When the controlling PID saturates high (valve fully closed to the bypass), the cooling tower sequencer starts turning on the towers one at a time, at minimum speed, until all towers are commanded on. As the condenser water temperature increases (more cooling is required from the towers), all towers are modulated by a single PID to maintain the condenser water temperature setpoint. This strategy maximizes energy efficiency, since the fan power consumption increases with the cube of its speed.

Open Loop Control

Three control modules are involved in open loop control. CW Bypass Valve Control modulates the Cooling Tower Valve output CTV-O command to maintain the Condenser Water Supply Temperature (CWS-T) to maintain Condenser Water temperature setpoint (CW-SP).

Open Loop Tower with Low Limit controls the Cooling Tower Speed Command based on the Current Chiller Plant Capacity. It contains the Near Optimal Open Loop Tower Control algorithm to provide a single command to all operating towers. If the condenser water temperature falls below the condenser water temperature setpoint, the open loop algorithm is overridden and a single PID modulates all operating towers to maintain the condenser water setpoint. The Open Loop algorithm is used when the condenser water temperature is between the condenser water temperature setpoint and high limit condenser water temperature setpoint. Similarly, if the condenser water temperature rises above the high limit condenser water temperature setpoint, the open loop algorithm is overridden and all operating towers are commanded to 100%.

The Cooling Tower Selector module commands towers on and off based on the CW Commanded Flow. Typically, this is one tower for each condenser water pump.