I wrote a program in my building's automation system (Metasys) to reset the ECWT to be seven degrees above the wet bulb temperature of the air entering the towers. Seven degrees happens to be the design approach temperature of my towers; others might vary.
Originally Posted by chad_nc
How it works is thus: I have a temperature and humidity sensor out in the cooling tower room. It feeds both readings to the Metasys, which then converts this data to wet bulb temperature. That number then runs through a program that links to the ECWT set point. As the wet bulb temperature drifts up or down, the ECWT is reset accordingly. Within the program is an upper and lower limit, so the towers are not run too warm or too cold. I also added a bit of buffering to reduce constant resetting of the ECWT set point due to fluctuating wet bulb temperatures.
NOTE: I could do this because my towers have VFDs on the blowers. The McQuay Chiller Plant Design Guide also discusses the wet bulb reset strategy as one method used for optimizing chiller plant operations. This guide is available on the internet.
Here's a screen shot of the logic (if the attachment works):
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.