Cooling Tower optimization

The simplest function of a cooling tower is to cool water to a desired temperature. For evaporative towers, the difference between the cold water temperature leaving the tower and entering air wet bulb temperatures is the approach.

The tower approach in conjunction with ambient wet bulb temperature determines the theoretical minimum temperature a cooling tower can provide at that time. Each tower style and size has a rated approach that is provided by the manufacturer.

Many Building Automation Systems that control cooling towers will target a static cooling tower water leaving temperature regardless of the ambient conditions or rated approach. 

PTS has worked to develop a control sequence that monitors both ambient wet bulb as well as rated approach in order to reduce electrical consumption of tower fans and reduce unnecessary wear on components. 

After Implementation of this control strategy our clients have observed significant electrical savings in summer months where ambient conditions drive the theoretical  minimum CTW temp below the normal target

The primary consumer of electricity in a cooling tower system is the main fan that pulls new cool air through the tower. With this in mind maximizing the overall system efficiency requires a understanding of some basic fan laws, which apply to both propeller and blower

type fans. 

The capacity (CFM) of a fan varies directly as the speed ratio. (A fan turning at 50% of design speed will move 50% of its volumetric air flow.)

However, the horsepower required to drive the fan varies as the cube of either the speed or the capacity ratio. (a fan turning at 50% of design speed will require 12.5% of design input hp) 

This relationship can be seen in the graph shown to the left, with these characteristics in mind PTS has developed and implemented a cooling tower sequencing strategy that will prioritize operation at lower speeds. This allows the overall system to take advantage of the efficiency of additional cooling towers at lower fan speeds before ramping to higher speeds to meet demand. 

If you have cooling towers operating at your facility in these upper ranges with additional towers on standby, there is efficiency gains to be had with a control strategy like this. 

Cooling towers continuously require makeup water due to evaporative losses, without the addition of makeup tower basins would run dry causing operational issues. 

Additionally as basin water is evaporated, minerals such as sodium & calcium are left behind. If the basin water becomes too concentrated crystallization can form within the tower on surfaces supporting heat transfer & overall operation of the tower. This concentration is monitored via electrical conductivity and controlled by blowing down some water to drain to be replaced via makeup.

Most Automation systems controlling basin levels use a "dead band control strategy" in this application a low level and high level switch are used to maintain the water level. When the low level is reached, a 2 state valve ( 100% On or Off) is used to re fill the tower to the high level switch location.

 Dead band control is an effective and cost effective way to control level within a vessel however it does create some inefficiencies.  The incoming cool basin water does have an effect on the cooling towers capacity to control temperature as well as drops basin conductivity.

Tower basin conductivity is often controlled in parallel and is blowing water to drain to maintain water chemistry, with dead band control every fill event drives conductivity down significantly causing blowdown to overshoot its target frequently leading to unnecessary water usage.

PTS has recently worked with a client to replace dead band control with continuous makeup via analog basin monitoring, a variable makeup valve, and PID control logic.  

The benefits of this implementation were significant, losses due to blowdown were reduced and water chemistry has been steady without the sudden influx of new water. Additionally, an increase in capacity has been observed due to the continuous addition of cold water & load on the upstream water distribution system reduced.