How water efficiency is measured in a cooling tower

As cooling towers do their jobs and evaporate water away, any impurities that were inside of the circulating water will be left behind and build-up over time. To prevent damage to the cooling system, the tower “bleeds off” or “blows down” to purge these impurities. Water lost to evaporation and bleed is then replaced with makeup water. While water treatment cannot control how much evaporation is needed to adequately cool a building, it does impact how often the tower needs to bleed.

Measuring water efficiency as a ratio of makeup water volume to blowdown volume is important because it allows us to track performance regardless of how much evaporative load is placed on the tower. This is known as the concentration ratio, or cycles of concentration. If we only measured efficiency by the amount of makeup added, we would be penalizing ourselves for hot weather, or getting unfair credit for cool weather.

With this in mind, to achieve higher water efficiency, we want to create conditions that will require the tower to bleed off less water, and therefore require less new makeup water to be added. This is where industrial water softeners come in.

Hardness is the #1 barrier from achieving higher water efficiency

While there are many impurities that are managed by the bleed process, the controls are most often set with the primary goal of preventing scale precipitation and accumulation, which can be disastrous when uncontrolled. Therefore, lowering the scale forming potential is the first place we should look in trying to boost water efficiency (meaning to send less blowdown to sewer, and add less makeup water). While different impurities can cause scale, enemy number one is “hardness”, defined by the level of dissolved calcium and magnesium ions.

Attack hardness at the source

Although water treatment chemicals such as scale inhibitors do an excellent job of keeping hardness in a dissolved, non-scale forming state, they are limited when new makeup water reintroduces hardness. To truly achieve higher water efficiency, we need to cut off hardness at the source. One of the easiest and most cost-effective ways of doing this is by installing a water softener on the makeup line, which removes hardness before it enters the tower.

Water softeners are vessels filled with a simple form of ion exchange resin beads that swap the calcium and magnesium ions for sodium ions, which are harmless. The harmful ions adhere to the beads before entering the cooling loop. Once the beads become saturated, they have to be rinsed and regenerated with a salt solution.

With calcium and magnesium ion content reduced in the recirculating water, the tower can usually run for much longer before the scale-forming potential of the water becomes high enough to trigger a blowdown. If there is less bleed off, then there will also be less makeup required.

For more ways to fight scale, check out: “Tackling Cooling Tower Scale: Strategies and Solutions.”

Softeners need careful management

Although adding a water softener may be a smaller investment than other strategies for controlling scale, they do require active management to run efficiently and safely. To help with this, we’ve put together a separate article outlining the top five questions you should ask your water treatment vendor when considering a new softener.