Water Management and Winter Operation of Cooling Towers

When dealing with a cooling tower one of the main focus points is proper water management. Let’s face it: You have a cooling tower because you have a heat load that needs to be dissipated, and your cooling tower is the most efficient way to do that. However, a key detail inherent to the process is water management. You have water that contains the heat, and all you really want to get rid of is just the heat. Ideally, you’d like to keep all of the water since it’s a valuable resource, but through the “magic” of a cooling tower and evaporation, if you’re willing to give up a small fraction of the water, your efficiency at heat rejection is greatly enhanced. Therefore, since you already have a cooling tower, you’ve embraced and enjoyed this benefit.

But what are the other important aspects of water management? In general, water is lost from a cooling tower via evaporation, blow down, drift, and splash-out. While the amount of water lost via drift and splash-out are generally several orders of magnitude less than evaporation, the effects of their losses can cause notable impacts; from concerns ranging from Legionella bacteria dispersion or the generally corrosive and chemically charged nature of the circulating water within a tower to the problems of standing water on a rooftop or ice accumulation on and around a cooling tower, if you have drift or splash-out problems they can become daily headaches and safety hazards that inhibit your ability to focus on your business.

Drift Eliminators (DE’s) exist to keep the circulating water in the tower by capturing water droplets that become entrained in the exhaust air stream. With Brentwood providing drift eliminator options yielding drift removal efficiencies ranging from 0.005% – 0.00025% of circulating water flow, you can save water and prevent the many problems that drift causes.

If your tower doesn’t have an extended Cold Water Basin (CWB) that simply catches splash-out by being far enough outside the tower structure to enclose the trajectory zone, then your tower has inlet louvers to cover that trajectory zone. Inlet louvers generally work by providing a surface that catches the splash-out and directs it to the CWB via a sloped surface and gravity. Inlet louvers are typically either a cellular type where the sloped surface is formed by closed cell designs, or a slat type where the sloped surface is created from angled sections of plywood, corrugated FRP, or cement board at the air inlet.

Water management also affects the operation and performance of your cooling tower. In counterflow towers, two key factors in water management are the water flow rate into the tower and providing seals and diverters to ensure that the water goes where it is intended (the fill and then the CWB). If the water flow rate through the tower is too far off from the design flow rate, then the nozzles will not perform as intended and you could have dry spots allowing air bypass and non-use of fill (low flow) or excessive wall water and potential excessive drift and splash-out (high flow). In crossflow towers, the amount of water also affects the water distribution pattern and subsequent effective use of the installed fill, and it can have an impact on drift and splash-out as well. Diverters and seals to prevent water escaping the “wet” section of the cooling tower are also paramount in crossflow towers.

As we approach winter in the Northern Hemisphere, these water management concerns can reach a greater level of importance due to the effect that freezing temperatures can cause on the water you’re trying to cool. On one hand, lower ambient temperatures allow for “free cooling” which is when you can achieve your desired heat rejection without the use of the chiller. The cooling tower and low ambient temperatures are able to provide cold enough water to satisfy the internal system cooling requirements. However, if tower operation is not handled properly during winter operation, that cooling may not be as “free” as you’d like it to be.

Freezing ambient temperatures combined with airflow can lead to ice build-up and potential structural failure in the fill, louvers, tower structure, or different combinations of the three. For this reason, proper water management is key to making sure that winter operation does not cost you unnecessarily. During freezing weather operation, the general idea is to ensure that you either have a constant heavy flow of water closest to the air inlets (if your tower design incorporates this feature) or that you maintain a constant heavy flow of water over the entire fill section so that the elements of the tower located in the coldest air are bathed in warm water. This helps prevent ice accumulation. In multi-cell cooling towers this may mean shutting down cells and maintaining tower design flow over the remaining operating cells.  For more detailed information on winter operation, the Cooling Technology Institute (CTI) offers Chapter 4 of the Cooling Tower Manual, titled, “Recommendations for Winter Operation of Water Cooling Towers.” Please note that it is critical to inspect your cooling tower frequently during periods of extreme cold weather to be sure that your operational strategies are effective!