Here in the greater Phoenix area, harsh water conditions require smart water treatment strategies to properly keep equipment water-cooled. Proper management of the cooling tower sump water characteristics along with maintaining good overall tower hygiene accomplishes at least four positive things.

1) Avoid wasting excess water.

2) Inhibits scale formation.

3) Control biological growth.

4) Reduces the corrosion rate of the metallic parts of the tower.

Let’s take a look at how a cooling tower works to understand why proper water treatment is important.

Most valley residents are familiar with an evaporative cooler. The water from the sump of the evaporative cooler is circulated by a pump over the pads in the evaporative cooler and the outside air is drawn through the pads. As the air passes through the wet pads, some of the water evaporates and cools the air. The cooler air is then circulated to the space where it is desired to cool. In the process of air cooling, the water that is recirculated through the pads is also cooled.

That is exactly the same principle that is used in the cooling tower, but on a much larger scale. Rejecting large amounts of heat from a building’s mechanical system requires a lot of water to evaporate. For example, a 100-ton water-cooled chiller operating at full capacity for 24 hours would require the evaporation of more than four thousand gallons of water. That brings us to the issue of make-up water characteristics. Make-up water is the water supply that replaces the water that evaporates in the cooling tower.

Here in the metropolitan area, water conditions vary widely because our water comes from several different wells, as well as from surface sources. Water quality can change rapidly in a short period of time because different sources are used for water supply. Each well has different water characteristics and often varies greatly from one side of the city to the other. Water that comes from surface sources, such as the Central Arizona Project, will generally have significantly different characteristics than well water. Surface water quality can also be influenced by climatic conditions such as drought or increased runoff.

As the water evaporates in the cooling tower, all non-volatile components remain in the cooling tower sump. In reality, there is much more to water than what we would call hardness (carbonates). There are also chlorides, suspended dust particles and biological microorganisms. As more and more water is added to replenish the evaporating water, these dissolved and suspended components in the sump water continue to accumulate. If steps are not taken to control the concentration of these components in the water, the solution will eventually increase in concentration to a point where “things” begin to come out of solution. This “material” ends up depositing on the surfaces with which the water comes into contact.

Have you ever seen an evaporative cooler where the pads haven’t been changed for a long time? I have seen them so encrusted with mineral deposits that the blower could no longer draw air through them. I have also seen them produce an abundance of biological growth in the sump water. That is exactly what will happen in a cooling tower without proper attention to hygiene and a proper water treatment strategy.

When solids that dissolve in water come out of solution, they deposit first on heat exchange surfaces and on surfaces where water evaporates. Heat exchangers, water-cooled condensers, drift eliminators, tube bundle in closed loop cooling towers, and fill in open cooling towers are some examples of surfaces where this occurs.

Deposition of mineral scale, dirt, and biofouling on any heat exchange surface can reduce heat transfer, reduce tower efficiency, and increase energy costs. While it is important to reduce the deposition of these with respect to the cooling tower, it is absolutely critical to avoid fouling or fouling in the water-cooled condenser. Scale and scale in the condenser significantly reduces heat transfer capacity and will seriously affect energy costs, performance, and reliability.

Generally, a two-part strategy is used to manage the mineral content of the sump water. The first is to keep the pH of the sump water within allowable limits and feed the correct type and amount of chemicals to help the water keep more dissolved solids in solution. The second part is to intentionally send some of the sump water down the drain (purge). The purge reduces the highly concentrated mineral content of the sump water by diluting it with fresh make-up water that is added to replace intentionally wasted water.

Biological growth can also become a significant health risk depending on the particular organism involved. Allowing sludge and biological growth to accumulate in a cooling tower sump can accelerate sump corrosion and shorten the life cycle of the cooling tower. It can also provide a haven for microbes to escape the effects of a biocide.

A proper water treatment strategy and good cooling tower hygiene are not a one-size-fits-all solution. The quality of the make-up water will require an adjustment of the type of chemicals and biocides used. It may also require changing the feed and purge rates for proper control. Additionally, these requirements can be altered by the characteristics of each individual cooling tower installation.

According to Alan Bateman of DB Water Technologies, there are several things that a good cooling tower water treatment program must address to be effective. They are total dissolved solids (TDS), hardness, pH, chlorides, suspended solids, an appropriate method for biological control and an adequate purge strategy. Every cooling tower manufacturer publishes recommendations for maintaining proper sump water conditions. Advice from a qualified water treatment professional is recommended to ensure that each item above is included in your overall strategy for cooling tower water treatment.