How to Protect Your Tower
Cooling water chemistry and treatments have changed during the last 10 to 15 years. Years ago, acid, chromate and chlorine were the primary treatments used in larger systems, and other biocides were used in smaller systems. Today, acid is not often used due to the availability of effective scale inhibitors. Like-wise, chromate has often times been replaced with phosphorous- and nitrogen-based corrosion inhibitors, which also can serve as nutrients for microbiological organisms, and bromine-based biocides or other biocides.
In the past, water treatments were designed to protect the heat exchangers and chillers, not the cooling tower. Experience has shown that to ensure a long-lasting cooling system, you must protect the tower as well. How you treat the tower depends on its materials of construction as well as its components.
WoodAlthough other local woods are used occasionally, redwood and Douglas fir are the most commonly used woods for structural members in cooling tower fabrication. Redwood has been used for many years, and while it is still used, that use is on the decline due to high costs and limited availability. Redwood contains a natural preservative that prevents wood decay, which explains its long use as a cooling tower structural component.
Douglas fir is a relatively recent redwood replacement. Although it has no natural resistance to wood decay, Douglas fir can be impregnated with a preservative. Even with the added cost of preservative treatment, Douglas fir has a lower cost than redwood and is used in most wood cooling towers today.
Treated Douglas fir generally utilizes copper chromate arsenate (CCA) or, more commonly, acid copper chromate (ACC) preservative treatments. Impregnation does not mean penetration throughout the entire wood member. Actually, the preservative goes to a maximum depth of 0.5" in the sides of the Douglas fir lumber; in the ends of timbers, it can be sucked to a depth of 10 to 12" or more.
How does this affect water treatment? Depending upon water chemistry and treatments, preservatives leach out of the wood. New wood loses much more initially, then less with time. This results in the presence of copper and chromate in the cooling water. In the case of CCA, arsenic also can be present in the cooling water. Chromate, arsenic and even copper can result in toxic cooling water blowdown.
Copper has been found in tower water at levels as high as 40 ppm on an all-wood cooling tower and up to 8 ppm in a wood-structured tower with plastic fill and mist eliminators. Similar levels of chromate and arsenic have been found. Copper is particularly bad and will create severe pitting corrosion when plated on mild or galvanized steel cooling system components such as piping and heat exchangers. Yes, preservative-treated wood can cause equipment corrosion. Your water treatment program can be adjusted to tie up the copper and minimize or prevent copper-caused corrosion. New treated wood should be water rinsed prior to installation to reduce initial high levels of copper.
MetalMetal cooling towers generally are fabricated from galvanized steel or stainless steel. Galvanized steel corrodes. Known as white rust, the corrosion can occur quite quickly and severely, requiring repair or replacement in just a few years. White rust results from water chemistry or improper water treatment; it also can result from improper startup and lack of protection of new galvanized steel cooling towers. It is important to select the proper water chemistry, including pH, hardness and water treatment chemicals such as scale inhibitors, to extend galvanized cooling tower life expectancy.
Stainless steel cooling towers are quite resistant to corrosion unless high levels of salt and corrosive microbiological organisms are allowed to accumulate on surfaces. Water chemistry treatment must keep the stainless steel surfaces clean to allow oxygen to repair and maintain the protective oxide film that makes stainless steel "stainless." Water chemistry should include good biological deposit control, maintenance and layup practices.
Plastic and FiberglassPlastics such as polyvinyl chloride (PVC), polyethylene and fiberglass-reinforced plastics are most resistant to almost all water qualities and chemical treatments. However, ozone use can increase the deterioration of fiberglass, polyethylene and even PVC. Also, water temperatures above 125oF (51.7oC) can soften and distort PVC and polyethylene. Certain microorganisms are known to attack plastics, so it is important to maintain clean plastic surfaces through the use of an effective microbiological control program along with periodic washings of plastic surfaces.
Cooling Tower Component DesignFill design has changed dramatically in recent years from wood or plastic splash-type to plastic film-type. Splash fill consists of a series of wood or plastic slats that are designed to break up water droplets for good evaporation. Slats are spaced 3 to 12" apart and can tolerate deposits with little or no impact on performance. Film fill is a densely packed plastic -- often PVC. It creates a large wetted surface that increases evaporation but is prone to deposit buildup and plugging. Plugging can be due to microbiological organisms, oils, scale, mud, corrosion products and some water treatment inhibitors. Film fill that is partially plugged can reduce cooling tower efficiency. Severely fouled film fill has been known to collapse into the cooling tower basin.
Water treatment must be designed to prevent film fill deposits as well as to protect all other water-contacted equipment within the entire cooling water system. This means a good biological and deposit control program must be effective through -- not to -- the film fill.
The newest cooling tower development is use of high efficiency drift or mist eliminators. They are similar to film fill: made of plastic (PVC), densely packed and prone to plugging, particularly by microbiological organisms, algae, slime and possibly legionella bacteria if inadequate water treatment is used. Again, water chemistry must be improved to effectively prevent deposit buildup on these eliminators. This also requires regular inspections and washings with detergents and biocides.
Cooling towers have changed, with greater tendency to plugging of film fill and eliminators. There also is the threat of white rust and preservative leaching, all of which increase the demand for improved water treatment programs and maintenance inspections.