How Wet Cooling Towers Work:

Open recirculation cooling systems feature “wet towers,” where cooling water comes in direct contact with upward airflow. Water from the heat exchanger is pumped evenly over the top deck of the cooling tower.  It cascades down and is broken into tiny droplets as it passes through a series of splash plates, called cooling tower fill.  This fill can be corrugated plastic sheets, wooden slats, or other devices that increase surface area, thereby enhancing evaporation.  As the water droplets bounce off the cooling tower fill, the hottest molecules break away from the water and are carried up and out of the tower as “drift”.  The remaining cooled water collects in a tank at the bottom of the tower, called the basin.  This cooled water can now be pumped back into the heat exchanger.

COOLING TOWER SCALING AND CORROSION

Advantages of Recirculating Cooling

A recirculating cooling tower design is preferred for promoting large decreases in coolant temperature and where feed water is an expensive resource.

Risks of Open Recirculating Cooling

One drawback to wet cooling towers is that repetitive evaporation concentrates dissolved and suspended solids. These solids eventually reach a saturation point and begin to precipitate and deposit, leading to scaling or corrosion.

Scaling

When insoluble materials, like calcium carbonate, calcium phosphate, magnesium salts and silica, precipitate out of cooling water, they form solid deposits called scale. A layer of scale as thin as 1/64 inch can reduce heat exchanger efficiency by 15%. Reduction in heat exchanger efficiency leads to wasted energy and increased operating costs. Scaling may be promoted by variations in pH, temperature, alkalinity, and concentration of scale-forming constituents in water.

Corrosion

Most metals used in cooling tower systems are very susceptible to corrosion. Surfaces must be kept clean to prevent under-deposit corrosion, which can lead to system failures. Corrosion can be uniformly distributed throughout the system, or it can be localized.

COOLING TOWER WATER TREATMENT

Water may be treated with chemical inhibitors to extend the saturation point of insoluble materials and prevent scaling and corrosion. Water treatment with cooling tower chemicals is preferred to supplying fresh water due to environmental and cost concerns.

Cooling Tower Blowdown

Cooling tower chemicals are useful to increase saturation point in cooling water. However, there is a limit to the solids concentration or cycle of concentration that can managed by a chemical inhibitor.  For this reason, dissolved solids in cooling water are reduced by removing or “blowing down” a percentage of water from the system and replacing it with fresh water.

Cooling Tower Blowdown Control

Measuring and controlling conductivity allows accurate calculation of blowdown quantities and timing. Measuring and maintaining conductivity is critical, because poor blowdown control may lead to:

• Scaling and corrosion (when conductivity is too high) or
• Wasted resources, including water and cooling tower chemicals (when the system could tolerate higher conductivity feed water)

ADDITIONAL CONSIDERATIONS

Risks of Closed Recirculation Cooling

Unlike wet cooling towers, dry cooling towers have a relatively low temperature profile and do not rely on evaporation. This reduces the risk of scaling in these systems. However, the risk of corrosion is still significant in dry cooling towers, due to the high solubility of dissolved oxygen at low temperatures. Additionally, low temperatures may promote microbial fouling due to the ability of bacterial populations to survive in the cooling tower environment.

Microbiological Deposits

Makeup water and wind can carry microorganisms into a cooling water system. Under certain conditions, microbial communities multiply in the system, causing problems. Microbiological stress can affect any component in a cooling system. In fact, microbiological stress is often the most significant stress on cooling systems, as it can promote other types of stresses. For example:

• Corrosion occurs under the bacterial slime layer
• Inorganic molecules become trapped in the slime layer, leading to scaling

Preventing Microbiological Deposits in Cooling Towers

An effective microbial control program focuses on the specific microbial communities damaging a given cooling system. Appropriate biocides may be selected, perhaps in conjunction with dispersants to penetrate and remove deposits. Chlorine oxidizing biocide is commonly used in biocontrol along with some non-oxidizing biocides. Bromine or chlorine dioxide is used instead of chlorine for heavy stress systems. Microbiological treatments may require control of pH and other water quality parameters to ensure maximum effectiveness.

Other Types of Cooling Tower Fouling

Solid material other than scale, like airborne debris, corrosion products, process in-leakage and suspended solids, accumulates in the system and contributes to loss in efficiency and equipment deterioration. A variety of factors influence the level of fouling. You may have to understand the type of fouling and the source in order to determine the best way to control it. This may involve recommendations for mechanical improvements, clarification of the makeup water or the addition of dispersants specific to the fouling problem. Side stream filtration can also be a solution for severe fouling stress systems.