FAQ on Cooling Tower Treatment

Physical debris which resides within cooling towers can cause operational problems by blocking strainers etc but the major cause for concern is that the debris creates areas where bacteria can replicate. These bacteria may include Legionella which can be a health hazard.

Cooling water treatment chemicals are used to protect systems from the four problems encountered by such systems namely, scale, corrosion, suspended solids and bio-fouling. These chemicals help improve system efficiency and maintain safe working conditions within the cooling system itself.

Most closed-loop water systems use a sodium nitrite based inhibitor and a biocide to prevent biological contamination. These inhibitors will protect both the ferrous and non-ferrous materials in your piping system. Vendors can supply the inhibitor with a colorant, which allows the treatment to be visually monitored.

Closed water systems are carefully designed to create precise and adaptable environmental conditions within buildings or process plant. The design necessitates the use of small bore control and regulating valves. If corrosion debris, installation debris or microbes are present these orifices may become blocked or restricted and the system will not operate to design parameters. Ultimately if systems are not cared for the pipe work and fittings may fail prematurely due to corrosion and other contributory factors. Water quality and water treatment are therefore very important.

Because evaporative towers scrub the air that passes through them, they are prone to collecting debris from the air. This debris can accumulate and cause flow restrictions; as well as aggravate corrosion. Also, after the water evaporates, dissolved minerals are left behind and accumulate rapidly. For these reasons, a properly engineered and administered water treatment program must be employed continuously with the cooling tower.

In an open tower cooling system, the water quality must be regularly monitored and treated to control the following conditions:

1)  Lime scale and other water mineral deposits.

2)  Corrosion of all types.

3)  Micro-biological growth, such as algae, bacteria, fungus and molds.

4)  Suspended solids accumulations, such as airborne dirt and debris that is washed into the cooling tower water.

Dry Coolers recommends consulting a local water treatment supplier that is familiar with your local water quality to monitor your treatment program.

There are many different types of bacteria many of which are harmless. Some bacteria can however cause problems if they get into the wrong place. In closed systems bacteria which cause biofilm are problematic since this biofilm can reduce the efficacy of flushing (leaving debris within pipe work), affect thermal transfer, create conditions for under deposit corrosion and in association with other debris, cause blockages or restrictions in control and regulating valves as well as strainers. Other undesirable bacteria are associated with microbial corrosion or loss of corrosion inhibitor (in the case of nitrite based inhibitors).

Closed-loop systems are less susceptible to scaling because pretreated water (RO/DM/Softened Water) is used in these systems. For this reason they are often left untreated for scales. However, closed-loop systems must have corrosion protection. An untreated closed-loop water system can cause serious corrosion in your equipment.

The fan on a cooling tower draws in thousands of cubic feet per minute of outside air that contains sand, dust, insects, and fibers from vegetation. These airborne contaminants mix with the process cooling water and eventually these suspended particles find their way into heat transfer surfaces. After a period of time these surfaces become fouled and insulated causing equipment to run hotter and replacement or repair is necessary.

By removing 98% of these suspended solids mechanically (PSF/SSF), fouling is greatly reduced and chemical water treatment and bleed from the system can be reduced significantly.

Full stream filtration protects the system from dirt deposits such as winds blowing over newly plowed fields, chunks of scale eroding from steel pipe or foreign deposits encountered by adding new piping to an existing system.

As the terms suggest, forced-draft cooling tower employ an air distribution system that forces air into the tower, while an induced draft tower operates by pulling air through the tower. Forced draft towers are characterized by fans positioned on the side of the cooling tower. Induced draft cooling towers utilize fans located on top of the cooling tower.

The crossflow or counterflow designation characterizes the orientation of the airflow within the heat transfer media (fill) in the tower with respect to the direction of water flow. In counterflow towers air travels vertically upwards through the fill and makes intimate contact with water droplets falling down through the fill media. Hence the air and water travel in opposite directions. In crossflow cooling towers the air passes through the fill media in a horizontal direction, thus crossing the downward water flow. Counterflow towers are inherently more efficient than crossflow towers.

There are two primary mechanisms by which water is cooled inside a cooling tower. Sensible heat transfer takes place when the incoming air temperature is lower than the temperature of the incoming water, thus heat from the water is absorbed by the colder air. If that were the only cooling that took place inside a cooling tower, the cold-water temperature would be limited by the ambient temperature. However, the bulk of the cooling that takes place inside the cooling tower (>80%) is driven by evaporation of the water itself. Evaporation requires energy (heat), so when water is evaporated within the fill media in a cooling tower, heat is removed with the water vapor and leaves in the exiting air stream from the top of the tower. The result is that the remaining water is cooled significantly, even to temperatures below the actual ambient temperature.

Simply stated the entering wet-bulb temperature (EWBT or WBT) is a measure of the level of humidity in the ambient air entering the cooling tower. In general, the higher the wet-bulb temperature, the more moisture that exists in the air. The wet-bulb temperature is a key parameter in the designing/sizing of a cooling tower, since it determines the degree to which more water can be evaporated. Cooling towers operating in high wet-bulb temperatures require a larger tower than those found in lower wet-bulb regions of the country.

Blowdown is the term given to water discharged from the cooling tower system to control the buildup of dissolved solids, such as salts or other impurities that occur in water as well as suspended solids that are “washed-out ” of the entering air. As a pot of tea gets concentrated if it continues to boil, so the water in a cooling tower becomes concentrated with salts and other impurities as water evaporates. In addition to blowdown, the predominant loss of water from a cooling tower system is through the planned and desirable evaporation that takes place. When water is evaporated to pure water vapor, it leaves behind many impurities which redissolve in circulating water or even deposit on cooling tower internals. Make-up water is introduced to the system to compensate for water losses, but the circulating flow continues to become increasingly concentrated with these impurities as more water evaporates. If the dissolved solids level becomes too high, accelerated scaling can occur inside the cooling tower and reduce the efficiency and or capacity of cooling in the tower. Blowdown of the circulating flow is implemented to keep this dissolved solids level below that saturation level.

In every cooling tower, there is a loss of water to the environment in the form of pure water, which results from the evaporative cooling process. This evaporated water leaves the tower in a pure vapor state, and thus presents no threat to the environment. Drift, however, is the undesirable loss of liquid water to the environment, via small unevaporated droplets that become entrained in the exhaust air stream of a cooling tower. These water droplets carry with them minerals, debris and microorganisms and water treatment chemicals from the circulating water, thus potentially impacting the environment. High drift losses are typically caused by fouled, inefficient or damaged drift eliminators, excessive exit velocities or imbalances in water chemistry.

Minimizing drift losses in a cooling tower reduces the risk of impacting the environment with potentially corrosive water treatment chemicals. Drift is usually responsible for damage to property near the cooling tower yard, i.e. buildings, cars, etc. Water use and chemical consumption are also reduced since more remains in the circulating flow, thus generating savings in operating costs. Last but not least, excessive drift losses pose serious health risks, not only because of the chemicals released, but because of microorganisms that can be transmitted through drift, most notably L. pneumophila, the bacterium that causes Legionnaires’ disease.

Certainly, the most effective means of reducing drift is to install high-efficiency drift eliminators in your tower. The drift eliminators are your last, but most critical line of defense for mitigating drift. Maintaining a balanced water chemistry is also very important. Certain chemicals used specifically for cooling water treatment can reduce the water’s surface tension, thus interfering with the normal agglomeration of water droplets that occurs in the drift eliminators. The result is that water droplets are smaller and more easily entrained in the exiting air stream. There is no substitute for a well maintained water treatment program. Finally, periodic inspection of spray distribution systems and drift eliminators is recommended. A clogged spray nozzle, fouled drift eliminator or even an improperly installed drift eliminator can cause excessive drift in a cooling tower.

Both Liquid Scale Dissolver and Imperial Scale Remover (Season Start) contain a “pH color indicator” that will guide you through the process. When the acid/water cleaning solution is at proper strength, it will be green-to-light blue in color. When it is neutralized and unable to dissolve scale, it will be purple. When it turns purple, another dose of acid equal to the first must be added. The system is usually clean when you can maintain the green color for 20-30 minutes.