Cooling Towers

Soft Water For Cooling Towers

The majority of cooling towers in use today draw upon raw, hard water as top-up or make-up water to replace the water lost through the cooling process. This make-up water can be sourced from municipal supplies, plant wells and treated municipal wastewater. These quality of the water gathered from these sources varies with a lesser or greater degree of concentration of dissolved and suspended solids present in the water.

One of the things to look out for when introducing water into the cooling tower is the presence of materials that can form scale.

Mineral Scale is a fundamental problem in cooling water systems. The primary cause of mineral scale that form on heat transfer surface found in cooling towers is calcium hardness, also referred to as lime scale. This lime scale creates a reduction in heat transfer efficiency due to the fact that calcium salts of carbonate form an insulating barrier on heat transfer surfaces.

In addition to creating in insulating barrier that reduces system efficiency, scale can also restricts water flow, exacerbate corrosion, and creates an environment where microbiological growth can flourish.

What Causes Scale Deposits?

The formation of scale deposits takes place when the dissolved minerals exceed the solubility of the cooling water. This process happens as a percentage of the water in the cooling tower evaporates. Because the evaporated water is pure, it leaves behind dissolved minerals, increasing the ratio of dissolved minerals in the remaining water. As makeup water is added the concentration levels of scale forming minerals increases in the recirculating water.

When left unchecked, the solubility of the dissolved minerals will eventually be exceeded. If this happens, the formation of an adherent scale deposit will occur. If scale does not develop then the mineral impurities can instead produce an unattached sludge that fouls the tower fill, basin or distribution piping.

There are several conditions that promote the formation of scale in cooling waters such as temperature, pH levels and the quality of the water added to the cooling tower for makeup. As the temperature of heat exchangers and other heat transfer equipment rises, the solubility of mineral salts decreases. Common scales like calcium carbonate are less soluble at higher temperatures, increased alkalinity and elevated pH. These factors directly affect both the quality and amount of scale deposit formation in heat exchangers, cooling towers and evaporative condensers.

One very good way to reduce scale in cooling tower water is to improve the condition of makeup and recirculating water. This is best achieved through the use of softened and treated makeup water. Improving the condition of makeup and recirculating water will reduce the growth of mineral scale deposits on heat transfer surfaces, reduce corrosion inside the cooling tower and conserve water, resulting in a reduced running costs of the cooling tower.

Types of scale that form in cooling towers

There are various types of scale classified by their chemical composition. The most common mineral scale found in cooling towers is a scale called calcium carbonate. Calcium carbonate thrives in cooling towers because it is the least soluble of the scale-forming minerals found in water supplies used for makeup. Other common scales such as calcium sulphate, silica and calcium phosphate also form either alone or in combination with calcium carbonate.

Calcium carbonate: Sometimes called lime scale due to its shared chemical composition with limestone. The chemical makeup of calcium carbonate is calcium hardness (Ca) and carbonate alkalinity (CO3). Calcium carbonate (CaCO3) scale forms when calcium associated with bicarbonate alkalinity (HCO3) present in the makeup water rises in temperature and reacts to thermally decompose the bicarbonate alkalinity (HCO3) into carbonate alkalinity (CO3). If the pH levels of the water are not maintained to the correct levels calcium carbonate will form.

Calcium sulphate: Also known as gypsum, can form in cooling water systems with a high level of sulphate (SO4). Water supplies in some areas are naturally high in sulphate. Some towers use sulfuric acid for pH control, and thus create the environment where gypsum or calcium sulphate can form. Calcium sulphate, like calcium carbonate, is less soluble at higher temperatures, but unlike calcium carbonate, becomes less soluble as pH levels drop.

Silica: An almost invisible glass-like coating that can form on metal surface. Unlike calcium carbonates which become more soluble at lower temperatures and at lower ph levels, Silica becomes more soluble  as pH levels and temperatures increase. 150 ppm (as SiO2) is a conservative maximum solubility limit for silica in most cooling water systems. However, due to the fact that silica solubility increases with pH, silica can tolerate pH levels as high as pH 9 and up to 250 ppm. As a result of silica being more soluble in higher temperatures it is more commonly found in the fill and distribution piping and not so much in the heat transfer areas of the system.

Calcium phosphate: Calcium phosphate forms a dense deposit similar to calcium carbonate. Formation of calcium phosphate is the result of a chemical reaction between calcium hardness (Ca) and orthophosphate (PO4). It is more likely to form at higher temperatures and pH and when the orthophosphate concentration exceeds 10 ppm. The phosphate component of calcium phosphate finds its way into the cooling tower water system as a phosphate-based chemical additive, because polyphosphate is commonly used to inhibit corrosion of materials used in the cooling tower. Another source of Phosphate found in cooling towers is municipal water supplies used for cooling tower makeup.

Legionella and Legionnaires’ Disease

Legionnaires’ disease is a respiratory disease attributed to a bacterium belonging to the genus Legionella. In some cases the bacteria can cause what is essentially a type of pneumonia or lung infection which is termed Legionnaire’s disease. In other instances the bacteria can cause a less serious but still troublesome infection called Pontiac Fever with symptoms analogous of a mild case of the flu. The most common form of Legionnaires’ disease associated with air-handling and water-cooling systems is Legionella pneumophila serogroup 1.

There is no recorded evidence of Legionellosis being passed from person to person. All documented evidence seems to suggest a person acquires Legionnaires disease through contact with environmental water habitats, particularly placings that intensify the growth of Legionella bacteria like those found in water-cooling systems including cooling-towers and evaporative condensers where water and air are used in combination.

Cooling Towers and Legionella

Cooling towers are a major component in air conditioning systems used in large buildings in cities and towns or anywhere evaporative cooling systems are used. These cooling towers are an energy, cost and space-effective solution for the removal of heat created in the evaporative air cooling process. Water is used in an evaporation process to pull the heat from the system and move it outside, away from the space being cooled.

Due to the location and open nature of these cooling towers the basin that captures circulating water is exposed to dust and other debris. When left unchecked bacteria can flourish, creating the ideal breeding ground for harmful Legionella.

Specific factors contributing to the growth of legionella in cooling towers include:

• Water temperatures ranging from 20°C to 50°C. Legionella’s optimal growth range is between 35°C to 46°C.
• Stagnant, non-circulating water
• pH levels between 5.0 and 8.5
• Various forms of sediment the feed the growth of microflora conducive to the spread of legionella
• The presence of certain microorganisms, including algae and Pseudomonas, which contain essential nutrients required for legionella growth.

How to control Legionella Growth In Cooling Towers

There are four basic steps, which, if you take them, can control or prevent the growth of legionella in cooling towers. If you don’t take these steps, the likelihood of legionella growth is high.

I’ll mention the 4 steps here and then expand on each in more detail. The 4 steps to controlling legionella are: Regular maintenance of the cooling tower, system wide filtration, treating the water, and ongoing monitoring.

1. Regular Cooling Tower Maintenance

Cooling towers are constantly exposes to particles or debris in the air in their immediate environment. As a result, it’s only natural that contaminates will blow into the cooling tower. Screens can sometimes be used to reduce debris from blow-in but it’s impossible to eliminate debris completely. This means your regular maintenance routine needs to include removing blow-in materials that settle in the tower and particularly in the basin.

2. System-Wide Filtration

Even with frequent cooling tower maintenance dust and debris will continue to gather in the cooling tower basin. As a result, the water requires continuous filtration to maintain a satisfactory level of cleanliness. An effective filtration system will:

• Arrest organic composites and biological organisms
• Remove food and materials that nurture the growth of bacterial
• Eradicate dust and debris drawn into the cooling tower

3. Treatment of Water

The water in the cooling tower and the makeup water entering the cooling tower require treatment. Each cooling tower has its own set of unique features including location, size, source of water, and so, and therefore the water treatment program will vary from one cooling tower to another. But in general, a water treatment program that stops or reduces the chance of legionella will include:

• Minimizing scale formation
• Eliminating biological contamination  
• Monitoring and controlling the water pH levels
• Reducing the instance of corrosion and fouling

4. Ongoing Monitoring

An ongoing program of system wide monitoring and plant commissioning will keep the system functioning and maintain water quality levels. An optimum monitoring program will utilise technology, people and systems to achieve the following:

• maintaining water quantity and quality by monitoring temperature, pH levels and solubility of dissolvable materials
• Maintaining historical records and seasonal patterns effecting water quality
• Alerting maintenance personnel to significant chances effecting water quality

Follow the steps above and you’ll achieve two things: 1. you’ll maximise the efficiency of your cooling tower and 2. You’ll minimise the chance of legionella growth 

Cooling Tower Water Problems and Solutions

Four common problems associated with water cooling towers.

1. Corrosion in Cooling Towers

Certain materials commonly used in cooling towers, such as mild steel are highly susceptible to corrosion. Corrosion is an electrochemical process that naturally takes place due to material in the cooling tower being in constant contact with the chemicals in the water. When corrosion takes place, the corroded materials lose thickness and strength which can lead to process fluid leaking through the corroded material into the cooling water or leakage of cooling water into the process fluids.

2. Scale in Cooling Towers

Scale forms in the cooling tower when the level of dissolved ions in the water surpasses the solubility of a given mineral.

Examples of relatively insoluble minerals found in cooling towers include calcium carbonate, calcium phosphate, and magnesium silicate. The constant circulation, increasing and lowering temperatures, rising and falling PH levels, and evaporation of water in the cooling tower creates the right environment for these insoluble minerals to precipitate out of the water to form scale deposits.

Four primary factors contribute to the production of scale formation in water:

1. Changes in temperature

2. Changes in Alkalinity or acidity (pH)

3. The Amount of scale-forming material present at any given time

4. The introduction of other dissolved materials, regardless of whether these materials are scale-forming or not

How These Factors Increase (or decrease) the Formation of Scaling?

As any of the above mentioned factors increase or decrease, scaling tendencies also increase or decrease. For example, for the most part, salts increase in solubility as temperatures rise. With that said, some salts, for example calcium carbonate, become less soluble as the temperature rises, and will therefore cause scaling deposits at higher temperatures while others cause scale deposits are lower temperatures.

A change in pH or alkalinity is another cause of scale formation. For example, Calcium carbonate decreases in solubility as the pH or alkalinity levels increase, and eventually precipitates on system surfaces. Other materials, such as silica (SiO2 ), are less soluble at lower alkalinity. So it’s a balancing act. Of course, the presence of insoluble materials doesn’t automatically produce scale. Scale occurs when the amount of scale-forming material dissolved in the water exceeds its saturation point. The higher the levels of scale-forming dissolved solids present in the water the greater the chance of scale formation.

3. Fouling in Cooling Towers

When solid materials other than scale, such as dirt, sand, silt, aluminium phosphates, corrosion products and microbial masses accumulate in the cooling tower they can cause the deterioration of plant equipment. When this happens it is termed fouling.

Why does fouling occur?

Most water contains suspended materials that don’t dissolve in water. Naturally, the lower the instance of these suspended materials the lower the chance of fouling and the higher the instance the higher the fouling potential.  If these suspended materials are allowed to remain in the water they can cause a significant fouling problem. The amount of fouling that occurs on system surfaces depends entirely on the amount of suspended material found in the system.

What Influence Does Microbial Growth Have on Fouling?

Because of the nature of microorganisms they can form deposits on any surface. Because microbial colonies act as a collection site for silt and dirt, they trap and cause deposit sites for a range of foulants.

How Temperature Accelerates Fouling in a Cooling Tower?

As the water temperature rises it increases the tendency toward fouling. This is due to the fact that heat transfer surfaces are hotter than the cooling water, thus they create an environment that accelerates fouling in a cooling tower.

Can Corrosion Increase Fouling?

Yes, because the insoluble corrosion products can migrate through the cooling tower and mix with other process contaminates and debris, or microbial masses to increase fouling.

The Effects of Process Contamination on Fouling?

When material from the process side of any heat exchange equipment leaks in to the cooling water it can cause serious fouling problems. Example of process side contamination include:

• Microbial growth brought about by leakages of process contaminates which provide nutrients for microorganisms

• Insoluble product deposits

• Process contaminates reacting with scale or corrosion inhibitors

4. Biological Contamination in Cooling Towers

If growth of microorganisms is left uncontrolled this too can lead to the formation of deposits that add to corrosion, scale and fouling.

Microbial Slimes

When masses of microscopic organisms and their waste products clump together we refer to them as Microbial Slime. These slime deposits are effective at trapping foulants present in the bulk water as they are usually sticky.

What Effect do Layers of Slime Have on the Cost of Operating a Cooling Tower?

The main effect Microbial Slime has on the cooling tower is the reduction in efficiency in heat transfer. This is due to the insulating nature of microbial slime which creates a higher level of insulation than other common deposits found in a cooling tower. The problem is made even worse because of the sticky nature of microbial slime and the deposits it can capture.

How Microorganisms get into the Cooling Water System

There are two main ways microorganisms can get into cooling water systems. The first is through the makeup water supplied to the system. If the water added to the system contains microorganisms they are introduced into the system in this way. The second way is via airborne microorganisms that can travel into the cooling tower through the air that is drawn into the tower.

What Factors Encourage Microbial Growth?

Typically, the main culprit contributing to microbial growth is the amount of microbial contamination introduced into a system as top up water is added. Next are:

1. The amount of nutrients present in the cooling tower such as hydrocarbons or other carbon sources that serve as a major food source for slime-forming organisms.
2. The atmosphere is another major contributor because the organisms common in cooling towers depend upon the availability of oxygen or carbon dioxide for growth.
3. The location of the cooling tower effects factors such as the amount of light and moisture available to encourage microbial growth.
4. The temperature inside and surrounding the cooling tower can contribute to microbial growth as the organisms that make up slime tend to flourish in temperatures ranging from 4 to 66°Celsius.