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:
- 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.
- The atmosphere is another major contributor because the organisms common in cooling towers depend upon the availability of oxygen or carbon dioxide for growth.
- The location of the cooling tower effects factors such as the amount of light and moisture available to encourage microbial growth.
- 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.
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