Cooling Tower

As its name suggests, a cooling tower is used for cooling. Equipment used in industry for heat transfer is one of the important equipments, among which the cooling tower is effective and practical for the continuation and continuity of operations and other processes. Many operational units require cooling. This cooling is done by fluids such as water, oil, air, etc. In the cooling tower, water comes into contact with air and cools down. In other words, water that has been heated in various processes and its temperature has increased is cooled down again and returned to the units.

A cooling tower is made of different parts and sections. It also has different classifications and types that are selected according to its application in the industry, but the method of operation in all of them is more or less the same.

Formation of scale and corrosion in cooling towers

The water that enters the cooling tower from processes contains amounts of pollutants due to contact with various contaminants. Therefore, by continuously entering the tower for a long time, scale is produced on the parts inside the tower that are in direct contact with water. The formation of scale in the equipment causes an additional resistance, which in turn causes a sharp decrease in the heat transfer rate. On the other hand, these contaminants cause the corrosion phenomenon in the tower to intensify and cause maintenance and operating costs to increase. Therefore, the two factors of scale formation and corrosion in cooling towers are always the biggest problems facing these towers. Many solutions have been proposed and tested to prevent or stop these two phenomena.

Problems caused by scale formation in cooling towers

Scaling is one of the most important problems of cooling towers. The formation of scale in a cooling tower creates additional resistance to heat transfer and severely reduces the efficiency of the tower. On the other hand, scale causes corrosion in the parts and pipes in the tower.

What types of scale are formed in cooling towers?

1. Calcium carbonate
2. Calcium phosphate and zinc phosphate
3. Silica and magnesium silicate
4. Calcium sulfate

Condensation cycle in cooling towers

Cooling towers use a concept called the condensation cycle, which is the ratio of the electrical conductivity of the hot water entering the tower to the electrical conductivity of the make-up water. The higher the numerical value of the condensation cycle in the tower, the lower the operating costs and the better the environmental performance. When the tower operates at a high condensation cycle, the amount of scale in the tower increases. Therefore, anti-scale materials are used to prevent the formation of scale in the tower.

Use of anti-scale solutions in cooling towers

Anti-scale materials, as their name suggests, act as a barrier to scale and stop or reduce the scale process. Table (1) lists the anti-scale material for each scale in the tower.

Table (1): Types of scale and its anti-scale material

Function of scale inhibitors in cooling towers

The mechanism of action of these inhibitors in the tower is different and is generally divided into three categories:

1. Preventing the creation and formation of crystal nuclei
2. Preventing the growth and development of scale crystals
3. Preventing the aggregation and accumulation of scale and the formation of larger crystals

Effect of anti-scale solution on different types of scale

The effects of anti-scale materials vary according to the type of constituent material and the type of scale. This is related to the quality of the anti-scale material, some of which are mentioned in Table (2).

Table (2): Some important scales and their anti-scale effects

Factors affecting the rate of sediment growth in a cooling tower

Some factors in a cooling tower cause changes in the rate of sediment growth. Among the most important of these factors are three:

1. pH:The solubility of sediment decreases with decreasing pH. In other words, the amount of sediment in the tower generally increases with decreasing pH. Of course, there are also limitations.
2. Temperature:The solubility of sediment-forming compounds in water decreases with increasing temperature. In other words, the amount of sediment in the tower increases with increasing temperature.
3. Velocity:The amount of sediment decreases with increasing water velocity. Hence, sediment formation is greater in dead spaces in the tower where the water velocity is low, such as ponds.

Biological sediments:

The second category of cooling tower sediments is biological sediments, which result from the accumulation and settling of mud and silt and the growth of microorganisms and bacteria in the cooling tower. This type of sediment, in turn, reduces the rate of heat transfer and corrosion.

Types of Biological Sediment

Basically, biological sediment is divided into two groups: mud and silt. Table (3) describes this type of sediment.

Table (3): Types of Biological Sediment and Their Quality