The second law of thermodynamics says that to produce work in a cyclic process, waste heat have to be rejected to the external surroundings. One equipment normally used to release heat back to the environment is through a closed loop cooling water system using a tower and a heat exchanger.
Waterside fouling and/or scaling can seriously affect the heat transfer efficiency and increase operating costs. Scaling can result when the dissolved solids reached saturation in the water. In cooling tower, water is evaporated to cool its temperature when latent heat is absorbed by the water when it changes state. The solids are left behind during this process. With the addition of make-up water to replenish the amount evaporated, the amount of dissolved solids increases. Blowdown helps control excess dissolved solid buildup. However, blowdown by itself is not the solution to scaling problem as several compounds in solution will have exceeded saturation level and precipitate out of solution on the heat exchange surfaces.
In the absence of any treatment, the primary scale forming compound would be calcium carbonate. Unlike most of the solutes, the solubility of calcium carbonate in water is inversely proportional with the temperature. Thus they tend to form on the heat exchanger surfaces transferring heat to the cooling water system. To minimize fouling, the water need to be treated chemically. The chemical can either be dosed directly on the circulating water (as scale and corrosion inhibitor) or treatment of the make-up (i.e. softener, etc.)
However, the more common cause of reduced heat transfer is microbiological fouling. Our cooling water system provide an ideal environment for microbes in terms of temperature, water availability and source of organic materials. Bacteria can easily grow on the surfaces of heat exchangers and cooling tower fill. Fungi thrives on the cooling tower wood while algae would grow on the wet cooling tower components exposed to sunlight.
The problem with microbes (bacteria in particular) starts when they secrete a protective polysaccharide film whenever they settle on a surface. This film will then collect suspended solid particulates and grow ever thicker, increasing the resistance to heat transfer. We then have a varied community of micro-organisms with aerobic microbes colonizing the surface and anaerobic microbes flourishing underneath. These in turn secretes waste products comprising of acid and other chemicals that directly attack the metal. The deposit also act as concentration cells where the lack of oxygen below the deposit causes it to develop an electrical potential that is anodic to the exposed areas resulting to pitting corrosion and failure well before the expected lifetime of the material.
Worse some bacterial species (i.e. Legionella) naturally living in the water can cause serious respiratory disease when dispersed through the cooling tower drift and inhaled in the lungs. Fungi will attack the cooling tower wood in an irreversible manner and will eventually cause structural failure. Algae will foul cooling tower spray decks and may impair its performance and possibly cause failure.
Microbiological control and treatment is accomplished by feeding biocides into the water and kill any organisms before they can settle on the tube walls, cooling tower fills and surfaces. The most popular treatment is by using an oxidizing biocide like chlorine and/or bromide. Commercial method currently use a supplemental feed of non-oxidizing biocides such as DBNPA (2,2,-dibromo-3-nitrilopropionamide), glutaraldehyde, isothiozoline, quaternary amines, etc.
Hydrogen peroxide is also an alternative but not very popular due to its short lifespan and tendency to escape from solution. Growing in popularity is the use of ozone through on-site generation.
Careful evaluation of the microbial species in the cooling water is necessary to determine the most effective chemical to use. Other factors to consider in determining the right treatment to use are costs, government regulations and safety considerations.
Waterside fouling and/or scaling can seriously affect the heat transfer efficiency and increase operating costs. Scaling can result when the dissolved solids reached saturation in the water. In cooling tower, water is evaporated to cool its temperature when latent heat is absorbed by the water when it changes state. The solids are left behind during this process. With the addition of make-up water to replenish the amount evaporated, the amount of dissolved solids increases. Blowdown helps control excess dissolved solid buildup. However, blowdown by itself is not the solution to scaling problem as several compounds in solution will have exceeded saturation level and precipitate out of solution on the heat exchange surfaces.
In the absence of any treatment, the primary scale forming compound would be calcium carbonate. Unlike most of the solutes, the solubility of calcium carbonate in water is inversely proportional with the temperature. Thus they tend to form on the heat exchanger surfaces transferring heat to the cooling water system. To minimize fouling, the water need to be treated chemically. The chemical can either be dosed directly on the circulating water (as scale and corrosion inhibitor) or treatment of the make-up (i.e. softener, etc.)
However, the more common cause of reduced heat transfer is microbiological fouling. Our cooling water system provide an ideal environment for microbes in terms of temperature, water availability and source of organic materials. Bacteria can easily grow on the surfaces of heat exchangers and cooling tower fill. Fungi thrives on the cooling tower wood while algae would grow on the wet cooling tower components exposed to sunlight.
The problem with microbes (bacteria in particular) starts when they secrete a protective polysaccharide film whenever they settle on a surface. This film will then collect suspended solid particulates and grow ever thicker, increasing the resistance to heat transfer. We then have a varied community of micro-organisms with aerobic microbes colonizing the surface and anaerobic microbes flourishing underneath. These in turn secretes waste products comprising of acid and other chemicals that directly attack the metal. The deposit also act as concentration cells where the lack of oxygen below the deposit causes it to develop an electrical potential that is anodic to the exposed areas resulting to pitting corrosion and failure well before the expected lifetime of the material.
Worse some bacterial species (i.e. Legionella) naturally living in the water can cause serious respiratory disease when dispersed through the cooling tower drift and inhaled in the lungs. Fungi will attack the cooling tower wood in an irreversible manner and will eventually cause structural failure. Algae will foul cooling tower spray decks and may impair its performance and possibly cause failure.
Microbiological control and treatment is accomplished by feeding biocides into the water and kill any organisms before they can settle on the tube walls, cooling tower fills and surfaces. The most popular treatment is by using an oxidizing biocide like chlorine and/or bromide. Commercial method currently use a supplemental feed of non-oxidizing biocides such as DBNPA (2,2,-dibromo-3-nitrilopropionamide), glutaraldehyde, isothiozoline, quaternary amines, etc.
Hydrogen peroxide is also an alternative but not very popular due to its short lifespan and tendency to escape from solution. Growing in popularity is the use of ozone through on-site generation.
Careful evaluation of the microbial species in the cooling water is necessary to determine the most effective chemical to use. Other factors to consider in determining the right treatment to use are costs, government regulations and safety considerations.
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