Raw or filtered makeup water contains dissolved minerals and insoluble matter that pose a serious threat to efficient cooling. Microbiological organisms, dirt or silt, dissolved minerals and gases, if left untreated, can concentrate and cause serious reductions in heat transfer efficiency, increased maintenance problems, or even a total system failure.
As the cooling water evaporates, contaminants are allowed to concentrate in the system. Contaminants enter the system either through the makeup water or from the air via the cooling tower. If left untreated, high concentrations of impurities in open recirculating systems can lead to a number of serious problems, including:
1. Scale
2. Fouling
3. Microbiological growth
4. Corrosion
1) Scale:-
The most serious side effect of scale formation is reduced heat transfer efficiency. Loss of heat transfer efficiency can cause reduced production or higher fuel cost. If heat transfer falls below the critical level the entire system may need to be shut down and cleaned. Once scale becomes a serious threat to efficiency or continued operation, mechanical or chemical cleaning is necessary.
Scale in cooling water systems is mainly composed of inorganic mineral compounds such as calcium carbonate (which is most common), magnesium silicate, calcium phosphate and iron oxide.
These minerals are dissolved in the water, but if left to concentrate uncontrolled, they will precipitate. Scale occurs first in heat transfer areas but can form even on supply piping. Many factors affect the formation of scale, such as the mineral concentration in the cooling water. Water temperature, pH, availability of nucleation sites (the point of initial crystal formation) and the time allowed for scale formation to begin after nucleation occur.
Dissolved mineral salts are inversely temperature soluble. The higher the temperature, the lower their solubility. The most critical factors for scale formation are pH, scaling ion concentration and temperature. Precipitation is prevented under these conditions by the addition of a scale inhibitor.
2) Fouling:-
Waterborne contaminants enter cooling systems from both external and internal sources. Though filtered and clarified, makeup water may still hold particles of silt. Clay, sand and other substances. The cooling tower constantly scrubs dirt and dust from the air, adding more contaminants to the cooling water. Corrosion by-products, microbiological growth and process leaks all add to the waterborne fouling potential in a cooling system.
Waterborne fouling can be controlled by a combination of mechanical and chemical programs.
3) Microbiological Contamination
Cooling water systems are ideal spots for microscopic organisms to grow. "Bugs" thrive on water, energy and chemical nutrients that exist in various parts of most cooling water systems. Generally, a temperature range of 21-60 Degree C and a pH range of 6-9 provide the perfect environment for microbial growth. Bacteria, algae and fungi are the most common microbes that can cause serious damage to cooling water systems. Microbiological fouling can cause:
1. Energy losses
2. Reduced heat transfer efficiency
3. Increased corrosion and pitting
4. Loss of tower efficiency
5. Wood decay and loss of structural integrity of the cooling tower
4) Corrosion
Corrosion is the breakdown of metal in the presence of water, air and other metals. The process reflects the natural tendency of most manufactured process metals to recombine with oxygen and return to their natural (oxide) states. Corrosion is a particularly serious problem in industrial cooling water systems because it can reduce cooling efficiency, increase operating costs, destroy equipment and products and ultimately threaten plant shutdown.
Deposits in the system caused by silt, dirt, debris, scale and bacteria, along with various gases, solids and other matter dissolved in the water all serve to compound the problem. Even a slight change in the cooling water pH level can cause a rapid increase in corrosion. Open recirculating systems are particularly corrosive because of their oxygen-enriched environment.