Water Treatment in a Cooling Tower System
Evaporation of water is a highly endothermic reaction and can therefore be used in cooling systems. Air in motion evaporates a portion of the water, which results in cooling the remaining water. Makeup water is added to the basin to replace the water that has been lost to evaporation. A valve controls the water level in the basin. Evaporation removes only water and not dissolved or particulate materials and therefore cannot be done indefinitely without associated “bleed” or “blowdown.” The concentration of solids in the water increases as evaporation increases. During the evaporation process, the atmosphere also contributes a variety of particles, gases and microorganisms into the cooling water (Flozone, 2018).
The main objectives of cooling water treatment are to protect the equipment that is in contact with the cooling water and to optimize the efficiency of the cooling water system. In order to fulfil these objectives, corrosion and microbial growth on cooling equipment have to be controlled and deposition should be minimized. Environmental compliance, water conservation and safety requirements must be addressed by treatment programs (Guyer, 2014). Measures taken to control scale are:
• Controlling cycles of concentration (CoC)
• Chemical scale inhibitor treatment
• pH adjustment
• Softening of cooling water
Fouling is the deposition of materials which are normally suspended in water. Airborne material scrubbed from ambient air and process contamination by the cooling tower can result in deposition. Contamination can result from suspended materials or oil leakage into the makeup water. A polymer dispersant that is soluble in water (such as poly-crylate) is used in cooling water that is fouling from mud, silt and other corrosion products. To prevent foulants from being deposited on the heat exchange surface, they are dissolved (Meitz, 2000).
There are three types of microbial organisms that occur in cooling towers: algae, bacteria, and fungus. The growth of these organisms raise problems which inhibit the operation of the cooling tower. Bacterial diseases such as Legionnaire’s Disease are waterborne and are a major health and safety concern associated with the operation of cooling systems. They cause problems such as fouling, corrosion, and result in decreased operation efficiency. Deposits on metal surfaces by biological organisms act as a heat insulator. Biocides are used to control the spread and growth of these organisms. Oxidizing and non-oxidizing biocides are a form of microbial control that are specific for the type of bacteria (Keister, 2008).
Corrosion is the chemical deterioration of a metal (through electricity) which is in contact with cooling water. There are two methods used to protect the metals from corrosion in cooling tower systems. These include raising the pH of the cooling water and the use of chemical corrosion inhibitors. Another major concern is white rust. It is a serious problem primarily found in newer galvanized steel cooling towers and related components (Keister, 2008). White rust is accelerated by high pH and softened water. Often the chemicals used to control bacteria, scaling and biofilm, contribute to chemically induced corrosion.
Poor cooling water treatment and control results in destruction of equipment and facility damage which in turn raises the operation costs. Adequate resources should be directed towards water treatment to lower costs and increase operational reliability. The water treatment program must have compliance and safety considerations. A well designed program should achieve equipment protection at the lowest possible cost. Operational features and parameters of the cooling water system must be measured continuously to select the proper scale and deposit inhibitors. There parameters are such as minimum velocities, maximum temperature, usage patterns, and the occurrence of layup periods.
Works Cited
Flozone, 2018. The Basics of Cooling Tower Water Treatment. [Online]
Available at: https://www.flozone.com/wp-content/uploads/2014/07/basics_cooling_tower.pdf
[Accessed 6 February 2018].
Guyer, J. P., 2014. An Introduction to Cooling Tower Water Treatment, New York: Continuing Education and Development.
Keister, T., 2008. Cooling Water Management, Basic Principles and Technology. Pensylvannia: ProChem Tech International, Inc.
Meitz, A., 2000. Water Treatment for Cooling Towers, s.l.: Nalco Diversified Technologies.
