Introduction to ION Generation
A cooling tower functions to cool a circulating volume of water. The tower acts as a heat
exchanger by driving ambient air through fidling water, causing some of the warmed water to evaporate (evaporation gives off heat, providing cooling), and then circulating cooler water back
through whatever equipment needs cooling (such as a chiller condenser). Typically, chemicals such as chlorine and chelating agents are added to cooling tower water to control biological growth (called
"biofilm") and inhibit mineral build-up (called "scale"). The control of biofilm and scale is essential in
maintaining cooling tower heat transfer efficiency. As the water volume in the tower is reduced through evaporation and drift, the concentration of these chemicals and their byproducts increases. Cooling
towers also pick up contaminants from the ambient air. To maintain chemical and contaminant concentrations at a prudent level, water is periodically removed from the system through a process
called "blowdown" or "bleed off". The blowdown water and the water lost through evaporation and
drift are replaced with fresh "make up" water (which will also contain minerals and other impurities).Blowdown water must subsequently be discharged to a local wastewater treatment facility or
discharged on site to the environment. Blowdown water from a cooling tower can be sent to a municipal drain, or it may require on site pretreatment prior to disposal to a drain. In some cases,
blowdown may be stored on site and then retrieved by a disposal service. If water and sewer services are purchased from a municipal or public utility, reducing blowdown and make-up water requirements
will trigger a series of resource and cost savings in those utilities. If the site operates its own water treatment and wastewater treatment facilities, reducing blowdown and make-up water requirements
will allow the facility to realize benefits. The blowdown water typically contains organic material, and the local wastewater treatment facility will charge extra sewage fees for accepting the water. These
costs can be significant in the overall costs of operating a cooling tower. Discharge of the blowdown water to the environment on site is coming under increasing EPA regulation due to the contaminants
typically found in blowdown water. Cooling tower water is continuously exposed to airborne organic materials, and the buildup of
bacteria, algae, fungi, and viruses present hazards to the tower system and to the health of humans encountering the water. For example, Legionnaire's Disease is caused by the bacterium Legionella,
pneumophila that frequently thrives in cooling tower environments. High levels of bacteria can also lead to an increased risk of microbially influenced corrosion. Sulfate-reducing and iron-metabolizing
bacteria can destroy iron piping in as little as nine months. Moreover, a biofilm coating on heat exchanger surfaces reduces heat transfer efficiency. Scale and biological deposits reduce the ability of
refrigerant condensers and industrial-process heat-exchangers to transfer heat. Another phenomenon requiring treatment in cooling towers is mineral buildup. Minerals such as
calcium and magnesium, which are common dissolved solids in water, are deposited by two different mechanisms, thermal and biological. As the water in a tower evaporates, dissolved solids concentrate
in the recirculating water. Biofilms also start to form on the walls and other components of the tower. In essence, the biofilm acts as an adherent for mineral micro-crystals. Over time, deposition of organic
and inorganic matter increases scale thickness. One operating concern of a cooling tower is the gradual corrosion of various parts of the tower. Much of the corrosion in cooling towers is associated with
bacteria that create conditions favoring micro-biologically induced corrosion. |
