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 falling 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.
