A cooling tower is a system that uses the latent heat of vaporization to transfer heat to the atmosphere and away from another process. The principle is similar to how our body dissipates heat: sweat glands to spread water over our skin and our body cools as the water evaporates. A basic cooling tower sprays hot water at perforated metal plates that break the water up into a fine mist of tiny droplets. A fan is used to move cool air across these tiny water droplets, resulting in evaporation and a decrease in temperature at the metal plates.
The cooled water is then pumped back into the cooling system to the heat exchanger to be reused.
Water treatment is a necessary requirement of evaporative systems to prevent scaling, corrosion, and biological fouling. Any of these can cause the efficiency of the cooling tower system to decline.
Scale formation has its root in the evaporation of water; as water evaporates and leaves the system as a vapor, solids that were present are left behind. Replacement (or make-up) water introduces more solids. Without treatment, the system would reach a point where the concentration of solids in the water would exceed the solubility point, and the solids would precipitate out as scale. Lime build-up is a common example of this in homes.
Corrosion is the antithesis to scaling. Generally speaking, scaling helps prevent corrosion. Corrosion helps reduce the effects of scaling. Corrosion must be treated in conjunction with scaling.
All water contains living organisms in some form. Recirculated water and wetted surfaces provide an excellent place of growth for bacteria and algae which can foul heat exchanger surfaces and, in some cases, attack system components. The degree in which biological agents are present depend on local conditions. For example, make-up water reclaimed from sewage treatment plants can be rich in nutrients. Cooling towers located near bakeries tend to have a higher concentration of biological material due to the molds and yeast in the air.
Chemical treatments, such as chlorine, iodine, and bromine, are used to address biological issues separately from scale and corrosion. Ozone may be used to control both biological agents and scale at the same time.
System Controls - Bleed vs. Make-up
Water treatment controls are set up to maintain chemical levels to prevent scaling and corrosion and to control biological fouling. Systems use either a 'continuous bleed', where a portion of the recirculated water is constantly wasted; or a make-up system to introduce fresh water to replace that which has evaporated and bled (functions like a toilet tank float and valve). In either case, new water mixes with system water and dilutes the solids concentration (controlling scaling and corrosion) and reducing the presence of bacteria and algae (controlling biological fouling).
Continuous bleed is generally less expensive (initially) than make-up, but it is often illegal. The bleed system is normally set to drain water at a rate necessary to maintain system control at maximum usage. Since cooling towers are typically not used at maximum capacity, this design tends to waste a lot of water.
A conductivity monitor is used to signal when water needs to be cycled in a make-up system. The conductivity of the water is directly proportionate to the solids concentration. First, the device is used to measure conductivity of the water. Then, a make-up cycle is initiated when the conductivity reaches a pre-defined limit. The duration of the make-up cycle is also defined by the technicians. Liquid chemicals (Scale and corrosion inhibitors) are typically injected into the system as it bleeds. Hence the term: feed and bleed.