A cooling system exists to remove heat from a process. This process may be a physical, chemical, or mechanical process. Heat is picked up by a recirculated fluid from a heat exchanger. A heat exchanger, shown in Figure 1, is a device in which the recirculated water is separated from the hot process fluid by a thin heat- conducting surface. Heat moves from the hot process fluid through the exchanger surface raising the temperature of the cooling water. From the heat exchanger the hot cooling water goes to the top of the cooling tower, shown in Figure 2, it is sprayed over the fill and slowly falls to the sump. The fan at the top of the tower induces a draft, which causes water evaporation and cooling. From the sump cool water is pumped back to the heat exchanger.
Due to high temperatures and permanent scrubbing of nutrients, cooling towers shape a perfect environment in which microorganisms can thrive. The circumstances in cooling towers are ideal for the growth of Legionella bacteria. Controlling and preventing the growth of microorganisms in a cooling tower system is extremely important for health reasons and required to keep the system running under optimal operating conditions. The presence of the microorganisms may cause the system to breakdown, decrease the efficiency of the heat transfer and hence need to be replaced more rapidly.
For total cooling tower treatment, a variety of chemicals is available which are typically applied as a cocktail. These chemicals have specific characteristics that individually address respective chemical and microbiological needs in the water circuit of the cooling tower. Most commonly used chemicals in the cocktail are scale and corrosion inhibitors, biocides, algaecides, slimicides, bio-dispersants and pH regulators. The most ideal solution would be to employ a biocide that incorporates all these chemicals in one product for cooling water treatment. Typically oxidizing biocides such as chlorine and bromine or non-oxidizing compounds like glutaraldehydes have been used to treat cooling tower water. Unfortunately, these chemicals are highly reactive with other chemicals and organics present in the water. Once these reactions take place, these biocides lose much of their ability to eliminate the microorganisms present.
• Buildup of odor episodes and slimes caused by an increased microorganism population;
• Loss of heat transfer, due to the low thermal conductivity of the biofilm and inorganic deposition;
• Increased corrosion rates, due to electrochemical cell formation in the biofilm and blocking of contact of any corrosion inhibitor with the metal;
• Increased pumping energy required to circulate the cooling water in the presence of a biofilm which has a high friction factor;
• Lack of microbiological control the water circuit may impose unacceptable health risks, such as the formation of Legionella species, which in turn may lead to an outbreak of Legion- naires’ disease, a frequently fatal form of pneumonia.
Contrary to chlorine, chlorine dioxide is very non-reactive to other items found in the water and fully retains its biocidal efficacy. Likewise it is also a superior biocide for removing the biological film layers, “slime layers” found within the cooling tower system. It is the formation of theses biological film layers that lead to the most predominant challenges that all cooling towers face.
Corrosion takes place beneath the biofilm layer. Anaerobes are a type of bacteria that exists and rapidly grows in an environment without the presence of oxygen. Underneath the biofilm layer such an environment exists, and these anaerobes secrete acidic by-products which corrode the metal and cause pitting. Once the pitting process has begun it is very hard to stop because of the layer of protection that is provided by the biofilm layer.
Scaling and Deposits
The growth of the microbiological organisms can lead to a mineral scale formation with the towers. All scaling begins at a site where nucleation can occur. The biofilm later provides such a site, and its existence can lead to large scaling problems in a tower.