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Legionella in Cooling Towers

Legionella is the bacteria that causes legionnaires disease, a serious form of pneumonia. Although legionnaires disease is not spread through person-to-person contact, it is transmitted through the inhalation of aerosolized water containing the legionella bacteria. Aerosolized water containing legionella can be spread for miles by cooling towers if they are not properly maintained.

Cooling towers work by evaporating water to remove heat. This heat is then lost to the atmosphere. Water circulating through the system will cool chillers, heat pumps, compressors, condensers, heat exchangers, and other equipment. Along with the heat, aerosolized water is also released to the atmosphere by the cooling towers. If legionella is present in the water, this bacteria will also escape the system and cause the rapid spread of legionnaires disease.

There are two types of cooling tower systems: open circuit cooling towers and closed circuit cooling towers. Open circuit cooling towers utilize a fairly simple design. These systems are less complicated than their closed system counterparts, however every part of their design must be compatible with oxygen. Open circuits will expose the process cooling water to the oxygen. This is a direct contrast to the isolated design of the closed circuit cooling towers.

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Open Circuit Cooling Tower System

Closed Circuit Cooling Tower System

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Closed circuit cooling towers utilize two circuits, one internal and one external. The flow of heat moves from the internal circuit to the external circuit through the use of a heat exchange coil. The purpose of this heat exchange coil is to prevent the fluid from touching the atmosphere. Instead of the heat exiting to the atmosphere directly from the process fluid like in the open circuit design, this heat exchange coil is in place to maintain a closed loop and keep the fluid uncontaminated from the atmosphere. The process fluid flows within this heat exchange coil and acts as the internal circuit. Spray water flowing over the coil makes up the external circuit. Heat is transferred from the process fluid in the internal circuit through the heat exchange coil to spray water which then partially evaporates into the atmosphere.

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One benefit of using a closed circuit cooling tower system would be the minimal maintenance due to the isolation from the environment. The only significant routine maintenance that is required for these systems would include the maintenance of the heat rejection equipment. However, no matter how reliable these systems are, it is still incredibly important to be watchful regarding the signs of legionella.

Some common maintenance problems in cooling towers would include the corrosion of internal structures, water quality problems, and mechanical damage. Sediment, biofilm, temperature, water age, and disinfectant residuals are all key factors that affect legionella growth in water systems. It is always important to look for the signs of bacteria growth or calcium deposits on any cooling tower.  A water management plan must be in place in order to keep track of any maintenance activities. This would include tracking the temperature of the water, any changes in pH, as well as routinely testing for the presence of legionella. In the circumstances that legionella is present in a cooling tower, you must consult ASHRAE guidelines for remedial response. 


Cooling towers work by spreading aerosolized water into the atmosphere. This can be dangerous when legionella bacteria is present in the water of the cooling tower system. While closed circuit cooling systems provide a more isolated system, they are still at risk for legionella and must be adequately maintained using proper water treatment routines and equipment management plans. Key factors affecting legionella growth are sediment, biofilm, temperature, water age, and disinfectant residual. 

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