Cooling Towers And Legionella

Cooling Towers

A cooling tower is designed to reject heat. It works by extracting waste heat into the atmosphere by cooling a stream of water to a lower temperature. Simply, it works by means of evaporation. Cooling towers are used where it is necessary to provide much lower water temperatures than can be achieved by air cooled or dry heat rejection apparatus. If you consider occasions where you have seen something rapidly cooled by pouring water on it, you will see how much more quickly cooling is achieved over blowing with cool air. Take an overheated car radiator, it is cooled rapidly by pouring water over the surface and the heat generated by the radiator evaporates the water and cooling is achieved. The cooling effect of a “wet” surface is much greater than a dry one. Another example of this kind of cooling is if you have ever seen a blacksmith working with red hot steel. To cool the metal once it has been shaped, it is plunged into cold water for rapid cooling.

Cooling towers are commonly used to provide cool water for air conditioning systems, especially where land and water are in short supply. Small cooling towers that may be seen on small buildings handle a stream of water of only a few litres per minute whereas large cooling towers for industrial applications such as electricity generation handle thousands upon thousands of litres per minute.

Air enters the cooling tower from the sides and passes through the water stream. As the air passes through the water, a heat exchange takes place and some water evaporates. The evaporated water and heat exit the tower as a fine mist. The water that has been cooled in the process collects at the bottom of the cooling tower and is then pumped back into the cooled water is collected at the bottom of the tower and pumped back to be reused.

Legionnaires Disease

Many people are concerned by the risks of contracting Legionnaires Disease from air conditioning systems that use cooling towers. It has been the case that early outbreaks in recent times were identified as having been acquired from systems of this type.

In 1976, a respiratory disease was contracted by delegates at a convention in Philadelphia. The conference was held by the American Legion of Pennsylvania and the name “Legionnaires’ Disease” was invented. The bacteria causing the disease was isolated and named Legionella pneumophila.

There are around 35 different species of the Legionella bacterium and they are commonly found in wet environments. The bacteria survive for several months and can multiply if algae and other organic matter are present.

Legionnaires’ disease is treatable with antibiotics and is identified by laboratory tests. Work done by scientists on transmission of the disease has revealed that the presence of the Legionella bacterium is normal in soil and water and does not usually cause a disease outbreak. In order to produce disease, the Legionella microbe has to reach the lungs. Inhalation of small particles of soil or contaminated water seems to be the cause although new evidence shows that any fluid and particles going into the lungs by mistake such as during a coughing fit can be enough to allow microbes to enter the lungs. The disease is not transferred from person to person.

Good Engineering Practice and System Maintenance

Prevention of Legionnaire’s disease has been taken very seriously by the engineering profession and the introduction of good engineering and maintenance practices have reduced the threat of the disease. Recommendations include annual inspection and cleaning of evaporative condensers and cooling towers. Maintenance work ensures corroded parts are replaced and any algae and scale accumulation is removed. This process will control the germs and maintain efficiency of the units.

The cooling water is constantly treated with water treatments to ensure the quality of the circulating water. Chlorine and other disinfectants are used which have proven effective in tests.

Best practice for new installations ensure that air intakes are not built close by to cooling towers to avoid any contaminated airborne particles entering the ventilation systems. Air filters will be cleaned, inspected or replaced and tested for leaks. The siting of a cooling tower and the cleaning of hot water tanks are also key to keeping the system contaminant free.

Since the first outbreak of Legionnaire’s disease, every step has been taken to ensure the environment is hostile to the bacteria responsible. All air conditioning engineers will supply the appropriate maintenance tasks periodically to keep a cooling tower system safe.


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