The Legionella bacteria thrive in any warm, non-sterile water. The danger arises when contaminated water becomes aerosolised to a size between 1 and 5 microns, which the nasal passage filters cannot catch, and it is inhaled into the lowest part of the lungs.
It has been shown repeatedly that the Legionella bacteria is present in many water supplies including potable water, yet its presence in water is not known to create a problem. The bacteria must be transmitted as an aerosol in sufficient bacterial densities to be potentially infectious.
The minimum droplet size that the bacteria require for transmission as an aerosol is 1µm. The main mechanisms by which aerosols are created are water streams breaking or striking a surface, or bubbles bursting at a water surface. Air movement can spread these aerosols hundreds of metres from their source. The temperature of the water plays a crucial role: the bacteria develop between 25 and 46ºC and the optimum temperature for growth lies between 30 and 37º C. This explains why legionella is seldom found in cold-water installations and public waterworks. The bacteria are killed in temperatures above 50ºC.
Evaporative cooling does not cause legionnaires disease and varies substantially in operation and design from cooling towers and evaporative condensers. In natural evaporation, hot air is blown over water causing an adiabatic change of state. The water is vaporised into a gaseous state and the resulting air has increased levels of relative humidity. The total energy of the process is the same but the practical working of this natural phenomenon is a reduction in the sensible heat of the air. Thus through this evaporative process air is cooled so that it becomes a refreshing breeze.
Evaporative coolers contain large filter pads, which are kept wet by a water distribution system. Hot air is drawn through the pads by a powerful fan and is cooled by the evaporative process. In this way, the air is cooled without introducing any water droplets through aerosols, to provide a constant flow of naturally, cooled, fresh air, without any air being recirculated.
The water used in evaporative coolers is usually from potable supplies in addition to wells, rivers, lakes and reservoirs. The air supplied to the cooler may contain a variety of contaminants including dust, dirt and bacteria, soluble gases and nutrients. The water scrubs out these impurities so they are not passed into the building. Only pure water is evaporated in the adiabatic process so these impurities do not get a chance to be entrained into the air stream. As the water in the evaporative cooler is at or about the wet bulb temperature, typical UK temperatures in the summertime would mean that the growth of any bacteria present would be severely inhibited. Maintained regularly, drained, cleaned and dried out the possibility of Legionnaires' disease originating from evaporative coolers is removed.
As with all microbiological organisms, Legionella bacteria require nutrients and optimum water quality to proliferate. Evaporative Cooling reduces the nutrients required for the development of legionella with a cleaning cycle. This cycle is pre-programmed to automatically activate on a regular basis when the system is in use, draining stale water from the reservoir and replacing it with fresh water to maintain optimum water quality. In addition, when the evaporative cooler is not in use the drain valve automatically empties the water reservoir, leaving it completely dry.
There have never been any cases of Legionnaires' disease attributed to evaporative coolers, there can be no room however for complacency. With the much greater longevity of the latest generation of evaporative coolers plus the year-round use of evaporative cooling for industrial and commercial applications, preventative measures must be addressed. The following simple maintenance procedures will improve cooler performance, prevent musty odours and prolong the life of the equipment. Establish a balance between the amount of water bled off or dumped to drain and that replaced by fresh mains supply.
A regular autumn service to clean and drain the sump prior to the winter shut down, and a corresponding service in the spring is necessary. However some units have an automatic drain facility, but others require the sump to be emptied and allowed to drain for 24-hours. When the system is shut down allow the fan to operate long enough to completely dry the unit out. Maintain system cleanliness and develop a maintenance checklist.
Correctly installed evaporative air conditioning systems are completely safe from the risk of legionella, economic to fit, cost effective to run, and in addition to consuming less energy than conventional air conditioning systems enable doors and windows to be left open. Because a constant natural flow of 100% fresh air is drawn into the building and then expelled, odours, germs and airborne contaminants are removed and not recirculated around the building. |