Removing the Legionella threat: Peter Robinson looks at how evaporative cooling can help remove the problems of Legionella.
Wet cooling towers are often implicated in outbreaks of Legionnaires disease. They are designed to remove unwanted heat from air-conditioning plant or from process cooling in industry. Many central air conditioning systems cool the air through chilled water coils for which the water is chilled by refrigeration equipment. The building's heat is removed at the refrigeration condenser, which may be air-cooled or, more commonly, water-cooled through a cooling tower. The water used for cooling the condenser is pumped to the top of the cooling tower and gravitates via a filler pack into the reservoir below. Large amounts of outside air are circulated through the filler pack, which evaporatively cools the water. The process is particularly vulnerable to being contaminated with Legionella bacteria because of the favourable water temperatures involved. The high air velocities easily entrain fine water droplets in the fast vertically rising discharge air that are ideal for carrying the Legionella bacteria. These droplets disperse into the cooling tower vicinity where casual passers-by can inhale them to their detriment if their immune systems are low.
In July 1976, 182 delegates to an American Legion convention in Philadelphia were struck down with a mysterious acute respiratory illness. The search began for what was, six months later found to be the bacterial species Legionella Pneumophila a particularly small bacterium that thrives in warm, non-sterile water and is also found in damp soils. Legionella has probably been around for hundreds of years but has only recently been isolated because of the new environments, like cooling towers, which support its growth.
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 1[micro]m. 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 20 and 46[degrees]C and the optimum temperature for growth lies between 30 and 37[degrees]C. This explains why legionella is seldom found in cold-water installations and public waterworks. The bacteria are killed in temperatures above 50[degrees]C and are dormant at temperatures below 20[degrees]C.
Direct Evaporative Cooling
Evaporative cooling uses the adiabatic principle to take hot, ambient air, and cool it by up to 150C by passing it over wetted cooling pads. Adiabatic air-cooling is a natural process used Worldwide--it is only now that equipment based on this principle has become available to reflect both the business needs and weather patterns in the UK. This technology avoids the use of environmentally damaging refrigerants.
Direct 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 moisture. The practical outcome of this natural phenomenon is a reduction in the sensible heat of the air and thus its temperature, thereby producing a refreshing cool breeze. Evaporative coolers contain large cooling pads that are kept wet by a pump and water distribution system. Hot air is drawn through the pads by a powerful fan and is cooled by the evaporative process, providing a constant flow of naturally cooled, fresh air, without any air being recirculated. No aerosols are produced in such equipment.
The water used in evaporative coolers is usually potable water supplied from central reticulation systems. This water normally contains dissolved minerals and suspended solids. The air supplied to the cooler may contain a variety of contaminants including dust, dirt and bacteria, soluble gases and nutrients. Many of these impurities are scrubbed out within the air cooler and do not pass through the cooler into the cooled zones. As the temperature of the circulating water in the direct evaporative cooler is quite low (at or about the wet bulb temperature), typical UK temperatures in the summertime mean that the growth of any Legionella bacteria present would be greatly suppressed. 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. Modern direct evaporative air coolers keep systems clean with a special valve, which reduces the build-up of dust and other airborne impurities that may get into the system during warmer weather. The systems are pre-programmed to automatically activate on a regular basis when the system is in use, draining stale, contaminated 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 clean and dry.
How Evaporative Coolers work
In an evaporative cooler a pump circulates water from a reservoir (tank) to the top of absorbent cooling pads. The pads become saturated with water as the water gravitates back into the reservoir. Powerful centrifugal fans draw warm air from outside through the wetted pads. As the air passes through the pads it is cooled by evaporation and the cooled air is then circulated through the building.
Evaporative air-cooling systems do not re-circulate air as this will result in hot and humid conditions. Air from an evaporative cooler is used once only; it is forced from the cooler, into the area to be cooled and then out of the building. Constant cool air movement pushes heat out along with stale air, smoke, odours and pollution. Therefore sufficient natural or mechanical venting facilities must be provided to ensure the air can escape to the outside.
Evaporative air-cooling is particular suitable for commercial and industrial applications, where compressor-type air conditioning is prohibitively expensive. Quick and easy to install, coolers are fitted to the exterior of the building, either on the roof, the sidewalls or at ground level. A system of ductwork and air diffusers is then used to channel the cool air to where it is needed. Rigid pre-fabricated metal ducts are the most common, but for some installation flexible ducts can be used. For spot cooling in factories and warehouses, vertical ducts with plenum boxes are also common.
The Advantages of Evaporative Coolers
Drawing fresh air from outdoors, evaporative coolers offer a great health advantage over refrigerated air conditioning, which re-circulates the same stale dry air over and over.
* Outside air often contains dust and pollen. Evaporative coolers filter the incoming air, removing most airborne dust and pollen particles. The wet cooling pads trap most of these particles, which are washed down to the unit reservoir by the circulating water.
* Evaporative coolers do not dry out the air like refrigerated air conditioning and do not require an airtight building structure.
* Generally installing evaporative cooling costs up to 50% less than refrigerated systems and can save up to 80% of energy costs.
* Evaporative cooling does not use environmentally harmful CFC gases
* Door and windows can be kept open
It's not just during the summer that evaporative cooling is needed. High temperatures in factories, warehouses and office complexes effect workers and therefore productivity throughout the year. Evaporative cooling provides businesses with a cost effective solution that helps increase production, decrease downtime and improve workers' morale.
* For further information please visit: www.breezair-europe.com
* Peter Robinson is European technical manager at Seeley International (Europe) Ltd.
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|Title Annotation:||INDUSTRIAL COOLING|
|Publication:||Plant & Works Engineering|
|Date:||Mar 1, 2008|
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