Legions of reasons to beware HVAC blind spots
October 1st 2009

Preventing bacteria, and particularly legionella, thriving in water systems is essential for health and safety Alfa Laval offers a solution

Legionnaires' Disease was first recorded in 1976. The bacteria that caused the disease were named after members of the American Legion attending an annual convention in Philadelphia who became its first victims.

Scientists have isolated and named no fewer than 46 species and 70 serogroups under the generic term Legionella. They know that Legionella bacteria occur in low numbers natural environments such as rivers, lakes and reservoirs: That they can survive temperatures as low as 6°C and as high as 50°C. And that they are killed by temperatures above 60°C.

The problem is the bacteria can thrive in man-made water systems. Enclosed, warm storage vessels, complex pipework with lots of blind spots and under-used water systems with stagnant water provide the ideal habitat, particularly if sludge, sediment and scale are present to provide food for the bacteria to feed on.

Typical sources include cooling towers, large central air conditioning systems, evaporative coolers, hot water systems, showers, whirlpool spas, architectural fountains, room-air humidifiers, ice making machines, misting and other equipment that draws upon a public water supply. For Health & Safety purposes, this can include not just the main vessels but all associated pipe-work, pumps, valves and other ancillaries including heat exchangers and chillers.

Treatment & solutions A number of chemical based systems and techniques have been developed to prevent Legionella, including biocide treatments such as chlorine dioxide and ionisation of the water using copper and silver ions.However, the one certain way to kill the bacteria is prolonged exposure to temperatures above 60°C.

Maintaining the right temperature in hot water systems There are two keys to an effective thermal treatment regime. The first is a temperature programme that heats the water to the right temperature to kill the bugs yet cools it to avoid scalding at the tap. The second is to keep the water in movement to prevent scaling and dead spots developing. In an energy-conscious age, such a thermal system should also involve low energy input or be designed to recycle the heat to minimise energy usage.

Calorifers and other conventional vessels can provide a breeding ground for Legionella. During periods of peak demand when hot water is drawn from the tank bottom, temperature stratification occurs within the water which, in turn, can encourage bacterial growth.

A Domestic Hot Water heating system based on compact heat exchangers, on the other hand, meets both criteria extremely well. The AquaProtect , for example, which provides hot water in hospitals, hotels, schools, leisure centres and similar buildings, disinfects the water by heating and maintaining it at 70°C for six minutes to ensure total sterilisation. After disinfection, the system cools the water to the optimum temperature for comfort and safety prior to use. At the heart of the system is a high efficiency heat exchanger, which uses the hot water used for disinfection to pre-heat incoming cold water, thus keeping energy losses to a minimum.

How it works Water enters the system at ambient temperature and is heated to 70°C by the heat exchanger. It then flows through a reaction tank for six minutes, during which time all Legionella bacteria are killed. From the reaction tank, the sterilised water flows into a storage tank in the normal manner.

When a tap is opened, water, at 70°C, passes through another compact heat exchanger where it pre-heats incoming cold water and, has its own temperature reduced in the. Consequently, all of the energy used for disinfection is recovered.

Water circulates continuously through the system to prevent the development of dead spots and avoid scaling and fouling. Once hot water has been drawn off, circulation is reversed and stored water returns to the heat exchanger for the disinfection process to start again. It enters the storage vessel at the top, thus avoiding stratification.

Prior to start-up the entire hot water system is disinfected by circulating high temperature water for a specified time and the same process is used throughout the life of the installation to keep it bacteria-free.

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