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Legionnaires’ disease & evaporative cooling

The recent outbreak of Legionnaires’ disease in Edinburgh must be a cause for concern for the managers of any data centres which use evaporative cooling. Legionella, the bacteria which cause Legionnaires’ disease, occur naturally and so can be present in water used for evaporative cooling systems. Also, the legionella bacteria thrive at temperatures between 25°C and 40°C – just the range of temperatures that occur in evaporative cooling systems in summer. So is the concern justified?

The legionella bacteria, along with any other impurities in the water, are left behind when the water evaporates so the evaporative cooling process is not, in itself, a means by which the bacteria are transmitted. To be infected, a person has to inhale contaminated droplets of water and so the first question is: what type of evaporative cooling system do I have?

For water to evaporate its surface must be in contact with the air and so a large surface area is desirable. This can be achieved in one of three ways:

1. Spray water into the air in millions of small droplets
2. Cascade water over a large flat surface
3. Soak a suitable porous material in water and pass the air through it

Of these, the first has the greatest potential for distributing the bacteria because the tiny droplets can be carried many miles. Thus these systems therefore rely heavily on water treatment to kill any bacteria.

The second and third systems should not, in theory at least, pass anything other than pure water vapour into the air and so should not be a problem. However “carry over” of water droplets can sometimes occur, particularly if air velocities are high. This can be prevented by the use of “eliminators”.

But how do I know which one I’ve got?

Traditional Cooling Towers

Legionnaires’ disease was first identified in the 1970’s and was so named because the outbreak occurred at a convention of the American Legion. The cause was attributed to a nearby air conditioning system and it was quickly realised that cooling towers in particular (and to a lesser extent some humidification systems) were to blame. The result was that cooling towers became very unpopular and for a time few new ones were installed and many existing ones were taken out. In recent years their efficiency has brought them back, and with good water treatment and “drift eliminators” to remove droplets they should not be a problem. If you operate a large data centre then you will probably have a chilled water system and the heat rejection may well take place via a cooling tower.

Adiabatic Coolers

Although cooling towers use adiabatic cooling, the name has is more often used for the type of evaporative cooler that either directly cools the air going into the data centre (direct fresh air free cooling) or cools the data centre via an air-to-air or water-to-air heat exchanger (indirect free cooling). These types of cooler have been around for centuries but are currently enjoying a new-found popularity with data centre designers. They can employ any one of the three methods mentioned above, i.e. water spray, cascade or wet media. If your data centre has such a system and you don’t already know which method it uses you will need to check your O & M manual or ask the supplier.

Of course every data centre which uses any type of water system will have treatment regimes in place to kill bacteria (not just legionella) and best practice designs will include periodic draining and flushing of pipes and equipment to prevent any risk of infection.

A well designed system, of whichever type, will not be a problem as long as the maintenance is done properly.

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