LEGIONELLA - World Health Organization

5.1.1 Cross-flow cooling towers
As shown in Figure 5.1(a), cooling towers are heat exchangers; they act by cooling water that is in
direct contact with the air moving through the tower. Most towers use a medium, referred to as
“fill” or “pack”, to maximize the surface area of water in contact with air and therefore available
for evaporation.
Water from the cooling tower is piped from the tower to a condenser (or other heat source),
where it is heated. Warm water is distributed via a spray, or a trough and gutter system at the
top of the tower, and falls down over the fill against the countercurrent of air. The warm water
is then piped back to the cooling tower to be cooled, and the process is repeated. There may
be tens or even hundreds of metres of piping between the tower and the point where the source
is cooled. The piping circuit can be quite complex in some industrial settings, where several
devices may be cooled.
5.1.2 Counterflow evaporative condensers and cooling towers
As shown in Figure 5.1(b), counterflow evaporative condensers and cooling towers are similar
to cross-flow cooling towers, except that the warm fluid that is being cooled is contained inside
a tubular matrix and does not come into direct contact with the air.
The water has only a short circuit from the sump at the base to the distribution system at the
top. It then flows down over the tube bundle, in the opposite direction to the airflow, thus
cooling the fluid within the tubes. Figure 5.1(b) shows how vapour enters and liquid exits the
condenser coil.
5.1.3 Links to outbreaks of legionellosis
Cooling towers and evaporative condensers have been implicated in many outbreaks of legionellosis.
As discussed in Chapter 1, L. pneumophila serogroup 1 MAb2-reactive strains are the primary
legionellae associated with outbreaks of disease from these systems. The causative organism
has been readily isolated from many of these devices, usually as a result of neglect or insufficient
maintenance (Fields, Benson & Besser, 2002), as illustrated by the example given in Box 5.1.
A significant proportion of outbreaks of legionellosis have been attributable to the start-up of
stagnant systems without adequate chemical treatment (Bentham & Broadbent, 1993).
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS