LEGIONELLA - World Health Organization

Chapter 5 Cooling towers
and evaporative condensers
Barry Fields, David F Geary, William McCoy, Richard Bentham, John V Lee
This chapter describes how a water safety plan (WSP) can be applied to assessing and managing
the risks associated with Legionella in cooling towers and evaporative condensers.
It should be read in conjunction with Chapter 3, which discusses the different elements that
make up a WSP, and shows how a WSP fits within the framework for safe water quality developed
by the World Health Organization (WHO).
As explained in Chapter 3, a WSP has 10 steps that fit within the three main areas of system
assessment, monitoring and management and communications (see Figure 3.2). A WSP must
be comprehensive, and all 10 steps should be implemented in assessing and managing the
risks associated with Legionella. However, this chapter focuses on parts of the WSP where
information specific to cooling towers and evaporative condensers is needed.
5.1 Background
Cooling towers and evaporative condensers (also known as evaporative fluid coolers or closedcircuit
cooling towers) are heat-transfer devices in which warm water is cooled by evaporation
in atmospheric air (see Figure 5.1). These devices are used:
• to provide cooling for a wide variety of industrial processes
• for refrigeration plant used in cold stores
• to cool water for air-conditioning to buildings.
Air movement through the tower or condenser is produced by fans or, occasionally, by natural
convection. Aerosols generated by the operation of cooling towers and evaporative condensers
can transmit legionellae to susceptible hosts (Broadbent, 1996; Geary, 2000).
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS