aerosol a suspension in a gaseous medium of solid particles, liquid particles, or

solid and liquid particles having negligible falling velocity. In the context of this

document, it is a suspension of particles which may contain legionella with a typical

droplet size of <5μm that can be inhaled deep into the lungs.

algae a small, usually aquatic, plant which requires light to grow, often found on

exposed areas of cooling towers.

bacteria (singular bacterium) a microscopic, unicellular (or more rarely

multicellular) organism.

biofilm a community of bacteria and other microorganisms, embedded in a

protective layer with entrained debris, attached to a surface.

calorifier an apparatus used for the transfer of heat to water in a vessel by

indirect means, the source of heat being contained within a pipe or coil immersed

in the water.

cooling tower an apparatus through which warm water is discharged against an

air stream; in doing so part of the water is evaporated to saturate the air and this

cools the water. The cooler water is usually pumped to a heat exchanger to be

reheated and recycled through the tower.

deadleg pipes leading to a fitting through which water only passes infrequently

when there is draw-off from the fitting, redundant or abandoned legs of pipework.

drift circulating water lost from the tower as liquid droplets entrained in the

exhaust air stream; usually expressed as a percentage of circulating water flow, but

for more precise work it is parts of water per million by weight of air for a given

liquid to gas ratio.

drift eliminator more correctly referred to as drift reducers or minimisers –

equipment containing a complex system of baffles designed to remove water

droplets from cooling tower air passing through it.

dry/wet cooling systems dry coolers with the capacity to employ evaporative

cooling when required either due to high ambient air temperature or when cooling

demand is high.

evaporative condenser a heat exchanger in which refrigerant is condensed by a

combination of air movement and water sprays over its surface.

evaporative cooling a process by which a small portion of a circulating body of

water is caused to evaporate, taking the required latent heat of vaporisation from

the remainder of the water and cooling it.

fouling organic growth or other deposits on heat transfer surfaces, causing loss

in efficiency.

legionnaires’ disease a form of pneumonia caused by bacteria of the genus

legionella.

legionella a single bacterium of the genus legionellae.

legionellae the name of a genus of bacteria which includes over 50 species and

belongs to the family Legionellaceae. They are ubiquitous in the environment and

found in a wide spectrum of natural and artificial collections of water.

Legionella pneumophila one of the causative organisms of legionnaires’

disease.

legionellosis any illness caused by exposure to legionella.

microorganism an organism of microscopic size including bacteria, fungi and

viruses.

nutrient a food source for microorganisms.

Pontiac fever a disease caused by a species of legionella, an upper respiratory

illness less severe than legionnaires’ disease.

risk assessment identifying and assessing the risk from exposure to legionella

from work activities and water sources on premises and determining any necessary

precautionary measures.

service provider companies or individuals or their sub-contractors who are

involved with providing advice, consultancy, operating, maintenance and

management services or the supply of equipment or chemicals to the owner or

occupier of premises.

sludge a general term for soft mud-like deposits found on heat transfer surfaces

or other important sections of a cooling system. Also found at the base of calorifiers

and cold water storage tanks.

stagnation the condition where water ceases to flow and is therefore liable to

microbiological growth.

strainers a coarse filter usually positioned upstream of a sensitive component

such as a pump control valve or heat exchanger to protect it from debris.