2What is an air cooling tower?2 A wet air cooling tower is an “air/water” heat exchanger in which the water to be cooled is in direct contact with ambient air. The hot water is sprayed onto the upper part of the air cooling tower and streams onto the heat exchange body. The air circulates through the streaming system and is discharged into the atmosphere. Cooling mostly results from water evaporation; system efficiency is linked to the design and maintenance of the air cooling tower as well as atmospheric conditions (temperature and moisture).
One of the recognised contamination sources is the dispersion of legionella into the atmosphere by wet air cooling towers operating by spraying water into the air. 2Plume2 Steam-saturated air creates a cloud when coming out of wet air cooling towers. This cloud, called “plume”, is made up of:
Anyone operating an industrial installation, public building (shopping centre, hospital etc.), office building, residential building etc. can operate this type of air cooling tower. These towers are mostly used for the air conditioning of large premises, IT rooms or the cooling of heat-generating industrial processes. These installations come under the permit or declaration system in accordance with decree 2004-1331 of 1st December 2004; operators are required to report to the departmental Prefect. The list is kept up to date by the regions.
NOTE: these towers should not be confused with dry air-conditioning processes, which do not involve water spraying and do not present any risks of legionella disease (such as air conditioning systems in cars or individual homes). 2Examples of air cooling towers2 Open air cooling tower
Closed air cooling tower
2Different types of refrigerating circuits2
Open-circuit cooling installation:
Installations other than those with a closed primary circuit are installations for which the water circuit in contact with air circulates from the tower to a heat exchanger or a process distant from the tower (not adjoining the tower).
In this case, the volume of water in contact with air, and in which the concentration of legionella must be controlled, is significant and requires a larger channelling surface where biofilm can form than closed primary circuit installations.
A hybrid tower (operating by dry/wet process) can be a closed primary circuit installation if the tower is closed (the water circuit in contact with air is restricted to the tower) or not if the tower is open, which is the case in the third diagram below.
Open tower: the water of the circuit to be cooled down is directly dispersed onto the heat exchange body of the air cooling tower. Part of the water evaporates to ensure water cooling, while the other part is recovered in a receptacle then returned to the process that needs cooling.
Open tower + a heat exchanger not adjoined: an intermediate plate heat exchanger is positioned between the circuit to be cooled down and the circuit of the tower equipped with a heat exchange body. The operation of the tower is identical to that of the open tower with an independent water circuit.
Open hybrid tower: this type of tower is made up of a dry cooling coil and a heat exchange body onto which process water streams: the fluid that needs cooling initially circulates through a dry cooling coil situated at the top of the air cooling tower. If dry cooling is not sufficient, the fluid is dispersed onto a heat exchange body, partly evaporates and then returns to the process at the desired temperature.
Closed circuit cooling installation:
Closed primary circuit installations are installations for which the water circuit in contact with air is limited to the tower, whether a closed tower or an open tower cooling a heat exchanger adjoining the tower.
For these installations, the volume of water in the circuit in contact with air is lower. The conditions favourable to the development of legionella in the circuit are restricted by the limitation of channelling surfaces prone to biofilm formation, but the risk of legionella proliferation still exists.
Open tower + adjoining heat exchanger: the intermediate plate heat exchanger is physically attached to the tower equipped with a heat exchange body. The operation of the tower is identical to that of the open tower with an independent water circuit.
Closed tower (with tubular heat exchanger inside the tower): the fluid that needs cooling circulates through a tubular heat exchanger positioned in the air cooling tower and replacing the heat exchange body. A secondary water circuit within the tower implements the evaporating cooling process.
Generally speaking, the risk of proliferation is easier to manage when:
The Guide of good practices (June 2001) “Legionella and air cooling towers” is the result of an inter-ministerial work (ministries in charge of Health, Industry and the Environment). Its objective is to contribute to preventing the risk associated with Legionella in air cooling towers. It is divided into two sections:
Guide for the analysis of the legionella proliferation risk (February 2005)
This methodology guide, analysing the risk of legionella proliferation in cooling installations dispersing water into an air stream, was drawn up by a working group led by the Ministry for Ecology, Sustainable Development and Spatial Planning.
Different water cooling processes in industrial and tertiary installations (February 2005) The CETIAT guide (Technical Centre for Air and Thermal Industries) presents the different cooling processes in industrial and tertiary installations (PDF format).
Training guide for the management of the legionella proliferation risk in cooling installations dispersing water into an air stream.
This guide is a training support in the management of the legionella proliferation risk in air cooling towers. It is aimed in particular at the operators of these installations and is divided into four sections:
Guide: “Treatments to manage the risk of legionella proliferation in cooling installations”
This guide presents the different existing treatments to control the risk of legionella proliferation and indicates the good usage practices for each type of treatment
Decree no. 2004-1331 of 1st December 2004: creation of classified installations section 2921
Ministerial order of 13 December 2004 for permit-holding installations
Ministerial order of 13 December 2004 for installations with a declaration obligation
Ministerial order of 10 December 2007 certifying bodies to monitor cooling installations dispersing water into an air stream