Cooling towers and evaporative condensers are widely used in air conditioning, industrial refrigeration and process cooling systems. They range in size from small packaged units to the large hyperbolic cooling towers seen at power stations. There are several types of configurations of cooling towers and evaporative condensers but they all work on the same principle of evaporating a small portion of the water circulating within the tower to achieve the cooling effect.

In an open circuit cooling tower the water to be cooled is distributed over a fill pack whilst air is blown or drawn through the packing. A small quantity of the water is evaporated and this causes the remainder of the water to be cooled (Fig 1).

Closed circuit cooling towers and evaporative condensers (which directly condense the refrigerant in industrial refrigeration plants) incorporate a closed loop heat exchanger within the tower instead of a fill pack. A secondary flow of water is distributed over the heat exchanger and, by the same evaporative process, heat is extracted from the fluid or refrigerant circulating inside the heat exchanger (Fig 2).

On rare occasions cooling towers have been the source of outbreaks of Legionnaires’ Disease and it is imperative that owners and users are aware of their responsibilities. No well maintained and clean cooling tower has been known to cause Legionnaires’ Disease; however following widely available technical guidance and instructions are essential for safe operation.

In the UK the HSC Approved Code of Practice and Guidance (L8) covers the important aspects of the design and management of cooling tower and evaporative condenser systems.

The key construction requirements for cooling towers and evaporative condensers are:

• high efficiency drift eliminators to minimise the release of water droplets;
• suitable louvres, screens and covers to reduce the effects of windage loss , prevent the entry of birds, leaves or other debris and avoid sunlight shining directly onto the water in the tower pond;
• a water distribution system to minimise the production of aerosols and a pipework system that avoids deadlegs and loops which are difficult to drain;
• all parts of the tower to be readily accessible for maintenance and cleaning with a fill pack that can be removed and dismantled for cleaning purposes;
• materials of construction that do not support microbial growth;
• location of cooling towers as far away as possible from ventilation inlets, opening windows and occupied areas taking into account the prevailing wind direction;

The guidance on management of cooling towers covers commissioning, operation and maintenance, all of which need to be carried out in accordance with the manufacturer’s instructions and good engineering practice.

Effective water treatment is essential both to prevent conditions suitable for legionella multiplication and also to keep heat exchange surfaces clean to maintain system efficiency. A complete programme that addresses corrosion, scale formation, fouling and microbiological activity is needed, which are covered in turn in L8. These conditions are inter-related and failure to control any one may lead to all occurring simultaneously, resulting in an environment that encourages the growth of legionella.

Whenever possible, the water treatment system needs to be semi or fully automatic and in all cases the treatment programme must be designed to take into account the quality of the make-up water to the cooling tower system.

Chemical water treatment is the traditional method for cooling towers. However there are other non-chemical or physical methods available. It is imperative that a competent water treatment service provider is consulted to establish a suitable and complete programme.

The choice of biocide is particularly important and must have a proven efficacy in preventing the proliferation of legionella bacteria. In the UK it is normal to dose an additional secondary biocide to ensure there is always biocide circulating throughout the cooling system.

To ensure the continued effectiveness of the treatment programme, L8 offers guidance which describes the monitoring checks required. These normally comprise weekly tests by cooling tower system operators, as well as other regular tests by qualified water treatment practitioners or accredited laboratories.

In the UK it is mandatory, at least quarterly, to test specifically for the presence of legionella. Testing of these water samples must be carried out by an accredited laboratory. If legionella is detected at significant levels, immediate cleaning and disinfection of the system is required.

L8 describes how cleaning and disinfection should normally be carried out and highlights other occasions when this procedure is recommended. This includes when a system is first put into service, after a shut down or if monitoring indicates a microbiological problem. In addition cooling tower systems should be cleaned and disinfected at least twice a year in normal operation.

Cooling towers and evaporative condensers handle large quantities of air and contaminants are ‘washed’ out of the air and deposited in the tower requiring manual cleaning and de-sludging to be carried out as necessary. Cleaning methods that create excessive aerosols such as high pressure water jetting should be avoided.

Manual cleaning can be reduced by the use of a filtration system for the recirculating water, also clean water is an added advantage for water treatment efficacy.

The technique for disinfecting the system using an oxidising biocide, normally chlorine, is described in L8. PPE is needed to protect staff carrying out the cleaning and disinfection, and precautions taken to ensure heavily chlorinated water is neutralised and discharged in compliance with Local Authority permissions. Over chlorination should be avoided as this can damage the materials of construction leading to corrosion.

In the UK, an Employer or the Responsible Person must inform the Local Authority (usually the Environmental Health Department) of any cooling towers or evaporative condensers on their premises. The Notification of Cooling Towers and Evaporative Condensers Regulations 1992 require written information about the installation and notification of updates if units are added or removed.

Article by:
Robert Macleod-Smith
Director of B.A.C.Balticare
Email: rmacleod-smith@balticare.co.uk
Tel: +44 (0)1895 814777