A cooling tower is responsible for cooling the water used in processes such as heat exchangers in factories and air conditioning in commercial buildings. You may recognise them from power stations like this:
These are huge cooling towers and are unlikely to be found anywhere other than at a power station. Cooling towers will vary in size depending on the work required but may look a bit more like this:
There are a number of different cooling tower designs but the following diagram should give you a better idea of how a typical cooling tower works:
Correct water treatment is a high priority when maintaining a cooling tower on a site as they can release an aerosol (fine, airborne particles). It’s in this aerosol that the Legionella can escape the water system and be inhaled by people in the area. These particles can travel significant distances from the tower. As you can imagine this means that cooling towers need to be carefully, designed, maintained and treated in line with strict regulations to ensure that this does not happen.
There are four major problem areas associated with cooling tower treatment – scale, corrosion, solids and microbiological fouling (including Legionnaires disease). Cooling towers are treated by chemicals and other water conditioning processes to prevent these problems.
The water from a cooling tower should be regularly sampled and tested for Legionella bacteria to ensure the treatment regime being used is working effectively. Cooling towers should also be cleaned at least 6-monthly to remove fouling.
When talking about the operation of a cooling tower, common phrases such as “drift” – the water driven or drawn out of the tower with the main flow of air, and “windage” – the water driven out of the cooling tower due to wind. With windage the droplets are often quite large, making it wet on the outside of the tower. Losses due to both drift and windage can reduce the chemicals in the water, increasing the risk of a Legionella outbreak.
These losses must therefore be taken into consideration along with evaporative losses and controlled bleed (see below), which together account for the required total make-up of a system.
Most modern towers have a more enclosed design and specially designed air inlet louvres to reduce this. In the diagram above you can clearly see drift eliminators.
As previously mentioned Legionella is not the only reason for chemical treatment in a cooling tower. Scale and corrosion are also factors that need to be taken into consideration in order to protect the cooling tower and ensure it is working efficiently. Cooling towers have a “purge” or “bleed” to remove some of the recirculating water to keep the risk of scale and corrosion at bay.
All aspects of water treatment should be considered and actioned together with a regular monitoring programme to ensure good, efficient, economic and safe operation.