If cooling towers looked like people in 2021, drift eliminators would look like face masks. Both humans and cooling towers exhaust moisture, they both have the potential to propel it at high velocity and the expulsion from both may carry micro-organisms that are potentially harmful to those who come into contact with them.
As with people and their face masks, drift eliminators should be viewed as the last line of defence against the escape of dangerous bacteria from their cooling towers.
Drift eliminators are an extremely important piece of equipment in the fight to prevent Legionnaires’ disease, while simultaneously improving water efficiency and arresting chemical losses. Their primary purpose is to minimise the amount of aerosols escaping from cooling towers.
During normal operation, cooling tower water leaves the distribution system and splashes gravitationally into the internal fill pack where it is cooled by an opposing mechanical air stream before cascading into the sump below. The movement of the air passing through the water causes the production of aerosols. An aerosol is defined as a substance enclosed under pressure and released as a fine spray by means of a propellant, in this instance the propellant is the air stream generated by the cooling tower fan. Cooling tower aerosols are known as ‘drift’ and have the potential to harbour bacteria and pathogens such as Legionella.
Drift emitted from the tower can have a detrimental effect on surrounding equipment and the environment. More importantly though, drift can be inhaled by people as far as 1km from the source depending on the prevailing wind, sometimes with deadly consequences.
To minimise the emission of drift, drift eliminators were developed. Excessive amounts of drift will occur if the cooling tower is not fitted with drift eliminators or if the drift eliminators do not meet the prescribed standards. Drift eliminators will not work if they are not fitted correctly or if they are damaged, scaled, dirty, corroded, or brittle.
The Australian Standard that addresses microbial control in the design, installation and commissioning of air-handling and water systems of buildings is AS/NZS 3666.1:2011. This standard requires all new cooling towers to be designed to achieve a condenser water system loss rate of not more than 0.002% due to drift. Older cooling towers are not exempt and must be retrofitted with functional drift eliminators that achieve the same outcome.
The design criteria for both new and retrofit systems must satisfy the certification method as prescribed by AS 4180.1-2008 Drift loss from cooling towers— Laboratory measurement Part 1: Chloride balance method, or AS 4180.2-2008 Measurement of drift loss from cooling towers Part 2: Lost chloride method.
There are a significant number of certified cooling towers with drift eliminators fitted that cannot meet the stipulated criteria. Many towers are certified but continue to release aerosols that are visible to the naked eye. It is vital that the drift eliminators perform their function effectively. They are the last line of defence should there be a failure in the ability of the water treatment program to control the proliferation of Legionella bacteria.
Drift eliminators have come a long way since they debuted as a simple arrangement of timber or galvanized iron slats. Modern versions are complex arrangements of thin profiled PVC blades that have been engineered to bring a higher level of efficiency and longevity.
In a counterflow style cooling tower (Fig. 1), the drift eliminators are fitted horizontally near the top of the tower. In crossflow cooling towers (Fig. 2), they are fitted vertically on the inside of the tower. In both instances their shape is designed to cause a sudden change of direction in the air flow, trapping almost all the small water droplets and aerosols. Whilst it is impossible to prevent the emission of all drift, any small volumes of aerosols emitted to the atmosphere will generally evaporate rapidly.
Drift eliminators should have safe and easy access points to facilitate regular cleaning with most manufacturers recommending at least an inspection every three months. Drift eliminators are increasingly being identified as impasses in the Risk Management Plan and auditing process by local and state authorities.
Eliminate doubt, seek the advice of an experienced HydroChem water treatment specialist.