In evaporative cooling applications, the word drift is used to describe the small water droplets that escape the cooling tower within the air stream. Drift is not to be confused with plume, which is the cloud- or mist-like result of the evaporation process, commonly seen being emitted from cooling towers. While plume is condensing water vapor from evaporation, drift are actual droplets, and these droplets contain the chemicals and solids that are present within the circulating water.
To give you an idea of how small drift droplets are, they typically range in size from 10 to 2,000 microns. The average human eye can only see particles down to 50 microns, so we’re not even capable of seeing some of these droplets.
Why should we eliminate drift?
There are several reasons why eliminating drift is important:
- Health and safety: A cooling tower’s circulating water can be expelled through drift, so any dirt, debris, or bacteria that is within the water is also going to be dispersed along with the drift. At the forefront of these health concerns is Legionella, which is a respiratory infection that strikes individuals exposed to the Legionella bacteria. An individual could inhale droplets containing the bacteria, and they are small enough to be deposited deep within the lungs. This is especially a concern in big cities where towers are often installed atop of buildings.
- Fan blade erosion: Fan tips can see speeds up to 11,000 feet per minute, and the impact of the water droplets on the leading edge of the fans can be detrimental to the performance and the lifecycle of the fan.
- Nearby equipment impacts: Drift can create a nuisance for surrounding areas within the drift spray radius. This can lead to accelerated corrosion of nearby equipment as well as spotting on cars, windows, and buildings.
- Water loss: Reducing drift will also help to reduce unnecessary water loss from a cooling system. Required makeup for a cooling tower is a combination of the water loss from evaporation, blow down, and drift. So, reducing drift also helps to reduce your make up.
- PM10 emissions: The particulate matter that is contained within a drift droplet is left behind once the water that forms the droplet evaporates, possibly qualifying as a PM10 emission. While only one factor of a complex equation, reducing drift rate can also help reduce your tower’s regulated emissions.
Cooling tower drift and drift emissions are best controlled by using drift eliminators, an essential component in every cooling tower.
How do drift eliminators work?
The main principle behind drift eliminators is inertial impaction. Modern drift eliminators have three impact zones, or directional changes. So, as the airflow and drift mixture come up and approach the drift eliminator plane, they reach the first directional change or impact zone. The mass and subsequent inertia of these droplets cause them to diverge from the air stream, impacting the eliminator and collecting in the form of a water film that then drains down the surface of the drift eliminator and flows back into the tower. The same occurs at the remaining two impact zones. Having three impact zones increases the scrubbing efficiency of the lighter and smaller droplet sizes.
In the 1970s, drift eliminators were used to achieve drift loss rates at 0.01% tower water flow. Today’s drift technologies have advanced to match tighter government regulations. The most current standard for drift loss rate is 0.0005%. This standard is 1/20th of the drift loss percentage from that of the 1970s and is a testament to how far drift eliminating technology has come.
What are the different types of drift eliminators?
The earliest form of drift eliminators consisted of wood slats that were stacked on angles in order to provide a barrier to the air flow and water droplets. These early systems were known as herringbone drift eliminators. However, this simple shape of a wood slat had limited area for drift removal. Additional slat layers, or passes, could be added to increase the drift capability of the herringbone eliminators. This came at the cost of greatly increased pressure penalties across that component.
The next evolution of drift eliminator technology was the blade-style eliminator, which is still used today. The blade-style eliminator consists of individual blades that are assembled into panels. Initial iterations of these drift eliminators had abrupt directional changes and relatively wide spacing between the blades. Since then, improvements have been made to create a rounded blade profile, optimized blade spacing, and a faster overall manufacturing process.
The latest technology for drift elimination is the cellular-style drift eliminator. Cellular eliminators have taken the blade style to another level by providing more pathway surface area for droplet impingement. They offer greater removal capabilities and have the advantage of ease of manufacturing and installation over other designs.
Brentwood offers various blade- and cellular-style drift eliminators. Please reach out to Brentwood’s Cooling Tower Team with any additional questions regarding the topic of drift!