To answer this question, we must first start with the basics of drift eliminators. Drift eliminators aid in the removal of water droplets from an airstream in a cooling tower. They work by changing the direction of the airflow and causing the entrained water droplets to impact the walls of the eliminator. When this happens, the droplets are removed from the airstream and run along the walls to drain back into the wet section of the tower.
Since drift eliminators function through inertial impaction of the drift droplets on the eliminator walls, there are different aspects as to how they work across varying operating airflows of a cooling tower:
- Slow airflow: At low air velocities through the drift eliminators, the droplets will meander through without getting hung up and scrubbed out.
- Fast airflow: At high air velocities, you will get very good droplet impaction on the walls. However, if the airflow is too high, the droplets will not be able to drain back down into the wet section of the cooling tower; instead, they will be sucked out of the drift eliminators, become re-entrained into the exiting airstream, and be expelled out of the cooling tower. (It’s a very surreal moment when you see this happening first hand because you are watching it “rain” upwards)! This is known as “breakthrough.”
- General operating regime: Air velocities between these extremes, normally about 450-700 ft/min (2.3-3.5 m/s).
It is also important to note that proper plenum height allows for a more uniform air distribution across the drift eliminator plane. This means that consideration needs to be given not only to the average air velocity through the drift eliminators but also to the possibility of localized velocities that may be caused by the tower design. Additionally, nozzles have a signification effect on drift eliminator performance. Droplet size generated, distance from the nozzle to the drift eliminator plane, and the surface tension of the circulating water are significant factors in understanding potentials for drift.
For prime drift eliminator efficiency, we want to be somewhere between the airflow extremes with proper plenum height and nozzle configuration. The operating range, as you can see in the graph below, is where we want to be for the drift eliminator to work most effectively and meet its published drift rating. In some cases, you may get even better drift reduction than the published levels but the manufacturer, so it’s important to know how each manufacturer rates its drift eliminators.
A good way to begin analyzing drift eliminator effectiveness for counterflow towers is to model the tower in Brentwood’s S.T.A.R. program. This program will help you to determine the average airflow through the drift eliminators, which you can then use to estimate a drift rate based on the published drift rate curve for a given product. These curves can be downloaded by creating a myBrentwood account and navigating to the Document Center. The S.T.A.R. software can also provide an alert to potential velocity imbalances from a short plenum based on a low fan coverage value.
If you’d like to discuss performance data or results from S.T.A.R., you can always call and talk to our Application Engineering team at 610-236-1100. And if you’d like to receive more information like this from Brentwood, sign up here for our quarterly cooling tower newsletter!