Drift is composed of entrained particles of water in the exit air stream of the cooling tower. Unlike the evaporated water that is pure, the drift particles will contain whatever soluble constituents are present in the circulating water. This could include minerals, water treatment chemicals, and biological organisms. The substances in drift pose potential health hazards for people near the tower and could cause corrosion of nearby equipment. For these reasons, preventing drift is an important aspect of cooling tower operation.
Modern drift eliminators are very effective in eliminating fine drift particles; however, one characteristic of water that must be examined is the surface tension. This is where the topic of surfactants comes into play.
Surface tension is the measure of how water will wet a contacted surface. If the water has high surface tension, it will tend to bead up. Lowering the surface tension of water causes it to film out and spread across the material. The most common surfactant we use on a daily basis is soap, which can lower surface tension by a factor of 3 or more. Soap allows the materials to get “wetter”, and therefore, allows the water to clean better. Surface tension is typically measured in dyne/cm. Pure water has a surface tension of 73 dyne/cm at room temperature and 68 dyne/cm at 120°F.
Although surfactants have many beneficial uses, they will have an adverse effect to preventing drift from cooling towers. Drift eliminators work by inertial impaction of the water droplets. The shape of the drift eliminator causes the air to rapidly change direction, and heavy water particles have too much inertia to make the direction change. The water droplet impacts the surface of the drift eliminator and drains back into the cooling tower. The larger the water droplets, the more effective the drift eliminator is in removing it from the air stream. When surfactants are added to the circulating water, reduced surface tension increases the population of very small droplets produced by the air-water interaction within a cooling tower. The smallest of these droplets can pass largely unimpeded through the drift eliminators.
The surfactants that may be present in cooling towers include bio-dispersants, scale inhibitors, and some non-oxidizing biocides. Surfactants, and the resultant surface tension, can vary quite a bit. The amount of treatment chemicals can vary seasonally or may be added in large quantities periodically as a way to ensure harmful organisms have been completely eliminated. Blowdown and make-up water will also change the concentrations of surfactants in the water. Because surfactant amounts can change rapidly, the amount of drift the tower is producing can vary quickly as well. A drift issue being periodic in nature is one sign that surfactants are at work, in contrast to a poor installation or a product quality issue that will tend to be continuous. Consultation with your water treatment company will help identify what could be causing the issue and when it is likely to occur.
Based on Brentwood’s experience, the surface tension of the circulating water must not fall below 60 dynes/cm for our products to meet our published drift rate curves. If a drift emissions test is to be performed, all surface-active additives should be discontinued 72 hours prior to, and throughout, the entire test period to ensure a representative test result is gathered.