4.7: Drift Eliminators
4.7 Drift Eliminators
In the use of wet-cooling towers for heat rejection, drift or mist refers to the small droplets of circulating water carried out of the cooling tower by the saturated exhaust air. The droplets contain chemicals and other impurities that pollute the environment. Inertial impaction separators, known as drift or droplet eliminators or separators, are employed to remove the water droplets from the warm exhaust air and conserve water and chemicals for corrosion control and algae control.
In this type of separator, the two-phase exhaust flow is forced to change direction abruptly. This causes the dense drift droplets to hit the eliminator walls and become trapped inside the cooling tower.
Drift eliminators have evolved from the early single-pass wood lath to multiple-pass wood and then to sinusoidal wave shapes. These were followed by combinations of sinusoidal and honeycomb shapes reported by Holmberg. Currently, various styles of cellular drift eliminator packs are constructed from thermoformed sheets of polyvinylchloride (PVC) shown in Figure 4.7.1.
Figure 4.7.1: Drift Eliminator
The performance of these drift eliminators is measured by two criteria: droplet collection efficiency and pressure loss.
Chilton constructed an apparatus to test different counterflow fill and eliminator schemes for resistance to airflow and drift elimination. Foster et al. investigated the variation in collection efficiency both theoretically and experimentally with drop size for two types of eliminators. Chan and Golay developed a numerical model to calculate the collection efficiency for various droplet sizes using different eliminator geometries. They also compared the results...