Cooling Tower Drift: Causes, Risks & Eliminator Design

What is Drift?

Drift is the entrained water droplets that are carried out of the cooling tower by the exhaust air stream. Unlike evaporation (which is pure water vapour), drift contains all the dissolved solids, chemicals, and microorganisms in the tower water. Drift is the primary pathway for Legionella bacteria to leave the tower and reach people.

Drift is measured as a percentage of the circulating water flow. A well-maintained tower with good drift eliminators has a drift rate of 0.001–0.005% of circulation. A tower with damaged or missing eliminators can have drift rates of 0.1% or more — 100× higher.

Risks of Excessive Drift

• Legionella transmission — drift droplets can travel up to several hundred metres from the tower, carrying bacteria into air intake systems of nearby buildings
• Chemical exposure — drift carries water treatment chemicals (biocides, scale inhibitors) that can affect people and vegetation nearby
• Water waste — drift is water lost from the system, increasing make-up water consumption
• Visible plume — drift creates a visible mist that can cause complaints from neighbours
• Corrosion and staining — drift deposits dissolved solids on nearby structures, causing staining and corrosion
• Environmental contamination — drift can carry Legionella and chemicals into the surrounding environment

Drift Eliminators

Drift eliminators are baffles installed at the top of the tower (in the air discharge path) that force the air to change direction. The inertial force causes water droplets to impinge on the eliminator surfaces and drain back into the tower. Modern eliminators can achieve 99.9% drift removal.

• Blade-type eliminators — parallel blades that create a tortuous air path. Simple but less efficient.
• Cellular (honeycomb) eliminators — corrugated sheets that create narrow channels. Very efficient but more prone to blockage.
• Material — typically PVC or polypropylene. Must be compatible with the water treatment chemicals.
• Installation — must be sealed at all edges. Any gap between the eliminators allows drift to bypass.

Causes of Excessive Drift

• Damaged or missing drift eliminators — the most common cause. Eliminators degrade, crack, or are removed for maintenance and not replaced.
• Excessive airflow — if the fan is running too fast or the fan pitch is wrong, air velocity through the eliminators is too high, and droplets penetrate the eliminators.
• Poor water distribution — if the nozzles are blocked or misaligned, water pools in certain areas and creates more droplets for the air to carry.
• Excessive water flow — pumping more water than the tower is designed for overloads the eliminators.
• Gaps between eliminators — eliminators must be installed with no gaps. Even a small gap allows significant drift bypass.
• Scale or biofouling on eliminators — reduces the channel size, increasing air velocity and drift penetration.

Maintenance

• Inspect drift eliminators monthly — look for damage, gaps, scale buildup, and biofouling
• Clean eliminators during annual shutdown — remove scale and biofilm with low-pressure water (not high pressure, which can damage them)
• Replace damaged eliminators immediately — never operate the tower with missing or broken eliminators
• Verify all eliminators are properly seated with no gaps at the edges
• Check fan speed and pitch — verify the airflow is within the design range
• Maintain proper water distribution — clean nozzles regularly to prevent pooling