Why Wet Stacks Can Produce Droplet Rainout Even When the Plume Looks Normal

Wet scrubber exhaust often leaves the stack:

• warm;
• humid;
• close to saturation.

Operators may focus on the visible plume.

But another environmental problem can occur even when the plume itself does not look unusual:

droplet rainout.

Liquid droplets leaving the stack can fall onto:

• nearby roofs;
• equipment;
• roads;
• surrounding land.

The droplets may contain:

• salts;
• acid;
• alkaline chemicals.

Rainout is therefore different from ordinary water-vapor condensation in a visible white plume.

Understanding the difference is essential when evaluating mist eliminator performance.

Rainout Requires Actual Liquid Droplets

A gas can contain water vapor without containing liquid mist.

Water vapor leaving a stack may later condense into very fine atmospheric droplets and create a visible plume.

Those droplets may evaporate or remain suspended.

Rainout typically involves larger liquid droplets capable of settling to the ground.

Possible sources include:

• scrubber carryover;
• wall drainage;
• stack condensation;
• internal liquid film stripping.

A Clear Stack Does Not Prove Zero Droplet Carryover

Large droplets may be relatively few in number.

They may not create a dense visible plume.

Yet because they are large enough to settle, they can produce local fallout.

Therefore, visual plume appearance is a poor standalone test of demister effectiveness.

Residual Large Droplets Can Be Environmentally Important

Mist eliminator outlet specifications often focus on total mass.

Rainout behavior also depends on:

• droplet size.

A relatively small mass fraction of large droplets can fall rapidly.

A much larger number of ultrafine droplets may remain suspended.

Therefore, downstream environmental impact cannot always be judged from total liquid concentration alone.

Stack Wall Condensation Can Create New Liquid

Saturated scrubber gas can cool while traveling upward through the stack.

Liquid condenses on the wall.

The resulting film drains downward unless gas velocity shears it from the surface.

If the film is stripped into droplets near the stack outlet, rainout can occur even though the upstream demister performed correctly.

This is a downstream liquid-generation mechanism.

Stack Geometry Matters

Variables include:

• stack diameter;
• outlet velocity;
• height;
• wall temperature.

High velocity can entrain liquid from wet stack surfaces.

Low velocity may allow larger droplets to settle internally.

There is no single stack velocity that guarantees zero rainout.

The complete wet-stack design needs review.

Chemical Composition Can Identify Scrubber Carryover

If fallout liquid contains the same characteristic salts as scrubber circulation liquid, physical carryover becomes more plausible.

If it is much cleaner water, stack condensation may be a larger contributor.

Sampling and composition analysis can therefore help distinguish the two mechanisms.

Rainout Can Be Directional

Wind affects where droplets land.

But persistent fallout concentrated on one side of the stack may also reflect:

• asymmetric outlet flow;
• local wall wetting.

Site observations should include:

• weather;
• wind direction;
• operating load.

One isolated wet surface is not enough for diagnosis.

Mist Eliminator Overload Is One Possible Cause

If rainout increases at high scrubber gas rate, the demister may be approaching:

• re-entrainment.

Other clues can include:

• higher wet DP;
• increased downstream drain flow.

Reducing gas load may improve the problem.

But the stack itself still needs investigation before assigning all rainout to the separator.

Demister Wash Cycles Can Produce Temporary Rainout

Online washing can create short periods of higher outlet liquid carryover.

If those droplets survive to the stack, site fallout may occur primarily during wash cycles.

Plant observations correlated with wash timing can reveal this.

Duct Low Points Can Release Liquid Slugs

Condensate can accumulate in horizontal ducts between:

• scrubber;
• stack.

If a pool is suddenly picked up by gas, the stack receives a temporary burst of droplets.

This can produce intermittent rainout that has little relationship to average demister efficiency.

Duct drainage should therefore be inspected.

Why Increasing Mesh Density May Not Solve Rainout

If the droplets are created downstream by:

• stack condensation;
• wall-film stripping,

a denser demister upstream cannot remove them before they exist.

Increasing separator resistance may add:

• fan power

without solving fallout.

Location of droplet formation must be identified first.

Useful Diagnostic Questions

Ask:

• Is fallout continuous or intermittent?
• Does it correlate with gas load?
• Does it occur during demister washing?
• What is the liquid chemistry?
• Where does condensation begin?
• Are downstream ducts draining correctly?

These questions divide the problem into:

• separator carryover;
• downstream liquid generation.

Final Engineering Perspective

Wet-stack rainout is a liquid-droplet problem, not simply a visible-plume problem.

Droplets can originate from the scrubber or can form downstream through condensation and film stripping.

Reliable diagnosis therefore follows the liquid path from demister outlet to stack tip, rather than automatically blaming the separator.