Why Scrubber Mist Carryover Can Damage a Downstream Induced-Draft Fan

Wet scrubber mist carryover is often discussed mainly as an emissions problem.

But if an induced-draft fan is located downstream of the scrubber, the fan may be one of the first pieces of equipment affected by poor mist elimination.

The gas leaving the scrubber can contain droplets carrying:

• water;
• acid;
• alkali;
• salts;
• slurry;
• suspended solids.

When these droplets strike a rapidly rotating fan impeller, they can contribute to:

• erosion;
• corrosion;
• deposits;
• imbalance;
• vibration.

The mist eliminator therefore acts as a protective barrier between the wet process and the rotating equipment.

Why Fans Are Sensitive to Liquid Droplets

Fan blades move at high tip speed.

A liquid droplet entering the fan can impact the blade at high relative velocity.

One droplet causes little damage.

Millions of droplets over months or years can create significant mechanical effects.

The severity increases when the liquid contains:

• abrasive solids;
• corrosive chemicals.

Therefore, downstream fan reliability depends not only on gas chemistry but also on how much liquid is physically carried with the gas.

Droplet Impact Can Cause Erosion

Clean water droplets can produce erosion under sufficiently severe conditions.

Slurry droplets are even more aggressive.

A droplet containing:

• ash;
• mineral solids

acts like a small abrasive projectile.

Repeated impact can wear:

• leading edges;
• blade surfaces.

The fan gradually loses its intended aerodynamic profile.

Efficiency can decrease.

Mechanical balance may also change.

Corrosive Mist Creates a Combined Damage Mechanism

If the liquid contains:

• acid;
• chloride;
• caustic,

the wetted fan surfaces may corrode.

Mechanical droplet impact continuously removes:

• corrosion films;
• deposits.

Fresh metal is exposed.

Corrosion and erosion can therefore reinforce each other.

A corrosion-resistant fan material helps, but reducing liquid carryover upstream is often equally important.

Deposits Can Cause Rotor Imbalance

Not every entrained contaminant erodes the fan.

Some materials stick.

Examples include:

• salts;
• slurry;
• sticky process residues.

Deposits rarely accumulate perfectly uniformly on all blades.

One region gains more mass than another.

The rotating assembly becomes imbalanced.

Possible symptoms include:

• increased vibration;
• bearing load;
• reduced reliability.

Repeated cleaning may be required.

A well-performing mist eliminator can reduce the amount of deposit-forming material reaching the fan.

A Fan Can Re-Atomize Liquid

If large droplets or wall liquid enter the fan, the rotating impeller can break them into much finer droplets.

Downstream of the fan, the liquid may therefore appear as a fine aerosol.

This can complicate emissions diagnosis.

The original problem may have been relatively coarse scrubber carryover.

The fan transformed it into smaller droplets.

A downstream observer then incorrectly concludes that the scrubber inherently produces very fine mist.

Fan Suction Can Influence Demister Flow Distribution

The relationship works in both directions.

The downstream fan does not only suffer from demister carryover.

Its suction can also affect gas distribution through the separator.

If the fan inlet or outlet duct arrangement creates a strong asymmetric flow field, one region of the mist eliminator may carry more gas.

This can promote local re-entrainment.

Therefore, the fan and demister should be considered parts of one gas-flow system.

Outlet Duct Geometry Matters

If the mist eliminator is followed immediately by:

• a side outlet;
• elbow;
• fan inlet,

gas may accelerate unevenly.

The region closest to the outlet experiences higher velocity.

Local carryover can increase.

Providing enough transition space can improve separator utilization.

A correct mist eliminator can perform poorly if the downstream duct pulls gas unevenly through it.

Fan Material Upgrade Is Not Always the Best First Solution

When fan corrosion appears, the first response may be:

“Use a more expensive alloy.”

That may extend fan life.

But if the root cause is excessive liquid carryover, the fan is still being subjected to:

• unnecessary wetting;
• deposits.

The better system solution may include:

• improved demister performance;
• drainage correction;
• reduced upstream entrainment.

Material upgrade and separator improvement may both be appropriate, but they solve different parts of the problem.

Pressure Drop Creates a Design Tradeoff

The mist eliminator protects the fan.

But it also creates resistance that the fan must overcome.

A very dense separator may reduce carryover but increase:

• fan static-pressure requirement;
• power consumption.

As the demister fouls, the resistance rises further.

The correct design therefore balances:

• liquid removal;
• sustainable DP.

The fan must have enough pressure capability across the expected operating cycle.

Carryover During Wash Cycles Can Reach the Fan

Online demister washing introduces temporary additional liquid.

If the wash system produces:

• re-entrainment;
• flooding,

the downstream fan may receive a short high-liquid-load event.

Operators may notice vibration or wet deposits after wash cycles.

The wash procedure should therefore consider fan protection as well as separator cleaning.

What Fan Damage Pattern Can Tell You

Inspection of the fan can provide evidence about upstream carryover.

Directional erosion may indicate:

• droplet-laden flow entering from one region.

Chemical deposits can reveal:

• composition of entrained scrubber liquid.

If the fan repeatedly develops the same deposit after every cleaning, upstream carryover should be investigated rather than treating the fan as an isolated maintenance problem.

Mist Chemistry Can Confirm the Link

Compare fan deposits with:

• scrubber circulation liquid;
• demister deposits.

If they contain similar characteristic:

• salts;
• chemicals,

the process-liquid pathway becomes more plausible.

This can help distinguish scrubber mist from unrelated:

• condensation;
• environmental contamination.

Fan Vibration Trends Can Provide Early Warning

A fan gradually accumulating deposits may show a slow increase in vibration.

This can occur before severe downstream liquid is visually obvious.

If fan vibration increases together with:

• demister DP;
• scrubber carryover,

the separator condition may be contributing.

Mechanical monitoring and process monitoring can therefore reinforce each other.

Vane Versus Mesh Selection Depends on the Upstream Duty

For heavy slurry carryover, an open vane separator may provide strong liquid handling and fouling tolerance.

For finer clean droplets, mesh may provide better polishing.

If the fan is highly sensitive to liquid, staged separation may be justified.

The correct choice depends on:

• droplet size;
• liquid load;
• solids;
• pressure-drop budget.

The fan protection requirement should be included in the separator design basis.

What Should Be Defined?

Useful information includes:

• fan location relative to scrubber;
• gas flow range;
• droplet carryover target;
• liquid chemistry;
• solids loading;
• demister DP limit;
• fan material;
• downstream duct geometry.

Where fan erosion or corrosion has already occurred, photographs and deposit analysis can provide valuable evidence.

Final Engineering Perspective

A downstream induced-draft fan converts mist carryover from a simple separator-performance issue into a rotating-equipment reliability problem.

Entrained liquid can cause erosion, corrosion, deposits, imbalance, and vibration, while the fan suction can also create local demister overload.

The mist eliminator and fan should therefore be designed as parts of the same gas-handling system.