Why Mist Eliminators Protect Downstream Heat Exchangers From Fouling and Corrosion
A heat exchanger downstream of a wet process is designed to transfer heat.
It is not normally intended to function as a liquid separator.
If mist passes through the upstream gas-cleaning equipment, droplets can enter:
- tubes;
- channels;
- fins.
Those droplets may contain:
- salts;
- acid;
- alkali;
- solids.
As water evaporates or temperature changes, contaminants remain on the exchanger surface.
A mist eliminator can therefore protect exchanger:
- heat-transfer performance;
- pressure drop;
- corrosion life.
Droplets Carry Nonvolatile Material
A wet scrubber droplet is not always clean water.
It can contain:
- dissolved reaction salts;
- suspended solids.
When it reaches a hotter exchanger surface, water can evaporate.
The nonvolatile material remains.
A thin liquid carryover can therefore create a large deposit problem over long operation.
Deposits Reduce Heat Transfer
Scale creates an additional thermal resistance between:
- process gas;
- heat-transfer surface.
The exchanger needs a greater temperature difference to achieve the same duty.
Eventually:
- outlet temperature changes;
- energy efficiency falls.
Cleaning frequency increases.
Preventing droplets upstream can reduce this deposition mechanism.
Wet Salts Can Accelerate Corrosion
Some deposits absorb moisture.
They create concentrated local electrolyte films on metal surfaces.
Chloride-bearing droplets are a common concern.
Even if bulk gas chemistry appears acceptable, a wet salt deposit can create a much more corrosive local environment.
Mist control can therefore influence corrosion as well as cleanliness.
Gas Coolers Can Also Create New Liquid
The relationship is not one-way.
A heat exchanger may cool gas below its condensation point.
New liquid forms inside the exchanger or downstream.
An upstream mist eliminator cannot remove vapor before it condenses.
Therefore, two questions must be separated:
- Is liquid entering the exchanger as mist?
- Is the exchanger creating new condensate?
Different problems require different solutions.
Hot Exchangers Can Bake Deposits
If the downstream surface is hot, a droplet can evaporate rapidly.
Salts or solids can become:
- hard;
- adherent.
Repeated deposition creates a difficult scale.
Preventing carryover before the exchanger is often easier than removing baked deposits later.
Fin-Type Exchangers Are Sensitive to Plugging
Compact exchangers can have narrow gas passages.
Deposits consume a large fraction of this open area.
Pressure drop rises quickly.
A small upstream mist problem can therefore become a major exchanger airflow problem.
Corrosive Acid Mist Can Attack Tube Surfaces
Acid droplets may produce localized corrosion where they impact.
If the exchanger surface is cooler than the bulk gas, condensation can further increase wetness.
The combination of:
- entrained acid;
- local condensation
can create severe attack.
Material upgrades help, but reducing unnecessary liquid carryover can also extend life.
Mist Eliminator DP Must Be Balanced Against Exchanger Protection
Adding a fine separator upstream creates pressure drop.
The system therefore needs to balance:
- protection benefit;
- fan/compressor pressure budget.
A staged arrangement may be useful where heavy droplets can be removed with a low-resistance vane and finer droplets polished only where necessary.
Exchanger Deposit Chemistry Can Diagnose Carryover
During cleaning, analyze deposits.
If they contain characteristic species found in upstream scrubber liquid, mist carryover becomes a strong suspect.
If the deposits instead arise from compounds that condense only after cooling, the heat exchanger itself may be generating the liquid phase.
Composition helps identify the source.
Load Dependence Is Another Clue
If exchanger fouling accelerates when plant gas throughput increases, upstream separator re-entrainment may be involved.
Compare:
- gas load;
- demister DP;
- downstream deposit rate.
A strong correlation can support the diagnosis.
Wash Cycles Can Send Temporary Liquid Bursts
Online demister washing can increase downstream liquid briefly.
If the exchanger is sensitive, these short events may still deposit chemicals.
Wash-system operation should therefore be considered in downstream equipment protection.
What Should the Demister Requirement Be Based On?
Instead of using a generic:
“99% removal”
target, ask how much liquid and contaminant the exchanger can tolerate.
For high-value or difficult-to-clean equipment, a stricter outlet requirement may be economically justified.
What Data Should Be Provided?
Useful information includes:
- gas flow;
- temperature;
- upstream liquid chemistry;
- droplet loading;
- heat exchanger type;
- downstream temperature;
- fouling history;
- pressure-drop budget.
This allows separator duty to be linked to actual equipment protection.
Final Engineering Perspective
A downstream heat exchanger can turn a small liquid carryover into a large long-term deposit problem.
Mist eliminators protect not only gas purity but also heat-transfer area, pressure drop, and corrosion resistance.
The key is to distinguish liquid entering the exchanger from liquid created by condensation inside it.