What Causes Re-Entrainment in a Mist Eliminator?
A mist eliminator can capture droplets successfully and still allow liquid to leave the separator again. This failure mechanism is called re-entrainment, and it is one of the most important reasons a demister may underperform even when the original collection mechanism is working.
Re-entrainment occurs after droplets have already contacted the mist eliminator. The liquid collects on wire, blades, or internal surfaces, coalesces into larger drops, and should drain away. If the aerodynamic force of the gas becomes strong enough, however, part of that liquid is stripped from the separator and carried downstream again.
This means mist-removal efficiency depends on more than initial droplet capture. A successful separator must complete three steps:
- capture the droplets;
- coalesce them into drainable liquid;
- remove that liquid without allowing it to return to the gas stream.
If the third step fails, downstream carryover can remain high.
How Re-Entrainment Begins
Inside a wire mesh mist eliminator, fine droplets strike the knitted wire and form a liquid film. Small droplets combine into larger drops. Gravity then pulls the accumulated liquid toward the drainage side of the pad.
In a vane separator, droplets impact blade surfaces and flow along the profile toward drainage channels.
During normal operation, the liquid moves away faster than the gas can remove it.
When gas load or liquid accumulation becomes excessive, this balance changes.
The gas begins to shear liquid from the wire or blade surface. Large droplets may break into smaller droplets and become airborne again.
The separator is no longer simply capturing mist. It is also generating new entrainment.
High Gas Velocity Is Only One Cause
Re-entrainment is often associated with excessive gas velocity, but velocity alone does not explain every case.
The operating limit depends on several interacting parameters:
- gas density;
- liquid density;
- liquid viscosity;
- surface tension;
- liquid loading;
- separator geometry;
- drainage efficiency;
- fouling condition.
A system that operates safely with a low liquid load may begin to re-entrain when liquid carryover from the upstream process increases.
Likewise, a clean mesh may operate normally while a partially fouled mesh experiences local velocity peaks and unstable drainage.
This is why a demister should not be evaluated using only one nominal velocity number.
Liquid Loading Can Push a Separator Toward Re-Entrainment
Every mist eliminator has a finite drainage capacity.
As liquid loading increases, more liquid must travel through or across the separator.
If the collected liquid cannot drain quickly enough, liquid holdup rises.
This creates several effects:
- more of the mesh or vane surface becomes wet;
- available gas-flow area decreases;
- local gas velocity increases;
- aerodynamic forces on the liquid become stronger;
- liquid films become easier to strip.
Eventually, the separator reaches a point where additional liquid loading causes more carryover rather than more successful drainage.
This condition may occur even if the original droplet collection efficiency was high.
Poor Drainage Can Create Re-Entrainment at Normal Gas Load
A separator may be correctly sized hydraulically and still experience re-entrainment because the liquid cannot leave properly.
Typical causes include:
- blocked drain paths;
- incorrect separator orientation;
- poor pad support that creates low spots;
- excessive compression of wire mesh;
- vane drainage channels installed incorrectly;
- solids deposits;
- scale accumulation;
- insufficient clearance below the separator.
When liquid becomes trapped, the local wetness of the separator increases.
The gas then interacts with a much larger liquid inventory.
In severe cases, the downstream face may visibly spray or spit liquid.
Fouling Can Create Local Re-Entrainment
Fouling does not always block an entire demister uniformly.
Deposits often accumulate more heavily in certain areas.
Gas then avoids the high-resistance region and passes through cleaner regions.
The average vessel velocity may still look acceptable, but local velocity through the remaining open area may become much higher.
Those local high-velocity zones can strip liquid from the mesh.
This explains why an old demister may suddenly show increased carryover even when total gas flow has not changed.
The problem may be flow redistribution inside the pad, not an increase in overall throughput.
Separator Thickness Does Not Automatically Prevent Re-Entrainment
A common reaction to poor mist removal is to install a thicker mesh pad.
This may increase droplet interception, but it does not automatically solve re-entrainment.
If the real problem is high velocity or poor drainage, a thicker pad may hold even more liquid.
Pressure drop can increase and drainage can become slower.
The correct response must address the actual failure mechanism.
Possible solutions may involve changing:
- active cross-sectional area;
- mesh structure;
- drainage arrangement;
- separator type;
- operating load;
- support geometry.
Simply adding more material is not a reliable engineering solution.
Downstream Carryover Can Be Misdiagnosed
Not every liquid problem downstream of a demister is caused by poor capture.
Liquid may come from:
- re-entrainment from the separator;
- wall flow bypass;
- upstream flooding;
- leakage around the demister edge;
- spray nozzles located too close to the separator;
- condensation downstream of the demister.
These failure modes require different corrective actions.
If a demister appears intact but carryover increases mainly at high throughput, re-entrainment becomes a strong candidate.
If carryover occurs only in one region, bypass or local hydraulic problems may be more likely.
Practical Indicators of Re-Entrainment
Operators should investigate re-entrainment when they observe:
- carryover increasing sharply above a certain throughput;
- unstable pressure drop;
- downstream liquid droplets becoming larger;
- excessive wetness at the downstream face;
- liquid discharge or dripping from unexpected areas;
- acceptable performance at low load but poor performance at high load.
The most useful investigation compares process behavior at different gas and liquid loads.
A sharp performance deterioration above a repeatable operating point often indicates that the separator has reached a hydraulic capacity limit.
Engineering Review Points
When evaluating possible re-entrainment, check:
- actual gas flow at operating temperature and pressure;
- gas density;
- liquid density and viscosity;
- liquid loading;
- separator active area;
- mesh or vane condition;
- drainage paths;
- fouling distribution;
- pressure drop;
- support and installation geometry.
Do not rely only on vessel diameter or nominal flow rate.
Final Engineering Perspective
A mist eliminator does not succeed merely by capturing droplets.
The captured liquid must remain separated long enough to drain from the gas stream.
Re-entrainment is therefore a hydraulic stability problem after successful collection.
Understanding this distinction is essential when troubleshooting carryover.
If the separator captures liquid but the gas strips it away again, improving collection efficiency alone will not solve the problem.