How to Size a Mist Eliminator in a Rectangular Duct or Non-Circular Vessel

Many mist eliminator discussions assume a circular process tower.

The active area is calculated from vessel diameter:

A=πD24A=\frac{\pi D^2}{4}

But industrial mist separators are also installed in:

• rectangular ducts;
• horizontal scrubbers;
• ventilation systems;
• non-circular process vessels.

The basic separation principles remain the same.

The engineering challenges do not.

Rectangular systems are often more sensitive to:

• gas maldistribution;
• corner flow;
• support blockage;
• module arrangement.

Simply multiplying width by height and calculating an average velocity may not be enough to ensure reliable performance.

The Basic Area Calculation Is Simple

For a rectangular gas passage:

A=W×HA=W\times H

where:

• WW = clear width;
• HH = clear height.

Superficial velocity is then:

V=QactualAV=\frac{Q_{actual}}{A}

This provides the basic face velocity.

But the calculation assumes that gas is distributed evenly across the entire rectangle.

In real ducts, that assumption may be poor.

Duct Flow Is Often Highly Nonuniform

Rectangular ducts frequently contain:

• elbows;
• transitions;
• dampers;
• fans;
• side inlets.

These can produce strong velocity profiles.

After an elbow, gas may concentrate toward one side.

After a rapid expansion, recirculation zones may form.

Therefore, an average velocity of 3 m/s could hide regions operating at:

• much higher velocity;
• much lower velocity.

Mist eliminator performance depends on the local velocity—not merely the arithmetic average.

Corners Can Behave Differently

The four corners of a rectangular separator may experience different gas flow from the central region.

Depending on upstream geometry, corners may become:

• low-flow zones;
• deposit locations;
• bypass paths.

A low-flow corner may accumulate liquid or solids.

A high-flow region elsewhere carries more of the actual gas.

The full nominal separator area is then not being used effectively.

Flow Straightening May Be Required

If the mist eliminator is located too close to:

• an elbow;
• inlet transition;
• fan discharge,

gas may not have enough distance to redistribute.

Possible engineering solutions can include:

• longer straight run;
• perforated flow distributors;
• turning vanes;
• baffles.

These devices create their own pressure drop and must be designed carefully.

The objective is to present the separator with a reasonably uniform velocity field.

Rectangular Vane Packs Are Often Modular

Vane mist eliminators in ducts are commonly divided into rectangular modules.

Module dimensions should consider:

• access opening;
• support beams;
• installation sequence.

Each module must align correctly with its neighbors.

Gaps between modules can create direct bypass.

A large rectangular face containing many modules therefore needs careful joint design.

The separator should behave as one continuous active surface.

Support Beams Can Remove Significant Area

A wide duct may require several vertical or horizontal structural members.

These supports occupy part of the cross section.

Gas accelerates around them.

The actual open area is therefore smaller than:

W×HW\times H

if structural blockage is significant.

This should be considered when determining the practical velocity through the separator.

Horizontal Ducts Create Drainage Requirements

If gas flows horizontally through a demister, gravity acts downward.

This can be advantageous because collected liquid drains perpendicular to the main gas flow.

However, a lower collection system is required.

Liquid must be removed from the bottom of:

• mesh;
• vane pack.

If the bottom drain becomes blocked, liquid accumulates and can be re-entrained.

Drainage troughs and collection channels are therefore an important part of rectangular horizontal separators.

Vertical Mesh Panels Need Mechanical Support

Wire mesh installed vertically does not behave exactly like a horizontal pad.

Its own weight and collected liquid load act downward along the panel.

Without sufficient framing, the mesh can:

• settle;
• sag;
• create top gaps.

Gas then bypasses the active media.

A vertical mesh module therefore requires a frame that maintains its geometry over time.

Pressure Drop Should Be Checked Across the Complete Face

One pressure measurement may not reveal local maldistribution.

A large rectangular system can have:

• one blocked region;
• one overloaded region.

The average DP remains acceptable.

For critical equipment, multiple measurement points or flow surveys may provide valuable information during commissioning.

The need depends on system scale and performance sensitivity.

Spray Position Is Especially Important

Rectangular scrubbers may use multiple spray headers.

If one nozzle bank delivers more liquid to one side, the mist eliminator receives uneven liquid loading.

One section becomes wetter.

Its resistance rises.

Gas shifts toward drier sections.

The original liquid maldistribution then creates gas maldistribution.

This interaction can become self-reinforcing.

Replacement Projects Need Field Dimensions

Large duct systems may not match old drawings exactly because of:

• liners;
• corrosion;
• field modifications.

Measure:

• clear width;
• clear height;
• support positions;
• access size.

Do not manufacture modules only from nominal duct dimensions.

A small mismatch can create large continuous bypass gaps along the edge.

Module Size Should Balance Access and Hydraulics

Many small modules are easier to carry.

But more modules create:

• more frames;
• more joints;
• more potential bypass points.

Very large modules reduce joints but may be impossible to install.

The segmentation should balance:

• access;
• mechanical strength;
• open area;
• sealing.

What Data Should Be Provided?

Useful information includes:

• duct clear width and height;
• actual gas flow;
• temperature;
• pressure;
• flow direction;
• upstream elbow/transition geometry;
• liquid loading;
• access opening;
• support layout;
• drainage location.

A general statement such as “rectangular demister 2000 × 3000 mm” is not enough for a critical system.

Final Engineering Perspective

Rectangular mist eliminator sizing begins with width × height, but reliable design requires much more than average face velocity.

Gas distribution, corners, support blockage, modular joints, drainage, and upstream duct geometry all influence actual performance.

The separator should therefore be treated as part of the complete duct flow system, not simply as a rectangular panel inserted into an opening.