How to Size a Mist Eliminator for an Annular or Obstructed Vessel Cross Section

Many mist eliminator sizing calculations assume a simple open circular vessel.

The active area is then calculated from:

A=πD24A=\frac{\pi D^2}{4}

But some process vessels contain a central obstruction such as:

• large pipe;
• draft tube;
• shaft;
• internal chimney.

The gas therefore flows through an annular or partially obstructed cross section.

Using the full vessel diameter can overestimate available separator area and underestimate gas velocity.

This creates a hidden hydraulic risk.

What Is an Annular Flow Area?

An annulus is the area between:

• outer circular diameter;
• inner circular obstruction.

For a centered circular obstruction:

A=π4(Do2−Di2)A=\frac{\pi}{4}\left(D_o^2-D_i^2\right)

where:

• DoD_o = outer available diameter;
• DiD_i = inner blocked diameter.

This is the basic geometric area available for gas flow.

It can be significantly smaller than the complete vessel cross section.

Why Using Vessel Diameter Alone Is Wrong

Suppose a vessel has a large central pipe.

If an engineer calculates:

Q/(πDo2/4)Q/\left(\pi D_o^2/4\right)

the blocked center is being treated as though gas can pass through it.

The calculated face velocity becomes artificially low.

The actual gas velocity through the open annulus is higher.

This can reduce:

• re-entrainment margin;
• pressure-drop margin.

Support Structures Reduce Area Further

The annular geometry is only the first correction.

The real active area may also be reduced by:

• support beams;
• frames;
• drain pipes;
• mounting brackets.

Therefore, geometric annular area and hydraulic active area may still differ.

A large central obstruction combined with heavy supports can make this difference substantial.

Gas Distribution Around the Annulus May Not Be Uniform

Even if the obstruction is perfectly centered, gas does not necessarily divide uniformly around the ring.

Upstream inlet geometry can send more flow through:

• one side.

A side inlet or elbow can create high local velocity in part of the annulus.

Therefore, average annular velocity is only the first hydraulic check.

Local distribution still matters.

Off-Center Obstructions Are More Difficult

If the central pipe or chimney is not centered, the available gap width varies around the circumference.

One side has a narrow passage.

The other side has a wider passage.

Gas resistance differs.

This can create strong:

• maldistribution.

The narrow region may experience high local velocity or, depending on upstream pressure field, the wider region may attract more flow.

A simple annular-area calculation cannot fully describe this geometry.

Demister Segmentation Becomes More Complex

A circular mesh pad can be divided into:

• wedge-shaped;
• strip-shaped segments.

An annular separator also requires a central cutout.

The segments must fit both:

• outer vessel wall;
• inner obstruction.

This creates two perimeter sealing requirements.

Gas can bypass around:

• outer edge;
• inner edge.

The central perimeter therefore deserves the same attention as the vessel wall.

Inner-Edge Bypass Is Easy to Miss

Engineers naturally inspect the outside circumference of the separator.

But an annular pad has a second circumference around the central object.

If this gap is open, gas can travel directly beside:

• pipe;
• shaft.

The bypass area may be small geometrically but attractive hydraulically because it has very low resistance.

The separator can show poor efficiency despite correct outer sealing.

Thermal Expansion Can Change Both Gaps

The outer vessel, central pipe, and mist eliminator may use different materials.

During operation, each expands differently.

The annular fit can change at both boundaries.

For large high-temperature equipment, differential thermal movement should be reviewed.

A pad that fits both edges perfectly cold may be:

• compressed;
• loose

when hot.

Drainage Around the Inner Obstruction Matters

Liquid collected near the central boundary needs a clear path downward.

A poorly shaped inner frame can create a horizontal shelf.

Liquid pools.

Gas passing nearby can re-entrain it.

The inner edge should therefore provide:

• sealing;
• drainage.

Mechanical fit alone is not enough.

Vane Packs Can Be More Challenging Geometrically

Vane modules are normally easier to fabricate as:

• rectangular blocks.

Installing them inside an annular area can create complex unused regions unless the support system converts the annulus into suitable module shapes.

Every inactive region reduces hydraulic area.

The actual open separator face should therefore be calculated from the final module layout—not merely from vessel geometry.

Wire Mesh Offers Greater Shape Flexibility

Knitted wire mesh can be fabricated into:

• curved;
• irregular segments.

This can help match annular vessel geometry.

However, complicated segmentation creates more joints.

Each joint is a potential bypass location.

Installation quality becomes especially important.

Access Can Control Segment Size

A ring-shaped separator may need to enter through a small manway.

Segments must be small enough to pass through the opening.

But excessive segmentation creates:

• more joints;
• more frames.

The design should balance:

• access;
• sealing;
• structural integrity.

Replacement Projects Need Field Measurements

Do not rely entirely on old nominal drawings.

Measure:

• vessel inside diameter;
• central obstruction OD;
• eccentricity if any;
• support positions;
• lining thickness.

Corrosion linings or field modifications can change the actual annular dimensions.

A few centimeters of mismatch can create a continuous bypass gap.

Pressure Drop Should Use the Real Open Area

Once the final support and module arrangement is known, use the real effective area for hydraulic review.

This is particularly important where:

• obstruction ratio is large.

A calculation based on full vessel area can significantly underpredict:

• local velocity;
• DP.

What Should Be Included in the RFQ?

Provide:

• outer clear diameter;
• central obstruction diameter;
• obstruction position;
• gas flow;
• flow direction;
• support arrangement;
• access opening;
• operating temperature.

A drawing is especially useful because irregular geometry can be difficult to communicate through dimensions alone.

Final Engineering Perspective

Mist eliminator sizing should use the gas area that actually exists.

In an annular or obstructed vessel, the full circular vessel cross section is not available.

The central obstruction changes:

• velocity;
• segmentation;
• edge sealing;
• drainage;
• gas distribution.

Reliable design therefore requires treating the separator as an annular hydraulic system with both inner and outer boundaries.