How Wire Mesh Demisters Work

A wire mesh demister (mist eliminator pad) removes entrained liquid droplets from a vapor stream. It is not a particulate filter: vapor must pass with low pressure drop while droplets collide, coalesce and drain.

Follow one droplet through the pad

Separation is a sequence, not a single impact. A droplet must first contact a wire, remain on the wetted surface, merge with other liquid and then drain against the vapor flow. If any step fails, captured liquid can be re-entrained. This is why wire diameter, mesh geometry, liquid surface tension, viscosity, liquid load, pad orientation and support details all matter.

The wire surface does not act like a sieve opening. Droplets smaller than the apparent mesh opening can still be collected because their inertia or trajectory brings them into contact with a wire. Conversely, a dense-looking pad can perform poorly when liquid cannot drain, vapor bypasses the edge or the pad is overloaded.

Three separation mechanisms

1. Inertial impaction — Larger droplets (often on the order of tens of microns) cannot follow vapor streamlines around the wires. They hit the wire, wet the surface and merge into a film.
2. Direct interception — Mid-size mist droplets follow streamlines that pass within about one droplet radius of a wire and are captured. This is a major contributor in the few-micron range where knitted mesh is commonly applied.
3. Brownian diffusion — Sub-micron fog wanders randomly and can still contact a filament, but standard knitted mesh alone is usually weak here. Co-knit (wire + fiber) pads can improve capture of fine mist; predominantly sub-micron duties require specialist evaluation and may require a fiber-bed system. Co-knit is an upgrade over standard mesh, not a like-for-like fiber-bed substitute.

Exact cut sizes vary with velocity, wire diameter and liquid properties. Industry references (for example mist-eliminator design manuals) typically treat knitted mesh as strong on mist droplets of a few microns and above, not as a substitute for fiber-bed fog elimination. A pad that works in a knock-out drum can still flood in a high-liquid-load scrubber.

What efficiency claims mean

Statements such as “99%+ removal” are only meaningful with a stated droplet cut size and operating velocity. For preliminary sizing, engineers often use the Souders–Brown approach to set a maximum vapor velocity before re-entrainment. Final design still needs vessel geometry, inlet distribution and drainage review.

The familiar form is v = K × √((ρL − ρV) / ρV), where v is allowable superficial vapor velocity and ρL and ρV are liquid and vapor density at the operating condition. Because the density ratio is dimensionless, K carries the same velocity unit as v; an imperial K value cannot be entered into an SI calculation without conversion. K is not a material constant and is not supplied by the equation. It represents the selected separator construction and operating margin; pressure, liquid load, foaming, fouling, orientation and downstream consequence can all change the project value. The result is a velocity check, not a guaranteed collection efficiency.

For an efficiency statement to be reviewable, ask for all of the following:

• droplet size or size distribution at the demister inlet;
• inlet liquid loading and the basis for that estimate;
• minimum, normal and maximum vapor cases at operating pressure and temperature;
• allowable pressure drop and the condition at which it applies;
• whether the quoted result is a calculation, a test on the offered construction or a general catalogue claim.

Drainage, flooding and bypass

Liquid collected on the mesh must leave the pad. As vapor velocity or liquid loading rises, liquid holdup increases until drainage becomes unstable. Re-entrainment can then rise sharply even though the same pad performed well at a lower load. Foaming liquids, viscous liquids and solids can narrow the stable operating range.

Mechanical fit is equally important. Gaps at the vessel wall or between sections provide a lower-resistance route around the mesh. A pad can therefore have the specified density and thickness yet fail because vapor bypasses it. Review the support ring, hold-down grid, section joints and drainage clearance with the media selection.

A buyer’s specification should connect media to duty

Do not buy only “150 mm thick stainless demister.” Freeze the operating cases, material grade, wire or monofilament basis, finished pad density and thickness, flow direction, grid and tie-wire materials, section map, wall seal, support interface and required inspection records. If performance is contractual, state the droplet basis, inlet loading, operating envelope, allowable pressure drop and acceptance method in the PI rather than relying on a product name.

For preliminary comparison, use the demister sizing calculator with a documented project K value. Then use the mesh pad versus vane pack guide to check whether the liquid load and fouling mechanism make knitted mesh the right separator family.

Practical takeaway

If your mist is mostly coarse entrainment at moderate velocity, an ordinary knitted metal pad may be a reasonable first candidate; the alloy still depends on chemistry. Fine mist may call for co-knit mesh; fouling slurry or very high liquid load may point to a vane pack or a combined mesh + vane arrangement — not simply a denser pad. Predominantly sub-micron acid mist requires specialist evaluation and may require a fiber-bed system: see the sulfuric acid mist guide for that selection boundary.

Engineering sources and limits

Sulzer’s Gas/Liquid Separation Technology and Koch-Glitsch’s Mist Elimination and Phase Separations describe the same mechanism hierarchy and the different operating envelopes of mesh, vane and fiber-bed devices. They are useful engineering references, but their proprietary style performance is not automatically transferable to another construction. Project sizing still requires the offered media geometry, operating data and a stated design basis.