Co-Knit and High-Efficiency Demister Pads

Standard knitted metal mesh is strong on inertial capture of larger droplets. When the mist is dominated by very fine droplets, fabricators use co-knit constructions: metal wire knitted together with a fiber that raises fine-mist capture.

What “co-knit” does and does not specify

Co-knit describes a construction family, not one repeatable grade. The metal wire provides structure while the second filament changes available surface area, wetting, droplet interception and liquid holdup. Fiber material, filament size, proportion, knitted geometry, finished density and pad thickness can all change the result. Two quotations that both say “wire + fiber” are therefore not necessarily equivalent.

The added filament also has its own chemical and thermal limits. A broad name such as PP, PTFE or glass fiber does not establish a complete service envelope. The named grade, continuous and upset temperature, chemical exposure, mechanical loading and support arrangement need review.

What changes

• Potentially higher collection of fine mist at comparable face velocity, subject to a stated droplet-size basis
• Usually higher pressure drop
• Different fouling behavior — fibers can trap solids more readily
• The applicable sizing K factor must be checked separately from standard mesh. Fiber construction, pressure drop, liquid holdup, drainage and the required operating margin may support a lower project value, but the direction and amount are design-specific; do not reuse the standard-mesh K without an engineering review

Diagnose the carryover before upgrading

Co-knit helps only when fine droplets are the limiting mechanism. First separate four possible causes:

1. Vapor bypass — wall gaps, open section joints or a displaced pad let untreated gas pass.
2. Hydraulic overload — high gas velocity or liquid loading causes flooding and re-entrainment.
3. Upstream upset — foaming, distributor damage or liquid slugs overwhelm an otherwise suitable pad.
4. Fine-mist penetration — the existing pad is mechanically sound and hydraulically stable, but the inlet droplet distribution is below its useful capture range.

Only the fourth case directly supports a finer medium. For the first three, adding fiber can increase pressure drop or holdup without removing the root cause.

When to consider co-knit

• Persistent fine acid mist — only after droplet-size and emission review
• Product purity limits driven by fine-mist carryover — for true sub-micron aerosols such as acid mist or SO₃ fog, specialist evaluation of a fiber-bed system may be required. Co-knit is an upgrade over standard mesh, not a fiber-bed substitute. Fiber-bed systems are outside Meshium’s current supply scope.
• Cases where a standard pad “meets velocity” but still shows carryover

When not to

• Heavy solids / gypsum service (evaluate vanes first)
• Undefined temperature/chemistry for the fiber component
• Buyers chasing “high efficiency” without a droplet size basis

Questions an engineer should answer

• What liquid is being carried over, and is it a free droplet, aerosol or condensable vapor?
• What inlet droplet-size distribution or field evidence supports a fine-mist diagnosis?
• What are minimum, normal, maximum and upset vapor rates at operating density?
• What liquid load, foaming tendency, solids content and viscosity reach the pad?
• How much clean and end-of-run pressure drop is available?
• Can collected liquid drain freely, and is an online or offline wash system present?

Where no measured droplet distribution exists, document the inference. Downstream staining, mass balance or carryover sampling can justify further testing, but appearance alone does not establish a cut size.

Procurement controls

If you request co-knit, state the metal alloy, fiber family and named grade, their proportions or offered style identification, finished density, thickness, support construction, flow direction and temperature limits. Require the supplier to identify the offered construction behind any performance curve. Efficiency marketing without a cut-size definition is not a specification.

If performance is contractual, freeze inlet loading, droplet basis, operating range, allowable pressure drop and acceptance method in the PI. If these inputs are unavailable, buy against the agreed construction and inspection evidence without turning a catalogue statement into a guarantee. Material or batch traceability should be listed in the quoted scope; special certificates and third-party inspection remain optional and must be arranged before procurement or production.

Use the wire-mesh separation guide for the mechanism boundary and the mesh-versus-vane guide when solids or high liquid load may point to a different device.

Engineering source and scope boundary

Sulzer’s gas/liquid separation guide describes composite mesh media optimized around inlet droplet distribution, liquid holdup and drainage. It also separates such mesh from fiber-bed equipment used for predominantly sub-micron aerosols. The source supports the need for a construction-specific review; performance claimed for a proprietary Sulzer style is not a transferable default for an unnamed co-knit pad.