Type 4
Wire mesh demisters
Like a vane type separator, a wire mesh demister works on the principle of mass inertia. When droplets to be separated are significantly smaller than 10 µm, a wire mesh demister is the right choice. The demister is built up from thin wire knitted into a three-dimensional mat, which can be made in almost any shape and used in vertical-flow applications. Smaller diameters are produced as one piece; larger surfaces are split into segments so the demister can be removed through a manhole.
The mat composition determines efficiency: thinner wire packed more densely separates the smallest droplets. Rinsing a wire mesh demister is difficult, so its use is limited to liquids that do not foul the mesh.
Typical performance is 99.9% separation for limit droplet sizes of 5–20 µm. Pressure losses are 1.5–4 mbar — appreciably higher than vertical vane separators — and depend strongly on the liquid load. Because they clog easily, wire mesh demisters serve only as a fine-separation stage.
Droplets pass through the mat, collide with the wire surface due to inertia, agglomerate at the wire intersections and fall back as larger droplets. Performance increases with gas velocity; above the optimum, flooding sets in and droplets are re-entrained. Through special construction, deposit sizes down to 3 µm can be achieved with 99.9% fractional separation.
Wire diameters typically vary between 0.1 mm and 0.5 mm; specific surface areas range from about 100 m²/m³ to 1,100 m²/m³. Specially designed support and cover grids provide roughly 90% free flow area. The mat must fit snugly to the column wall to prevent bypass. Standard mat heights are 100–150 mm; condensation-driven fine droplets may require taller or multi-layer constructions.
Sizing — the Souders–Brown equation
The same fundamentals lead directly to a sizing procedure. In a given application, a droplet separator has an optimal operating range. Above it, performance is hampered by entrainment and flooding; below it, efficiency for small droplets drops faster than for large ones until the smallest droplets fall to an unacceptable level — the lower velocity limit.
A widely used formula generalises performance from air/water to other gas/liquid systems. The Souders–Brown equation has long been the standard for the maximum allowable vapour velocity in a packed column:
The K-factor expresses an effective gas velocity adjusted for liquid and gas density — a single parameter that lets the separator be sized from collected air/water data. Design K-factors for both horizontal and vertical separators apply only at low to moderate liquid loadings (up to ~0.1 vol-%, equivalent to about 0.5 l/min·m² at 3 m/s); above that, K must be corrected. Lamella separators are less sensitive to liquid-load effects on capacity.
Source: M. Souders and G. G. Brown, Design of Fractionating Columns. I. Entrainment and Capacity, Industrial & Engineering Chemistry 26 (1934), pp. 98–103.
