Demister pads

Demister pads or mist eliminators may be either of mesh type or of vane type. The most popular demisters are wire meshes knitted in the form of a pad. The knitted pad has about 97 to 98% void space and it collects particles primarily by impingement. A liquid particle striking the wire surface, which it does not wet, flows downwards where the adjacent wires provide some capillary space. At these points, liquid collects and continues to flow downward. Surface tension tends to hold these drops on (he lower face of the pad until they are large enough for the downward force of gravity (o exceed (hat of the upward gas velocity and surface tension.

The efficiency of the pad is a function of pad thickness, wire diameter, closeness of weave, etc. The thickness of the pad is not very critical since droplet coalescence is completed in the first one or two inches and the extra thickness is mainly for catching the re-entrained material. Thickness usually provided is between 100 mm and 300 mm.

A vane type demister consists of a number of parallel corrugated plates. When gas passes between the plates it has to change direction a number of times. The heavier particles are thrown out and get collected in the pockets. Coalescence of small particles into bigger particles is provided by agitation and surface contact. The surface of the element is usually wet and particles striking it are absorbed into the liquid film. The efficiency of the vane type demister is a function of pressure drop since closer plates introduce more pressure drop.