Mist Extraction - Page 2

Gas flow in separators is seldom laminar due to the very low viscosity of gas, so Stokes Law does not apply directly. An empirical equation based on Stokes is used and assumes all particles down to an acceptable size are separated.

V = Allowable gas velocity through separator, ft/sec
k = Empirical constant

 The velocity is easily calculated. It is the actual volume of gas flowing in ft3/sec (not standard ft3/sec) divided by the cross-sectional vessel area devoted to gas, sq. ft.

The "K" value is empirically determined and is approximately:

0.16 x (sep. length, ft) 1/2 -

Horizontal separators to 20' long with parallel plate vanes

0.35 -

Vertical separators

0.35 -

Wire mesh in vertical separators or columns

With this formula and the "K" values, it is possible to size vertical and horizontal separators to separate bulk liquid and mist particles from gas.

However, if the steam is cooling it may be loaded with a fog of very tiny particles which do not settle with gravity. These particles of liquid will wet any solid surface they strike. Placing a large surface area in the flow path in such a way that the chances of a collision with a particle is almost certain as a means of fog removal. This large area could be, in its most common form, a pad of wire mesh, 4 to 8 inches thick. One commonly used knitted and crimped mesh has a density of 9lb/ft3, consisting of stainless steel x .011" O.D. wires of stainless steel, and 85 sq. ft. wire surface area cu. ft.

Mesh will coalesce virtually all of the 10 micron particles, and larger, and perhaps half of the 3 micron particles. Mesh also collects paraffin, hydrates or solid particles, if present, resulting in eventual plugging of the mesh. For this reason mesh is used more often in clean gas streams, such as in the top of a glycol-gas contact column. The collection efficiency of mesh is calculable by a method described in a paper by B.J. Warner and Frank Scauzillo of Mobile and published in the 1963 Proceedings of the Gas Conditioning Conference of the University of Oklahoma.

The impingement technique can be used with much less plugging possibility by using a pack of closely spaced vertical plate assembly vanes shown in previous figure 1. This configuration creates many flow direction changes causing centrifugal force to drive the stream to the outside of each turn while creating a drain path on the inside of each turn. An 8" section of plate assembly vanes has a particle removal capability similar to mesh, and has far better drainage characteristics, resulting in less plugging.