Vapor Velocity in Mist Eliminators

Mist eliminators, consisting either of knitted wire mesh pads (demisters) or corrugated sheet metal vanes, are often used in critical services to improve separation of entrained liquid. Droplets that are too small to settle strike the wires or the vanes, collecting to form larger drops. The larger drops run down the wires or vanes and fall from the bottom. Generally, the wire mesh type is preferred because of its better collection efficiency for droplets in the 1 to 10 micron size range. A vane-type device usually has a higher vapor capacity and is more likely to resist plugging by solids and viscous liquids.

A demister may be included in the design of a new separator on the basis of good experience in similar service. A demister may be added to an existing separator to improve removal of liquid from vapor, provided that it is known that poor separation is not the result of poor level control, plug flow, wave action, or some other factor.

For most applications, use a wire mesh demister, Yorkmesh Type 431 or equivalent, 6 inches thick. Optimum superficial velocity (ignoring the space taken up by wires) is given by:

V = Optimum actual vapor velocity, ft/sec

Wire mesh demisters are reported to work well at velocities 30% to 110% of optimum; therefore, the design velocity is usually 75% of optimum (from Equation 300-3). Note that this design velocity is still above Vc from Equation 300-2 (use of a demister does not influence the vessel diameter).

Vane-type mist eliminators are also sized with Equation 300-3 but with higher velocities usually allowed. For its product, York recommends a constant of 0.40 (in place of 0.35) for upflow and 0.65 for horizontal flow. This factor appears adequate; however, the Company has little experience with vane-type mist eliminators in applications where their performance can be accurately determined.

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