Vertical Knockout Drum – Wire Mesh Mist Eliminators

Generically known as demister pads, wire mesh has been used for decades as a contacting media for coalescing liquid aerosols. First introduced as a woven wire screen, the construction was relatively stiff and usually applied as a thin, single layer. The technically superior design now predominantly used is a thick knitted wire mesh pad.

Knitted mesh pads possess high void volume and contact surface area. The result is high removal efficiencies for aerosols smaller in size than what can be removed by vane mist eliminators. Figure P-2 and P-4 illustrate these efficiencies.

The variables that control void volume, area and density of the mesh are:
1. Wire size – typically .002″ to 0.020″ in diameter
2. Size of knitted loops
3. Amplitude of wire crimping for added surface area.

Pad density varies from 2 to 24 lb per cubic foot and can possess surface areas from 44 to 1200 square feet per cubic foot of mesh. Separation efficiency is governed by the relation of wire diameter and density. As wire diameter decreases, density and efficiency increase. Practical application requirements and cost narrow the range of styles readily available, although a large array of styles and materials are still available.

As an example, a commonly used selection would consist of 0.011″ diameter wire mesh possessing a void volume of approximately 97%, have a density of 12 lb/cu.ft., and a surface area of approximately 110 sq. ft./cu. ft. The mesh is typically available in thicknesses of 4″, 6″ or 12″, or can be rolled into a much thicker pad configuration for special applications.

Mesh pads can be made from almost any material that can be extruded or drawn. In addition to carbon steel alloys, some examples of other available materials are:

• 300 and 400 Series Stainless Steel
• Titanium, Tantalum
• Copper, Aluminum
• Synthetics (Polypropylene, Teflon)

The material of choice depends on the known characteristics of the contaminants in the gas. Representative samples of the gas for analysis are recommended prior to making design changes or specifying a new purchase. Mesh pad manufacturers and CRTC materials specialists are excellent resources for consultation on material choice.

Figure P-7 is a guide of K values for standard demister pad and vane styles offered by Otto York Company. Special conditions which change this value are discussed in the Section, “Effects of Pressure on K Value.” Similarly, other manufacturers should offer recommended K values for their products.

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