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TEX-MESH Technical Bulletin 102

The size of a knitted mesh mist eliminator is dictated generally by one of three constraints: vapor velocity at the system limit for gravity separation, vapor velocity at which re-entrainment occurs, or pressure drop. In most cases, the system limit vapor velocity is the design constraint. However, if the volume fraction of entrained liquid is large (for example, greater than 0.1%), then re-entrainment load becomes the design constraint Although pressure drop for a TEX-MESH mist eliminator is small (typically less than two inches water for a six inch pad), pressure drop occasionally becomes a design constraint.

The collection efficiency of a mist eliminator increases with vapor velocity up to the reentrainment point.

Efficiency also increases with pad thickness. The standard TEX-MESH pad thickness is six inches. In some cases, the improvement in efficiency justifies the increased cost. However, there is definitely not a direct relationship.

For efficiency, pressure drop, and reentrainment constraints, refer to TEX-MESH Technical Bulletin 101 which contains performance curves for the most popular models of TEX-MESH mist eliminators.

The system limit for gravity separation of liquid droplets in vapor is the critical constraint in most cases for sizing knitted mesh mist eliminators. The entrainment velocity is a flow condition for which the upward drag force on a liquid droplet is greater than the force of gravity. As droplet size increases, the entrainment velocity increases. At a point called the system limit, more droplets rise with the vapor than fall. This point was characterized mathematically for distillation columns by Sounders and Brown. Consequently, entrainment system limit has come to be characterized by a system of independent velocity called the Sounders-Brown velocity (also called a "K" factor). The Sounders-Brown velocity is defined by:

VSB=VV*(RHOV/(RHOL-RHOV))^0.5
where VSB = Sounders-Brown velocity = K factor, ft/sec
VV=Vapor Velocity, ft/sec
RHOV=Vapor Density, lb/cuft
RHOL=Liquid Density, lb/cuft

For most vapor, countercurrent systems below a pressure of 90 psia, the entrainment systems limit occurs at a Sounders-Brown velocity of approximately 0.4 ft/sec. Consequently, the design K factor for mist eliminators low pressure service is 0.35 ft/sec. This basis provides about a 12% margin of safety. Moreover, the design vapor flow usually has a safety factor already.

At higher pressure, a smaller factor should be used. The particular value depends on systems properties such as surface tension and critical pressure. An experienced designer should be consulted for high-pressure applications.

Equation 1 can be transformed into a design equation by expressing the Sounders-Brown velocity by a K factor of 0.35 ft/sec, the vapor velocity as a volumetric flow rate divided by the cross-sectional area, and rearranging.

A=CFM*(RHOV/(RHOL-RHOV))^0.5/(0.35*60)
where A=Cross-sectional area flow, sq/ft
CFM=Vapor volumetric flow rate, cuft/min

This equation, which uses a K factor of 0.35 ft/sec, can be used to determine the size of up flow mist eliminators in low-pressure systems with a liquid entrainment load less than 0. 1 % (v/v). For wire mesh mist eliminators in horizontal flow, the maximum K factor is 0.5 ft/sec.

Vane type mist eliminators have different drainage characteristics than wire mesh mist eliminators. Consequently, vane units can be sized according to a K factor of 0.5 ft/sec for vertical up flow (or 0.65 ft/sec for horizontal flow). Vane units are discussed in more detail in Tex Mesh Technical Bulletin 104.

Final determination of mist eliminator size should be coordinated with TEX-MESH professionals.