Procedure 5-11 Design of Demisters

Demister pads or mist eliminators, are important internals in process vessels. Anytime there is a continuous two phase flow, vapor and liquid, there is the possibility for liquid entrainment. If it is desirable to separate the liquid and vapor, or prevent liquid carryover in the vapor stream, the velocity must be kept sufficiently low to allow flie liquid droplets to fall out of the vapor stream. Demister pads are effective entrainment separators which allow operation at vapor velocities that would otherwise be excessive. 

Demister pads are used in a wide variety of process vessels. Good performance can be achieved at velocities of 30% to 110% of optimum. 

Demister pads can be used in vertical or horizontal vessels. The mesh pad orientation can be either vertical or horizontal in a vertical or horizontal vessel. 

A wire mesh demister consists of a knitted wire mat mounted on a light weight support grid. The mesh is made by weaving small diameter metal wire into a mesh. Pads may be made out of metal or plastics. Metal type include carbon steel, stainless steel, monel and others. Plastic materials include PTFE, HDPE and PP. 

Pressure drop is usually extremely low, in the range of 1 inch of water column maximum. The pressure drop is normally so low that it is ignored in design. 

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