Demisters slope

Effectiveness of Internals in Controlling foam. Originally, the separators contained limited internals—an inlet disengaging device, surge baffles, an anti-foam baffle, and a demister at the outlet. After severe foaming occurred, however, a bank of parallel plates (Dixon plates) was filled in each separator on one of the Ninian and one of the Brent trains The plates provide a large surface area and are sloped so that the separated oil flows dow n to the surface of the liquid. The installation is illustrated m Ftg S. The parallel plates arc 4 in. apart and extend from near the top of the vessel in the vapor space down to the normal liquid level. 

To further improve the steam quality, the experienced design engineer places a dome, as shown in Fig. 15.4, on top of the kettle. The dome will have a sloped demister mesh pad. A demister acts to coalesce small droplets of water into larger droplets. The larger droplets easily settle out of the flowing steam. The demister is sloped, to promote liquid drainage from the surface of the pad. 

On many boiler steam drums, there is a sloped demister to suppress entrainment and hence improve steam quality. However, if this demister becomes partly fouled with hardness deposits, it probably does more harm than good. In such cases, I would probably discard the demister rather than replace it. If you feel you have been doing a good job on level control in the boiler, but steam quality is still poor, you should consider this possibility. 

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