Demisting mesh

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. 

Data for the efficiency of a cyclone bank were obtained in an experimental model of a gas scrubber, incorporating also an inlet vane and a mist mat or demisting mesh under the cyclone bank , which consisted of two cyclones operating in parallel. A simple diagram of the scrubber is shown in Figure 13.7.2. 

Impingement demister systems are designed to intercept liquid particles before the gas outlet. They are usually constructed from wire mesh or metal plates and liquid droplets impinge on the internal surfaces of the mist mats or plate labyrinth as the gas weaves through the system. The intercepted droplets coalesce and move downward under gravity into the liquid phase. The plate type devices or vane packs are used where the inlet stream is dirty as they are much less vulnerable to clogging than the mist mat. 

The rectifying section contains three or four bubble cap (wine) plates in the top section of the stiU to produce distillates up to 160° proof. Whiskey stills are usually made of copper, especially in the rectifying section, which often yields a superior product. Additional copper surface in the upper section of the column may be provided by a demister, a flat disk of copper mesh. Stainless steel is also used in some stills. 

Performance of Wire Mesh Demisters , Bulletin 635, Otto H. York Co., Inc. 

The amount of hardness deposits in steam is a function of entrainment of water, as well as the TDS of the boiler feedwater. A well-designed boiler, then, often is equipped with a mesh-type demister pad to remove entrainment from the produced steam. 

FIG. 14-110 Typical impingement separators, (a) Jet impactor. (b) Wave plate, (c) Staggered channels. (Blaw-Knox Food 6- Chemical Equipment, Inc.) (d) Vane-type mist extractor. (Maloney-Crawford Tank and Mfg. Co.) (e) Peerless line separator. (Peerless Mfg. Co.) (/) Strong separator. (Strong Carlisle and Hammond.) (g) Karbate line separator. (Union Catbide Corporation) (h) Type E horizontal separator. (Wright-Austin Co.) (0 PL separator. (Ingersoll Rand.) (/) Wire-mesh demister. (Otto H. York Co.)... 

The steam may be purified by Droplet Separation techniques such as knitted wire mesh demister pads, wave plate separators and fiber pad separators using PTFE fibers. It can also be purified by scrubbing devices such as packed columns, venturi scrubbers, and irrigated sieve plates. Sometimes a combination of Droplet Separators and Scrubbers are used because AN emissions from neutralizers are very difficult to remove due to the very fine particles103. 

York, O. H. (1954) Chem. Eng. Prog. 50 (Aug.) 421. Performance of wire-mesh demisters. 

Demister on wash tank exit Gas temperature too high and demister blocked with tars readily Demister removed and replaced with appropriately sized mesh for duty required... 

Knitted mesh demisting pads are frequently used to improve the performance of separating vessels where the droplets are likely to be small, down to 1 pm, and where high separating efficiencies are required. Proprietary demister pads are available in a wide range of materials, metals and plastics thicknesses and pad densities. For liquid separators, stainless steel pads around 100 mm thick and with a nominal density of 150 kg/m would generally be used. Use of a demister pad allows a smaller vessel to be... 

Demister Separators. A demister separator is fitted either with a vane demister package or with a wire mesh demister mat. The latter type is much preferred, although it is unsuitable for fouling service. The wire mesh demister is a widely applied type of separator, and is adequate for all gas-liquid flow regimes over a wide range of gas flow rates. 

Knitted wire mesh pads are a common impingement device for removing liquid entminment from vapor or gas stresma. The Otto H. York Cosepany pioneered this concept with the Demister". Several vandors are now active in the field. 

Internal demisters are expensive, have a high plugging tendency, and their sections may disintegrate. The author is familieir with one incident where chunks of wire mesh finm an internal demister tore loose and found their way to a draw pan above and to the bottom pump. For these reasons, they should only be installed in clean services experiencing entrainment problems that cannot be eliminated by alternate means. It is a good practice to install an internal demister only in the problem regions rather than above every tray in a column. In one case (278), it was foimd that demisters installed in the nonproblem regions did little to improve performance. 

Typically, an internal demister in the intertray space is about 2 in thick and contains wire mesh. Attention must be paid to correctly specifying the mesh type, size, and materials. In a refinery vacuum column, it is also important to keep the mesh wet (eg, by spraying wash oil above the demister) in order to inhibit coking. AVhen the service has a high plugging tendency, chevrons rather than wire-mesh may be the best demister choice (270). 

Mist Eliminator (or Demister) A wire mesh pad held in place between two light grids. The mist eliminator disengages liquids contained in the vapor. 

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