Mesh pads pressure drop

Figure 6a is a report of a wire mesh pad performwice using air-water mist mixture. In Figure 6a, wire mesh pad pressure drop versus air velocity is plotted at several mist (water droplet) loading. Ihese curves show that (1) There is a vapor velocity operating range 3 to 11 ft/sec) for a wire mesh pad to perform well. Wire mesh pad turn down ratio is about 4 to 1. (2) Pressure drop thru wire mesh pad is small, less than 1 water column (wc). 

Filters for mists and droplets have more open area than those used for dry parhcles. If a filter is made of many fine, closely spaced fibers, it will become wet due to the collected liquid. Such wethng will lead to mathng of the fibers, retenhon of more liquid, and eventual blocking of the fiter. Therefore, instead of fine, closely spaced fibers, the usual wet filtrahon system is composed of either knitted wire or wire mesh packed into a pad. A looser filtrahon medium results in a filter with a lower pressure drop than that of the filters used for dry parhculates. The reported pressure drop across wire mesh mist eliminators is 1-2 cm of water at face velocihes of 5 m sec T The essenhal collechon mechanisms employed for filtrahon of droplets and mists are inertial impachon and, to a lesser extent, direct intercephon. 

Pressure drop through wire mesh units is usually very low, in the order of 1-inch water gauge for a 4-inch or flinch thick pad. For most pressure applications this is negligible. If solids are present in the particle stream, then... 

Impingement type mist extractor exemplified by knitted wire mesh pads are widely used in oilfield liquid and gas separation equipment. Their liquid and gas handling capacities are good with high liquid droplet removal efficiencies with small gas pressure drop. 

A wire-mesh pad in a horizontal drum greatly enhances separation and significantly reduces drum size. For lower pressure drop across the pad. the requirement for a high-performance pad can be relaxed by designing so the vapor space removes larger droplets, reducing the load on the pad. 

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Figure 6.6 shows a design for a decanter. After the two-phase mixture enters the decanter at the feed nozzle, the liquid jet must be diffused to prevent mixing of the two phases and promote settling of the dispersed phase. One way to accomplish this is to insert two closely spaced, perforated parallel plates across the jet, as shown in Figure 6.6. The first plate drops the pressure of the jet, and the second plate decreases its velocity. Jacobs and Penny [17] recommend that the flow area of the first plate be 3 to 10% of the decanter flow area, and the second plate 20 to 50% of the decanter flow area. Another way to disperse the entering liquid jet, and at the same time enhance coalescence of the dispersed phase, is to use a wire-mesh pad in front of the feed nozzle.

Frequently vacuum gas oils are condensed in packed sections. If several zones are used, there will be opportunity for better heat efficiency. Structured packing, sheet-metal type, has largely displaced trays for contact in vacuum pumparound zones because packing requires less pressure drop. Thus, flash zone pressures can be lower and less vacuum pitch can be produced. Flash zone pressures as low as 25Torr (millimeters of mercury absolute) have become routine. For heat efficiency, a corrosion-resistant de-entrainment mesh pad is often installed above each pumparound packed zone. 

Pads of fine wire mesh induce coalescence of impinging droplets into larger ones, which then separate freely from the gas phase. No standard equations have been developed for the pressure drop across wire mesh because there are no standardized mesh pads. However, as a rule of thumb, the pressure drop of a wire mesh is AP = 1.0 in H2O. Every manufacturer makes a standard high efficiency, very-high efficiency, or high-throughput mesh under various trade names, each for a specific requirement. 

Pressure drop across a mesh pad is usually less than 1 in. H20 and is of little concern except in vacuum operations or systems involving large flows with fans or blowers at near atmospheric pressure. Pressure drop is estimated by Ap pc,. where Ap is the pressure drop (in in. H50), Vc is the gas velocity (in fris), and pc (he vapor density (in Ibn/ft. C = 0.2 for a standard-density 4 in. thick pad, and C = 0,12 for a low-density 6 in. thick pad. For situations where pressure drop is critical, methods presented... 

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