to the vertical. This angle is large enough for good drainage of liquid, avoiding stagnant pockets and regions of liquid accumulation, and small enough to prevent gas from bypassing the metal surfaces.

Surface features. The surfaces of a few structured packings are smooth (e.g., Fig. 8.10a,g). Most structured packings have a roughened or enhanced surface that assists the lateral spread of liquid, promotes film turbulence, and enhances the area available for mass transfer. Laboratory measurements of absorption rates showed that both mass transfer efficiency and wetted area are enhanced by texturing metal surfaces (28). The extent to which mass transfer was improved varied with the type of texturing used. Texturing employed by common structured packings includes grooving, lancing, shallow embossing, and deep embossing {Fig. 8.106 to e). One structured packing is fabricated from expanded metal (Fig. 8.10/").

The surfaces of most structured packings contain perforations (an exception is Fig. 8.10c0. The holes serve as communication channels between the upper and lower surface of each sheet. If the holes are too small, or nonexistent, both sides of a sheet will be wet only at low liquid rates. At high liquid rates, "sheeting" or "blanking" will cause liquid to run down the top surface with little liquid wetting the bottom surface (23). This may cause a reduction in efficiency as liquid flow rates are raised. A counterargument by suppliers of packings that contain no holes is that the holes impede the spread of liquid across a sheet, thus adversely affecting the surface action (27). Usually, the holes are of circular shape (Fig. 8.10a to f), but one design (Fig. 8.10^) employs slits in the shape of a W, with the tabs formed by the cuts bent to direct liquid to the opposite side of the sheet.

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