Perforated area

The area available for perforation will be reduced by the obstruction caused by structural members (the support rings and beams), and by the use of calming zones. 

Calming zones are unperforated strips of plate at the inlet and outlet sides of the plate. The width of each zone is usually made the same recommended values are below 1.5 m diameter, 75 mm above, 100 mm. 

The width of the support ring for sectional plates will normally be 50 to 75 mm the support ring should not extend into the downcomer area. A strip of unperforated plate will be left round the edge of cartridge-type trays to stiffen the plate. 

The hole sizes used vary from 2.5 to 12 mm 5 mm is the preferred size. Larger holes are occasionally used for fouling systems. The holes are drilled or punched. Punching is cheaper, but the minimum size of hole that can be punched will depend on the plate thickness. For carbon steel, hole sizes approximately equal to the plate thickness can be punched, but for stainless steel the minimum hole size that can be punched is about twice the plate thickness. Typical plate thicknesses used are 5 mm (3/16 in.) for carbon steel, and 3 mm (12 gauge) for stainless steel. 

When punched plates are used they should be installed with the direction of punching upward. Punching forms a slight nozzle, and reversing the plate will increase the pressure drop. 

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Prompt attention to, and covering of, damaged or perforated areas of skin. 

Assume a tray layout downcomer areas, non-perforat-ed area perforated area. Base downcomer requirements on bubble cap tray information of Figure 8-100. 

Percent hole area = 12.8% (of perforation area only) as shown in Figure 8-143. 

Square and equilateral triangular patterns are used triangular is preferred. The total hole area as a fraction of the perforated area Ap is given by the following expression, for an equilateral triangular pitch ... 

Use same diameter above and below feed, reducing the perforated area for plates above the feed. 

COATING PAN Pan dimensions. Par speed. TaMet charae- Bed porosity. Surface area Mixino baffles. Perforated area. Bed temperature. 

The alternative philosophy (equal flow path lengths) provides more bubbling and perforation areas in the central panels of Fig. 14-21d and less in the side panels. To equalize the L/G ratio, less liquid needs to flow toward the sides, which is readily achieved, as the center weirs are naturally longer than the side weirs. Usually there is no need for swept-back weirs, and only minimal picket-fencing is required at the center weir. 

Specimens that contain materials with very different ion-milling rates, such as metallic multilayers grown on silicon substrates, often tend to form bridges of material across the perforated area. Ion-milling at very low angles of incidence ( 1-2°) in a direction parallel to the interface can sometimes be used to overcome or at least alleviate these bridging problems. Finally, it should be noted that the use of a crystalline substrate such as silicon provides a convenient reference material for specimen orientation purposes in the TEM. Examination of the substrate EDP can be used to ensure that the substrate normal is aligned exactly perpendicular to the electron beam direction. The thin-film microstructure can then be easily determined. 

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