Calming Zones

As noted, the weir crest is calculated on an equivalent clear-liquid basis. A more realistic approach is to recognize that in general a froth or spray flows over the outlet weir (settling can occur upstream of the weir if a large calming zone with no dispersers is used). Bennett et al. [AIChE J., 29, 434 (1983)] allowed for froth overflow in a comprehensive study of pressure drop across sieve plates their correlation for residual pressure drop /jf in Eq. (14-87) is represented by Eqs. (14-115) through (14-120) ... 

Decide plate layout details calming zones, unperforated areas. Check hole pitch, if unsatisfactory return to step 6 (Section 11.13.11). 

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. 

Use cartridge-type construction. Allow 50 mm unperforated strip round plate edge 50 mm wide calming zones. 

Note that the group on the left side of Eq. (14-171) is dimensionless. When turbulence promoters are used at the inlet-gas section, an improvement in gas mass-transfer coefficient for absorption of water vapor by sulfuric acid was observed by Greenewalt Ind. Eng. Chem., 18, 1291 (1926)]. A falling off of the rate of mass transfer below that indicated in Eq. (14-171) was observed by Cogan and Cogan (thesis, Massachusetts Institute of Technology, 1932) when a calming zone preceded the gas inlet in ammonia absorption (Fig. 14-77). 

Allow a 60 mm unperforated strip on the outer edge, and a 100 mm calming zone between the downcomers and the active zone. 

Even though froth actually flows over the weir (unless calming zones are nsed), h is expressed on an eqnivalent clear liqnid basis, assuming that the Francis relationship also represents froth flow. Later, in connection with mass transfer in tray froths, we will discnss a relative froth density ... 

It has been common practice to provide a blank area between the inlet downcomer or inlet weir and the hole field, and another blank area between the hole field and the outlet weir (Fig. 6.3). These blank areas are termed calming zones. 

The minimum width of the outlet calming zone is often limited to about 2 in hy the presence of a tray support beam (73). 

A splash baffle is recommended when liquid flow rate is less than 0.1 gpm per inch of outlet weir (144). One successful application of tailor-designed sieve trays with splash baffles at liquid rates ranging from 0.01 to 0.07 gpm per inch of weir has been described (374). Splash baffles should only be used at low liquid loads, because they restrict the downcomer inlet area and can lead to premature downcomer choke. A calming zone upstream of the baffle is sometimes used to minimize this problem (374). Splash baffles also increase tray pressure drop and froth regime entrainment (31). 

A recessed seal pan distributes liquid to the tray with an upward vertical motion rather than a horizontal motion containing a vertical downflow component. This results in better aeration at the inlet edge, increases both capacity and efficiency, and avoids precipitant weeping. Using a recessed seal pan can also be exploited to reduce the tray s inlet calming zone. 

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