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7.6.3. Weir heightand weir length
The depth of liquid on the tray is maintained by installing a vertical flat plate, called weir. Higher weir height (hw) increases the plate efficiency. But it increases plate pressure drop, entrainment rate and weeping tendency. Weir heights from 40 to 90 mm are common in applications for the columns operating above the atmospheric pressure. For vacuum operation, Lw= 6 to 12 mm are recommended. The weir length (Lw) determines the downcomer area. A weir length of 60 to 80% of tower diameter is normally used with segmental downcomers. The dependency of Lw on downcomer area is calculated against the percentage value of 
( [3] page 572 ).
7.6.4. Calming zones
Two blank areas called calming zone, are provided between the inlet downcomer or inlet weir and the perforation area, and also between the outlet weir and perforation area. Inlet calming zone helps in reducing excessive weeping in this area because of high vertical velocity of the entering liquid in the downward direction. Outlet calming zone allows disengagement of vapor before the liquid enters the downcomer area. A calming zone between 50 to 100mm is suggested.
8. Stepwise design tray procedure
Iterative tray design approach ( [3] page 566 ) is listed below. The design is performed separately both above feed plate (top section) and below feed plate (bottom section) for single feed two product distillation column.
Step #1 : Determine the number of theoretical plate and vapor and liquid flow-rates separately both in top and bottom sections.
Step #2 : Obtain the physical properties of the system
Step #3 : Select a trial plate spacing
Step #4 : Estimate the column diameter based on flooding considerations
Step #5 : Decide the liquid flow arrangement (reverse, single-pass, or multiple-pass). A guideline is provided in Figure 11.28 ( [3] page 568 ).
Step #6 : Make a provisional tray layout including downcomer area, active area, perforated area, hole area and size, weir height, weir length
Step #7 : Check the weeping rate, if not satisfactory go back to step #6 and reselect tray layout
Step #8 : Check the plate pressure drop, if too high return to step #6
Step #9 : Check downcomer back-up, if too high go back to step #6 or #3
Step #10 : Decide plate layout including calming zones and unperforated areas and check hole pitch, if unsatisfactory return to step #6
Step #11 : Recalculate the percentage of flooding based upon selected tower diameter
Step #12 : Check for entrainment, if too high then return to step #4
Step #13 : Optimize design: repeat steps #3 to #9 to find smallest diameter and plate spacing acceptable to get the lowest cost for the specified application
tep #14 : Finalize design: draw up the plate specification and sketch the layout