Sieve trays with downcomers pressure drop through downcomer

Dual-Flow Trays These are sieve trays with no downcomers (Fig. 14-27b). Liquid continuously weeps through the holes, hence their low efficiency. At peak loads they are typically 5 to 10 percent less efficient than sieve or valve trays, but as the gas rate is reduced, the efficiency gap rapidly widens, giving poor turndown. The absence of downcomers gives dual-flow trays more area, and therefore greater capacity, less entrainment, and less pressure drop, than conventional trays. Their pressure drop is further reduced by their large fractional hole area (typically 18 to 30 percent of the tower area). However, this low pressure drop also renders dual-flow trays prone to gas and liquid maldistribution. 

Sieve tray tower Design is based on a coalesced layer of 1 inch h = 2.54 cm) with 50% of pressure drop through perforations and 50% through downcomer. An 18-inch tray spacing is used. 

Perforated plates that fit into bubble columns like sieve trays are still used in cocurrent bubble columns (11). Usually the resultant free area of the openings is very small, i.e. 1 - 5 % of the total plate area. These plates may totaly suppress backmixing by forming a gas buffer beyond each plate which prevents liquid backflow. Such plates cannot be used in countercurrent flow in as much as the required pressure drop through the holes prevents the liquid downflow. Several possibilities have been suggested for the utilization of perforated plates in countercurrent bubble columns, i.e. special kinds of downcomers with or without control valves (12) (13) or additional pulsing... 

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