For most trays, liquid flows across an “active area” of the tray and then into a “downcomer” to the next tray below, etc. Inlet and/or outlet weirs control the liquid distribution across the tray. Vapor flows up the tower and passes through the tray active area, bubbling up through (and thus contacting) the liquid flowing across the tray. The vapor distribution is controlled by (1) perforations in the tray deck (sieve trays), (2) bubble caps (bubble cap trays), or (3) valves (valve trays).

Trays operate within a hydraulic envelope. At excessively high vapor rates, liquid is carried upward from one tray to the next (essentially backmixing the liquid phase in the tower). For valve trays and sieve trays, a capacity limit can be reached at low vapor rates when liquid falls through the tray floor rather than being forced across the active area into the downcomers. Because the liquid does not flow across the trays, it misses contact with the vapor, and the separation efficiency drops dramatically.

Trays are generally divided into four categories: (1) sieve trays, (2) valve trays, (3) bubble cap trays, and (4) high capacity/high efficiency trays.

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