Chimney trays are used to provide good vapor distribution to the internals above, liquid collection and mixing for drawoff or internal reflux, and liquid holdup. In columns containing packing or grid, especially in vacuum towers where pressure drop is critical, good chimney tray design is essential to provide uniform vapor distribution while minimizing pressure drop. If trays are used above the chimney tray, vapor distribution is not critical since the pressure drop of the trays will normally even out the vapor flow.

A chimney tray should be used when:

• Appreciable liquid holdup is required, as for product surge or for water settling, 

• When leakage cannot be tolerated for process reasons, such as, in water wash sections where leakage into amine or caustic solutions is unacceptable