The amount of riser area is a function of vapor velocity and the allowable pressure drop across the total draw-off tray. Does this unit have a higher L/V ratio than some other units? It is especially important that the riser covers be designed to ensure the liquid leaving the cover is protected from the vapor exiting the riser so it is not entrained up the tower. A book by Henry Z. Kister, “Distillation Design”, gives some guidance for designing total collectors and the residence time required to clear the liquid of vapor before it is withdrawn. Also the size and the configuration of the piping to the nozzle outside the tower are also important and can affect the ability to draw the liquid off of the collector.