Residual gas pressure head (hr) :
The residual pressure drop results mainly from the surface tension as the gas releases from a perforation. The following simple equation can be used to estimate hr with reasonable accuracy ( [3] page 575 ).
 |
(7.10) |
Downcomer backup (hb) and downcomer residence time:
The liquid level and froth in the downcomer should be well below the top of the outlet weir on the tray above to avoid flooding ( [3] page 576 ).
 |
(7.11) |
Head loss in downcomer:  |
(7.12) |
Lwd = Downcomer liquid flow rate, kg/s
Am = Smaller of clearance area under the downcomer apron (Aap ) and downcomer area ( AD)
The average density of aerated liquid in the dowmncomer can be assumed as 
of the clear liquid density. Therefore, half of the sum of the plate spacing and weir height should be greater than the downcomer backup.
 |
(7.13) |
Downcomer residence time (
) should be sufficient for the disengagement of liquid from vapor in the downcomer to minimize entrained vapor. The value of >3 s is suggested. Downcomer residence time is given by ( [3] page 578 ):
 [s] |
(7.14) |
, mm