RC Al wrote:Ive seen slots like that at the top of a downcomer - in theory to break the surface tension of the bubbles so they collapse better before entering, without the bubbles we could use a much smaller tube - running bubble free water through it isnt a good test
While looking for a flow rate chart, I may have solved the length riddle too - there's a minimum length for a pipe to overcome the viscosity of the fluid vs the drag on the walls, check out the attached chart, the thing that would affect those numbers is the difference in viscosity of alc to water - alc is aprox 20% thicker so there would be a change in those numbers (by a different amount, beyond my paygrade) so all the way down to the next plate could make it more efficient.
This applies to tubes that are full of liquid. A downcomer has liquid running down it's sides and then pools in the cup below, before overflowing to the next plate. The maximum amount that can go down a downcomer is given by the RC, as condensed vapour drops down and fills the top plate, then overflows into the downcomer and then to the plate below and so on. The heat input and length of RC required to provide the condensate to keep a 3/4" downcomer completely filled would be excessive.
I don't think there's any reason to have long downcomers.