by TuMeke » Mon Jul 22, 2013 7:39 pm
That pic should work Kiwi - no reason for vapour to even try to go through that hole. Vapour is only going to move toward a lower pressure.
The same vapour pressure should exist above and below the weep hole as drawn in this, so add in the weight of fluid in the cup above the hole, and it shouldn't be a problem.
:-B
I figure downcomers should exist in one of two states:
1) cup empty/filling (startup) - at this point who cares which way the vapour goes - the plate above hasn't filled yet.
2) cup full, and over flowing (reflux) - we don't want vapour going up the downcomer because it could impede the flow of condensate coming down and lead to a flooded section.
There are a couple of 'fail' states too.
1) cup flowing at capacity, and downcomer backing up - this is the first stage of a flood, just hasn't made it to the plate above yet. - in this state vapour won't be going up the downcomer wherever you put the weep hole - there's just too much liquid coming down (need some less restrictive downcomers/cups to run that much power and reflux)
2) vapour pushing up the downcomer - this would indicate that the volume of vapour coming into the section exceeds the volume able to push through the plate above at the given chamber pressure.
The excess chamber pressure must exceed the force required to lift any fluid in the downcomer by the distance between the lip of the cup, and the top of the 'hole'.
Think about the vapour lock on the top of your fermenter - the pressure in the fermenter forces that liquid up until the CO2 can get around the corner. Put more water in the trap, and the fermenter pressure has to be higher before it'll bubble. So by increasing the depth of your cup, you'll have to work harder to lift it's contents.. ordinarily it's easier for the vapour to go through the plate.
Add in a weep hole, and you potentially have a third 'bad' but not necessarily fail state. If the weep hole is on the downcomer side of the vapourlock, then vapour pushing through it would most likely try to travel up the downcomer. It still bubbles through a layer of fluid, so from a re-enrichment point of view that's no problem, BUT you've bypassed the vapour-lock, so it's now down to the size of the hole to make it harder for vapour to pass. Trouble is that if you hit that critical pressure and start blowing vapour through that weep hole it has the potential to restrict the flow of downcoming liquid which could result in fail state 1 (bad).
By putting the weep hole where kiwi has indicated in the pic, it's on the chamber pressure side of the vapour lock, so not only will vapour not bother trying to go that way (there's no lower pressure to head towards) but even if it did, it'd bubble up outside the downcomer, so wouldn't screw with the downcoming flow anyway.
One more thing I haven't thought through yet, and it's an 'edge' case anyway:
If your weep hole can drain faster than your reflux is coming down, then the weep hole is going to drain the downcomer cup. At some point the downcomer vapourlock will fail, the chamber pressure will vent up the downcomer, and the plate above will collapse.
:teasing-blah:
Nuff thinking for a Monday night.
Tu.