weep holes in dowucomers and mixing bubble cps &perf plates

by **kiwikeg** » Fri Jul 19, 2013 7:22 pm

I want to drill a 1.5 mm hole in my downcomers to make them self draining. I am following macs design for the perf plates. With a 25/15 reducer and a 25mm end cap for a vapour lock.
Will this cause problems.
Also can you mix bubble caps and perf plates if so I am thinking a bubbles caps first plate then perf plates for the upper plates.

by **Dominator** » Fri Jul 19, 2013 7:48 pm

I have been thinking the same thing about down commers. I am just about to order bits for my bubbler and when they arrive I will be trialling a 1.5mm hole in the bottom of my down commers. It will be a few weeks until I have results, but I will let you know how it goes.

by **Yummyrum** » Fri Jul 19, 2013 8:09 pm

I'm pretty sure WineGlass tried this and it gave him no amount of problems.(Weep hole )
Saw this over on HD .Pretty sure he said the same thing on this forum too but can't remember where

by **kiwikeg** » Fri Jul 19, 2013 8:16 pm

Yeah I've seen a lot of people complaining of problems from weep holes but I think they have the design wrong.
If the weep hole is not directly in the vapour path I think it should be sweet?
Not sure? ??

by **Dominator** » Fri Jul 19, 2013 8:29 pm

How do you mean not directly in the vapor path?

by **punchy21** » Fri Jul 19, 2013 9:01 pm

If it doesn't work, a dab of solder and the hole is filled... :handgestures-thumbupleft:

by **Dominator** » Fri Jul 19, 2013 9:09 pm

punchy21 wrote:If it doesn't work, a dab of solder and the hole is filled... :handgestures-thumbupleft:

Yep, that's what I was thinking too.

by **MacStill** » Fri Jul 19, 2013 9:35 pm

Smaller than 1.5mm is the go, drill a 1mm hole and it will drain..... vapour takes the path of least resistance :handgestures-thumbupleft:

by **TuMeke** » Sat Jul 20, 2013 6:55 pm

I've been thinking about this too - I think the captive depth in the downcomer is possibly more the key factor. If the depth of retained fluid above the weep hole is equal to, or greater than the bath depth on the plate above, it should work just fine as it'll be easier for the vapour to go through the plate.

The same thing must be true of the downcomer cup I'd think - too shallow and vapour is going to try pushing up the downcomer because there'll be less head of pressure than going through the plate?

Could be totally mistaken of course!

by **db1979** » Sun Jul 21, 2013 8:49 am

That's assuming that the only liquid in the downcomer is in the cap. In order for the downcomer to overflow onto the plate below there must be more liquid in the downcomer than what the cap can hold. In my own tests with my plates I found that for an equivalent flow of water (from the kitchen tap) to what the still puts out with NO reflux, the downcomer fills up completely within a matter of about 10 seconds (after the plate overflows) and the downcomer works effectively. In other words there was water from the bottom of the downcomer to the top of the downcomer (too much flow though and the plates flood). So I think it is more likely that the depth of volume of liquid in the downcomer could well be close to the total height of the downcomer from the height above the plate to the distance below the plate.

So I think your on the right track in thinking about the liquid depth in the downcomer compared to the depth of liquid in the plate, but I don't think it is limited by the size of the cap. I know some builds on here have had downcomers without caps on the end but some other design that restricted flow and they worked well.

by **bt1** » Sun Jul 21, 2013 10:12 am

Howdy,

Read else where the issues with weep holes and suited to j traps, reckon there a few issues there especially if using other downcomer types.

How does a downcomer be filled or equal to the bath depth on the plate...it won't! ...once the fluid level exceeds the cup height(or bath level if cupless), add in surface tension issues etc the downcomer should like any fluid scenario flow to an equalised height, in other words it over flows. There would be a few mills in it at best.

bt1

by **kiwikeg** » Sun Jul 21, 2013 12:19 pm

Dominator wrote:How do you mean not directly in the vapor path?

Like this.. if vapour trys to push through it ends up in same chamber. Weep hole marked by red arrows

IMAG1143.jpg

arrows.

by **Dominator** » Sun Jul 21, 2013 12:24 pm

Ahh yep, that makes sense.

by **bt1** » Sun Jul 21, 2013 6:35 pm

Ahh yep, that makes sense.

How does it make sense?

by **Yummyrum** » Sun Jul 21, 2013 7:08 pm

bt1 wrote:Howdy,

Read else where the issues with weep holes and suited to j traps, reckon there a few issues there especially if using other downcomer types.

How does a downcomer be filled or equal to the bath depth on the plate...it won't! ...once the fluid level exceeds the cup height(or bath level if cupless), add in surface tension issues etc the downcomer should like any fluid scenario flow to an equalised height, in other words it over flows. There would be a few mills in it at best.

bt1

Only one way to prove it bt1 .... And you're the man... Glass downcomer :handgestures-thumbupleft: Tongue in cheek of coarse :teasing-neener:

@db1979. I once tried flow experiments with water .The surface tension can do some fucked up things .Try the same tests with Metholatyed spirits ( Ethanol ) and the results are completely different .Not saying what you did was incorrect just that ethanol and water behave very differently

by **db1979** » Sun Jul 21, 2013 10:38 pm

Yummyrum wrote:
@db1979. I once tried flow experiments with water .The surface tension can do some fucked up things .Try the same tests with Metholatyed spirits ( Ethanol ) and the results are completely different .Not saying what you did was incorrect just that ethanol and water behave very differently

I seem to recall that feeling that I was putting my foot in my mouth... I should listen to it next time.

Heck, I can't even see my downcomers! :laughing-rolling: 8-} :violence-smack:

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.

by **CH3CH2OH** » Wed Aug 14, 2013 1:48 am

Not sure if I missed something in the post but couldn't you just drill the weep home in the side of the cup horizontal and right at the bottom of the cup?

should still drain the same, could use a little larger hole and vapour wont want to take that path

by **Dominator** » Wed Aug 14, 2013 6:55 am

CH3CH2OH wrote:Not sure if I missed something in the post but couldn't you just drill the weep home in the side of the cup horizontal and right at the bottom of the cup?

should still drain the same, could use a little larger hole and vapour wont want to take that path

You could do the weep hole in the side of the DC cup but I still would not go bigger than 1-1.5mm. Any bigger and you risk the DC draining through your weep hole too fast and you will lose your vapour lock.