Bubble cap design, critique requested

[Steve Broady]

I would appreciate a little advice, critique, criticism, or anything else you care to offer. I'm trying to design a bubble cap plate in a stainless steel ball (two halves of some bowls which I picked up cheap on Amazon). Here's what I have so far:

Bubble Plate topless.jpg

Bubble Plate section.jpg

The ferrules top and bottom are 2", to match the outlet of a keg. There are 10 caps, each consisting of a 1/2" riser and a 1" cap. If I've done my math right, that gives me the same cross sectional area through the risers as through the ferrules, if that matters. There's also one 1/2" downcomer, which is just another bubble cap mounted upside down and with an extended riser. As drawn, the notches in the caps are 3mm x 6mm, and the downcomer is 18mm above the plate surface.

I do plan on adding sight glasses, I just haven't drawn them yet. At the moment, I'm just trying to make sure I have a viable design. I've already spent a fair bit of time drawing this thing, and I think it's time to decide whether to move forward or start over.

[still_stirrin]

How do you plan to assemble the two bowls?

Sometimes bubble caps need adjustment (higher or lower) depending on the fluid bed depth desired (also a function of power input). If your bubble plate is installed inside the bowls and the bowls are then soldered together, any future modifications (or adjustments) would require desoldering the bowls apart.

So, if you have a means to disassemble/assemble (like using binder clips on the flange lips, for example), then you’ll be able to work on it (and clean it if necessary). Just an “engineering” suggestion. Hint - wrap the flange lips with PTFE plumbers tape and you’ll get a good seal between the two bowls.
ss

[Steve Broady]

How do you plan to assemble the two bowls?

To be honest, I thought about that for a while, didn’t come up with any great ideas, and figured I’d leave it for future me to sort out. And then forgot to think about it again.

I like the idea of a few wraps of PTFE tape and a slip fit. Or even flour paste, either temporarily or permanently. Of course, there’s nothing as permanent as a temporary solution.

[Steve Broady]

My plan is to use a 2” sight glass. I might try to be clever and put the downcomer directly behind the sight glass, which would give some access to it without full disassembly. Or maybe that’s a dumb idea. I have plenty of those!

[Yummyrum]

Steve , have you seen this topic from Swedishpride ?

viewtopic.php?t=62483

And this one by mashrookie

viewtopic.php?t=25727

[googe]

Half inch downcomers will more than likely give you problems.

[acfixer69]

Half inch downcomers will more than likely give you problems.

Funny how we both know that one googe. Good to see you active here again.

[Saltbush Bill]

Half inch downcomers will more than likely give you problems.

Funny how we both know that one googe.

Ive never tried 1/2 inch downcomers , but I can also see them being a problem, 3/4 inch minimum is what Id be looking at.

[Wildcats]

Well I'll be the first to say... I'm EXCITED!!! Let's get a plan and get this thing build and up and running!!!
I can't give ya much advice on your design. As I've never had any experience with plates YET. That won't always be the case. But I am rooting for you and ready to see this take shape.
Sounds like you're gonna need 3/4" downcomers... Love all the knowledge on this site.

[Oatmeal]

Here's some relevant discussions. Pfshine advocates for a larger single cap for what it's worth.

Fabricating caps is a fair bit of work.



(1/2 downcomer didn't work for another plate project I built)

[Steve Broady]

Hmmm… this has definitely given me some food for thought. Looks like I’m going back to the drawing board.

How important is it to keep the total cross section of the vapor path equal to that of the inlet? My gut says that more area means lower vapor speed means longer contact time and better separation. But I’m struggling to find a lot of solid comparative data in my searches. Lots of opinions, lots of “Doesn’t matter, it’ll just work and you’ll learn to live with whatever quirks you build in.” Which is probably true, and reassuring, but I’d like to do the best I can up front.

[Setsumi]

Remember in cross section calculations you should include the path through the cap not just the riser.

Myself would make the bath height lower rather than higher.

I cannot point you to a thread with hard evedance but searh for a thread by flyingductchman, variables in sieve plates. I know you do caps but there is a lot of good info there.

[Yummyrum]

1/2" down comer on it's own is not the problem . If you look at the amount of liquid returning , a 1/2" can easily cope . The problem is that its not a nice little pool of liquid , but rather a violent explosion of bubbles . Its the bubbles that stop the liquid going down a 1/2" .

If you increase the open area of the 1/2" ,by seriously flaring it out bigger or create a weir around it , the bubbles have a larger area to collapse into it .

[Saltbush Bill]

Correct Yummy, and why I built hybrid weir type downcommers on my last plated column.

[Oatmeal]

I also had trouble tracking down a solid breakdown of the wheres and whys of caps. Thus I remain ignorant still.

At the moment I conceptualize the balance as being less about vapor speed, than vapor to condensate contact area. A bunch of small caps provides more contact area for the phase change interactions.

I mean, you can run a thumper on 3/4" size pipe.

(Hopefully Yummy's right, because I flared my 1/2 downcomer but haven't tested it yet)

[hellbilly007]

Copper or stainless steel bowls are an easy way of making the two halves of your ball. Most have a lip around them that will lend to being a flange to clamp, allowing for future adjustments. Also will be easy mounting your plates.

If all of your downcomers are going to be center mounted, it would be a good idea to direct the fluid to the sides. This will prevent the reflux from dropping through multiple plates. I also agree with the others on upsizing to ¾"

[Steve Broady]

searh for a thread by flyingductchman, variables in sieve plates. I know you do caps but there is a lot of good info there.

Thank you for that, Setsumi. You’re right, there’s a lot of good information there. Here’s what I took away from that:

- It’s not that important to have such a large free flowing cross sectional area. Very roughly, more area means it can handle more power. Since bubble caps retain liquid even at zero flow, there does not seem to be a lower power limit as there is with sieve plates, aside from the power needed to overcome the passive condensation within the system.

- Fluid bed depth is slightly inversely proportional to efficiency, so within reason, less is more.

So I’ve got some thinking to do while I’m at work today, and I’ll see what I cook up.

[Setsumi]

You have summarized it 100%

[Steve Broady]

I’m putting more thought into a redesign, and one of the questions I’m trying to answer for myself regards the vapor path underneath the edge of the cap. It seems logical that it should equal the riser area (although maybe not? There has to be a limiting factor somewhere, so why not here?), but does it have to be crenelated or jagged as so many designs show? My rough calculations at work last night suggested that for a a 1/2” riser and a 1” cap, keeping the edge smooth and elevating it 2mm above the plate would give the same area as the riser.

I feel like I’m missing something obvious. I can’t think of a good reason why that wouldn’t work just as well, but there has to be a reason why everyone who has come before me has done it a different way.

[shadylane]

Half inch downcomers will more than likely give you problems.

My thoughts exactly.
Two 3/4" downcomers might be better on a plate that sized.
That and have the downcomers cups 2X deeper than the plate liquid level.

[shadylane]

I’m trying to answer for myself regards the vapor path underneath the edge of the cap.....does it have to be crenelated or jagged as so many designs show?

Just guessing
The notches on the bottom edge of a cap are to make vapor sizzle outward, instead of burping.
I did two identical plates except for the edges.
The bubble action looked slightly different, but I couldn't measure any difference.
Best advice I can give for a bubble cap plate is big diameter, lots of caps.
Then run it gently....

[Oatmeal]

Crenellations may provide a "nucleation" point?

Here's the best overview of why a plate below a column can be so effective.

In this instance, Larry suggests that a deeper liquid bed (ie shifting all the alcohol into the apparatus) is more ideal?

[Setsumi]

I’m putting more thought into a redesign, and one of the questions I’m trying to answer for myself regards the vapor path underneath the edge of the cap. It seems logical that it should equal the riser area (although maybe not? There has to be a limiting factor somewhere, so why not here?), but does it have to be crenelated or jagged as so many designs show? My rough calculations at work last night suggested that for a a 1/2” riser and a 1” cap, keeping the edge smooth and elevating it 2mm above the plate would give the same area as the riser.

I feel like I’m missing something obvious. I can’t think of a good reason why that wouldn’t work just as well, but there has to be a reason why everyone who has come before me has done it a different way.

Yes, it will work. BUT you want as much bubbles as possible into the liquid. Smaller bubbles are better. But then on a practicle side you have to notch the (bloody) cap. In the flute talk thread you will find caps with a gap from the plate - to prevent clogs from solids (batch distilations should be way past that) and then slots.

[googe]

Agree shady, flow in flow out. Ive done a few different plates, not detailed and engineered mind set like some here but from my experience big bubbles, deep bath=flavor. Fine bubbles, shallow bath=purity. Similar abv, maybe 3-5 abv on the shallow bath. Its very anal and not realy worth the time on our size stills. Just my opinion.

[Swedish Pride]

I had ¾ inch downcomers in mine.

Would put at least 1 inch if I did it again.

Look for kegjams build, he used a single 3inch cap and seemed happy with it.

If you are soldering the plate in I'd really advice you to to get a big way soldering iron, 150w worked good for me, got a 200w one now but the tip is a bit beefy so its actually harder to use.

[shadylane]

Definitely a big soldering iron for making plates.
I've got a prehistoric 500w iron, but mostly I use a torch.