Bubble cap design. Riser/downcomer sizes.

[warramungas]

I've looked but cant find the info I need to build this thing and rather than learn all about process engineering I thought I'd ask you knowledgeable people.
How important is the riser open area compared to downcomer size? Is it just a velocity increase without much issue.
My design for a 54mm column will be 32 mm caps over a 25 mm riser with a 12.5 mm downcomer.
Calculations show that with the 1/2" downcomer going through the riser it will allow the same vapour path as a 20mm riser. Will this be enough?
I also worked out that if increase the size of the downcomer to 3/4" I'll be left with a vapour path equivalent to an approx 16mm tube. Will this be enough as it would dramatically increase the downcomer path.
I know this is only a small column and I'm eying off a 3 meter length of 4" copper tube stashed at the back of the workshop I might be able to trade later for a standard mine currency payment (carton) for the column and stainless for the rest.
This is going to fill the gap until then I hope.
Cheers,
Warra

[Ulysses]

Do you have a sketch? I'm not sure if you mean downcomer in the same way I do. Downcomers channel liquid only from the tray above, and need a lot less area than the vapour heading up. The most important design parameter of the downcomers is that they are long enough to provide the liquid head to overcome the vapour pressure gradient across the plate. Ie, Tray spacing will determine downcomer length.The bigger the spacing, the greater the rate the column can be run at, but of course at greater cost and height requirements. The seal pot at the bottom of the downcomer also has to hold the same volume of liquid as the downcomer, otherwise the seal can be blown and the column will flood.

[dogbreath vodka]

Not sure if this is any use to you.
4" Bubble cap drawings

[warramungas]

Ok. Rough drawing shows design with 3/4" downcomer as wtdist reckons 1/2" is too easy to flood the column with.
Basically both caps will clamp opposite each other directly onto the plate with a small bolt down the center of the downcomer. The downcomer should be approx 2" long and hold approximately 141 ml of liquid when full which the vapour pressure from below would need to push out the top to over come it. Hopefully enough.
The 3/4" downcomer leaves me with a riser open area the same as a 16 mm (slightly larger than 5/8") tube which I think will increase the speed through the cap but hopefully not choke the still too much.
I'm trying to keep it simple and easy to pull apart if it needs adjustment or cleaning.

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The below photo is a rough sketch of what it would look like with a 25 mm rise into a 32 mm cap and 1/2" downcomer. Looks better but would probably choke too much.

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[WTDist]

The downcomer will hold 58ml. this is rounded up if you use 19.05mm pipe 50.8mm long it will have a capacity just under 58ml. its much less than 141ml

[warramungas]

Oops. My quick pie r squared calc went awry. Cheers.

[Doubleuj]

Ok. Rough drawing shows design with 3/4" downcomer as wtdist reckons 1/2" is too easy to flood the column with.
Basically both caps will clamp opposite each other directly onto the plate with a small bolt down the center of the downcomer. The downcomer should be approx 2" long and hold approximately 141 ml of liquid when full which the vapour pressure from below would need to push out the top to over come it. Hopefully enough.
The 3/4" downcomer leaves me with a riser open area the same as a 16 mm (slightly larger than 5/8") tube which I think will increase the speed through the cap but hopefully not choke the still too much.
I'm trying to keep it simple and easy to pull apart if it needs adjustment or cleaning.

wp_ss_20151224_0002.png

The below photo is a rough sketch of what it would look like with a 25 mm rise into a 32 mm cap and 1/2" downcomer. Looks better but would probably choke too much.

wp_ss_20151224_0003.png

Umm, won't that make the down comer higher than the riser? Are you hoping for the vapour to keep the liquid from falling back down the riser or am I missing something?

[warramungas]

Yep positive vapour pressure under the cap will stop it flowing back totally. I've looked at lots of pictures of bubble caps and a lot of them have the downcomer level or even a bit higher than the top of the cap. I think even Macs are level with the top of the caps. Means the riser must be lower than downcomer. Don't know the engineering behind it but they can't all be wrong surely? Figure the top of my riser will be about 5 mm lower than the downcomer overflow if I put it together right. Got the couplings done. Gotta sort out the caps. Take some photos when I get home.

[Doubleuj]

I'm not too sure about that but mac or Sam should be able to answer definitively or professor green just hot some mac bubble cap plate maybe he could measure the heights or comment?

[bluc]

Interesting thread did it go anywhere have been toying with the idea of a 2" bubbler or an inline thumper for a pot with the same downcomer through riser design. To overcome the downcomer lower than riser thing i was thinking of trenching the top of riser so the downcomer was lower than riser. Have not worked out how but might be a direction for you to think in. Bout the only way i can think to do it is milling copper from solid block :angry-banghead:

[scythe]

Bluc: paint me a picture as i have no idea what youean by "trenching".

[warramungas]

All my copper pieces are still packed in the large fermenter in the shed. Haven't unpacked it yet since the move. All the couplings have been made up but I still need to put together the top caps and cut the slots in the top and bottom caps. Hopefully in the next month or so. Still a lot happening at the moment.

[bluc]

Drawing is rough as guts and probably wouldnt work but an idea





Sizes are guestimate, 3/4 downcomer- riser 1 inch and cap 1.5 vapour goes up between 3/4 and 1 then out through seration on bottom or 1.5 inch was thinking once 1.5 inch liquid level was level with the downcomer "trench" vapour would continue in on the higher peaks of the 1.5 cap 1inch soldered to plate downcomer soldered to u channel wich is soldered to the trench of the 1.5 cap and the cap soldered to the plate with copuple small dobs of solder.. :think: Trying to think of ways to maximise riser and downcomer on small colum..

[warramungas]

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So far so good. Not the prettiest downcomers but they'll do.
Braze tommorrow night.
I was going to use the little flower cap arrangement in the bottom of the fitting to allow liquid gas exchange but there wasn't enough clearance. Hence the stainless clip that will give me 10mm clearance.
Don't worry about the downcomer length as they've been cut deliberately too long. They'll be pruned after they're stuck together