Bubble Cap vs. Perf Plate Designs

[akira7799]

Hi All,
I have some questions regarding bubble cap designs. I've been doing research and I'm down in the minutiae. I'm looking for some guidance regarding the "open area" for bubble caps vs. perf plates.
Generally, perf plates run at ~5%-7%, but a bubble cap system if it ran at that amount of “open area” would be incredibly restrictive and create a large amount of back pressure.
So my question is this: how is the “open area” calculated on a bubble cap system?

[Setsumi]

I do not remember a straight up discussion on open area of caps. Try the thread by Jack C on single caps with central downcomer or pro caps. viewtopic.php?t=78362. Larry commented a few in his thread.
In my Twins thread in my signiture I give dimensions also on pro cap style.

Balance the riser downcommer vapour path under the cap and slots.

Before you start to build caps think hard, perf plates are so much easier to build and little difference in product.

[Setsumi]

Bump to get you more responses.

[Yummyrum]

In my mind , open area calculation of caps would be to measure the area of the gaps between the teeth around each cap to find the open area . IE if there were 30 2mm slots 5mm high on a 40mm diameter Cap , that would be 30 X 2 X 5 = 300mm^2.

The active area would be the total plate area minus the area of each bubble cap minus area of down comers and weirs .

The 40mm Cap has area 1256mm^2

Say a 22mm downcomer ( on the plate , not built into cap ) has an area of 314mm^2

On a 100mm( 4”) plate with an area of 7850mm^2

That would leave an active area of 6280mm^2

(7850 - 1256 - 314 = 6280)

So the percentage of open area = 300/6280 X 100 = 4.77%

If you cut 10mm high slots instead of 5mm slots , you have doubled the open area to active area percentage up to 9.55%

All this is pretty typical and I see no real difference between this type of open area and what might be on a Perf plate .

I’m not sure why you think where would be more back pressure on a Bubble cap plate verses a Perf plate with the same open area to Active area proportion , assuming same liquid level.

[shadylane]

Hi All,
I have some questions regarding bubble cap designs. I've been doing research and I'm down in the minutiae. I'm looking for some guidance regarding the "open area" for bubble caps vs. perf plates.
Generally, perf plates run at ~5%-7%, but a bubble cap system if it ran at that amount of “open area” would be incredibly restrictive and create a large amount of back pressure.
So my question is this: how is the “open area” calculated on a bubble cap system?

Sort of comparing apples to oranges.
Open area % is important with perf plates. Bubble caps dance to different music.

[akira7799]

I do not remember a straight up discussion on open area of caps. Try the thread by Jack C on single caps with central downcomer or pro caps. viewtopic.php?t=78362. Larry commented a few in his thread.
In my Twins thread in my signiture I give dimensions also on pro cap style.

Balance the riser downcommer vapour path under the cap and slots.

Before you start to build caps think hard, perf plates are so much easier to build and little difference in product.

Setsumi,
Thanks for your response. As I understand it, bubble caps can be driven at a lower wattage and I was contemplating if I needed that or not. I'll definitely dig into your posts and take the perf plate recommendation under advisement.

I’m not sure why you think where would be more back pressure on a Bubble cap plate verses a Perf plate with the same open area to Active area proportion , assuming same liquid level.

Yummy,
I was not considering the open area as the slits/slats in the cap, I was thinking that the limiting area would be a small(er) diameter hole in the riser, and that would contribute to higher back pressure. If the hole in the riser is large, then yes, the slits/slats in the cap would be the limiting area and not contribute as much to back pressure.

Thanks for your reply. I'll be doing more research.

Sort of comparing apples to oranges.
Open area % is important with perf plates. Bubble caps dance to different music.

Shady,
While I understand the the apples to oranges comparison, I think that Yummy did a good job with his maths at showing the similarity in open areas between perf plates and bubble plates. So, maybe more of an "apples to crab apples" comparison?

[Saltbush Bill]

If you do consider building perf plates then you might find a helpful hint or two here.
viewtopic.php?p=7581256
Hole size and spacing can stay the same regardless of plate diameter.
Downcommers are always the bottle neck when using perf plates in my experience. They are often the main restriction on how much heat can be applied before the plates begin to flood.
The problem is not so much dictated by downcommer diameter but by the shape and size of the downcommer " mouth/ entrance " by using a combination of a weir style mouth/ entry point leading to a traditional downcommer this problem ceases to exist.

[bluc]

The wide running zone, sweet spot wattage range, has won me over if I build again it will definitely be caps again, I have run my plater everywhere from 1200w to 3600w plates always load..

[akira7799]

If you do consider building perf plates then you might find a helpful hint or two here.
viewtopic.php?p=7581256

Saltbush,
That's some beautiful craftsmanship! I can see the appeal of the perf plates for sure. 4L/hour is a nice takeoff, but I'm also not going to be building a 6" still.

The wide running zone, sweet spot wattage range, has won me over if I build again it will definitely be caps again, I have run my plater everywhere from 1200w to 3600w plates always load..

Bluc,
did you build your cap risers or buy them? Seems like a very nice operating range of wattage. What diameter still do you run?

[PalCabral]

If you do consider building perf plates then you might find a helpful hint or two here.
viewtopic.php?p=7581256

Saltbush,
That's some beautiful craftsmanship! I can see the appeal of the perf plates for sure. 4L/hour is a nice takeoff, but I'm also not going to be building a 6" still.

I totally agree. Impressive work, Bill.

[Bolverk]

I built my 4" perfs to be about 10% open (i think they are like 11%) but added 3/4" downcomers

[bluc]

If you do consider building perf plates then you might find a helpful hint or two here.
viewtopic.php?p=7581256

Saltbush,
That's some beautiful craftsmanship! I can see the appeal of the perf plates for sure. 4L/hour is a nice takeoff, but I'm also not going to be building a 6" still.

The wide running zone, sweet spot wattage range, has won me over if I build again it will definitely be caps again, I have run my plater everywhere from 1200w to 3600w plates always load..

Bluc,
did you build your cap risers or buy them? Seems like a very nice operating range of wattage. What diameter still do you run?

Bought the plates. 4" 4plater with 600mm packed section. 3600w.

[Saltbush Bill]

Perfs or caps are very much personal choice, i wouldnt swap my perf plates for any caps out there.
Be sure you know what your getting your self into before starting work akira.
It can also produce at a lot higher speed that 4.LPH, im just not in a huge hurry so its generaly fast enough for me.
Wow those are some big arse holes bolverk.
But they are out of your continuous are they not?

[Bolverk]

Wow those are some big arse holes bolverk.
But they are out of your continuous are they not?

Yeah

[akira7799]

Be sure you know what your getting your self into before starting work akira.

That's what I'm trying to do...

Just for the risers, I'd need to figure out if I want to fab or buy. Fab options:
buy a lathe and machine them out of 5/8" copper round stock
OR I could use 1/2" copper pipe and TIG a top on the riser and tap that with machine threads
OR get a machinist to make some number of risers

Buy options:
My problem is that I haven't been able to properly source bubble cap risers alone. StillDragons sells them complete for $10 a piece. At $60 per plate for caps alone (5 up, 1 down), it get pricey quickly.

I've seen some people fabricate risers from plasma torch cutting heads, but they're brass, not copper. Brass introduces the potential for other heavy "pot metals" including lead. So, I'm definitely leaning away from the plasma torch fab angle. The more research I do, the more checkboxes I put in the perf plate "pros" column.

The biggest "con" for me with the bubble caps is currently cost. Unless I can figure out a way to fabricate them for a LOT less, it seems that perf plate is going to be the avenue of approach.

The biggest "con" for me with the perf plate is ALL THOSE HOLES! That's a lot of time at the drill press.

Based on the Parent Site's Reflux plate calculator, a 4" 5 perf plate still has everything I need. I'll probably just end up going with perf plates. Now, to find the right build for me...and the time.

[Setsumi]

Akira, you sure have a lot of build options if you have lathe and tick options. A few holes in a 4" is not that difficult. If you look at Old dog's original plate with its large hole you will see. Later guys went much smaller holes.

If you want caps, seriously consider a single cap.

Or be the first here to build a tornado plate....

viewtopic.php?t=94895

[Saltbush Bill]

You seem to be taking a super complicated approach, lathes and drill presses are not needed.
Improvisation and simple tools like dremels , files , an electric hand drill , a jigsaw to rough out the plates before spinning them down to an exact size are all that is needed.
If your only making a 4 inch there are not that many holes to drill.
There are a lot of good build threads out there which give simple tips on ways to get it done.
Admitedly on my 6 inch a friend did offer to tidy up the plate diameter on his lathe, but Ive done the same thing before just as successfully without a lathe.
Copper is soft and easy to work with.

[Yummyrum]

Agree with Saltbush …. All can be done with a minimum of tools . I never had even a drill press when I made mine . All done with a battery hand drill .
Drilling holes in a 4” plate is a heck of a lot easier than fabbing up a bunch of bubble caps . Or as Setsumi says , just use one big cap per plate .

Also , iff’n you were to go multiple caps per plate , you don’t have to make them exactly the same as Still dragons or the other clones with a screw on fancy rise . Just braze the 5 or 6 riser tubes to the plate cut from 1/2” tube . Then soft solder the caps to the plate .
This also eliminates the worry you had about getting the holes in the riser big enough to prevent back pressure . You simply have a short riser inside a cap.

[akira7799]

You seem to be taking a super complicated approach, lathes and drill presses are not needed.

The other tools were for the riser/cap approach.

If I go the perf plate route, which is now what I'm inclined to do, I understand that basic tools will accomplish what I need.

Also , iff’n you were to go multiple caps per plate , you don’t have to make them exactly the same as Still dragons or the other clones with a screw on fancy rise . Just braze the 5 or 6 riser tubes to the plate cut from 1/2” tube . Then soft solder the caps to the plate.

Good looking out Yummy. I like that idea too.

Thanks for the guidance and thoughts all!