Pressure

[eStill]

What pressures can you expect inside your boiler? I never gave it any thought until I looked at the lengths some folks have gone to in making sure they lid is properly secured on their water urn boilers.

I've designed two heads for a still. One is a 700m x 22mm straight pipe. You might consider it a pot still head. it offers no resistance to airflow and its use would create little or no pressure. The other is a spiral riser from the boiler, 1m x 22mm, packed with Raschig rings. It's actually quite difficult to blow on and I can see that if I were to attach it to a coffee urn, there'd easily be enough expansion of vapour to pop an lid off if it weren't clamped down pretty firmly.

So, what pressures are we talking? Should the vapour be able to pass more freely through my riser, or should I expect some resistance?

[rad14701]

There should be virtually no pressure buildup within the boiler... It shouldn't even be measured in PSI, or very few at most... If you can't breathe through that packed spiral there will be too much back-pressure buildup and you need to rethink your design...

[still_stirrin]

...One is a 700m x 22mm straight pipe....It's actually quite difficult to blow on...should I expect some resistance?

Damn...if it's actually nearly 3/4 of a kilometer long....I guarantee you'll have resistance. That's a loooooong piccolo, buddy.

Assuming a typo (before editing), with only a 22mm riser tube I would believe you may actually see the potential for some back pressure, especially if you're pushing it hard. It wouldn't be unusual to measure a couple inches of water (250-350 Pa).

Most importantly, the attention for a good boiler seal is not to contain internal pressure. Instead, it is to ensure that no vapor leaks out of the boiler, with the vapor constrained to the vapor path (riser).
ss

[eStill]

There should be virtually no pressure buildup within the boiler... It shouldn't even be measured in PSI, or very few at most... If you can't breathe through that packed spiral there will be too much back-pressure buildup and you need to rethink your design...

I couldn't breathe comfortably through it. I think I'll unpack it and see what the problem is. There shouldn't be any obstruction.

[eStill]

Damn...if it's actually nearly 3/4 of a kilometer long....

You crack me up. I enjoyed that. I'm usually on the "giving" end rather than the "receiving" end!!

with only a 22mm riser tube I would believe you may actually see the potential for some back pressure, especially if you're pushing it hard.

I'm not pushing it hard. 1k6W at 60% for the hearts. The empty tube has no problems. It's teh packed tube that's struggling. I'll take it apart tonight.

It wouldn't be unusual to measure a couple inches of water (250-350 Pa).

Stone me. That's a fair bit, all the same.

Most importantly, the attention for a good boiler seal is not to contain internal pressure. Instead, it is to ensure that no vapor leaks out of the boiler, with the vapor constrained to the vapor path (riser).

I was sure of that - I had heard my new boiler lid hissing when I did a test run with water.

[shadylane]

Build one of these and you will know the boiler pressure.
This one is around 12 inches tall so it also works as a 1/4 psi PRV.
To answer the original question
My boiler pressure is around 1 inch of Water Pressure when running as a potstill
And around 3 or 4 inches WP when driving the packed column.

[The Baker]

So this has the clear section partly filled with water and the top of the water is level in the tubes; and you can 'measure' the pressure (OR VACUUM) if the water level rises (OR FALLS) in one side compared to the other.
And the top end of the tube is effectively open to the atmosphere.
It has the advantage of being effectively a safety valve in case of overpressure in the boiler; (AND IN CASE OF A VACUUM IN THE BOILER). And would be a BETTER safety valve if the tubes were bigger....
I have seen some pretty dramatic pictures of a still in the woods, where a sudden downpour has destroyed the vapour in the still and the resulting vacuum has just crumpled the copper still.
And a sudden failure of the heat could have a similar effect...
I imagine the thumper and maybe a slobber box have meant the boiler was effectively not open to the atmosphere through the condenser, and the liquid in the condenser itself too would not have helped...

Geoff

[Hound Dog]

There should be virtually no pressure buildup within the boiler... It shouldn't even be measured in PSI, or very few at most... If you can't breathe through that packed spiral there will be too much back-pressure buildup and you need to rethink your design...

This is not really accurate. While air can pass through the packing as you assemble it, the column will be in an aquatic state while running. We have seen this in videos of borosilicate columns running (thanks Skow!) So the vapor you are boiling off has to push through the column of liquid that is filled with packing. I have never measured this in a 4 foot high 3 inch column that I run but have to think it is more pressure than I could blow through. A pot still on the other hand can flow pretty freely.

In the end, you aren't talking about pressure that is not easily handled by light gauge copper sheet and solder though. It's really not a lot of pressure.

[goose eye]

Depends I'd you got a doublin keg hooked up.
That why you see rocks on the cap sometimes.
With them up right outfits don't reckon ole boys ain't got a clue.

So I'm tole

[Kareltje]

I am surprised you can fill and empty a spiral of 22 mm with Rashig-rings. I had a straight pipe of 50 cm long and 28 mm wide and sometimes filled it with small copper spirals and pebbles and even then I sometimes had problems getting them out again.
I still do not dare to fill my spiral of 2 m and 15 mm OD.

You can measure the pressure building when you connect both your filled spiral and an U-shaped plastic hose à la shadylane to the upper ends of a T and then breath through the leg of the T. So you do not have to build and run the whole still.

Edit: Hound Dog had a correct correction: when using it in real life the packing is wet. I doubt if that leads to great pressures, because this are only tiny columns of fluid. Albeit a lot of tiny columns, in total at most the height of your spiral.


Maybe cooling the spiral a bit is a alternative for the packing?

[Hound Dog]

Depends I'd you got a doublin keg hooked up.
That why you see rocks on the cap sometimes.
With them up right outfits don't reckon ole boys ain't got a clue.

So I'm tole

Very true Goose.

[HDNB]

i'm still testing a new 4 plate. it has a combination of a bubble plates and sieve plates, with an 11mm bath depth on each one.

the first water run the PRV i installed (3-5lb) huffed twice.scared the shit outta me. it must have still been finding it's seat. I have done a vinegar run pushing hard and refluxed the hell out of a low grade garbage wash and it has not let let out a peep since.

i had to test the vacuum breaker too of course, so on the alcohol run i trickled a bit of room temperature water in (thru CIP) at the end of the run (at about 205*) until it popped. holy shit!!! a few dribbles collapsed the vapour and she whistled like a train coming through!

that's a test i don't think i will repeat!

(i think i'll use the shop vac when it's cold...seems a LOT safer)