Double boiler still - temp over time

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Dman
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Double boiler still - temp over time

Post by Dman »

I've been making absinthe for as few years now and have a constant problem with scorching my herbs.

I'm thinking of having a double boiler sleeve fabricated for a 3 gallon "milk can" style boiler.
After some experimentation I've run into a bit of a dead end:
How do I design the boiler to prevent the water from evaporating too quickly. I need to process to sustain itself for around 9 hours. if I design it to accept additional water, I'll be cycling the heat to the boiler and unbalancing my column.
If I design is like a pressure cooker (adding an "O" ring gasket and lock mechanism between the 2 elements of the double boiler) I'm concerned I'll be risking an explosion.

Any ideas out there on how I might design a double boiler style still that can maintain an even temperature for a long period of time?

-Dman
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Re: Double boiler still - temp over time

Post by olddog »

Myles is the best guy to talk to about double boilers.


OD
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Thorn_veritas
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Re: Double boiler still - temp over time

Post by Thorn_veritas »

Yep ask myles.

But I think that the best way to do it would be to use oil of some sort I have seen a few post around here bout double oil boilers. Saftey number 1 prior oi ty woth hot oil though.

But yeah chat to myles.
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Re: Double boiler still - temp over time

Post by myles »

Well this is just my opinion but I would get rid of the water completely. :)

Water/steam is great in sealed systems but for home build it is a PITA - unless you have the luxury of stilling outdoors where the loss of a bit of steam is not an issue.

You could use a vegetable oil but I prefer to use food safe propylene glycol instead. Over here it is more expensive, but the boiling point is far higher than veg oil. As you run it at much lower than its boiling point it lasts longer than veg oil so the cost probably balances out long term.

However you do it you want to include a few options.
1. A drain so that you can drain off your fluid for cleaning.
2. A sight glass or level indicator so that you can see the colour of your fluid.
3. A good seal between inner and outer pot's to minimise vapour loss, and a refil port.

Also remember that towards the end of the run the inner pot will try to float as you have reduced the weight of the contents. The glycol is an inherent thermal buffer. With a well insulated boiler it retains heat for a considerable time after you stop heating. Stable temperature is a built in feature.

Excuse the graph but this was the results of a trial on an early glycol boiler rig. Wash and glycol temp in degC against time in mins. Also shows power input (green line 3kw element), full power to take the glycol to 120 degC then switched off.

Image
Dman
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Re: Double boiler still - temp over time

Post by Dman »

Thanks for the info Myles, that's really helpful.

A couple for things I'm still not fully clear on.
Considering the high boiling point of propylene glycol, should I expect enough fluid loss to warrant any form of refill during my distillation process?
My rig is relatively very small (3 gallon boiler). I'd rather remove the refill and drain valves from the rig for simplicity sake. If there is no need to handle the propylene glycol mid distillation, I can always manually flush and clean the base/sleeve pot after the rig cools down.

Also, are you familiar with the expansion rates of propylene glycol? Ideally, I'd like to have a full seal between the two pots (O rings and clamps). This will also address the issue of the the pot floating. My only concern is that I don't know if I'm risking over pressurizing.
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Re: Double boiler still - temp over time

Post by myles »

There should be no pressure at all. The real benefit of the glycol system is that you can get the same temperature that you would have with high pressure steam - but at atmospheric pressure.

Yes seal your boilers together for convenience, just add in a combined vent/refil port. Ensure that this extends below the surface of the glycol and it means the only area of glycol that is exposed to atmosphere is the internal area of the vent. You can also run this system with a remote expansion chamber if you wish.

A drain for the glycol makes life easier. If you heat up too fast the glycol will burn, just the same as any other fluid. It is worth ramping up the power to prolong the life of your glycol. This is also standard procedure with any other (solar power perhaps) thermal transfer fluid.

Steam is excelent, and I would love a 50 psi jacketed steam boiler, BUT THERE IS NO WAY THAT I WOULD CONSIDER A HOME BUILT OPTION. Steam under pressure is too dangerous to mess about with if you are not qualified to do so.
Dman
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Re: Double boiler still - temp over time

Post by Dman »

This sounds like a great plan.
I use a 1.5 Kw heater attached to a Variac transformer that allows me to control the power output without cycling the power (even though it seems the glycol can soak the cycling).
So you say the glycol can take the high heat without excessive burn as long as you heat it up slowly?
I'm sot quite sure what the ideal still temp is for Absinthe, but I can dial that in through trial and error.

Thanks again!
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Re: Double boiler still - temp over time

Post by myles »

Dman, you will never "boil" your propylene glycol. You might occasionally hear a single bubble. The boiling point is 188 degC or 370 degF. In reality you only warm it up. I used to run mine at 120 - 130 degC, I think I once pushed it to 140.

If you go much higher you will burn your wash anyway - so what is the point? The real benefit of the double boiler is the massively reduced power density. The entire immersed surface area of your inner pot becomes your heat source. You can put in a LOT of energy without the localised hot spots.

Or think of it this way, you put in the same amount of energy, but the contact zone with your wash is much bigger. Your inner pot becomes the equivalent of an ultra, ultra :roll: low density element.

The inherent time lag/stored energy, with the glycol can be a bonus when it comes to the end of the run. Turn off the power, leave the cooling running, and put a big tails collector under your condenser.
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Re: Double boiler still - temp over time

Post by vqstatesman »

myles wrote:There should be no pressure at all. The real benefit of the glycol system is that you can get the same temperature that you would have with high pressure steam - but at atmospheric pressure.

Yes seal your boilers together for convenience, just add in a combined vent/refil port. Ensure that this extends below the surface of the glycol and it means the only area of glycol that is exposed to atmosphere is the internal area of the vent. You can also run this system with a remote expansion chamber if you wish.

A drain for the glycol makes life easier. If you heat up too fast the glycol will burn, just the same as any other fluid. It is worth ramping up the power to prolong the life of your glycol. This is also standard procedure with any other (solar power perhaps) thermal transfer fluid.

Steam is excelent, and I would love a 50 psi jacketed steam boiler, BUT THERE IS NO WAY THAT I WOULD CONSIDER A HOME BUILT OPTION. Steam under pressure is too dangerous to mess about with if you are not qualified to do so.
Are you saying glycol can't be used under pressure or am I reading that wrong?

Also what would you consider too fast heat up time for the glycol? How long do you take?
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Re: Double boiler still - temp over time

Post by vqstatesman »

vqstatesman wrote:
myles wrote:There should be no pressure at all. The real benefit of the glycol system is that you can get the same temperature that you would have with high pressure steam - but at atmospheric pressure.

Yes seal your boilers together for convenience, just add in a combined vent/refil port. Ensure that this extends below the surface of the glycol and it means the only area of glycol that is exposed to atmosphere is the internal area of the vent. You can also run this system with a remote expansion chamber if you wish.

A drain for the glycol makes life easier. If you heat up too fast the glycol will burn, just the same as any other fluid. It is worth ramping up the power to prolong the life of your glycol. This is also standard procedure with any other (solar power perhaps) thermal transfer fluid.

Steam is excelent, and I would love a 50 psi jacketed steam boiler, BUT THERE IS NO WAY THAT I WOULD CONSIDER A HOME BUILT OPTION. Steam under pressure is too dangerous to mess about with if you are not qualified to do so.
Are you saying glycol can't be used under pressure or am I reading that wrong?

Also what would you consider too fast heat up time for the glycol? How long do you take?
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Re: Double boiler still - temp over time

Post by cob »

none of these guys has been here in years. look at the posting dates. it shows you are reading keep it up.
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Re: Double boiler still - temp over time

Post by rad14701 »

cob wrote:none of these guys has been here in years. look at the posting dates. it shows you are reading keep it up.
Yep... Resurrecting necro/zombie topics is an act of futility...

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FreeMountainHermit
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Re: Double boiler still - temp over time

Post by FreeMountainHermit »

rad14701 wrote:Yep... Resurrecting necro/zombie topics is an act of futility...
Not really as the information contained therein in these old threads for the most part is still valid.
Blah, blah, blah,........
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Re: Double boiler still - temp over time

Post by rad14701 »

FreeMountainHermit wrote:
rad14701 wrote:Yep... Resurrecting necro/zombie topics is an act of futility...
Not really as the information contained therein in these old threads for the most part is still valid.
Well, it is an act of futility if you're waiting for one of the original posters to respond... There's been a greater than average amount of this in the past week... At least myles has been here in the last month and a half which is better than posters in a couple of the other topics...
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Re: Double boiler still - temp over time

Post by shadylane »

I like resurrected old posts, we get the old timers info and new ideas. :lol:
Without having to start from scratch answering questions :lol:
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Re: Double boiler still - temp over time

Post by FreeMountainHermit »

The info is still valid even though some of the old heads have moved on to other venues or have even left this earth.

HD Google search had me diggin' a very informative thread the other night while reading that had to be close to 10 years old.
Blah, blah, blah,........
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Re: Double boiler still - temp over time

Post by shadylane »

vqstatesman wrote:
myles wrote:There should be no pressure at all. The real benefit of the glycol system is that you can get the same temperature that you would have with high pressure steam - but at atmospheric pressure.

Yes seal your boilers together for convenience, just add in a combined vent/refil port. Ensure that this extends below the surface of the glycol and it means the only area of glycol that is exposed to atmosphere is the internal area of the vent. You can also run this system with a remote expansion chamber if you wish.

A drain for the glycol makes life easier. If you heat up too fast the glycol will burn, just the same as any other fluid. It is worth ramping up the power to prolong the life of your glycol. This is also standard procedure with any other (solar power perhaps) thermal transfer fluid.

Steam is excelent, and I would love a 50 psi jacketed steam boiler, BUT THERE IS NO WAY THAT I WOULD CONSIDER A HOME BUILT OPTION. Steam under pressure is too dangerous to mess about with if you are not qualified to do so.
Are you saying glycol can't be used under pressure or am I reading that wrong?

Also what would you consider too fast heat up time for the glycol? How long do you take?
Just my drunken opinion, since the other oldtimers are somewere else :lol:
It's not necessary to heat the glycol to boiling, nor would it be a good idea because glycol is expensive and degrades with excess heat. You can boil water without scorching or burning it. Glycol not so much :lol:
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