ambitious pot still project

[Apsynt]

Heya
Now that we have one running bok and several keg based pot stills it's time for a real challenge! Here's what we have so far, but i would appreciate suggestions and advice from those who have built similar things!
Got hold of a few old copper water cylinders, they are not the thickest but all we could find with a limited budget

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So idea is to do a double walled boiler using the narrower taller cylinder as the inside and the fatter shorter one as the outside. the gap between the two we are thinking of filling with heat transfer fluid/cutting oil as it is not corrosive, has a very high boiling point and is not flammable. the two cylinders would be cut down to sice and welded together leaving the cavity sealed except for a bit of tubing acting as a vent so no pressure builds up and probably a tap near the bottom for emptying the fluid. the inside boiling chamber would have a piece of pipe of at very least 40mm, probably larger with a tap for emptying the wash going through as in the second sketch. The whole thing would then be directly heated on a gas burner.

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maybe also we'd use the smallest cylinder as either a thumper or as a chamber for the worm. As for the head and the joint, haven't even started thinking about it, saving those headaches for later

What do you think so far? how stable/easy to work with, are fluid filled double boilers? any major issues i'm likely to run into during construction?

[myles]

Well you did ask so hear goes Nice plan. Don't worry about the thin wall it will just help you with your warm up times. You will definitely need to reinforce / strengthen and insulate the outer boiler. You need to keep all that heat in.

Even though I use one, I would reccomend that you ditch the idea of a double boiler completely. Use the middle sized tank as your boiler, MOUNTED HORIZONTALLY, reinforce the wall where you will apply heat with both of the ends removed from tall one, and apply heat directly. It is much faster and easier to heat directly, but that thin copper wall could give you problems with hot spots. Building up the wall thickness would help spread the heat better and improve the rigidity of the tank. Those hot water cylinders are notoriously wobbly when they are not completely full.
The small tank that you have is too small to use as either a thumper or as the container for a worm. It WOULD be usefull to include it as an onion or gin head in your column. I would cut out the bottom of the small tank, and solder it straight into the middle of your boiler and use it as the first part of the column.
Use what is left of the tall one to build the remaining sections of your column and lyne arm. It is fine if they are a bit thinner than your boiler.

If you are going to build the double boiler. Solder the taller tank inside the short fat one as you suggested but mount them HORIZONTALLY. You will dramatically increase both your liquid surface area and your vapour production rates. I would have to say don't bother with any fancy fluids. Keep enough water in your outer boiler to stop it boiling dry and use the steam it produces to heat your inner pot - it is far more efficient and is proven technology to boot. If you wish, think about fitting a pressure vent. Either from a pressure cooker or a hot water system. LOW pressure just enough to lift your steam temperature a few degrees would help but is not essential. Lots of potential there however you go ahead. Document it for a build post it could be interesting.

[Apsynt]

Thanks for your reply! LOTS of new things to consider here. Reason i want a double wall boiler is because i intend to use this still for fruit only, i have other ones for liquid washes, and things like grape must or feijoa pulp burn really easily, even if heated very gently on a direct flame.
have considered mounting it horizontally, but am worried about the distance i;d have left between the wash and the head as fruit tends to foam a lot when cooking. Good poindt about the pressure cooker vent...though if i use cutting 'oil' wouldn;t that give me a lot higher temperature overall without need to top it up constantly as there will be no evaporation?
A lot to think about before i make the first cut!

[junkyard dawg]

I use a double boiler for fruit/grain mashes. Never had foaming problems with a double boiler. Use water only. Oil doesn't work like water. Do a little googling on direct steam injection and you will get a better understanding of the latent heat of steam and sensible heats and so on... and hopefully understand a little better why it works. Water is the way to go.

[Hack]

I have to disagree with myles a bit. I think the small tank would make a decent thumper. I use a five gallon thumper with my fifteen gallon keg and I could get away with a smaller one if I wanted.

I also agree that water would be best for your double boiler.

[schnell]

Avoid the oil in the heat exchanger. Water is plentiful, cheap, and above all EASY TO CLEAN up after spilling. And the water won't get rancid, or decompose, or be a fire hazard if the still is accidentally fired while empty.

Remember murphy's law when considering your heat exchange fluid. You're going to slosh, spill, or drip some of that on and around your still. Even if its food safe and only collects dust it'll get nasty messy.

[absinthe]

water is crap as a transfer fluid the boiling points are so close between the fluids you just cant get a good output (thats why steam is better holds much more energy) oil would be much better but yes it will go rancid and also slowly set and coat the inside of the jacket with a gooy mess thats very hard to remove.. you will only get about 60-80 (Btu/ft² hr °F) but steam gives 205 (Btu/ft² hr °F)

water does have a better heat transfer coefficient BUT the boiling point is the problem from what i have found using a double boiler.. if you can find something to increase the boiling point of the water is would run much more efficient (the water in the jacket WILL boil too) you could run the jacket with a small amount of pressure but that just a little bit dangerous (about 30 psi to get 130°C)

i would use oil but just remember it will turn into a gooy mess and clog your jacket up quite fast (have a look around a slightly dirty deep fryer and you will see the goo it turns into)

i would try for steam if i was you run the mid sized tank as the boiler (although a keg would be a better idea, stronger and already pressure rated)and pipe that through a coil inside your other tank and have the pipe exit and finish in open air (so you use the still boiler like a worm condenser for the steam) and don't forget a relief valve

[Usge]

Anything old and copper should also be checked well for "lead" solder.