tube diameter when distilling/condensing water?

[clepsydrae]

Hi -- first post here, and I'm already off-topic. If this is out of bounds for the board, please accept my apologies and mods, kindly remove the post.

I'm making a steam distillation setup for making distilled water, and I'm not sure where else to ask this question, so I have come to the experts on all things distillation. It will be a pressure cooker with an outlet hose running to a copper coil submerged in a container of chilled water, spiraling down to the bottom of the container and draining out the bottom to be collected. A pretty common setup.

My question regards the diameter of the tube; I'm planning to use 30-50 feet of tube (feel free to talk me off that cliff: the idea is to minimize the changes of chiller water necessary, and my thought was that a short length of tube would start passing steam out the outlet sooner than a long length which can take full advantage of the water bath.)

1/4" diameter seems reasonable, but I was actually planning on 3/16", under the theory that a smaller/longer tube increases surface area contact with the water. Presumably at some point the i.d. gets so thin that 30-50 feet would present appreciable back pressure to the system and flow-through wouldn't be good, so I'm wondering where the crossover happens. 3/16" 1/8"? 1/16"?

Anybody have an intuition as to how small you can go before it becomes dumb?

Backstory: I'm doing this because I'm working on some home chemistry projects (specifically, determining oxalate content of local wild plants) and there is something wrong with my methods. I'm trying to eliminate confounding variables and since the method calls for distilled water (rather than the filtered tap water I am using) I am going to see if that changes things. (I also prefer to DIY rather than buy plastic jugs of it.)

It was mentioned that reverse-osmosis water might be better than home-distilled water? In light of that, I considered getting RO water from my local coop and then distilling that for the best of both worlds?

Any thoughts are welcome, thanks!

[SW_Shiner]

1/2 inch minimum imo. Any smaller and it could cause trouble very easily.
What is the pressure cooker made of? A lot of them are aluminium, while you're not planning on drinking it, aluminium is a reactive metal and some may come over in the distillate.

[clepsydrae]

Wow, interesting, didn't expect 1/2" minimum. By "trouble" you mean it would be so thin that the flow resistance would choke things off, require too much pressure from the cooker, etc?

Cooker is stainless -- thanks for that caution.

In case it's interesting, this guy used 20' of 3/8 and it seemed to be OK:

[zach]

What volumes are you in need of? Is there a reason not to use an air still? For less than $100 US you can get one that will produce 1 liter per hour and has a 304 stainless coil for condensing.

[clepsydrae]

Yeah, funny that you mention that... I had considered those but thought I would DIY it for the sake of saving money. Now that I've spec'ed out all the fittings and copper and such, that device is looking a lot more tempting.

I think the DIY route might generate a lot more volume, but I profess lack of experience.

To answer your question: I'd probably need like a gallon or two a week at the most, so that appliance is looking pretty good...

[Saltbush Bill]

It probably going to cost you more to make distilled water than you can buy it for.

[clepsydrae]

Thanks -- I think I'll come out ahead? Electricity here is about $.16/kwh all-in, so a 750W device running four hours = $.48 for a gallon. If the machine lasts 5 years, and I make a gallon/week on average, that's 260 gallons, so buying used on eBay for $75 that adds about 0.25/gal, bump to to $.30 for charcoal filters, etc, so a total of $.78/gallon, as opposed to $1.30/gal in-store. (Assuming it doesn't fall apart in a month, which is an assumption...)

But honestly even if it was twice as expensive per gallon I'd go this route just to avoid buying the plastic bottles.

[Yummyrum]

I work as a Lab Technician and use an Airstill clone to make all our Distilled water . Previously , we would buy “demineralised “ water from the local “Big Green Shed” in 4litre containers . …. It was costing several hundred a year .

I can put that thing on at 8.30 am in the morning and by about 1.30pm , I’ve got close to 4 litres of distilled water .. no Futzing with coolant water or anything .

I full it with tap water . And my god , the Scungy stuff that remains in the boiler is actually quite horrific .

I would definitely recommend them

[clepsydrae]

Just ordered one! Much as I love a good DIY project it was hard to argue for it in this case. Thanks everyone for the input!

[shadylane]

Just ordered one! Much as I love a good DIY project it was hard to argue for it in this case. Thanks everyone for the input!

That was the simplest choice.
I'd have built a 3 or 4 gallon SS boiler with an immersion heater and used a fan cooled condenser.
Basically a big brother to an airstill.

[clepsydrae]

The output per hour was probably significantly higher, yeah?