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After a few years now running a simple pot still, and a few months prior to that with a Nixon-Stone, I've just acquired a 4" modular 6 bubble-plate still. It came with tubing, wye connectors, and valves for both the reflux and product condensers, but the plumbing parts are all really cheap and I'd like to upgrade them and make some improvements on the system. I have questions about how to design the plumbing system, and I know there are a lot of unknowable variables that limit the ability to conclusively answer. But I'd like to avoid making mistakes that would result in buying a bunch of fittings and valves and stuff more than once to arrive at a working system. So I'm asking for your experience with similar equipment ...
I want to stick with flexible opaque polyurethane tubing, but get somewhat better quality connectors and valves. Based on my reading here, there's a few other mods I'd like to make too. Please see the following diagram:
Also, since it's hard to find metric fittings in the US, I want to replace the metric tubing that came with the still with tubing sized in good ol' American inches. Along with that, I'm trying to avoid steel fittings. Plastic push-connect fittings look like the best option for ease of assembly / disassembly, reconfiguration, cost and weight considerations, etc. There are both cheap and decent quality push-connect components available and I'd like to stick with better quality stuff.
Most of the problems I've encountered in designing this have been around the question of the amount of water flow I'll need at full reflux. The dephlegmator is 4" dia x 8" L, and made up of six 15mm ID vapor tubes, and will be fed by garden hose, adapted down to the tubing. The still was supplied with 12mm OD / 8 mm ID tubing, so I figure that's considered sufficient.
So I guess my first question is what tube size is suited to this still? I'm thinking 3/8", but that might be dependent on power input so I should add here that I'm gas fired, and the best I can do is guess at power input levels. I'm running on a 15.5 gal beer keg as a boiler. Also, 3/8" tubing has a 1/4" ID, which is rather smaller than the 8mm ID I was supplied with. I could step up to 1/2" tubing, but then you run into the second problem ...
... which is sourcing a decent quality, plastic body needle valve for reflux control. McMaster-Carr has these:
which look like a good option, but they only accommodate 1/4 and 3/8 OD tubing. And assuming 3/8 OD / 1/4" ID is enough flow for full reflux, will that valve pass enough fluid when wide open? I'm struggling to wrap my head around flow coefficients, but it looks to me like its max flow rate is .4 GPM (its flow coefficient). Or am I way off base? (That seems likely at this point, as my head is spinning trying to parse the physics of fluid dynamics.)
I'm happy to take suggestions for flow control valves. Finding push-connect valves has been hard, so I might have to compromise and use a couple metal NPT adapters to make something work.
OK, so assuming 3/8" tube is sufficient, and the valve is fine, another problem is that the coolant water supply also feeds an impressive garden and lawn, and its irrigation needs. Sprinklers are constantly turning on and off through the day, which will affect the flow rate of coolant, which will result in constant fiddling with the needle valve to keep things in equilibrium. To avoid this, I'm thinking a simple flow regulator of the sort used in drip irrigation systems might help if placed before the reflux condenser inlet. But what flow rate would I need at full reflux? Again, this brings up a bunch of questions I don't know how to answer: how much heat am I putting into the system? How efficient is the dephlegmator? What temp is the water? And so on. Generally speaking though, with similarly sized stills, is 1 GPM enough? Closer to 5? 10? What's your experience?
And lastly, I plan on adding a bypass valve to the outlet side of the dephlag, so I can quickly put the system into full reflux. I'm thinking a simple quarter turn ball valve will do here.