thermo siphons, stainless heat riser, small downflow space between bell and riser

i have an r2d2 look alike--old coal fired water boiler about 13 inches diameter and 2 feet tall, connected with3/4 copper to a hot water tank about 2 feet away--plugged directly into the drain at bottom as cold leg and the opening for the pressure relief valve (yes, i know i'm working without a net but it really makes for a direct connection)

I've been using this for many years, with a funky homemade plumbing system that allows me to open a valve and as the water expands simply pushes it back toward the storage tanks--works great, and can fill a 30 gallon tank with HOT water in a half hr or so depending on fuel etc--the amount of fuel that can be used at one time is limited and i'm burning wood not coal, so must refill several times in 1/2 hr to hr

along comes my 6" rocket mass heater in the main house and i drape the 55 gallon barrel with 60' of 1/2" copper coil and then send it 13 feet through 1/2 " pex to a water heater

and yes, i'm getting the thermo-siphon effect, but very slow, will take hrs to heat the tank-- even with a pump that coil just doesn't transfer heat fast enough from the barrel to the water

i bought a longer body barrel, so the possibility now exists to stretch r2 to be 2 feet taller and replace the 55 gallon barrel

like i said r2d2 is about 12"diameter--if i use a 6" center heat riser, that turns into 10" od with 2 inches of insulation and only leaves an inch for the downflow space around the riser inside the bell (r2d2)

diameter of 10 inches is 31.3 inches circumference so approx 30+ sq inches

theoretically i need about 30sq inches to carry the 6" flow of gas, which this would provide with very little to spare

has anyone out there played with closer tolerances between the riser and bell? what are the pitfalls of cutting the downflow air space right to the bone?

maybe i should be looking at thinner insulation, abandoning that small barrel altogether?

also, would having that water jacket right at the top of the riser where the hot gas exits interfere with efficiency of the burn, taking too much heat out of the process?

i'm a little reluctant to take apart a working system with the possibility that the rocket won't work anyway

and on a separate note--- right now i did an expedient thing that i know will have to change sooner or later, used 6" stainless double wall for the heat riser in my rocket--how soon will i need to replace it and are there serious risks, or just inconvenience when it fails? slowly losing it's rocketiness and finally smoking back at me

thanks, bob

bob day wrote:
and on a separate note--- right now i did an expedient thing that i know will have to change sooner or later, used 6" stainless double wall for the heat riser in my rocket--how soon will i need to replace it and are there serious risks, or just inconvenience when it fails? slowly losing it's rocketiness and finally smoking back at me

Hi Bob,

There's a bit in podcast 267 where Paul refers to a conversation he had with Tim (no last name mentioned) about pocket rockets. Tim had observed that if simple ducting was used for the pocket rocket's down-tube, it only survives about 20 hours of use. Paul points out that this is not very economical, but more importantly, that metal may be ending up in the air. If you are using stainless steel in you heat riser, you may experience a similarly high rate of degradation, but in this case you may also be adding nickel and chromium compounds to your exhaust, some of which can be extremely toxic (check out nickel carbonyl for example). I don't know for sure about how fast you would have failure, i.e. perforation -- stainless steel should last better than the steel in plain ducting, but on the other hand, the temperature in an insulated heat riser would exceed that in a pocket rocket. If you get perforation in the heat riser it will mess with your burn efficiency and could lead to higher temperature gasses entering your mass if they can short-circuit the barrel. Component failure doesn't look like it should be you main concern here though.

i think i heard that one, they were talking about green smoke from the galvanized--ducting is an order of magnitude weaker than stovepipe which is another order weaker than double wall stainless, so i hope no major degradation happens before i get a heat riser cast

i put it in when what i was reading was uncertain about the durability of stainless for that application, but since then it seems there are stronger precautions against, thanks for the thoughts

well, i finally got around to doing an autopsy-- it didn't die, but i knew it wouldn't last forever with that stainless double wall stovepipe i used for a heat riser

when i first took off the barrel it looked perfect,, but when i took out the heat riser i noticed something fell out when i set it down on the floor-- it was the last bits of the inner wall--the insulation seems to be holding on to the outer wall pretty good, and probably would have continued working for some time to come, this time i'm trying a formed in place heat riser using an old piece of 6 " stovepipe and a newer 10" one with perlite and fireclay as the insulation in between--the inner 6" piece is rusted and already pretty thin, so it should burn out pretty quickly. my core is made from the same mix, and it is doing pretty good except for the firebox which i occasionally have to repair with regular clay from outside,,

i quit working this evening after some major overhauls to the thermal mass as well,, not sure if i will use the water tank to replace the 55 gallon drum or not--if i do it will mean reworking the exhaust manifold and ending up with some pretty close tolerances , but i should get a better fit for the copper tubing sliding over the unribbed side of the tank

the barrel is a major hassle having to expand the tubing first going over the top edge because it's almost impossible to get the tubing tightened back up to make really good contact with the 55gal barrel, if there was an easy way i would definitely just put the barrel back on

anyone have an easy technique for tightening the tubing after it's been expanded to get it down on the barrel?

Bob; quick suggestion, try using a piece of sonitube instead of a piece of stovepipe, for your inner liner. The sonitube will burn out with your first fire leaving a smooth bore in your riser. I've included two pics of mine , before the first burn and after 3 months of daily burning. Good luck Tom

thanks for the suggestion, not quite sure what sonitube is, but heat riser part of the job is already done--i think, will find out if it's too tall very soon.

the stove pipe was just laying around, too rusted to be useful for anything else, but i will keep the sonitube in mind for future projects

bob; sonitube is a premade concrete form commonly used for support pillars, it is readily avalable at most building supply. available in several sizes. Always measure before you buy as they are not uniform in size.