Tiny rocket for heating van, using pump housing

[Rectifier]

Hey rocket folks,

I've been messing with rocket stoves for awhile, and am trying to design one to run in my van right now.

The big question is, how small can you go?

I've come up with a design using a small grinder pump housing (scrap from work), and if anyone has ever rebuilt a grinder pump, they'll realize they look a lot like a rocket stove heater - the motor is held in a sealed bell chamber, attached above the impeller chamber by a small hole for the shaft, and the impeller chamber has a 90 degree bend coming up off it to the outlet (for the fuel feed). The whole thing is solid cast iron. All I have to add is a heat riser inside, and possibly bore out the shaft bearing journal to the size of the riser.

Obviously my small van (toyota) will require very little heat to get toasty. I would like to exhaust through a 1" steel pipe rather than stovepipe, as it's cheaper here, and seals up gastight. I'm scared of CO in my van, of course, though I do have a CO/LPG detector in there.

This means the stove could only have a 1" heat riser (right?). That seems like it might be pushing the limits of scalability. Has anyone ever built one this small? I would most likely be burning split pallet wood, softwood pellets and twigs.

It is an absolute NOT ALLOWED to use a flue smaller than the riser, right? However... what if the flue is gastight threaded pipe... then it could run at a slight pressure, right, because it can't leak CO? Or, would it choke the airflow and hurt combustion?

I'm going to start by drilling/tapping a 1" NPT in the pump housing just for experiments' sake, I think - can always drill it bigger...

[Donkey]

Hey.. Welcome to the board.
The smallest Rocket Stove I've heard of so far is in Denmark.. It's a coffee maker and is around the size you are describing. One inch is mighty small.. Feeding it could be interesting, though I see no reason why it couldn't work. I would insulate the heat riser extra - extra.

Correct on the flue/riser thing. It's an issue of air-flow. Common wisdom is (so far) not to allow the flue to be smaller than the riser. There are exceptions, but it starts getting rather technical and math-y. Best to just stick with the rule.
One place where you may bump into difficulty (pretty likely actually) is ash buildup at the feed tube. It's so tiny that it will tend to clog quickly. You may wish to set up some kind of ash drop.

Your project sounds like fun. I see no reason why it shouldn't work. Might be best to set it up outside first while working out the bugs.

Keep us up to date on yer project as it progresses. Looking forward to it!

[Rectifier]

Actually, the ash issue is a non-issue because the pump's original intake hole is right below the impeller chamber, which is now the burn chamber. All I have to do is put a little slide door there and the ash will drop right out!

Another neat thing about the pump is that the outlet (now the inlet) is set off-center to the burn chamber, so the flame flow should be spiralling and turbulent as it enters the heat riser.

I did an initial test burn in the unmodified pump during break at work today with no heat riser, to see how the flow is through the bearing journal (turns out to be only 3/4"). The feed tube is so much bigger than that outlet hole that it would not draw much at all, even when I cheated and preheated the bell with the oxy torch

I had it running a bit like that but then the wind blew out the flame, and also cooled the bell too much. Of course, the only windy day this week is when I'm playing with fire...

Also, it will require very small fuelwood. I had it way overfuelled with a couple beat up chunks of pallet wood. It only sucked down about 1/4 of the flame, but the wood diameter was too large to sustain a smaller flame. Also, the intake : heat riser diameter is way off (intake 2", riser 3/4")

Plans for tomorrow:

- Bore out riser hole to 1", add thin steel riser pipe inside the bell, still uninsulated for now except for recirculated gases down the bell.

- Cap hole in top of bell, prop it up so it exhausts around the bottom lip of it, so the heat riser pipe will get nice and hot. These two changes should improve the draw a lot over just using the 6" cast iron bell, which did not really get hot at all.

- Cover up intake to improve intake velocity

- Smaller wood

[Donkey]

It would be helpful for me, perhaps others as well to see images of this pump and your project as it progresses. Do you have web-space for this?

I've been meaning to cast about for a free service that allows remote links (off topic, sorry). It's probably finally time to just go ahead and do it.. Any ideas wold be welcome.

[Rectifier]

Yeah, I'll drag a camera to work and take some shots of the thing when it's in any sort of shape. I've been working on coffee breaks, so each post here is like... 15-20 minutes worth of work - not much time for documentation.

For a picture of the pump though, you can see one on this page: www.buttspumpsandmotors.ca/femyers_pg8.htm
it has an intake hole in the bottom (now a covered ash cleanout) and the outlet is pointing up on the right (now the fuel/air feed). The top bell is the motor chamber, now minus its motor and with the heat riser inside, propped up so it exhausts out around the rim of the bell for testing.

I tried it today with those changes in the above post (minus boring the hole bigger, because I didn't have time).

It now draws enough to keep the flame going, if the sticks are placed inside the inlet, but not standing upright in it. As expected, feeding it is very hard to do, especially due to the lack of a proper rocket blast. The exhaust is now running with a minimum of particulate, but still a very distinct fire smell.

However, it's still not a real "rocket". I need to bore that riser hole out to get better flow. Also, the pipe I used for a heat riser is still way too thick (like, nearly 1/4" thick steel... way too thick. I went with what was lying on the shop floor.) - it needed signifigant torch preheating to draw, and never got up to the proper reburn temperature.

It needs a proper insulated riser - well, because that's what makes it a rocket stove, right! Now that I know the pump can sustain a flame inside it, the next step is probably a perlite-filled, thin steel heat riser.

By the way, it's great being able to post here without having to create an account!

[canyon]

I love this forum and the fellow tinkering pyros sharing via posts. Thanks rectifier. Registering is not a big deal at all. You are obviously more than a guest, member up!
canyon