Another Rocket Stove

[Paul99]

I unexpectedly got this week off work and so set to making a simple stove for the shed. A rocket stove of some description seemed like the best option given the materials / scrap to hand. It started out more as an experiment using literally complete rubbish and then later on having developed it and learnt a bit along the way I'd make a decent one using gas bottles etc at a later date. As ever I probably put more time and effort in than was necessary but as it started coming together it wasn't looking too bad for a first attempt and with a few changes / modifications tomorrow it should be good enough for the rest of this winter. It was also a chance to use the plasma cutter - what a revelation that machine is - god only knows how I ever coped with grinders, hacksaws, jigsaws etc.

Mk 1:

The first burn getting rid off paint oil etc.

Obviously I need to add legs and internally the flue needs insulating - I was too lazy to dig the vermiculite from the back of the lockup. It also needs extending so it finishes closer to the underneath of brake drum lid as the draw is so good there's more heat going up the flue than I'd like and the top is reaching only 100C or so.

I'll take a few pictures of the inside tomorrow.

 

[the artfull-codger]

I unexpectedly got this week off work and so set to making a simple stove for the shed. A rocket stove of some description seemed like the best option given the materials / scrap to hand. It started out more as an experiment using literally complete rubbish and then later on having developed it and learnt a bit along the way I'd make a decent one using gas bottles etc at a later date. As ever I probably put more time and effort in than was necessary but as it started coming together it wasn't looking too bad for a first attempt and with a few changes / modifications tomorrow it should be good enough for the rest of this winter. It was also a chance to use the plasma cutter - what a revelation that machine is - god only knows how I ever coped with grinders, hacksaws, jigsaws etc.

Mk 1:

The first burn getting rid off paint oil etc.

Obviously I need to add legs and internally the flue needs insulating - I was too lazy to dig the vermiculite from the back of the lockup. It also needs extending so it finishes closer to the underneath of brake drum lid as the draw is so good there's more heat going up the flue than I'd like and the top is reaching only 100C or so.

I'll take a few pictures of the inside tomorrow.

 

[the artfull-codger]

Paul, I made mine with 100 x 100 box for the ''j'' tube & haven't insulated it & it burns o-k so I've not bothered, I put a centrifugal fan on the top when it's hot & it blasts out like an electric fan heater.
graham

 

[Wallace]

Addictive aren't they? I had a play about with one just to get a bit of hands on after reading up on them. I used a small 14kg butane bottle and some 2" box section uninsulated and with some assistance from a little compressed air I managed to get the cylinder top around 300°C, steam out of the flue and no smoke. The tubing was too small to sustain the temperature on it's own without the compressed air.

 

[Paul99]

Paul, I made mine with 100 x 100 box for the ''j'' tube & haven't insulated it & it burns o-k so I've not bothered, I put a centrifugal fan on the top when it's hot & it blasts out like an electric fan heater.
graham

I'd certainly be using something like 4" box if / when I make another - the 3.5" x 7" I used was all I had to hand but it's too big imo.

The shed / workshop is only small and I'm sure it would easily keep it warm as it is already but the thought of making it more efficient is too tempting to just leave it as it is. I've always tried to stick to the if it isn't broke don't fix it rule but as making a few alterations won't cost nothing but a bit of time (well I suppose electric, wire and gas too) and I may learn more along the way then I'll give it a go I'll stop at the point it starts to cost anything. I do have a few old centrifugal extractor type fans that could be used for something like that though that could be overkill for my small shed especially when I get the rest of roof insulated.

Addictive aren't they? I had a play about with one just to get a bit of hands on after reading up on them. I used a small 14kg butane bottle and some 2" box section uninsulated and with some assistance from a little compressed air I managed to get the cylinder top around 300°C, steam out of the flue and no smoke. The tubing was too small to sustain the temperature on it's own without the compressed air.

I do have a couple of cylinders of various sizes ready to make a good one I've learnt a few things already and hopefully will learn a bit more before I start attacking a cylinder. I also want to make a sawdust burning version as again we have plenty that ends up on a bonfire which seems a waste if it could be put to good use - a very simple trial with a 5l paint tin and a length of flexi chimney liner was very promising and will look to be able to swap the innards to be able to use either fuel.

 

[Wallace]

From what I have read the general census is to use firebrick for the J tube when the efficiency is good as the steel erodes due to the high temperatures. If I could get away with no smoke I would be happy due to the the neighbours washing lines.

 

[Paul99]

Didn't get around to posting Friday but I've spent a bit more time playing with the rocket stove. I extended the riser tube closer to the underneath of the top plate, insulated it with vermiculite:

you can also see I placed a vortex generator in the bottom of the riser tube - it's only loose so can be taken out easily if it didn't work.

The last change I made was to add secondary air to the bottom of the riser tube via holes cut in the side of the J tube and small lengths of 30mm box:

I'm not convinced that the secondary / preheated extra air is really doing anything and may in fact be a nuisance until the whole thing gets up to temperature as it reduces the draw it seems - easy enough to add throttle plates too at some point though so they can be opened / closed as needed.

The other changes however have been a major success:

Can't be a shortage of mixing now and absolutely no smoke at the end of the flue now either. The top plate now happily sits at 350C so more than hot enough to boil a kettle.

A quick video showing the vortex working well - at this point it was still not up to running temperature the vortex is much more fierce now and flames extending much higher up the riser tube:

Obviously the smoke does not normally rise out of the feed tube - but does with the top plate removed.

Still work to do on it but it definitely looks like it's going to do the job it was made for.

 

[Wallace]

The vortex generator works well, I wonder if the exhaust flue was also offset the swirl effect would continue going down the chamber?

 

[Paul99]

The vortex generator works well, I wonder if the exhaust flue was also offset the swirl effect would continue going down the chamber?

That's an interesting idea that I can't say I've seen / noticed anyone try before. The flue exit will need to be changed (for a 90° bend) when it comes to fitting the stove in it's permanent place so I may just play with that idea a little and see what happens.

Looks like it's getting more than hot enough for complete combustion - though I'm not sure how long the thin / pitted / rusty MS votex generator will stick the punishment:

Looks like I'll need to add a shelf / grating along the bottom of the j-tube to maintain a constant air flow if I intend to feed it the rubbish bark + splinters that I had intended (you get loads of using a log splitter that gets wasted) - it doesn't burn away quickly enough and builds up like coal which eventually chokes the J tube up, reduces the draw which then causes the flames to burn upwards through the feed tube - not a major problem to overcome though.

 

[Shox Dr]

Cool in a hot way.

I really need to get my act into gear and build one of these. On the plus side, I keep modifying it in my head.

Has anyone tried using salt glaze pipes to form the inner burn chamber, or any type of building pipe?

 

[Paul99]

Cool in a hot way.

I really need to get my act into gear and build one of these. On the plus side, I keep modifying it in my head.

Has anyone tried using salt glaze pipes to form the inner burn chamber, or any type of building pipe?

I had considered using salt glaze pipe as well as cast iron pipe and fire bricks cut to size as I had all available but in the end opted for steel simply because it was so easy to use and could be cut, re welded and cut again until it was right. I'm sure the steel will eventually give up but this one was only meant to be experimental and made from total scrap that was not good enough for anything else anyway so if it only lasts one winter it's no loss.

I spent ages thinking about the design, looked at countless hours of you tube videos etc and like you I kept changing my mind and thinking of things that just had to make it better. In reality - especially when I gathered together what I had to work with steel wise the design had to change. To get a basic heater is simple it seems - how much better you can get it is just a matter of how much of a perfectionist you are and how much time you want to spend on it IMO. There's a few other things I want to play with before calling it finished but progress might be slower now I'll have to disrupt workshop time with work .

 

[Spark plug]

I plan on building one with a off cut of 16'' x 1/2'' box selection I've been promised when an old chap I know gets to the end of the length. Then line it with fire bricks for the burn chamber.

 

[Paul99]

It's been a while since the last update and I've made a few changes in an effort to improve the stove. As suggested by Wallace I attempted to produce rotation of gases within the flue by offsetting the flue - Using an airline to blow fine ash up a short dummy flue shows that air is spinning as it's emerging from the end of it - whether this does any good - I don't really know. It's certainly doing no harm but I probably wouldn't bother when I make the next one as I'm not sure it's worth the effort. I also added some very crude flaps to open close the secondary air supply - mainly to help draw upon initial lighting.

For a while I've been struggling to stop flames creeping up the feed tube - 99% of the smoke is drawn drown through the fire but the odd puff was emerging from the top of the feed tube - I could have lived with that but it was more of a problem when the whole feed tube was well alight from top to bottom it would take just a few minutes to block up the horizontal section of J tube with half burnt lumps of wood causing a much more serious amount of smoke and flames to be forced up out of the feed tube. I experimented with grates and plates in the bottom of the j tube to try and cure the blocking problem but failed miserably. The blocking was the symptom though and I needed to cure the problem of the flames creeping upwards....

That's where the lid came in - it stopped it instantly the feed tube can be filled to the brim and the lid closed, Air intake is controlled by the access door at the bottom, the wood burns only at the bottom of the feed tube and as it's meant too the fuel now drops down as it's consumed from underneath. The biggest upside of using the lower access door to control air is that it's now very controllable and within a few minutes you can go from a slow long burn to an incredibly fast fierce burn which you can see in this video - first half it's shut down and the second half, taken literally a few minutes later, it's opened out a bit (though far from flat out !)

In this video I was burning only lumps and strips of bark which usually ends up wasted as it's hardly worth the bother storing for the woodburner. A later run with very well seasoned ash split into 3inch cubes or so produces temperatures over 400C on the top plate and flames that reach the very top of the riser tube inside.

There's still a few more things to add like a tray to catch ash / embers and an access door to the offset flue box . I've found lighting a small piece of firelighter directly underneath the flue then sliding the flue onto it's socket before lighting the stove itself starts a draw off that sucks away initial smoke and gets it going with much less fuss - an access door would simplify the process.

 

[Caledonian]

Hi Paul

Just caught up with the thread, looking good and somewhere to boil the kettle can't be bad
Can I ask? What is that flexi you are using as the exhaust/flue? I'm trying to get round to building one myself

Jack

 

[Paul99]

Hi Paul

Just caught up with the thread, looking good and somewhere to boil the kettle can't be bad
Can I ask? What is that flexi you are using as the exhaust/flue? I'm trying to get round to building one myself

Jack

Hi Jack,

The plan is eventually to be able to boil a kettle on it. Might be a bit optimistic but I'm wondering if it will be capable of melting down aluminium in a crucible that will sit down inside the riser tube - got plenty more to play with before I get onto trying that though.

As for the flexi flue - It is a thin 5" single walled SS flexi liner - I am guessing it would be a gas flue liner as opposed to a solid fuel or oil liner (which would be thicker and double skinned to cope with higher temps and corrosive flue gases. It would also be the cheapest liner to buy due to it being so thin. This particular piece came from an ex B&B (closed early 90s) that we worked on last year building a big extension and renovating the roof. Each shower cubicle had an extractor fan on the ceiling with a length of this flexi on. All these lengths of flexi terminated in a wooden box in the attic - presumably there would originally been one pipe from this box running to a vent out through the wall. Seemed like a very odd and damn expensive way to ventilate the shower cubicles as SS flue liner isn't exactly cheap. It was all ripped out to be skipped so I rescued the lot 4 or 5 lengths of about 3 - 4m in length - it's been sat waiting to be scrapped but if I can straighten out the flattened / kinked sections I'll hold on to it.

I've not long come in from the shed where having completed a very quick experimental sawdust burn which worked unbelievably well for what is literally rubbish that is a PITA to burn on a bonfire - maybe two buckets of sawdust lasted 2hrs running flat out and heated the top plate to over 300C - though temporary rockwool "seals" gave up /melted / burned away (never thought it would do that) inside of 30mins causing big leakage from the bottom of the riser directly into the flue box and excess air to get in where it shouldn't have the result was burning around the edge as well as up the middle which was a waste of fuel. A bit more work and I reckon that same amount of sawdust last 3hrs maybe even more - looking forward to trying it again tomorrow evening will try and take some photos and maybe a video if I get a chance before leaving for the pub.

 

[the snooper]

saw dust, mix it with old engine oil, not to wet and then watch it burn

 

[Paul99]

saw dust, mix it with old engine oil, not to wet and then watch it burn

Funny you should mention that I was showing my brother the stove running on wood yesterday (just after I had done the videos) and mentioned I was going to try a sawdust burn - he said the same thing why not wet it up with old oil - I thought that was a scary suggestion for a fireman but said I will try it at some point. I'm glad I didn't try it tonight though with burning going on where it shouldn't have been it was easy enough to scrape out the sawdust into a metal bucket to get it back under control that mightn't have been so simple with hot oil in the mix . It's definitely on the ever expanding "to try" list. Not only would be a good way to get shot of all sorts of waste oil but I bet such a mix would be capable of some extreme heat and might well be the easiest way to achieve aluminium melting temperatures too...

 

[the snooper]

I used to burn it in a duel fuel stove in a workshop I rented years ago, got very hot and no smoke, you only want enough oil for the saw dust to absorb and only just stick together to much and it starts dripping

 

[Caledonian]

Thanks for the info Paul I'll be watching this thread with interest

Jack

 

[Wallace]

Interesting that the offset flue worked, I want to get building again! I also had problems with burning in the feed tube and smoke when the air was fed through an auxiliary feed tube, the burn was better too with a lid on the feed tube. I wondered if the auxiliary air was blocked off just before the feed tube lid was opened the draw might stop the smoke exiting the feed tube, something like a lid that closes the auxiliary air before the feed tube is opened. Maybe the burn area needs insulating to reduce the losses and increase the temperature like the ones made from fire bricks. Great project and videos.