Another Rocket Stove

[Paul99]

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.

Well it worked blowing ash through the stove with the airline but is not anywhere near as impressive in terms of speed or smoothness of rotation as the flames in the riser. There is no evidence of rotation left at the end of the flue in use (not that I would expect it) I remain to be convinced it makes any difference in actual use but was worth playing with none the less.

I've had another go burning sawdust again this afternoon in-between work and managed to get a burn lasting 3.5hrs. With some better fire rope seals I learnt a bit more this time - the biggest thing being that standard 32mm waste pipe is too small to form the flue through the sawdust with - it doesn't allow enough air through and smoke ends up being pushed backwards out of the feed tube / air supply for a good 20mins before everything gets up to temp and the flue has increased in size a little. The second thing is that as the sawdust "flue" gradually increases in size (as it's meant too) it draws more air and obviously has exposed a greater amount of fuel too - the result is a slow but steady gain in burn rate as time goes on - a positive feedback loop which would definitely reduce burn time if not kept in check by limiting air supply as it occurs - this effect was no doubt made worse due to having too small a flue to start with and having to have air supply wide open to try and get it to draw right. I'm assuming that by increasing the sawdust flue diameter from the start the burn rate can be kept fairly constant by limiting air supply right from the off and additionally draw should be better and smoke drawn through properly too. Didn't get a chance for pics or videos today but will hopefully get some tomorrow afternoon.

 

[Paul99]

The plan of spending a large chunck of this weekend on the stove never happened and amounted to just one more test burn. I did manage to get a few more pictures and a short video too though so not all was lost.
This is currently how the stove looks with it's lid, ash tray and the offset flue box as mentioned before. I also made a new door which can be locked off in any position to control airflow:

This is how I've been setting it up for a sawdust burn, the smaller 6" woodburning riser has been replaced by a larger 8" flexi liner of cut to allow more sawdust to be packed in:

With the formers removed and a sand top added (to prevent the fuel burning from the top down) you are left with a "flue" running up the middle of the sawdust:

The horizontal former pipe allows air to be drawn from what was the J-tube feedtube / access door. This horizontal section it lit via the bottom access door:

Get the former size right and there will be enough draw to get an impressive flame at the top:

Note that most of that flame is due to the excess air entering from the top causing a secondary burn - I really need to add secondary air to the top of the sawdust riser as replacing the lid stops this secondary burn and the flue produces smoke - too inneficient for my liking and will be addressed when I get time.

An hour into the burn the temporary rope seal at the bottom of the riser gave up again causing the sawdust to burn around the inside of the SS riser again - it cut the burn time down to 2hrs. With only a tiny amount of air needed to sustain the burn I was hoping for a much longer burn this time. This video shows it shut right down but the top maintained a temperature of around 250C you can't quite make out that the sawdust has burnt away from the edges:

Burn over and top removed it was easy to see that the seal was indeed to blame - note the hole:

Most of the "ash" is actually the sand (which will be re-used next time) . Next job is to weld in a socket to form a more permanent seal and solve this problem

With everything still hot I slid in the smaller 6" woodburning riser in (leaving the larger 8" sawdust riser in place), replaced the top and had it running on wood again inside of 30seconds - which is what I was looking for right from the beginning something that could burn both fuels equally well yet not require a long process of having to switch internals. The woodburning side is about spot on... It'll get to 330C (the point at which the infra red gun quits) in almost 5 mins dead when opened out and fed constantly. Shut down to maintain about 250C it'll go for around an hour before needing to be topped up. The sawdust burning side needs a bit more work yet but I'm learning a bit more each time I test it.

 

[Paul99]

I've just about finished with this rocket stove for now, it's certainly more than good enough to heat the shed - it'll easily keep the shed at 22C (too hot to work in for me) when it's in single figures outside using very little wood - given how many drafts there are yet to sort and the fact the roof yet to be finished insulating I'm very happy.

I have done quite a bit more work on it since the last post - I noticed on clean outs that build up of ash was smothering far too many chunks of half burnt wood / charcoal for my liking - sieving it out brought me to the conclusion I needed to separate ash from the fire. Initial tests with a drop in re-bar fire basket ruled that out as a permanent solution as it cut the draw down and provided very little room to remove the ash from underneath it. So instead I came up with a grate in the very bottom of the feed tube - simply sections of re-bar sat in notches making it very easy to replace should / when they burn out. The video below shows the amount of charcoal being wasted and the temporary slip in fire basket I tried before adding the ash grate + box to the bottom. I also added primary air controls on the ash box door and the feed tube lid which it perhaps a bit OTT but its been interesting watching how the burn changes by altering where air is fed in. It was also a good excuse to get some practice in with the lathe whilst making something functional at the same time.

Given a successful test run I decided that it's good enough for now and gave it a coat of stove black - which is yet to be baked on:

 

[Wallace]

Looks good with a coat of paint, could be mistaken for something from the Manhattan Project now!