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Post by thewongs on Mar 12, 2012 15:13:52 GMT -8
I hope I'm in the right thread here. I see "Rocket stoves for cooking", "Rocket stoves for heating" but not "Rocket stoves for both"  Anyway... I've just started building a novelty grill/patio heater as a present for my sister. As it happens, it's my wife's birthday on the 24th and we are planning a barbecue, so the plan is to have it ready, or at least functunal by then. This is a taller order than it sounds due to my work load at the moment, but I'm going to give it a shot. It is going to be a very small item. The tank is from an old 25l compressor and is about about 10" in diameter by 20" long. The main burner will be made from 3" (inner diameter) steel pipe and the feed box from 3 - 4" box section (depending on what I can get). The plan is to have it as a grill when the lid is open and a heater when it's closed. For this project, I have no plans to use insulation unless it's really needed because of the small diameters and other various restrictions. Today I made the main burner and welded it together. Being impatient, I made a feed box from an old paint tin and fired it up. I must say that this was the easiest rocket I have ever started! With one small piece of paper and with one small blow it was soon making the rocket sound and burning very well indeed. I've done one of my famous paint pictures to show the basic idea and will post pictures of the progress as it develops. |
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Post by thewongs on Mar 12, 2012 15:49:29 GMT -8
Picture of the burn test. I only fired it for about 15 minutes. After that, I was convinced that it is going to be fine.
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Post by thewongs on Mar 12, 2012 15:50:49 GMT -8
Looking into the tin firebox.
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Post by thewongs on Mar 19, 2012 17:23:13 GMT -8
At the weekend, I welded on the firebox and mounted the main works into the cylinder (compressor tank), then fitted the exhaust. The design changed a little as it developed and what I have now is in the picture. I ran a test and came across a small problem...
When the lid is open, this thing BURNS! So, as a barbecue, it will be great. The problem comes when the lid is closed, making it into a patio heater. As soon as the lid is closed, it starts burning from the firebox. I.E. The "rocket" draught is lost. Is this because the exhaust is low down? Do I need to fit a chimney to create a "secondary draught" to get it to suck again? I was under the impression that the rocket was the main draught creator and what happens after that is not so important. Obviously, I'm wong???
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Post by Donkey on Mar 19, 2012 19:09:38 GMT -8
2 possible issues: The exhaust at the bottom there, pretty likely that there's not enough space for proper flow. See the helpful hints thread. It's unlikely that just sticking a stovepipe in the side there will do it. This one is the one I'd check first and my bet for the problem. It could also be the gap above the heat riser is too tight. |
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Post by canyon on Mar 20, 2012 0:06:54 GMT -8
Also, you really need to insulate the heat riser. |
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Post by thewongs on Mar 20, 2012 15:01:27 GMT -8
Also, you really need to insulate the heat riser. Yes. I said earlier that I was not going to use insulation, but I understand it is an important part. I've seen quite a few burners without insulation so thought it would work anyway. Silly me!  As you can see by my latest drawing, I have put the burn tunnel inside the tank. My thoughts were that it would be easier to insulate it if the need arose, so I'll probably raise the exhaust a bit, put some vermiculite in with a plate above and insulate the riser. As far as the riser being too close to the top.... I think there is about 2" at least to spare but I'll need to check it tomorrow. If I understand the exhaust issue correctly, I need to make the exhaust exit larger than the riser diameter? I assumed that it needed to be the same all the way through the system? Does this then mean that the exhaust can exit at the bottom without a problem? I would prefer to do it this way to keep it looking better and have the option to connect to a heated bench seat later (maybe the next project). I'll do some measuring and take a few pictures tomorrow too. Many thanks guys. Your input is greatly appreciated. ;D |
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Post by Donkey on Mar 20, 2012 18:42:19 GMT -8
In theory, all same same is good. In practice, it's simpler and more effective to provide a larger area at that point, then reduce. Peterberg describes how to math it on the helpful hints thread.I'll quote his opening shot: There's a difference between a stream opening and a stream profile. The main culprit is the gap between insulation canister and barrel. When you're only looking at the opening in the side of the barrel, it's easy to think you have to make the opening the same as system size. This is utterly wrong, because the gases are not streaming straight out of the opening. Instead, it is coming from left and right, and from the top. Moreover, in the corners two streams can't pass there at the same time, so you have to compensate for that. Maths can help here. Start with system size area, divide it by the gap between inner and outer barrel, add twice the size of the gap for the top corners and you've got the length of the stream profile. The profile length consists of the top rim and sides of the opening. For example: system size of 8" equals a little bit more than 50" square. The gap is, say, 2" wide, which will get us at 25", adding the gap twice will give us 29". Presuming the opening is one foot wide, the height need to be half of 17", which is 8.5 inches. The open area in this instance will be about 90" square. The resulting opening in the side of the barrel will look enormous, nevertheless this is absolutely the correct method. The recommended gap is smaller than in my example, so the profile length will be even larger. P.S. The top rim is rounded, so you have to measure it along the contour, not as a straight line. |
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Post by thewongs on Mar 21, 2012 9:29:53 GMT -8
Thanks Donkey! Yes. It all works in my head and I know what you mean. It makes sense to me now that the exit needs to be a lot bigger.
I did some measuring today and took a few pictures. I'm thinking of changing the design from the picture on the left (as it is now) to the picture on the right. This allows for insulation and closes the gap between the pipe and the tank. It also moves the exhaust higher and makes it a bit larger. In the picture, I've allowed for a 4" exhaust, but maybe 6" would be better?
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Post by thewongs on Mar 21, 2012 9:32:05 GMT -8
The mini burrner as it is now.
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Post by dkrocketstoves on Mar 21, 2012 14:28:13 GMT -8
I would put a flue on in to get it to draw, hot gasses do not like to go down and cannot be pushed, but must be pulled with a rising flue, when the exit is at the same hight as the inlet no draw will happen, increasing the height of the exit will help but I think it may have to have a larger difference in height between the two than you are proposing, If you think of hot gas flow as the inverce of water flow, the bigger the height difference the better the flow.
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Post by Donkey on Mar 21, 2012 15:14:52 GMT -8
Ah, but in a rocket stove, you get gas movement out of heat differential. The difference in temp (and density) between the heat riser and the barrel allows flow without a chimney. Anyhow, there will be plenty of heat left over to drive a chimney attached to the bottom. thewongs, I'd move the exhaust down as far as it will go. With the lid closed, you've got a fast radiant heater there.. Maximize by making the chamber as large as is practical and the exhaust as low as possible. Remember the "stream profile" thing.. It's important. You might be able to reduce pipe size if you can dump enough heat.. I'd do the experiment by building your gizmo straight system size and trying it with a reducer on the end. You can try it reduced or standard and see what works (or not). |
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Post by thewongs on Mar 22, 2012 1:44:12 GMT -8
I'm not great with maths, but I understand the principal, so am guessing that something like this may work?
Basically, I'm just making the exhaust almost 3 times the size of the riser diameter (If I make it square, it could be bigger?
This is the maximum size I could go to with this small tank without it looking even more silly.
The other option is to forget about the heated bench and cut the bottom out of the tank?
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Post by thewongs on Mar 22, 2012 1:47:00 GMT -8
Having just looked at my own post above, I don't need to forget about the bench. It could sit on top of the bench? Might even look funny with a few legs welded on!  Anyway... That's for later.......... |
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Post by Donkey on Mar 22, 2012 7:33:33 GMT -8
Ok.. that might work. You will need to reduce back to system size for the bench.
If you keep the pipe larger than system size you may have other troubles.
Also, don't see room for an insulated heat riser in your drawing.
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