Pre-Heating the air?

[thewongs]

I'm building a very small rocket for my sister's garden. The idea is that it will be a barbeque with the lid off and a patio heater with the lid on (I'm sure you all know how that works, or will work so won't bore you with the details).

One of the other things I wanted was for it to look cool, so came up with the idea of adding bent exhaust sections for legs and arms to make it like an abstract arty type of person.

THEN this idea hit me (I'm full of those!!!) and poses the folowing question....

If I use exhaust tubing for the arms, why not make them look like they are holding the feed tube and why not drill holes so the heat from the barrel (compressor tank in my case) can get to the feed tube or burn tunnel? I've done a silly drawing using MS paint to give a rough idea of what I mean. Actually, I drew it without the barrel, so you need to imagine it's there and the "arms" are connected to it.

I hope this all makes sense. I know what I mean though! ;D Anyone have any thoughts on this?

[Donkey]

There have been several attempts to do something like you suggest..
The results have been (so far) lukewarm at best, designs usually cause more headaches than they solve.
Nothing wrong with a new set of hands on the idea...

The air input to the stove should be as high in the feed as possible or it will cause smoke-back, air needs to flow over the sticks and into the fire from above. If you want to preheat ALL of the air, the tubing should have a cross section area of between 1/4 and 1/3 of system size. 1/4 area is easy, it's half of system pipe diameter, an 8 inch system needs a 4 inch air intake. My intuition says that if incoming air is hot, you may want to go with the larger size (1/3) as the air density will be low. 'Course, you don't want to pipe barrel contents back into the stove, there's nothing to burn left in it, so you'll need to bring air in from a different location and as low as possible or it will effectively reduce the height of the heat riser.

[thewongs]

Thank you Donkey. After I posted, I was thinking about it and thought it was silly of me not to mention that the air would be "fresh".

The system I'm doing will only be 3", due to the small compressor tank. I've just got my hands on some very nice (and thick) steel pipe. One of my friends repairs petrol station pumps and does the piping for new petrol stations. About 30 minures ago, he handed me a few lengths of it.

My initial thoughts were to have 1" - 1.5" car exhaust piping to supply the warm air, so I suppose I was not too far off the mark (remembering there will be two of them). After reading your reply, I may give it a try, but only after I've tested it without first. That way I can compare and of course let you know if there was any difference.

So... The second draught of the idea will have the warm air piped into the top of the feed box from a pipe as low as the fire tunnel with a lid on the feed so it will force it to take the warm air and not the masses of cold stuff trying to climb in.

[thewongs]

My second MS painting... If it makes sense? Again, you need to imagine the barrel / tank.

[Donkey]

Well, It's certainly worth a try. I don't see any problems with it at this point. Except maybe, you need to remember to NOT take heat from the riser, insulate your pre-heater from the riser and get yer heat from the barrel side only.
It might be good, for experimental purposes to have a shutoff or (better yet) bypass in it somewhere. To compare how it burns with or without preheated air. Nice to be able to select different air, but deliver it through the same port into the fire. I know it will add complexity to your comparatively simple design, but it will give you a better idea of what is working or not.

I'm looking forward to your results.

[thewongs]

DoH!

I'm glad I have you to keep me right! I was thinking of the manifold on the riser, then insulating around the whole thing and thinking it would work! What a **** I am! Of course the fresh air coming in is going to counteract the insulation.

Thank you! ;D