Rocket BBQ

[hpmer]

I built a Walker style bbq and have a few questions. I built a 6x6 core and thoroughly dried it out with a few burns before putting the barrel on and cobbing it up. The core draws pretty well and I get a decent, though not spectacular rocket sound.

After adding the barrel, cobbing it in and adding a chimney, the sound is gone, the draw is lazy and fire creep and smoke back are the norm. Not good. Also, the temp in the grill only got to 350*, well below what I had hoped, and I can hold my hand comfortably above the 5' chimney indefinitely.

I think I have several issues to address.

1. Wet cob. Even though the core is dried and fired, is it really possible that wet cob downstream can slow the stream enough to kill the draft, and the thing will improve as it dries?

2. My top gap is 4" due to a miscalculation. I could add an extension to the riser, but will try other things first.

3. Chimney too small. I think this is most of my problem. I'm using a 4" chimney for a 6" system. I assumed that with the hole in the barrel top to allow gases into the grill, and the heat shedding off the barrel sides that I would need a smaller chimney to compensate and keep the velocity up. I should have done the math first.

A 6" system with a 3" bbq hole leaves 27 sq" to exhaust. The square root of 27 / 3.14 is just over 5" (not 4" - doh!).

Since I'm using Matt's adjustable top damper idea to control grill temps, my question is should I go to a 5" or 6" chimney. Or specifically, is there any downside to using a slightly larger chimney, say 6" when a 5" will do the job in most cases? I suspect not and presume larger is the better of the two options since that will allow proper venting when the damper is mostly closed, but thought I'd ask to see if anyone has thoughts.

[mannytheseacow]

I would look straight to the exhaust size first. Try opening up your exhaust path all the way through and that should help. Unfortunately, flow rates don't respond to cutting sizes and redirecting them into two equally-sized CSA's.

I built a mock-up of my cook stove last year and made a dual riser to point the heat at the two hot plates. The core was 4" by 4" square and then I calculated that if I had two 3" round risers on it the CSA would be approximately the same. It ran really choked down though. After discussing it with some engineers at work they explained that gas flow is pretty much the same as water flow, and you can't just flow the same amount of water through two smaller pipes that add up to the same size.

So that should help the flow and get your rocket sound back. Disregard the opening for the BBQ and just size the main exhaust for your core.

I think you are on the right track with it being wet, though. It will improve as it dries. I also have to mention that fire creep and smoke back have been an issue on every stove I have built. From the limited work I've done, this seems to be more related to fuel. If I burn light wood like pine or basswood there isn't an issue, but very dense hardwoods like oak, hickory, and walnut always tend to creep and smoke back a bit. I thought this was bad stove design at first but the more I observe the phenomenon it appears that the wood is just so dense and packed with potential energy that it creates heat that burns up the piece before the center of the lowest part has fully combusted. I just cover the feed with a pot, though that pot will get damn hot!

Good luck. Throw a photo up of what you're building if you have time. Would love to see it.

[hpmer]

Thanks, Manny. Your experience confirms my thoughts. I'll post a photo once I figure out how to do it. I thought I could insert it with the "Img" button in the 'edit the post' screen, but I guess not.

[hpmer]

Ok, I upgraded to a 7" exhaust and that seemed to help the draft and stop the fire creep. The rocket sound is coming back if only faintly. Still can't get more than 350* though.

Lots of steam coming out from the cob. Can't tell if it's the cob itself drying out, or if there are holes in it letting out the exhaust gases. More of the latter I suspect. I'll put another coat on it next weekend and expect that should improve things even more.

Did cook dinner on it though. Fish steamed with vegetables in foil packets. I really think I'm going to like cooking on it once I get it up to speed.

[matt walker]

Hpmer, can we get some photos? How big is the hole in the weber? Go for 4" in both the barrel top and weber bottom. I run a big gap above the riser as well, 6" or more. Also, make sure you have enough space for the transition from barrel to exhaust. I will be home next weekend and will be able to offer more help then.

[hpmer]

Matt, let me know how to do photos and I will.

The holes in both the barrel and grill are 3", half the system size. I can increase the size of the holes, but in one of your threads you mentioned that you run yours with the baffle almost fully closed so I thought this would be more than sufficient. I'm hoping that as the thing dries out it will improve. My baffle is an electrical plate screwed on to a piece of flat stock and is only 3" wide, so I'd have to come up with some other idea for it.

I've thought about the transition to the exhaust as an area that needs work. I think there is sufficient volume there but I'm not happy with it for other reasons so I'll work on that part as well. The barrel mostly exhausts to the back and one side so the gases should be streaming fairly well already, but I suppose it wouldn't hurt to make a small bell just to be sure. The exhaust is only a five foot pipe so getting it up higher would be beneficial as well.

I think it's pretty close, but I would like to be able to get temperatures north of 500* (and ideally north of 700*) and also have a good draw and sound.

[mannytheseacow]

You have to host your photos at another site, like photobucket. Photobucket is free, and I'm sure there are others too.

Once you upload your photo to photobucket, when your looking at a picture there are a bunch of scripts on the right side. I click on "direct link" and copy that link. Then here in your post click on "Img" and just paste the link between the two "[img]" boxes so it looks like [img]your-link[img].

[matt walker]

How's it coming Hpmer? 3" is half system size, but the physics of flow mean that that leaves only a very small stream of gas, and most of it will be pulled out away from the smoker/grill. I'd upsize them both to 4" for sure.

[hpmer]

Made some changes to the stove this weekend. There are several potential issues to be addressed so I'm going to do them one at a time so I can learn something in the process.

First thing I did was to rip out the exhaust path and build in a small chamber. That seemed to help with smoke back and fire creep for a while, although after it had burned a while the issues resurfaced. There are several possible culprits, all of which are likely contributors to some extent or another.

Here they are in no particular order:

- The support barrel has metal handles, one of which points toward the feed tube. I suspect that as the handle heats up it raises the temperature of wood in its proximity enabling fire creep. I tried cobbing over the handle and that helped until the bottom of my patch fell off and, I think, confirmed this as a part of the problem. I plan to turn the barrel a quarter turn and get the handle out of the picture.

- I had a number of measurement failures with this stove and one was making the burn tunnel too long for the height of the riser. As the core was drying I "fixed" the problem by cutting back the burn tunnel by several inches, a few inches too many as it turns out as now there is no space between the front edge of the feed tube and the barrel leaving a hot metal surface right up against the wood in the feed tube. I'll likely cob both the inside and outside of the barrel at this point to help improve things.

- When I cut down the burn tunnel to shorten it, it had the effect of lengthening the feed tube making it much too long. To address this I covered part of it to choke down the feed and used a fire brick to do so. That thing got pretty hot and in the process raised the height of the heat in that general area. I tried putting a pot of water on the brick when the stove was running and that seemed to be helpful, so I'm going to cast a replacement out of an insulative mix.

- I incorporated Matt's oven idea opposite the burn tunnel and noticed flame, and therefore heat, swirling back around and into the oven. Since I don't think I'll end up using it enough, I'm going to close it up, probably with a removable plug.

- Matt's suggesting increasing the opening to 4" in both the barrel and the grill so I'll try that as well, but for the moment I'm more concerned about the quality of the burn than I am about the temperature. Interestingly I was only able to get to 300* (down from 350*) this weekend which I attribute to the fact that as the area surrounding the core dried out there was less cold wet cob to help cool the top of the feed tube and the true characteristics of the stove are shining through. Plus I recobbed the exhaust path so that needs to dry out as well.

In the end, the burn seems better at least initially, but with all of the above going on I suppose it's amazing the thing runs at all.

Still, it's usable enough to cook on and we enjoyed burgers and mashed potatoes with roasted garlic butter this weekend. I think promoting these things as a bbq alternative might find a larger target audience than a heating device. I know I, for one, am unlikely to ever have one in my living room, but this is a perfect use for me.

Thanks for all the suggestions. I'll post updates as I go and try to get some photo's up.

[mannytheseacow]