Bamboo Skewer NARB

[JHCC]

I've been very happy with my first NARB, but the building of my new gasser has provided the opportunity to build another with some variations. I plan to use the same T-burner, just with a different plenum and block.

JADF posted a couple of photos the other day of a button burner that looked very interesting, and it occurred to me that I could cast something similar using bamboo skewers for cores. So, time to do some calculations and tests.

JADF used 181 holes of 1/8" in diameter. By my calculations, 185 holes of a 3mm diameter (which seems to be the standard for skewers) would have the same total opening as the 19 crayon-sized holes of Frosty's original design, which gave me my starting point. Since I want a ribbon burner and not a button burner, I decided to try six rows of 12 holes each, spaced 3/8" on centers:

I welded up a new plenum from 2" square tube; 10-1/2" long, with a flap on each end folded and welded down.

As it turned out, the rows of holes were a bit too far apart, so they just barely fit on the plenum. Fortunately, I was still able to duct tape the block in place for testing, but I suspect that a couple of the holes on the edges were blocked.

There was a little trouble with some of the flames lifting off the block, but I suspect that this will be less of a problem if I put the rows a bit closer together and none of them are blocked by the edges of the plenum. I think I'm ready to move forward with this.

While I had the welder out, I added about 3/4" to the open edge of the plenum. In addition to making it look faintly evocative of a painting by Mondrian, this will help me better embed the edge in the block once cast.

The connection between the T-burner and the plenum is the threaded section off the neck of scuba bottle, by the way. Making good use of what's on hand.

[jlpservicesinc]

So far so good..  Grabbed the popcorn.. 

[JHCC]

Cut the base block for the mold and marked it for drilling. 

[JHCC]

And drilled the holes. Playing with ideas for the sides of the mold. 

[Latticino]

Very curious how this is going to work out for you John.  I saw the multi outlet "button burner" from the other post and was intrigued, as I still need to recast my ribbon burner block for my small forge.  I did some research on the Ceramic honeycomb and perforated tile style burner outlets and was disappointed regarding their typical output per square inch (the best I found was for a Majestic tile at around 26 MBH per 3.5 x 5.5 size block).  Ideally I'd prefer to see between 80 and 120 MBH.  I'm not sure a simple equivalent area calculation will work for this.  I think you are getting into boundary layer considerations with holes that small (the "dead zone" of high friction at the surface of the hole starting to be a more significant fraction of the open area).The flame may look good, but may not put out sufficient heat.

I'd be happy to be proved wrong though.  Please keep us posted on your results. 

Going to be fun packing the refractory down into those small gaps between the skewers.  My suggestion would be to pack the refractory in first, then cover the open side with a piece of pink high density foam insulation, flip the assembly, and push the skewers through from the other side of the block (drill all the way through).  If you spray the skewers with some release first you may even be able to pull them back from that same side once things set up a little.  Then grout the new block into the mixing box with Satanite.

[JHCC]

The first ribbon burner I made has the edges of the plenum embedded in the refractory block. That's worked well, and I plan to do the same here.

[Goods]

Would it be possible to reduce the friction by just reducing the thickness of the burners face? Obviously there would have to be some balance between the heat transfer in the face, thickness and cooling effect of the FAM moving through the block. With so many small holes cooling the face, I’ve been thinking through greatly reducing the thickness and coating the face with plistex(?) to reduce the heat absorption to make an interesting burner. I’ve even considered taking an old pizza baking “stone” to try it with, but I have no idea what temperature that type of ceramic can withstand. Honestly, I have so little time in the shop, I still have barely scraped the surface of what I can do with my solid fuel forge.

Just thinking out loud,

David 

[Frosty]

18 hours ago, Latticino said:

Going to be fun packing the refractory down into those small gaps between the skewers.  My suggestion would be to pack the refractory in first, then cover the open side with a piece of pink high density foam insulation, flip the assembly, and push the skewers through from the other side of the block (drill all the way through).  If you spray the skewers with some release first you may even be able to pull them back from that same side once things set up a little.  Then grout the new block into the mixing box with Satanite.

Ooh, good idea, thanks! 

Frosty The Lucky.

[JHCC]

I’m planning on using petroleum jelly as a release agent. 

[Latticino]

9 hours ago, Goods said:

Would it be possible to reduce the friction by just reducing the thickness of the burners face

Yes, and that is exactly why I plan on trying to keep my face down to around 1/2" thickness.  I'll use Mizzou, because that is what I have, but If I had 5# of Greencast 97, or similar high alumina castable I'd use that.  I've found Kasolite to be a lot more durable than I expected for a castable insulation, but balk at using it in such thin sections.

Hopefully thinner walls will also be less prone to thermal shock.

I like the idea of a Plistex or Metracoat wash as well.

[Buzzkill]

Hmmm.  My concern in this area has been the possibility of exposing the plenum to high heat.  If the burner block is only 1/2" thick you'd have to recess the burner a bit to alleviate the problem.  The difference in coefficient of expansion between steel and the casting material could be an issue as well.  I'm interested in the results.  Please let us know.

[ThomasPowers]

John; not going for Paul Klee? (Senecio, Castle and Sun, Sinbad,...?)

[JHCC]

Where I live in northern Ohio, the soil has all the Klee I can stand. (And yes, that's an a-Paul-ing pun.)

The current plan for getting the kastolite into all the nooks and crannies is to pack in what I can by hand and then mechanically vibrate the mold, repeating as necessary. I have an old orbital sander that belonged to my late father-in-law that is much too rattle-y for actual sanding use, and I think that should work well.

[Goods]

Buzzkill, you could cast it full thickness on the outside edges, leaving the face recessed on the backside. This could cause thermal stress where the thickness increases though.

[Latticino]

5 hours ago, Buzzkill said:

Hmmm.  My concern in this area has been the possibility of exposing the plenum to high heat.  If the burner block is only 1/2" thick you'd have to recess the burner a bit to alleviate the problem.  The difference in coefficient of expansion between steel and the casting material could be an issue as well.  I'm interested in the results.  Please let us know.

Actually my plan allows for this as well. The burner block is cast as an open box with the foam as the core, approximately 1/2" thick all around. It is tall enough to keep it at least 2" from the mixing block steel plenum section.  This latter has metal mesh welded on to allow something for the grout to hold onto.  The metal should keep cool enough not to expand.

Just waiting for John or the button burner guy to finish theirs so I can see whether the tons of 1/8" holes will heat a forge well or if I have to go more conventional size.

[Buzzkill]

Now I'm even more interested in the results.

[JHCC]

Long overdue update: I ultimately decided to go with beeswax as a release agent, dipping the mold bottom and skewers in melted wax before making up the rest of the mold. Mixed up the Kastolite as usual (1 lb dry mix to 3 oz of water) and rammed everything in place. Cured in a sealed high-humidity environment (trash bag with wet rag) for 48 hours, and unmolded.

Removed the rest of the mold from the ribbon block and fired things up. 

The wooden base didn't want to come off the ribbon block, so I split it off with the treadle hammer and a hot cut.

The skewers were stuck in tight,

so I trimmed everything down to the block with a hammer and chisel.

Brought out the blowtorch.

Which didn't seem to do much,

But actually melted the beeswax enough that I could push the skewers down into the plenum

And fish them out with needle-nose pliers.

On the first burn, I wasn't getting ignition in all the holes, so I reamed them out with a twist bit in a hand drill.

Which improved things considerably, but I was still getting some flames lifting off the block. I may ream out the holes a bit more later.

This is at about 15 psi. I think a lot of the yellow is from calcite in the ribbon block and some minor remnants of skewer and beeswax.

And this is throttled back to about 5 psi.

I put the burner into the new gasser:

And closed up the doors.

Which worked pretty well.

Although I think the doors give too much back pressure when they're completely shut. I could see that there were some flames forming outside the forge, as if the gas wasn't getting enough oxygen inside. I never got the burner and the forge up to full temperature, though, so that might change things.

[ThomasPowers]

I would think about putting the setup in a warm oven overnight and try to heat the entire block up to get the skewers wax to melt and release.  Below kindling temp of course.

Did you cover the face of the wooden block with a release?

[JHCC]

Other than the wax, no. That gave me the same glue/release issue as with the skewers.

As I noted over in Frosty's original NARB thread, I just realized that I made a HORRIBLE mistake setting this up. Somehow, although I calculated that I'd need 185 skewers to match Frosty's original 19-crayon layout, I somehow laid out the block for only 75. No wonder I'm getting liftoff. Now I'm working on figuring out if/how I can fit that many skewers into this size block. 

[WhiskeyTangoFoxtrot]

JHCC did you get your skewer burner dialed in? What size forge is that?

[JHCC]

Not yet; I've been using my original (crayon-sized hole) NARB. I should probably get back to this project, as I still have lots of Kastolite. The forge has an floor of about 9" square and an arch that's about 5" tall.

[WhiskeyTangoFoxtrot]

Anyone tried using kiln shelf as the burner face? Just thinking about how to get away from all the skewers when I do mine.

[Buzzkill]

I considered it, but there were 2 main drawbacks in my opinion.  1) It's difficult to drill a lot of holes in kiln shelf without breaking it, and 2) Most kiln shelf isn't very thick and a portion of that has to go down inside the plenum to get a good seal (unless you tried to do a surface seal).   So, you potentially expose the plenum to really high heat, which potentially causes premature ignition.

I'm on my second insulating firebrick burner head for my NARB now.  It's easy to drill.  I just used a piece of 1/8" welding rod that I ground to a chisel type point.  However, it is easy to crack/break these, especially when you get lots of holes in them.  The first one overheated and kind of self-destructed as it started to melt.  I'm still using the 2nd one, but I think I'm starting to see signs that I'll have to replace it in the near future.  On the upside it only takes me a couple hours to drill all the holes, remove the NARB from the forge, replace the burner head, and be back up and running.

[WhiskeyTangoFoxtrot]

Buzzkill have you been running your drilled IFB in a forge? I know I read another member was working with the same idea, but sounded like he hadn't been running it in a forge yet. You say you're on #2? How long do you think you're getting out of a brick before it deteriorates?