Castable core

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Castable core

Postby Carter » Sat Oct 12, 2013 6:42 pm

Hey guys my name is carter. New to your forum.
Matt I really like your new stoves, I saw your video the other day. Good work.
I was reading everything you guys talked about with core ingredients and would like to know what is working best sofar. I noticed some of you are now using vermiculite instead of perlite and it should make it stronger? I already have the fire clay, firestop and perlite but if I should try a different mix I will grab other stuff.

One more thing, I had the thought of building the core with split firebricks and casting around that. Is that a good or bad idea, I just want this thing to be as durable as possible.
Thanks guys.
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Re: Castable core

Postby matt walker » Sat Oct 12, 2013 7:22 pm

Hey Carter, welcome! Is this the Carter who PM'd me on another board? Did you get my reply? Just wondering....

Anyway, I still believe in that mix I share on the cast core video, but certainly there are quite a few approaches that will probably work just as well if not better. For sure a true castable refractory is the superior choice, but for lots of us it is unavailable locally, and can be expensive. Prescott did make a comment about vermiculite being a superior choice, and perhaps it is, but I've had no problems with the Perlite.

As for the bricks in the casting, if you look at Manny's build you'll find he cast regular bricks into his casting and we all learned a lesson there. The casting mix goes through quite a bit of shrinkage in the drying process and since the bricks don't they create cracks in the casting.
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Re: Castable core

Postby mannytheseacow » Sun Oct 13, 2013 1:49 am

Hey Carter! Glad to hear you're going to make a rocket heater. I'm stoked at how many people have been on this site lately planning to build. I can't wait to see some pics of what you come up with.

Matt's right, I would avoid bricks at all cost. His mix is about perfect if you can find the materials. You don't really need fire cement since your core will be buried in the bench. The basic mix works just fine. I recently added some sand and Portland cement to the mix to strengthen around the feed tube. I don't know how it would work for the whole core and the higher temps deep in the burn chamber but I really happy with the strength it has around the feed. Portland cement is really cheap in my area, not sure about yours. A lot cheaper than fire cement.
"Knowledge is power. Arm yourself."
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Re: Castable core

Postby Carter » Sun Oct 13, 2013 3:47 am

Yes matt this is the same carter from permies. I am sorry I thought I replied back to your message. I have been so busy with work and doing research on the RMH. Thanks alot for the info I was wondering about the shrinkage but still thought it would make the core stronger. Ok I will avoid using them.

One more for you guys. Both of you like having a round riser vs square? The round riser makes sense to me but I remember in one of Ernie and Erica's videos they commented on the square riser creating more turbulence and a cleaner burn, what's your take on that.

It will still be a min for me to gather the rest of my materials, a 30 gallon drum isn't that easy to find, 55 gallons are everywhere, just a little large for my spot. I am really excited about this project. It's going to be an L shaped bench in my dining room and I plan on building a dome on top of the barrel for baking and cooking.

I will keep everyone updated.
Thank you Matt and Manny.
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Re: Castable core

Postby matt walker » Sun Oct 13, 2013 5:54 pm

Carter, no worries on the message, I wasn't bothered. Just wasn't sure it went through. Glad to hear you got it, and glad to have you here.

As for the riser, I would take a lot of that stuff with a grain of salt, personally. Even folks with tons of experience would be hard pressed to be able to discern the small differences in performance gains once we are in the ball park of a well running stove. That includes me and everyone else. Without very expensive test equipment, it's all just guessing.

So, with that said, let the guessing commence! I have read, from a source with said test equipment, PvdB, that what we actually want in that part of the stove is a smooth flow. General consensus is that round is slightly better in this regard, but keep in mind that with a square section core, the "vortex" is actually a horizontal rotor spinning in the low pressure area made at the corner where the top of the burn tunnel transitions sharply into the riser. So, there's a rotor there, rolling along inside the base of the riser. In that way a square riser can hold that turbulence format just fine. I guess I'm saying I doubt it matters a whole heck of a lot if you've got proper sizing and insulating materials everywhere. At that point the gains are a matter of small percentages and, like I said, we'd be hard pressed to sense any significant difference.

I like round because it's easy to cast that shape using highly insulated materials. I would guess that they prefer square for the same reason, it's easy to build with their chosen material which is brick. I will say that I think heavy firebrick is exactly wrong for everything in the burn zone, core and riser included. That's my opinion, but I've verified enough times to be confident that I prefer an insulated build at the expense of a little less durability.
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Re: Castable core

Postby Carter » Sun Oct 13, 2013 8:16 pm

I do like the idea of a round riser, I was just about to make another mock version in the yard with rough dimensions of what my final core will be.
This will be a 6" system so I was thinking I would make the feed and tunnel 6" wide x 5" high. So my thought was the transition from the tunnel to the riser.
Wouldn't it be best to cast the pipe all the way to the bottom of the burn tunnel so the fire and gasses have the perfect flow from a rectangle to round. All I would have to do is cut the 6"pipe in half 5" high then support it so I can cast around it.
I hope you follow that.
I don't know if you have tried that. Plus that 1/4 cement board you use it does burn out correct?

I guess I do have one more question for anyone out there. How good does triple wall stainless pipe work for the heat riser, I found one the other day at the scrap yard for 2 bucks. I didn't know if I would use it but I know how expensive they are new so I grabbed it.

Thanks alot for your advice Matt. I plan to build my cast in a couple days.
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Re: Castable core

Postby matt walker » Sun Oct 13, 2013 8:35 pm

No problem I Carter, I absolutely love to help out with these things and am constantly learning right along with every conversation like this. I admit I get frustrated trying to help elsewhere as I don't feel like there is truly free exchange of ideas on heavily censored boards, so I don't participate as much as I would truly like to. Anyway...

Your riser casting plan sounds great, I do exactly that on some of my batch burning core castings. And yep, the backer board cracks out of there pretty quickly.

As for the triple wall, save it for your vertical exhaust. I put one on the core I made in the cast core video and completely destroyed it within an hour. The inner part warped and closed off about half the system CSA. They are really valuable though and fantastic for the vertical chimney at the end. That's a great score, I wish I could find some more for 10 times that price. That'd still be a deal.
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Re: Castable core

Postby matt walker » Sun Oct 13, 2013 8:38 pm

Oh, when you use flue pipe for the casting of the lower riser, make sure you stick some scrap wood in there across the diameter in a couple directions to brace the flue or it will be squished out of round and it's possible to get a weird shape there.
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Re: Castable core

Postby hpmer » Sun Oct 13, 2013 10:43 pm

Personally, I prefer the square riser. I cast them and build them around thin floor underlayment, reinforced inside with wood braces which double as attachment points for the form.

I find the square so much easier to repair as well, since it's easy to slather on a patch with a trowel, a putty knife or even a flat piece of wood. Much easier than try to match a curve. Also, easier to make csa even all the way thru.

I agree with Matt though. Probably not much real difference in performance so more personal preference than anything.
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Re: Castable core

Postby Carter » Tue Oct 15, 2013 1:00 pm

Thanks Matt and hpmer. I think it would be nice to go square for the riser and would be easier to make the transition but it does seem like the round would have much better flow. I know casting the pipe to the bottom of the core is a little bit more of a pain in the butt, but I would think the flame traveling through the tunnel and hitting the curved wall at the end would be alot smoother for the flow up the riser.
I could be wrong with that thought.
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