Sacrificial Metal tube for Batch Box heat riser

I'm finalising the design for my 5" sidewinder batch box rocket stove.

I've a length of 5" diameter stove pipe in the workshop and was going to use it as a sacrificial form for the heat riser  My plan is to surround the pipe with a vermiculite/high alumina cement mix. I'm sure the pipe will badly corrode in the riser (I know, metal is doomed... ) but I'm hoping the cast vermiculite/alumina cement will hold its shape and be able to cope with the riser temperatures once the enamelled pipe has failed. What do you think? One of my concerns is that sections of the corroded pipe may fall to the base of the riser and block the port entry from the firebox.

I like the idea of casting the insulated riser around a form. I've had great success using a 6" stove pipe surrounded by a vermiculite/alumina cement mix in my workshop rocket stove. I investigated the condition of the steel pipe in that heater today and noticed only minor corrosion near the base of the riser - mind you, this is in a 'J' tube set-up and it appears that higher temps can be achieved using a batch box.

Another alternative perhaps would be to make a 5" cardboard laminated 'tube' from several layers of corrugated packing card, use this as the form and then burn it out on first fire-up.

Thanks as always for any help or advice.

Hi John,
The thing with batch box rockets as compared to J-tubes is that the base of the riser is the combustion chamber. A J-tube sports a burn tunnel where most of the combustion takes place. A metal riser mold inside an insulating shell will corrode like mad in a batch box but it will also deform badly. Gas streams in the riser will become erratic and the clean combustion will be hampered badly, stretched over a whole season or more.

Better use a mold which can be burned out, this won't be complete combustion but it is controlled and temporarily.

Thanks very much Peter for your quick and informative reply. I'll use the method you recommend.

Thanks again.

Hi John; I have cast several risers using a round cardboard concrete form (sonitube). They are readily available at building supplies by several different names. They burn out with the first fire, works very well. The only time it was an issue, was when my cast mix was to wet and my inner form threatened to buckle after sitting all night . I lit it off early and was able to save the riser.

Thanks very much Thomas.

Over here in the UK these tubes seem to be marketed as 'sonotube' - I'll try and locate a dealer and get a price.

By the way, what mix did you use for casting your risers?

John, don't bother buying a sonotube. One layer of single skin corrugated card taped around your metal tube. Some cellophane wraped around. You cast your heat riser, even in place, then pull the metal tube. And either burn or remove the card. That's prety much how i would do it if i was casting.

Thanks Satamax for the tip. Just to confirm the layers :-

1. Steel flue pipe
2. Corrugated card
3. Cellophane (cling film?)
4. Insulating Mix.

Have I got it right?

Also, as Peter mentioned previously that the lower section of the heat riser in the batch box set-up is also the combustion chamber, do you need a much harder wearing insulation mix than usual to cope with the extreme temperatures in this area?

I use a fireclay and perlite mix , heavy on the perlite till the very top where i use more firelay to shape the top edge .  One 50# sack fireclay mixed with one 4 cubic foot bag of perlite should do it, although I always have an extra bag of fireclay on hand . I use a 16 gal barrel as an outer form on my risers ,makes them very durable as well as movable if you need to work on your core.

Thanks for the reply and for posting the photos Thomas. The barrel as the outer form is another great idea.

Hi John;  All my risers have been for J tubes.  As you mentioned, in a batch the riser is part of the combustion chamber. If it's available, insulating castable refractory cement might be a better choice than fireclay/perlite.  I haven't built a batch yet, I will one of these days. I only see a benefit in that you do not split down your wood as much.  Burn time is reported to be apx 1 hr... my J tubes consistently burn 40 + minutes easy between tending.  Heat output on a batch is reported to be very high especially in an 8" and I have watched matt walkers video of changing his 8" J tube over to a 6" batch box. I will build one, it is a great innovation.  Most likely an 8 " out in my auto shop.

As far as sonotube goes, I haven't seen it smaller than 8" nominal diameter (found a 7 5/8" o.d.) anyway, so getting it in 5" diameter might be a moot point.

Max's plan sounds like a very good one.

For the outer shell, I have found that multiple sections of black snap-together stovepipe work well. If you want 12" o.d., put two 6" stovepipe sections together. The combined diameter is equal to the sum of the component diameters, so you could get any desired diameter by using the right combination of standard size pipes. You do need to flatten out the curve somewhat before assembly to get the result to be smooth. Stovepipe typically comes in 2' sections, but staggering the joints can give you a longer reasonably solid length.