Use of ceramic glass in a batch box roof to preheat secondary air in a batch box.

Pyro's.

I'm contemplating using ceramic glass to form the roof of a batch box as a means of porting heated secondary air to the riser throat.

The plan is to have one sheet of ceramic glass as the ceiling of the batch box and use say, 1 inch of ceramic fiber board around the perimeter of the glass panel and then top it off with a sheet of ceramic fiber (or other suitable material) in order to create an air gap with a path through the system from the door to the riser throat.
If used in a Double Shoebox type setup, two panes of ceramic glass could be used so as to capture heat from the secondary as well as the primary firebox.

The working idea here is to capture as much of the early startup heat as possible and get the air in the airgap warming up as soon as possible to help fuel the secondary burn.  You might wonder why I would want to have the secondary air entering the system so much farther up from the port, (I can see Peter scowling at me  now) but the system I have in mind will be using a space resembling that of a cyclone separator in order to capture the hottest gases in a circular path that encourages turbulence through the use of (trip wires) on the ceiling of the chamber ring.  As the gasses expand and cool, they will spiral downward to the bottom of the cone, dropping their load of ash and particulates in the collection chamber at the bottom.  Once gases have reached the bottom of this cyclonic secondary chamber, they will spiral back up into the inside tube and make their way to the back end of the system, oven, bench, cooktop, radiant bell/drum, or with the use of dampers, maybe more than one.

I propose building the firebox and what I will call the Combustion Cyclone out of a aerated (foamed) mix of castable refractory cement, bulk ceramic fiber, and pearlite.  I expect to use some ceramic paper product with which to form the inside tube for the Combustion Cyclone.  Sodium Silicate will be used on the ceramic paper as it is rolled into form on a 8 inch diameter tube.  (8 inches as this is the size of the system I am targeting.)



I have followed the adventures of a certain Honey Do Carpenter on Youtube who has made a name for himself with his experiments in foamed Portland cement.  Seeing his success in that endeavor gives me confidence that same can be achieved to some degree with the 60% alumina castable refractory I have available.  This video shows how his aerated cement can be poured into panels. https://www.youtube.com/watch?v=MXs-tqfCSX0  
Such an aerated mixture could also be easily poured into molds to create more complex forms.

This is where you guys get to pick this apart and ask me questions about things I have overlooked.

It's high time I started this build, so I think I will start with the bench, that will give me time to get these other details sorted out.  I'll probably keep documenting this build here so if any of this intrigues you, then please follow along.

A diagram of how a cyclonic separator works.  Notice how the entrance leads into a circular path that crashes back into itself.  Also, since there are no moving parts, the entire form, with the exception of the exhaust port tube, can consist of negative space.

This is an interesting concept. Lots of people have designed explicitly circular vortex-generating combustion cores, but I don't recall one intended for ash collection as a major function. Have you built or used a standard rocket mass heater, either J-tube or batch box? Are you aware of the fly ash generation rate? RMHs typically need cleaning of fly ash downstream of the firebox/burn tunnel maybe once a year, so I wonder how important the cyclone is for that.

A major function of the heat riser is to keep the gases super hot until they finish combusting, and to generate draft by the principle of hot gases rising; I wonder how much draft this type of cyclone would generate. If you have a strongly drafting chimney, the combustion core does not need to provide the draft, but if not, you would want as much draft from the riser as you could get.

A cyclone to separate ash would depend on smooth flow so that heavier particles could settle out, which is at odds with the principle of turbulent flow for gas mixing.

Hi Glenn,

Nice to hear from you.   First of all, thank you for your input.  

I have not as yet built a standard J-tube or Batch rocket, this would be my first.  Although I have studied these boards and the material at Donkey's board for several years now, so I have a pretty good understanding how things are supposed to work.

I was not clear on how often the fly ash would need to be collected, once a year doesn't sound bad, but once every 5 or ten years sounds even better if the cyclone proves efficient.

My situation affords me a 34 foot chimney, mostly sheltered from the outside.  It provides good draft so I anticipate that it, working in tandem with the push generated by the batch box, will provide sufficient throughput to get the hot gasses through to the bench where I will have a bypass installed for quickly heating up the flue.

If you look at the cyclone drawing, you'll see that the mixing will take place at the top, as the gasses spiral down, they eventually are pushed to the bottom where they then vortex their way back up the inside tube and exit.  I've pondered how hot gasses would work in such as system v.s. room temperature gas, but it shouldn't matter, since all of the gas should remain close to the same temperature in this highly insulated enclosure.  I imagine adding (tripwires) if I am remembering correctly the way a ripple, or bump, was provided in the ceiling brick of a J-tube burn tunnel to promote mixing.  As of yet I have not landed on a particular plan for that.
As for the particles settling out, that is what the lower 3/4 of the cyclone is for.  I expect to have to find a way to mold this so the cone is very smooth.  Casting a block with a traffic cone has come to mind.

I spent some time last night chasing down parts to build the foam generator.  Here is a video by Jim White which shows how such a foam generator is built.  Jim has provided further links about how it works.  https://www.youtube.com/watch?v=P5EMmo4XNQk

Of course I'll be doing some testing on this foamed refractory cement before I commit to casting large parts.

Any advice you may have regarding the pitfalls of such a foamed refractory or the use of the ceramic glass in the burn chamber would be much appreciated.  Other threads seem to indicate the ceramic glass can handle a great deal of heat.  Probably even better if it has a constant flow of air over it to help keep it cool.   Others have hinted at mixing ceramic or other fibers and or pearlite in with the castable refractory though I have not seen the results of those experiments.
To be sure, I am not aware of anyone attempting to use a foaming agent to expand the castable refractory so I am out on my own in that regard.  Though hopefully, not entirely out in the weeds.

It sounds like you won't have to worry about draft
An 8" batch box is a beast - what sort of space are you looking to heat? 6" is the most common size for a batch box that I have seen.

If you want to experiment to find improvements on the tried and true, I think it would be very beneficial to test a standard design first so you can tell whether the more complex experimental design is worthwhile. A J-tube core is easy to make up and test, and if you use clay slip mortar, all the parts can easily be reused in another setup. A batch box can be connected to any kind of riser, so you could make  the firebox and test it with different risers, a standard style and your cyclone. A 5-minute riser (1" ceramic fiber blanket inside a sheetmetal tube) could be reused in or on top of a cyclone if you want to. Are you thinking of the cyclone cylinder being 8" diameter? Given the desired flow path, I think it would need to be distinctly larger to not be a bottleneck, say about 12". 8" would be appropriate if you only wanted the gases to spiral up.

As far as smoothness in the cyclone, I have seen others here who have investigated and found that no matter what, ash will accumulate on surfaces and cause some roughness, so beyond using generally smooth forms I don't think you would benefit from special measures in that regard.

I have a 2000 sq ft basement to heat.  But I am open to downsizing things if practicality points in that direction.  While you are probably being a good friend and doing me a kindness to try to talk me out of it, there are three innovations I wish to explore with this build.  Yes, I could go with a tried and true method, but being a tinkerer at heart, there wouldn't be nearly as much fun in that.  I stuck my head in the old fireplace today and removed what was left of the broken flue damper, so I guess you could say its officially started. Got a nice shower of soot and cinders all over my head to show for it too! I didn't measure it yet but the flue opening appears to be about 4 x 16 inches.  Just about perfect csa for an 8" core.  I've decided to build my bench and core foundation utilizing a bed of insulating perlite and whatever binder makes the most sense, and an assortment of solid concrete blocks.  Cost is about 4 cents per pound/ 72 dollars per ton.  Like you suggest, I will use a clay slip to bond them together.  My backyard consists almost entirely of dark gray clay that should work well for that.  Right now I can get my hands on everything I need except for that ceramic glass.  Put a shout out to my local FB group for unwanted electric glass topped stoves.  I'm sure one will make itself available to me in due time.  For now I'll be working on assembling that foam generator to make the expanded castable and making some observations about how well that chimney draws.   Wish I had a sketch or two to help illustrate my plans.  I've tried using sketchup but found it all rather frustrating.  I'm fantastic with conceptual visualization....putting visual ideas on paper?  Not so much.

Oversizing a mass heater is not the same problem as oversizing a wood stove - if it is more powerful than you need you can just burn less often, as long as the core and mass are properly sized to each other and to the chimney. An oversized wood stove is dangerous as it invites frequent throttling and heavy creosote generation with risk of chimney fires.

I think some clarity about the elements you will use and their relationship would be helpful. You plan a batch box; that is straightforward with the port location to be decided. Batch box dimensions have been thoroughly tested and the best proportions are given at batchrocket.eu. The question is how you will go from the firebox to your cyclone. The cyclone wants its supply horizontal at the top of the cylinder. This works with the standard port and with the sidewinder port, but the double shoebox does not seem to have a neat direct connection to the cyclone.

"but the double shoebox does not seem to have a neat direct connection to the cyclone."

But what if we turned the upper shoebox 90 degrees so it exhausts perpendicular to the core?

A word of warning here regarding the chimney size. It might be that the csa of the chimney looks like it's perfect for an 8" system. In my opinion it isn't, due to aerodynamic behaviour in that same channel. The hot and fast streaming core will be quite narrow and the corners aren't helping a strong draft, rather the opposite.

In this part of Western Europe the chimney sweeps are using a century-old formula to determine what diameter a comparable round channel would be.
It goes as follows: take twice the width, multiplied by the length of the chimney's csa and divide that by the sum of the width and length. The result is the diameter of the equivalent round duct.

2xWxL / (W+L) = D

Conclusion would be: your chimney is comparable with a round duct of 6.4", aerodynamically-wise.
An 8" system-by-the-book won't run well on such a chimney size. Let alone coupled to a cyclone which gobbles up a substantial part of the draft energy anyway.

I had assumed that the 4" X 16" was merely the throat opening, leading to a squarish chimney. If the whole chimney is 4x16, that is very different as Peter says. If this is an urban party wall brick building, a wide thin chimney is likely; if freestanding, I would think a square chimney would be more likely. You will need to determine the actual situation before proceeding.

Let me elaborate a bit further.  the chimney itself is at least 8 inch CSA, but the flue opening in the old fireplace, which is a 1950's style manufactured insert known as a Heatilator, is approx 4 x 16 inches.  I have wondered about how restrictive this might be and it would be easy enough, though with some discomfort, to take a cutoff wheel to it and make it larger by 3 or 3 times what it is now.  That would open up access to the full throat of the chimney opening above the smoke shelf.  That would minimize any restriction at that point.  I was curious about sealing off the hearth and mounting a small anemometer in an opening to measure draft.  Do you think that would be worthwhile.  

Glenn also made a point about the diameter of the cyclone.  Just in keeping with the 8 inch box plan,  then the inside, vertical exhaust tube and the exhaust opening of the cyclone would have a CSA of 8 inches.  The diameter of the intake and main cyclone cylinder would be proportionately larger, though I feel there is room to play with here as the gasses can descend to a lower point in the cyclone as long as the output side provides room for equilibrium. The cyclone combustion chambers I have seen others attempt always permit the hot gasses to flow freely upwards.  This one restricts their upward movement for a given time until they reach the inside exhaust tube where they then are free to ascent and exit.  I like Peter's idea of going smaller on the core and leaving room for back end restrictions, however they might be useful.

Now that you've got me thinking about it, I am uncertain of exactly what the dimensions of my flue tiles are.  They are terra cotta flue tiles with a square profile.  To be honest, I have always assumed they are 8x8x 24, which is common around here, but I could be wrong, they could be bigger, though I doubt they are smaller.  I guess that wins me a trip up to the roof to get the numbers.  There are 3 flues in my chimney.  One of which is used for the main fireplace upstairs which we enjoy often.  The second is the one to the old heatilator, which I will use for this build, and there is a third, which I believe was built into the system as a means of providing make-up air.  A system which as popular at the time but proved to not be very effective or efficient.

I'll be back with my findings.  It will be much warmer here in NW Ohio next week so I will wait until then as to avoid any icy roof accidents.

I haven't found flue tile smaller than 8" x 8" nominal, so I expect yours are at least that size. The ones I have are around 7" x 7" inside. (Tiles from two sources have slightly different dimensions.) 10" x 10" is the next larger square size.

Weather here is optimal so I jumped on a ladder and got up on the roof.  Unfortunately, I am unable to get as close to the chimney top as I'd hoped.  My chimney has a hardware cloth rodent screen with 1/2 inch square mesh so I was able to count the squares and come to a pretty close estimate.  I was wrong about the form though, it is 12" x 8" rectangular.  So I would suppose that still leaves me in a good position.  I also lit a small fire in the old fireplace today, seeing as how the broken flue is no longer in the way, the draw was instant and impressive.  No worries there. I'll have to find something to block it off with in the meantime while I'm building or I'll be heating the great outdoors.

I remember Glenn had said something regarding the cyclone about ash building up on surfaces no matter how well you try to polish them.  I don't suppose it's all that important, so long as those loose agglomerations of ash are able to break away under their own weight from time to time and fall to the bottom.  Probably all the better that way.

Ok.  I would like to report some progress on this project.  As I mentioned earlier in the thread, one of my objectives is to experiment with aerated refractory cement.  To do that I would need a foam generator.  I just finished that up tonight, perhaps tomorrow I will get a chance to leak test it and check it for function.  Next up is the miniature propane furnace for heat testing the aerated refractory mixes.  Very interested in doing a test on aerated geopolymer, but I'm not confident in my chemistry ability and not sure how to go about that.  Any suggestions in that regard would be welcome.

https://drive.google.com/file/d/1N4Vn5i8vn-QAd9Z1vpiciENXkKs7nk2m/view?usp=sharing