Glass "bricks" embedded in a Rocket Mass water heater...

I've been considering building an outdoor Rocket mass water heater, that I can use as an "outdoor" hydronic heating loop source.  I was thinking about embedding some clear glass jars, bottom side in, as an observation window for the interior of the heat bell portion.  I think that if I do this, with the original jar top accessible on the outside, I can use a small webcam to record video of the interior from every angle that I install such a jar.  I have a lot of jars...

But my question is not about that part.  My question is, if I were to put one or two of these on the top of the horizontal burn tunnel, would it survive?  Or should I expect it to not survive very long?

Hi Creighton;
I would expect them to not last long at all. At 700-900 F glass becomes like pudding.  Part way up your riser temps are known to be 1500F + I suspect that the burn tunnel could easily be over 700-900F
However all the other places they should work OK (I guess...)  Camera view would be convoluted (fish eye) I would think.

A normal jar will most likely crack (and then melt). But even if you used a fused silica window, the heat will still be far too much for the camera to handle. With a mirror maybe… but then you are getting to far away to see a lot. A small hole in the top works to "see" inside (but leaks exhaust gas).

Sebastian Köln wrote: A small hole in the top works to "see" inside (but leaks exhaust gas).

Hmmm, that makes me think that some kind of ceramic "plug" that could be removed for a few seconds at at time (with it's own spiral handle) would be the solution here.

I wonder if a basic clay pot could handle the heat.  If I had one clay pot that could fit pretty snug into another clay pot, the larger could be embedded in the top of the burn tunnel with it's bottom cut out.  The slanted sides might permit the smaller pot to work as a plug, filled with sand or fire-mortar, to be lifted out for a photograph or a quick look at the state of the burn.  Alternatively, the smaller pot could be filled with small chunks of wood to be turned into charcoal, and itself capped to force the smoke down through the small drain hole in the bottom and into the flow.

Pyrex flows 1510 degrees Fahrenheit......

Bill Haynes wrote:Pyrex flows 1510 degrees Fahrenheit......

Excellent point.  While a normal soda glass jar might not survive, a lab glass quality beaker should.

thomas rubino wrote: Camera view would be convoluted (fish eye) I would think.

That's actually what I want.  Distortion could be corrected in software, if necessary, but my goal is to observe as much of the interior of the heat bell during operation as reasonably possible.

What about borosilicate glass as used for wood-burning stoves? I've seen thick round industrial-grade castings that were made for observation ports in kilns...in fact, the last time I stopped in at the local Blackwoods they showed me a couple that they ordered in for someone years ago who never picked them up, and told me no reasonable offer would be refused. If funding comes through for a couple of pilot projects I've got in the works, I might just be able to justify nabbing one of them.

Phil Stevens wrote:What about borosilicate glass as used for wood-burning stoves?

Sure.  That's what Pyrex brand and "lab glass" are made of, BTW.  How much does one such unit cost?

Phil Stevens wrote: Blackwoods they showed me a couple that they ordered in for someone years ago who never picked them up, and told me no reasonable offer would be refused.

I'll fund them.  What is "reasonable" in this context?  How many are there? How large are they, etc?

There were two, and if my memory serves they were 15 cm diameter. I could ask if they'd let them go for $100 for the pair. Could even ship one to you, but getting it across the Pacific won't be cheap.

I've got a rocket oven in the planning stage and would really like to install one at the end of the burn tunnel where it transitions to the heat riser. Am I nuts for wanting to do this?

Phil Stevens wrote:There were two, and if my memory serves they were 15 cm diameter. I could ask if they'd let them go for $100 for the pair. Could even ship one to you, but getting it across the Pacific won't be cheap.

<sigh> Yeah, I didn't notice your city tag until just now.  That's probably not going to work out.  Unless I can convince my wife on a New Zealand trip! (hmmm)

Don't lose hope...my mother-in-law will be visiting next month and we can see if she would take it back to Oregon in her checked bags, then ship it on from there. First I have to see if Blackwoods will take my lowball offer ;-)

 I think what is used in windows on wood stoves is actually a ceramic glass and not pyrex/borosilicate.

Neoceram, robax, and another brand name exist but I can’t remember the last one.

 Another high heat option would be corning ware cook ware, but that tends to be thick and amber/oramge brownish colored.

 Ceramic glass would probably only be required in the hottest zones... Areas where you could not leave a camera directly exposed very long to the radiating heat through the window anyway... but i’m not sure what temperature range and differential capabilities of borosilicate are.

 I do know that when a college roommate tried to use a pyrex pie pan like corningware, placing it on the stove top with water in it, and turning the stove on to high, it made a godawful explosion of hot glass shards all over the kitchen when it gave up to the overwhelming heat induced internal stresses...  Thankfully no one was in the kitchen, as he had started it and walked away to let it get boiling, and none of the glass was hot enough to start a fire... may have been some damage to vinyl flooring and seat cushions, I can’t remember that part specifically, just our other roommate losing his S#!T because it was his prized pie pan, the noise was totally jarring even in the other room, and he was already less than thrilled with the roommate in question...

 Good luck, and I look forward to seeing the results of such observation windows!





 

These sight glasses are annealed, so they probably won't shatter from thermal shock. I found a Pyrex datasheet that lists the softening point as 821 C, but they're over 1 cm thick so I think they will be slow to deform. All in the name of science.

I will prowl the secondhand dealers and see if I can find a small window from an old wood fire. All the modern ones are big and I can't imagine trying to take a wet saw to one. Anybody done this?

Apparently mica windows are still a thing, too, although synthetic mica seems to have taken over from the mined product.

Phil Stevens wrote:I will prowl the secondhand dealers and see if I can find a small window from an old wood fire. All the modern ones are big and I can't imagine trying to take a wet saw to one. Anybody done this?

 I believe the clear ceramic glass used in wood stoves can be cut with standard “score and snap” techniques used in traditional glass cutting/window glazing...  just errors are significantly more costly.  A belt sander or diamond file can then be used to round off sharp edges for safer handling, if needed...

 I think the belts used in sanding the edges of plate glass are different from your normal “wood shop” sanding belts, but not sure.

 The one i’ve seen had a 4” wide belt in a bandsaw like setup that must have been 8-10 feet of belt total.  I didn’t get to inquire about the type of grit.

 It might be the same as high quality wood sanding belts and just longer to allow the belt to have a reasonable life.  Looked like the purple 3M “SandBlaster” belt material.

If you are thinking of the plate glass from fireplace inserts or the door sets that can be installed on the front of an open hearth fireplace, I believe those are normally borosilicate rather than the ceramic glass, because that much ceramic glass would cost well more than I see those “fireplace door” sets selling for.

I guess if I can get one for under 50 bucks at the secondhand traders, I should be willing to take my chances with the carbide wheel and the gentle press over the dowel. I've scored and snapped a few pieces of plate glass and hundreds of tiles, so at least going through the motions won't be any different.

Not much in the way of open fire front glass in these parts. Cast iron and plate steel log fires have been the norm in NZ for a long time and if someone retrofits an open fireplace, it's almost invariably to run the flue of a freestanding fire up the existing chimney.

Bumping this to see if you melted that sight glass, Creighton. Inquiring minds, etc etc.

I haven't done anything with it, yet.  It's sitting on a shelf in my basement.