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Post by pinhead on Jan 14, 2014 6:47:56 GMT -8
The importance of insulation in rocket stoves cannot be overstated. It is pretty much common knowledge within RS circles that the "core" must be as well-insulated as is practical. However, I've been wondering why I haven't seen anybody experiment with radiant barrier(s) in addition to the typical insulation. With the typical/cheap radiant barrier reflecting >97% of the radiant heat -- and higher priced barrier reflecting 99% -- it seems that an air gap combined with a radiant barrier around the core of a rocket stove would be a huge improvement in insulation value for a very small increase in physical size. I haven't wrapped my Peterberg Batch Box with radiant barrier yet; the high-mass nature of the red brick construction isn't insulated well enough to make enough of a difference to justify the work. However, I did put a sheet up between the extremely cold [uninsulated] rock wall and the barrel which made a huge difference in heat radiated into the space. Since I am only able to burn the stove in the late afternoon/evening and the building cold-soaks over night and all day, heating the mass of the walls is a waste of time. The radiant barrier is less than 2 inches away from the barrel and you might assume the foil would get hot at this proximity. However, with my IR thermometer reading 400°C radiating from the foil, the foil itself is actually cool to the touch; most of the heat is simply being reflected back into the room. If I could get my hands on some superwool and hardener, I'd construct a superwool heat riser with a 3/4-inch air gap and some of this radiant barrier... I suspect this construction would be getting pretty close to the law of diminishing returns. Attachments:
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Post by Deleted on Jan 14, 2014 9:44:52 GMT -8
White ceramic fibers in an insulation will reflect most of the infrared light. This also applies to the inner surfaces of white porous materials. I do not think an additional radiant barrier would have much effect.
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Post by satamax on Jan 14, 2014 9:53:12 GMT -8
Don't know what your radiant barrier is. But if anything like thin film insulators. My findings is, that in a house, they're crap
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Post by oboblomov on Jan 14, 2014 10:04:26 GMT -8
White ceramic fibers in an insulation will reflect most of the infrared light. This also applies to the inner surfaces of white porous materials. I do not think an additional radiant barrier would have much effect. Interesting and very useful to keep in mind, Karl. Thank you. I'm currently making a low-tech clay riser. Would wrapping it in aluminum foil add significantly to it's insulation? Thanks, Pinhead, for pointing this out. obob
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Post by pinhead on Jan 14, 2014 11:18:41 GMT -8
White ceramic fibers in an insulation will reflect most of the infrared light. This also applies to the inner surfaces of white porous materials. I do not think an additional radiant barrier would have much effect. Interesting and very useful to keep in mind, Karl. Thank you. I'm currently making a low-tech clay riser. Would wrapping it in aluminum foil add significantly to it's insulation? Thanks, Pinhead, for pointing this out. obob Food-grade aluminum foil makes for a very poor radiant barrier; the surface finish is too rough and it will radiate more heat than it reflects. In order for a radiant barrier to work there has to be an air gap - 3/4 inch and smaller is optimal, as the space is too small for convection to occur. WITHOUT AN AIR GAP, A RADIANT BARRIER IS USELESS. Don't know what your radiant barrier is. But if anything like thin film insulators. My findings is, that in a house, they're crap Not all radiant barrier is created equal and they're a bit more prone to installation error than old-school mass insulation; without an air gap radiant barriers are rendered useless. I installed it on the underside of the roof of my shop. It made a huge difference - the shop is now habitable during the summer months and heatable in the winter. In my opinion, the combination of a radiant barrier and an expanding spray foam insulation are pretty much the ultimate insulation combination. Add a little thermal mass within the insulation envelope and you have an extremely efficient structure. But I digress. The stuff I'm using is two radiant sheets with a sheet of bubble-wrap sandwiched between them. The stuff is extremely strong and easy to work with; I cut it with a pair of scissors or a pocket knife. Got it at The Home Depot.
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Post by oboblomov on Jan 14, 2014 13:05:35 GMT -8
Pinhead said: "Food-grade aluminum foil makes for a very poor radiant barrier; the surface finish is too rough and it will radiate more heat than it reflects.
In order for a radiant barrier to work there has to be an air gap - 3/4 inch and smaller is optimal, as the space is too small for convection to occur. WITHOUT AN AIR GAP, A RADIANT BARRIER IS USELESS."
Thank you, PH. I understand what you are saying re. aluminum foil. In the pic you show that the reflective plastic insulation (radiation barrier) is attached to the wall behind the stove. Looks like a very good application for the RB. Perhaps the RB surface is cooled by both radiation and convection (at the warmed surface.) In any case it would seem that the RB acts like a mirror, transferring heat to a much larger and cooler space (the room.) This material would probably not work well as a wrap for a (partially) insulated riser, where temperatures are likely to be higher.
I've read somewhere that crumpled aluminum works as an RB; at least to some extent, I would imagine.
obob
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Post by pinhead on Jan 15, 2014 5:47:16 GMT -8
On my wall there is an air gap on both sides of the RB, as it is double-sided and held in place using 1/2"-thick wood strips around its perimeter.
RB has high reflectivity as well as low emissivity - which means it doesn't readily radiate its own internal heat.
This means it most generally won't cool itself off through radiation; it merely reflects most of the radiation that hits it. For this reason there is very little "heating" of the surface of the RB film. This is why I can place my hand on the RB directly behind the barrel without getting burned (though the back-side of my hand nearest the barrel gets too hot to handle very quickly).
I experimented with a square of RB on the inside of the door of my stove -- taking the full brunt of the burn. The RB lasted until about 1/2-way through the "coal" stage where the radiant heat is the highest. At that point the aluminum melted and the bubble wrap burned. To be clear, this was expected. I was actually impressed, however, at just how long it did last. And the whole time the RB was intact I could place my hand on the steel door which was being protected by the RB.
I suspect with a decently-insulated heat riser tube, RB could easily be used around the perimeter of the riser to further insulate the burn works and be very effective; every time I find a new application for this stuff I'm amazed!
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Post by oboblomov on Jan 15, 2014 8:50:30 GMT -8
On my wall there is an air gap on both sides of the RB, as it is double-sided and held in place using 1/2"-thick wood strips around its perimeter. RB has high reflectivity as well as low emissivity - which means it doesn't readily radiate its own internal heat. ... I experimented with a square of RB on the inside of the door of my stove -- taking the full brunt of the burn. The RB lasted until about 1/2-way through the "coal" stage where the radiant heat is the highest. At that point the aluminum melted and the bubble wrap burned. To be clear, this was expected. I was actually impressed, however, at just how long it did last. And the whole time the RB was intact I could place my hand on the steel door which was being protected by the RB. I suspect with a decently-insulated heat riser tube, RB could easily be used around the perimeter of the riser to further insulate the burn works and be very effective; every time I find a new application for this stuff I'm amazed! Good information and experiments! Aluminum melts at 660 C (1221 F), and Mylar(TM)(polyethylene terephthalate) melts at 220 C (500 F). While not as inexpensive as foil, this RB material does appear to have potential for a secondary insulation for risers. I'll get some and try it out. Thanks much, PH, for bringing it to our attention. obob
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