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Post by hybrid on Dec 10, 2013 17:21:39 GMT -8
I have (4) 100# propane cylinders I'm going to build into rocket mass heaters. I'm highly curious though...
Is there any real benefit to using stainless pipe for the burn chamber? I know it resists heat like crazy but are people really burning through mild steel easily?
Stainless pipe in a 4" diameter with 120 wall will cost me nearly 600 a stick (20') and that really dampens the cheap benefit.
I just don't want to have to cut this apart later for repair.
Thoughts? Opinions?
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Post by satamax on Dec 10, 2013 19:39:10 GMT -8
Forget about metal altogether for the burn parts.   Thjat's the spalling after 14 burns. The metal on top of the gas bottlme started melting and droping down. |
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Post by hybrid on Dec 11, 2013 8:32:05 GMT -8
Is this purely a rocket feature? So many people build pot belly stoves from gas bottles that it has to be related to the fire intensity.
A good stainless though has to be capable. I guess I'm not understanding how car headers take flame abuse but not a stove.
I know car exhaust has plenty of fuel in it comparably but is that the answer to the longevity?
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Post by treeclimber1 on Dec 11, 2013 8:55:07 GMT -8
I can second that Look at this picture of my test burner after about two burns. Notice the blisters. this is mild steel but how many cycles can any steel handle when heated to around welding temp. I would think there are a lot more BTUS In a rocket stove than are coming out of a car and I doubt that they get very many hours out of a drag car's headers either. I too am looking at the different materials used to construct these.  |
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Post by satamax on Dec 11, 2013 10:38:34 GMT -8
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Post by bernardbon on Dec 11, 2013 11:29:22 GMT -8
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Post by satamax on Dec 11, 2013 12:18:45 GMT -8
I don't think this could hold for long. Would be good for low mass heat risers, and forming venturis, but i'm real dubious!
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Post by hybrid on Dec 11, 2013 15:43:20 GMT -8
It's kind of a bummer. I'm a much better welder than a mason.
I guess it saves me money on stainless but makes the unit a lot heavier.
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Post by satamax on Dec 12, 2013 1:12:03 GMT -8
It's kind of a bummer. I'm a much better welder than a mason. I guess it saves me money on stainless but makes the unit a lot heavier. Depending where you are, there's other ways. Some form heat risers with superwool and binder. Others use vermiculite boards. I use refractory tubes. You could also use metal on the outside, to hold some bricks. Check this drawing donkey32.proboards.com/attachment/download/730From this thread. donkey32.proboards.com/thread/951/shop-rocket-heater?page=2#page=1 Insulative firebricks held by metal, and dry stacked is a good idea. As you can replace them easily. A lot of stoves in europe, are made this way. |
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Post by ringoism on Dec 13, 2013 9:39:58 GMT -8
I have (4) 100# propane cylinders I'm going to build into rocket mass heaters. I'm highly curious though... Is there any real benefit to using stainless pipe for the burn chamber? I know it resists heat like crazy but are people really burning through mild steel easily? Stainless pipe in a 4" diameter with 120 wall will cost me nearly 600 a stick (20') and that really dampens the cheap benefit. I just don't want to have to cut this apart later for repair. Thoughts? Opinions? Spalling like Max depicts is pretty impressive but not actually surprising when we're talking about MS, which has very low-temp service limits. But Grizbach had a stainless riser last three years in his modular J-tube. Not bad. And a couple years ago, Peter suggested I try 310SS for internals. Check this out: "The high Cr and Ni contents [of 310S SS and 314 SS] enable this alloy to resist oxidation in continuous service at temperatures up to 2200F provided reducing sulfur gases are not present..." and, "Adequate resistance to scaling at temperatures up to 2000F requires a chromium content of at least 25%, such as types 309, 310, or 446... "Type 446 and the new proprietary ferritic chromium-molybdenum stainless steels have a relatively low coefficient of thermal expansion, which tends to enhance spalling resistance. Additions of silicon also increase resistance to oxidation, such as Type 314, but silicon tends to decrease high-temperature strength..." etc, etc, etc. Granted, 2000F might be on the lower end of what some rockets will produce - but then, keep in mind that (as per the comment on 314) some of the ratings / limits also have to do with decreases in strength at those temps, more than any rapid oxidation / spalling. And you don't need a lot of strength to support some riser insulation or the roof of a burn tunnel. Some of the charts I saw were related to changes over very long cycles of 1000 and 10000 hrs. In my own test stove (portable 3-1/2"), I can see the beginnings of spalling on even protected MS parts after less than 20hrs burning, while a very thin mystery grade SS that I lined the orange-hot top of the burn tunnel with has absolutely no signs of it (nor of cracking) over the same time period. I tend to think that until someone has actually tried some of these high-temp SS grades in an actual rocket stove, it's a little too early to just write off all metals for any rocket application. There is definitely a huge difference between MS and even "generic" grades of SS when it comes to oxidation / spalling resistance. Find a download of "High Temperature Characteristics of Stainless Steel" from the Designer's Handbook Series, produced by the American Iron and Steel Institute, and study it awhile. And then, I'd really like for someone to take up the challenge and actually experiment with this (so that I won't have to do it myself)... I think that Max's idea of lining a simple steel chassis with firebrick is good, though as I said, my MS was spalling even behind the insulation. For my next build, I'm going to use a mystery-grade SS chassis with brick or supercera board linings. I suspect that might actually hold up for quite awhile. -Eric |
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Post by ringoism on Dec 13, 2013 10:21:31 GMT -8
Whilst exploring this question (whether ones' stainless can take it) I came across discussions related to highly modified Mazda (Wankel) rotary turbo engines, i.e., for racing applications. Exhaust temps are much higher there (Mazda uses a "thermal reactor" exhaust manifold on production units for burning off pollutants, much along the same lines of the rocket's heat riser). For really reliable performance under the most extreme conditions, Iconel www.hpalloy.com/alloys/hightemperature.html was preferred. There are some Iconel grades that have service temps up to 2200F. Also Hastelloy. Seems that should serve a rocket stove well enough. So if you live near, say, a nuclear power plant or a NASA installation (or else have access to e-bay US and a little luck), you might be able to pick up some tasty offscourings to build a stove with... Not sure of the required welding methodology there... but someone should try this: a high-tech, lightweight rocket stove that does away with the mud / masonry / heavy cast components (mud is wonderful, of course... but heavy and messy and a little more maintenance-intense). -Eric |
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Post by satamax on Dec 13, 2013 10:27:10 GMT -8
Well, 2200F° is still a bit low for my liking. My batch rockat had all the insulating refractory bricks, the refractory tubes et behind red/orange most of today. I wonder what temperature that was. And also how stainless would have fared.  |
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Post by hybrid on Dec 13, 2013 11:42:37 GMT -8
I want to use stainless. I'm willing to try it. It's very very expensive though. 6-800 per 20' stick.
Sure I can make 4 with just a stick but I highly doubt anyone wants to buy them from me to recoup costs.
I was really hoping I'd find some salvage stainless pipe.
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Post by satamax on Dec 13, 2013 17:36:45 GMT -8
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Post by hybrid on Dec 14, 2013 18:07:06 GMT -8
Thanks for the link. Being US bound means I'll probably do brick. Would rather have a viable option that doesn't cost me a million bucks.
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