What @taitgarry00 said. A rule of thumb building a boiler is every 10 kW needs about 1 m3 of surface area (gas to liquid heat exchange).
Thanks for that JO. It allows me to argue my own opinion.
I suspect that that rule of thumb would apply to boilers where the water is not flowing over the internal surface. The old Babcock & Wilcox (B&W), fire tube, boilers did it that way.
But White, Doble, LaMont and a few others were starting to do things differently. The Besler brothers fitted a Doble boiler and engine to an airplane!!! The Besler Steam Plane - YouTube
I suspect that with a countercurrent, watertube, circulating boiler much less surface area would be needed. Perhaps our waterheaters are still using a long outdated basic concept?
Rindert
P.S. In Minneapolis I saw where an 80 gallon water heater had been struck by lightning. It was in the basement of a Victorian, three story house. It took off like a rocket, blasting through all three storries and out of the roof. It landed near a side walk. Happily no one was hurt. But there have been very many boiler explosions similar. A watertube boiler contains much less water so it is naturally safer.
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Thanks! Thatâs a good video. I never thought of steam power as suitable for aviation 
It rather makes you think of heavy equipment.
I guess the amount of surface area is all about what kind of hot water flow you expect and how much heat you will allow to escape out the chimney.
Yes, clean hot water tubes are more efficiant but exhaust tubes are easier to maintain/sweep.
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Yes. The the old steam locomotives weighed 2 or 3 hundred tons. And they needed all that weight for traction.
But steam power doesnât have to be heavy. Obviously, since someone was able to power an airplane that way.
US Navy Comander Walter Douglas Lamontâs boiler can even operate on seawater. Something barely dreamed of by steam opperators.
Rindert
PS. There is a decent description of Commander LaMontâs boiler in Wikipedia.
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Hi there
I disagree. You should have respect, but there is no need to be frightened.
I am a fireman and engine driver on historic steam trains and fireman an a coal fired steam-ship in my spare time. Respect the power of steam and take care is of sure mandatory, but a well desingend and maintained boiler is very safe.
Boilers exploded some times, thatâs right. In the beginning of the steam age, the steel of very varying quality, proper design rules were not known, leading to failures.
Later, boilers nearly exclusively failed due to maintenance or operating mistakes.
Even locos colliding or falling down an embankment nearly never had boiler explosions, even if they looked like a big deformed piece of scrap.
In western Germany, the last steam locos were in regular service until 1977 and in eastern Germany until 1988. They were mainly abandoned because of the high operating costs because of the required labour intensive maintenance an operation, high emissions and reaching the end of the live cycle. It was also regarded as outdated and obsolete technology with bad reputation. Never heared of high insurance costs for locos in Germany, and I know some old railway-men and drivers. Some of them even preferred steam to diesel or electric, because they were not alone and had company by a fireman or just because of the more fascinating nature of a steam loco. Sure they wouldnât have made that if they were frightened about their life.
With further technological development, some of the drawbacks, mainly labour intensity and emissions could have been minimesed. Boiler safety was never a main driving issue in this respect.
Look at the big power stations. Coal power plants use steam boilers and turbines with a much higher power output. OK, stationary plants are easier to built, but they use much higher temps and pressure. How often is one of this blown up?
This was a long reply and I donât want to annoy anyone with it. I just wanted to write down my point of view, as I have some experience with steam power, especially locos, by my own for more than 20 years now.
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Yes Til, that is perfectly correct. The dreaded âcrownsheet failureâ would usually occur just a locomotive started to move. Why? Usually because the crownsheet had been allowed to run dry. Thus the crownsheet would overheat and weaken, and when a wave of water, created by the the engines movement, would suddenly cover it. This would create a sudden increase in pressure and sometimes a rupture. This whole scenario happened far too often. It was operator error, but it happened far too often. The same friend who still makes whistles told me all this and that the âbig malletâ locomotives were retired from service simply because diesel-electrics were much less costly to insure.
He actually has a 50 hp boiler that came out of a dry cleaning shop. It was made in 1898. It has a valid ASME stamp. It is rated for 200 psi. The 2-1/2" steam line connection is simple NPT threads cut directly into the 3/8" thick shell. There is no half coupling or any sort of added thickness. And this is legal. Bloody right Iâm scared!!!
Rindert
I think it will work. I think Garryâs right about efficiency. If you can make the water pipe cross the air flow more it will help with heat transfer.
That said, our hot water in the winter comes from our wood heater. It has only 29 inches of 3/4" hard copper tubing at the top of the combustion chamber at the edge of the top of the oven box. only about 2/3 of the pipe surface is exposed to gasses at all. about 1/4 is laying against the fire brick/oven. And the rest is on the leeward side of the pipe & not exposed to much direct hot gas contact. Also, It is caked with at least 1/2" of creosote/soot/tar/etc. And we have all the hot water we need. The pipe comes in and goes back out. One side to the top of an electric water heater, the other to the bottom. All that to say that it doesnât seem to take much efficiency to get all the hot water that we need.
But if you want it to be more efficient, use a piece of copper tubing and make a coil that is slightly funnel shaped or bend it back and forth through the hot gas flow path.
I like the spinner. Have to make the hot gas molecules hit the pipe to make heat transfer. So maybe moving the gases to the pipe is as good as putting the pipe in front of the gases. In my experience, using the pipe (coil or similar) to make the spinner gets more heat transfered faster.
I think it is true that water moving inside the pipe and making more water molecules contact the inner surface of the pipe will increase efficiency of heat transfer and thereby reduce the need for surface area.
Also, agree that steam doesnât need to make us shiver. Just need a little respect and care. I wish we could bring back a lot of steam tech, water pumps, food driers, engines, etc,⌠Especially for development settings where petroleum is not as affordableâŚ
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But we have woodgas and chargas to replace petrolium. And steam is not a fuel but a working medium. Argh! I really donât know you well enoughh to follow your thinking. I am actually not opposed to steam based power. I think Thomas Newcomenâs condensing engine (1712) is great. Very safe and it sure lifted a lot water out of mines. Newcomenâs system uses a brewerâs copper opperated at atmospheric pressure. If the boiler were to start leaking there is no pressure to cause a catastophic rupture. I imagine Newcomenâs engine would actually be good in âdevelopment settingsâ as you put it.
I just have a problem with fire tube boilers. To my way of thinking the total quantity of high pressure steam/water should be kept to a minimum and its explosive and other deadly effects acnowleged with failsafe designs. I think watertube boilers go a long way toward making steam power save. My point.
Thanks Billy thatâs a really good data point. Is the hot water then stored in a tank? I think my water heater might work if I can figure out how to keep the condensate water from dousing the burner.
Rindert
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No reason why you couldnât have multiple vertical runs of tubing in the exhaust stack. If you could slide a wire bottle type brush in behind and between, cleaning would be simple, and more surface area for heat exchange. And use the removeable center swirl diffuser.
As for condensation, offset the burn area so it doesnât get dripped on?
Those atmospheric pumping engines were really cool, but their efficiency was almost non existent.
I think they appeared first because the materials and seals of the day were so crude, they needed large devices working on low pressure and speed. But for high volume pumping they held on for a long time. Makes me tempted to tinker with a solar heated one for well pumping, the temperature differential should be enough to use Boyleâs law. Unfortunately in lowland tropical areas thereâs less temperature differential.
The other proven approach for power without pressurization / explosion risk is an Erickson hot air engine, exactly what drove the rev Stirling to invent the concept. I think the old cast hot air engines could benefit from a re-examination, engineering concepts and materials, particularly stainless steel should enhance performance.
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There was quite a bit of interest in Stirling engines in the 1990s. Sunpower in Athens, OH https://sunpowerinc.com/ seemed to have he best ones, in my opinion. I had the idea to replace the sliding piston with a metal bellows, better sealing, and survives high temperatures, though not good at high pressure. Rollsocks donât do well at high temps. Apparently the best working fluid is hydrogen because of its heat capacty (cp).
Rindert
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I personally like the old school vertical cylinder cast iron Rider - Ericsson hot air engines, efficiency isnât great, but if burning biomass, practical shaft power is the real point, efficiency isnât the same issue if using a specially processed/ purchased fuel. Better materials and design tweaks, like heat exchangers, should improve specific power, maybe on-board air compressor, but the overall result is a machine that can be relied on in the real world, long term.
I do admire the Sunpower linear Stirlings, opposite end of the scale, but great reliability and power output.
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Man, I like the way you think. 
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We have a regular electric water heater behind the stove. The water circulates itself through system by convection.
When we are not using the stove, we can use electricity. Only issue is occassionally we have to trip the breaker because the stove tends to get the tank hotter than the electricity. In the winter when we are running the stove really hard it will blow off hot water through the safety valve. It can make some nice water sculpture on the back porchâŚ
Looks pretty cleaned up. I guess Erika cleaned it up for the summer. It serves as the tomatoe table this time of yearâŚ
!
we had 105 teenagers here this week (2-days) from Texas, Florida, Indiana,california, and Alabama learning about appropriate technology, etc. Everything from sanitation to gasification to water purification and rocket stoves and a lot moreâŚ
Not sure why that picture got in thereâŚ
As for steamâŚI want to make a low volume steam generator. I need to figure out some kind of ram injector or some other way to inject cold water into a low volume, relatively high-pressure environment. For an on-demand steam application. That, with some safety blow-off valves, would take most of the danger out of it.
The fact is, wood gas and char gas are still too complicated and costly for many applications. Right now we are working on a coffee drier for small scale farmersâŚRocket drum ovens for heat exchangers and steam driven fans for air movement.
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Wow, I am amazed at the level of skilled people we have on this site, and those who work outside of the box, and sometimes the planet for their thinking 
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I have created a group on MeWe for rocket stoves if anyone is interested come drop by. MeWe - The Next-Gen Social Network
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Hi Billy,
Huge thanks for info about your hot water system.
It seems that a lot of grain farmers here in the US are using solar heated air to dry crops. Because, âCrop drying is one of the most energy intensive operations of grain farmingâ. https://www.cleanenergyresourceteams.org/publication/heating-things-farm-crop-drying-solar-air-heat-central-minnesota
But it is possible to DIY a simple water tube (relatively safe) boiler too.
Rindert
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