Hi everyone,
I’m looking to build a batch box rocket stove which can also heat water. Is there a simple, safe and efficient way to do this?
I’ve read that a vented tank inside a bell with coils inside to transfer heat out to a domestic hot water cylinder is safe. Hast anyone had any success with this method?
Yes. Check out Matt Walker's boiler. There's a post about it with links and an embedded video here:
donkey32.proboards.com/post/37433/threadWith simple components that don't cost an exorbitant amount of money (like fire tubes / heat exchangers made of Inconel 600 or 300 Stainless Steel?)
Without complicated automatic flue-gas redirects for really baseline safety?
Nope. There isn't.
The thermomechanical issue faced by boilers since the dawn of the industrial revolution is that an extremely hot fire blasting against an iron, steel, or 304/316 stainless steel tank filled with water will be subjected to heating extremes (around 1100ºC-1400ºC — which are well beyond these materials' thermal stability limits) on the boiler vessel's outer surface — yet kept below 100ºC on its inner surface. The issue is a combination of
A) putting too much heat into the steel faster than it can dissipate that heat through itself and transfer it to the water, plus
B) the chemical composition of woodgas flames, which weaken the steel with carbon deposition and nitrogen embrittlement, on top of
C) the thermal expansion differential between outside, inside, and flame impingement areas, which
D) exacerbate the spalling of iron oxides in delaminating layers from the outside-inward, due to the excess oxygen not yet consumed by the still-burning flames, plus
E) the corrosively alkali mix of ash and water vapor given off from wood combustion. That's what causes those holes.
• Cast iron only has a thermal conductivity of about 40 W/(mK), with a working temperature limit of 815ºC
• Carbon steel is around 45 W/(mK), with a working temperature limit of 399ºC
• 304 stainless steel is only 15 W/(mK), with very different *continuous* vs *intermittent* working limits. (Thermal cycling up from ambient temps and back down —like is done with a rocketstove— limits stainless steel's *intermittent* temperature limit drastically)
The flame temp in a rocket stove can be expected to reach 1100ºC and can be as high as 1400ºC in peak burn on a 6" or 8" J-tube. Going bigger than that system size will be like trying to boil water with a welding torch. You'll blow holes through your boiler even faster.
When you're continuously dumping heat from direct-flame wood fire into those alloys, there's always going to be a damaging temperature gradient between the inside and outside of the boiler vessel, and it gets worse as the steel accumulates carbon and nitrogen, which further impedes heat transfer while also making it weaker and more spalling-prone. The fact that you can't turn the flame down without producing smoky, sooty pollution (like you can with natural gas or propane) means that the boiler service life in a woodburning appliance is much, MUCH shorter than in a gas-burning appliance.
The only alternative to mitigate this corrosion/erosion is shielding the metal boiler with refractory or placing the boiler farther downstream from the flames —which are quite effective at preserving your boiler vessel— but it lowers the "efficiency" of heat transfer from the fire to the water. Given that burning away steel puts micronized magnetite, chromium, and nickel into the air as neurotoxic pollutants which accumulate in brain tissue and lead to dementia, cancer, and all manner of neurologic damage, I'm of the opinion people should stop pushing for the highest "efficiency" of heat transfer, and stop burning out their rocketstove boilers. Besides, stainless steel boiler vessels and industrial-sized stainless stockpots / tanks are expensive. It's not an appropriate use of materials nor an "efficient" use of money to treat them as consumable items. They're in no way "clean" nor "renewable" when used that way — but wood IS. ...so just burn a little bit more wood —
cleanly— and have a little bit more patience. (That, or use a different heat source. For example: accumulated solar energy can heat water and keep it hot and on-hand remarkably well, [usually, in most climates for much of the year] even without having to convert it to electricity first. Then it requires very little additional energy input to bring that hot water up to an actual boil if you need to.)
Your brain matter [and everyone else's] will thank you.
The crucial, catastrophic-failure thing with directly-heated tube- or coil- hot water heating systems is your flow-rate of water through the tubing heat exchanger. If that flow rate is not high enough, the unit can explode… kinda like what happens with a nuclear reactor when the electricity fails. (Think: Fukushima.)
Or think of it more like an engine coolant than a water heater: If your flow rate / volume is too low for the amount of heat your “engine” (rocket) is producing, then it will overheat, flash to steam, blow the coolant water out of the coil, and then begin to melt down or spall, and that's even with an open-air, unpressurized system.
Everywhere I’ve seen water heater coils like this used, it’s always recommended to have the system A) open to air (
as in a coil-in-a-tank like Matt Walker's design) thus allowing for pressure expansion —and B) fed by gravity and thermosyphon
without relying on pumps — which will act independently without electricity.
If something goes wrong (like, say, your pump overheats, shuts off, or you lose electrical power) then you no longer have water running through the exchanger to cool the tank — and you have water trapped in the exchanger coil which will rapidly turn to steam and create a bomb.
Steam expands to 17,000 times the volume of liquid water— and in a setup like that, could do so dangerously quickly.
It’s extremely risky to run wood fired water-pump heat-exchanger systems powered by electricity. Almost all of the electrical pump systems are fired by gas or heated by electrical heating element for the critically important safety feature of
shutting off the fire if *when* the pump or the electrical system fail to work properly.
Gas flame can have automatic shut-off valves in case a certain, predetermined temperature in the exchanger is exceeded, or in case the pump goes off, or in case the electrical supply is interrupted.
You cannot shut-off a rocketstove in that way, and that makes them very dangerous with “water-coolant” / “water heater” exchangers.
And the risk isn't just limited to electrical outages or pump failures.
All boiler tubes can accumulate mineral deposits (like calcium/lime) but those deposits
in a rocket boiler are more likely flake off in the copper or stainless steel's expansion and contraction caused by the extremes of thermal cycling within a rocketstove, and then migrate to a bend or downstream chokepoint, suddenly blocking the tube — like the way high cholesterol will constrict an artery and a migrating bloodclot causes sudden blockage and cardiac arrest. You there again have a kaboom. That's even if both your pump AND your electrical system still happen to be working. The safety features which prevent this in gas burners cannot be applied to a woodburning appliance without significant complication and the attendant added risk of overly complex safety features' failure, plus the decreases in heat transfer "efficiency" resulting from having to place an automatic bypass damper between your fire and your boiler vessel / tube system.