Rocket Pool Water Heater ---- Need advice Please!!!!

I am working on building a rocket pool heater and can use some advice from the experts here....

Please review my drawing.....

I plan to build a standard rocket stove and run the riser tube through steel tank that I have.  This would expose the water to the heat of the riser and the outer chamber.  I plan to either pump the water from the pool or allow it to thermal siphon.

My biggest concerns are:
- Efficiency - is this tank method more or less efficient than the tubing wrapped around the riser that if seen?
- Over pressure - The water system will be pressurized by the steam / thermal siphon.  Can this be dangerous?
- Is my rocket stove itself a good method? 4" square feed tube / 6" square burn chamber / 4" round riser / perlite ins / steel drum outer chamber ?

Any advice, feedback comments, concerns, discussion would be appreciated.

Thank you,

Hi Robert.

It's best to use a constant CSA: ie 4" or 6" all the way. Switching from one size to another may cause drafting problems.

In your drawing the water in the tank will cool the heat riser and may impair complete combustion - it would be best to site the tank some distance above the top of the heat riser to let full combustion take place before it goes near the cooling effect of the water tank. You should also insulate the heat riser to promote a clean burn.

Looking again, I assume from the drawing that you propose to make the riser from metal so you can weld it into the tank? Before Satamax joins in "Metal Is Doomed" It may survive in your application if you stick to your original drawing design because the water would cool it down: however, I would strongly avoid cooling a riser - as mentioned earlier, your rocket won't burn cleanly.

In the drawing your tank is 'open vented' - it's using the pool as a sort of feed and expansion tank so it should be able to release any built up pressure safely. You may however get a flash steam effect in the water if the thermo syphoning doesn't work properly (which may squirt boiling hot water into your pool). Use the largest diameter piping you can from the tank to the pool (and insulate the tank and the warm feed pipe) and ensure there is a sufficient gradient to help the syphoning. If you have to use a pump and it fails, you could get in a 'boom squish' situation - I'd try and rely on the thermo syphon. There's less things to go wrong without a pump.

Thank you for the feedback, very much appreciated!!!.

I made some adjustments...

I have an old pancake compressor I can use for the water tank.  It will be snug the outer drum is about 14"ID and the tank is 13"OD.
I adjusted the Rocket stove to all 6" tube.  I will use steel tube and weld it all together.

The riser is only 18".  Is this tall enough? does height matter?
The taller it goes the deeper I need to dig it under ground in order to keep the tank level with the water line for thermal siphon. I really want to dig as minimal hole as possible.

Any input, comments, concerns, advice, discussion greatly appreciated.........

May I suggest that a slanted feed tube doesn't work as well one would expect?
As drawn, all the incoming air is primary air, throttling pyrolysis of the fuel to high levels. But the combustibles produced can't be completely ingnited because there's no provision of heated secondary air. In your design there's no tunnel, the fuel is directly at the bottom of the riser and the flames are going straight up immediately. Instead of moving sideways in a horizontal tunnel which is THE essential part of the clean burning system in a rocket mass heater.
Moreover, there has been a lot of talk about metal in the core of a rocket heater, with good reason. When surrounded by high-temp resistant insulation the steel will burn away very quickly. Not by melting as such but by corroding very rapidly. By leaving out insulation the steel would probably has a better chance to survive but the so sought after clean burning is killed off too.

It's your choice to build it like this or not. But I thought I could at least warn you being on the wrong path in my opinion.

In addition to Peter's cautions, I would add that making the riser the same height as the feed tube will kill the differential draft that powers the rocket effect. It will be just as likely to burn back up out of the feed as up the riser.

One thing to keep in mind regarding a thermosiphon system is that it really only works if the heat source is below the level of the water storage tank. This means that the cold water return must be higher than the highest point of the heat coil. Also, there is a practical limit to the horizontal distance from the heat source to the storage tank. As mentioned previously, larger transfer pipes will reduce the hydraulic resistance and allow the thermosiphon to work much more efficiently.

Hi

We have developed a Rocket Stove water heater that would certainly heat your pool. It runs on wood pellets. Check out www.perfectburn.co.uk

here is the PerfectBurn heating a hot tub