Philip Rothwell

Hi all!  My apologies if this has been discussed here before, but I have not come across it anywhere:

I am in a position to build another rocket stove (for a neighbor this time) and this has gotten the wheels turning in my head again.  I see two major sources of inefficiency in the RMHs built by myself and others, both related to draft:  1) most systems rely on buoyancy of hot (warm) exhaust gasses at the end of the system for reliable draft.  This obviously wastes heat.  2) In real-world practice, the stove rarely gets shut down at the appropriate time, and because of the buoyant draft at the end of the system, will continue to extract heat from the system (and house) and dump it outside.

This all has me thinking that a system relying on the heat-pump effect will have much better real-world efficiency.  The system exhausts horizontally out the wall, and when the fire burns out, the draft grinds to a halt.  I know that horizontal-venting systems are recommended against because any little wind will tend to cause a backdraft.  However, there's a more fundamental design problem that causes this condition:

If we figure our core temps are around 2000F and room temp is around 70F, the core gasses will be thermally expanded 4-5X compared to room gasses.  As the gas travels through the system, it cools back down to room temp before being ejected from the house.  This means that to maintain gas velocity through the system, the system exhaust must have 1/4 the cross-section of the intake (conservatively).  For an 8" system, this means the exhaust should be 4".  An exhaust with small cross-section and high draft will be far less susceptible to wind-induced backdraft.  This arrangement *will* increase load on the system, but this is easily overcome by appropriate scaling of the heat pump (riser and barrel combo).  

I have other ideas, including a passive thermal-bypass bell, and that a bell does *not* stratify heat but rater sets up a convection current, but I'll save them for another time.

If there is one thing my 38 years have taught me, it's that my bright ideas are rarely as bright as I think, and so I submit them to you all for picking-apart.  Do you think a system as described could be started without the assistance of a bypass valve or fan, etc.?

Thanks for your input,
--Phil

I have completed a second round of testing, this time with a neutral vane installed. I will post photos soon.

The neutral vane is an extension of the side of the burn tunnel that extends about an inch-and-a-half into the heat riser. It is cut at a 45 degree angle so the rotating gasses of the riser impact the incoming flame at 45 degrees instead of 90.

The change in performance is dramatic. At least 50% less soot is deposited while the stove is warming up (only a slight coloration at the top of the riser); This soot is burned off the first 10 inches of the riser almost immediately; Exhaust gasses are smokeless in something like 1/2 the time (2 or 3 minutes); The vortex is at least twice as strong at the top of the riser (an undeniable cyclone of fly ash); Heat output is nearly doubled (I can no longer hold my hand for a moment within a foot of the top of the riser), and the draft seems to be 50% stronger. These are not measured results-- merely my impressions-- but I'd consider it a big success.

It seems, however, that it may still take an hour of firing to completely burn off all the soot.

Thank you all for the input. Do these results seem more in line with your stoves? Perhaps double neutral vanes could be used to improve your double vortex.

-Phil

...Oh, and, in this mock-up made of only refractory fire brick and no additional insulation (and some small air leaks) the soot in the riser began to burn off after maybe 30 minutes of firing, and was completely burnt off an hour after that.

I'll be happy to take some photos on my next weekend. I am interested by this double vortex... do you think it provides better mixing?

My idea with the single vortex is to establish a counter-clockwise rotation of gasses that continues into the barrel, with assistance from the manifold, which will suck gasses out one side (like the burn chamber shoots them in one side). I hope this will achieve three things; 1) higher velocity gasses in the barrel which lower pressure and increase draw strength, 2) gasses spending more time in the barrel (cooling) and in the riser (heating) increasing the thermosiphon effect, and 3) mono-directional flow through the manifold, reducing drag, or bottlenecking. I get the impression that the traditional manifold, even when it's made large enough, will force gasses from all directions to collide, reducing velocity.

The flame in the bottom of my burn chamber makes one complete rotation, directly impacting the flame coming out of the burn chamber.

Thanks for your input
-Phil

I have heard it asserted by someone of rank in the RMH community (I cannot remember who) that in the burn chamber, the hottest gasses will rise to the top. This made a lot of sense, and was concerning to me because I am (in my first build) planning to use a tall (9 inch) and narrow (5 inch) burn chamber. I envisioned great difficulty in getting a proper mixing of gasses with this tendency for the hot ones to float way up to the top.

The purpose for my narrow burn chamber is that it allows my 8 inch system to shoot gasses into the right side of its 10 inch octagonal heat riser, producing a vortex without any complicated steel contraptions like I have seen others using. I have just completed a successful weekend of outdoor testing, and discovered that the mix is excellent without any tinkering.

As the stove was getting up to temperature, a film of black soot developed on the interior of the insulative firebrick riser. By looking down the riser, I could see a uniform stream of flame coming out of the whole height of the burn chamber. As the stove continued to heat, the soot began to burn off, starting at the bottom of the feed tube, and gradually progressing to the top of the riser. When this clean portion emerged from the burn chamber into the riser, it did so first at the lower-most point-- meaning that the bottom, not the top, of the heat riser is the hottest!

In hindsight, this makes a lot of sense. Embers from the feed tube fall out into the floor of the burn chamber, heating it and streaming off hot gasses along the bottom. Also, air intake is sucked in primarily along the path of least resistance at the top. This all means that the insulation below, not above the burn chamber is most important, and that mixing is inherent in the design, as cool intake air tries to swap places with hot exhaust on the bottom.

-Phil

I am preparing to build a RMH in my home, which is constructed with 2' thick stone walls and 4-6" thick mud plaster. Using interior walls themselves as part of the thermal mass is my goal, but these walls contain electrical wires embedded in the mud plaster. I understand that common household wire is rated for 194 degrees F; a temperature I am sure the stove's thermal mass can exceed. What do you folks consider to be the minimum safe distance between (un-additionally-insulated) electrical wiring and exhaust ductwork, assuming both are encased in the thermal mass? What (additional) wire insulation do you recommend when this minimum distance cannot be achieved? The area that concerns me most is the final vertical run of duct going to the roof, where electrical wire must pass within 6". This topic does not seem to be covered online, so I appreciate you all weighing in.