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RMH - How narrow is too narrow?  RSS feed

 
Posts: 13
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Greetings Permies Folks

I seek your counsel.

I am going to make a rocket stove for my home.  I am cold, skint and getting depressed and I am damn determind to do something about it!  The idea is to get some instant heat now with the option of adding the mass element later on.  I need to use some materials I have to hand in order to save money but this throws up a design problem.

I have 2 gas bottles that I want to weld together to make the 'barrel' element.  After some research and running out in the rain with a tape measure I have established that the ID of the gas bottles is only about 12.4 inches.

From research on this forum it seems that I should cast a heat riser with a minimum 6 inch ID and a 2 inch wall thickness; resulting in a heat riser with a 10 inch OD.  The trouble is that this results in a gap between the outside of the heat riser and the inside of the 'barrel' of only 1.2 inches all around.  Whereas I understand this gap should be at least 2 inches.

Further research suggests this may be able to be worked around by making the initial exhaust / plenum area bigger in size.  

Questions:

1.  Would making the exhaust / plenum area bigger actually work?............
2.  Assuming it would work, what's a good approach?
3.  Alternatively if I abandoned the idea of adding mass later on could I make a smaller diameter heat riser (plus rest of system) and still have a rocket stove that would function properly?

Thanks in advance.

Ez
 
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Location: latitude 47 N.W. montana zone 6A
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Hi Errol;  Welcome to Permies!

I see that you are in England.  I would guess that used brick might be easily available ?  If so I think you want some!

Stay inside from the rain ...  (I know its always raining) and think about not using your tanks and having to weld them.
Locate some "free" bricks and build a larger "bell"  with them. Your riser will fit inside easily and you can make your "bell" brick box as large as you have time and room for.  You build it with clay mortar so it can easily be reconfigured at a later date!  
This way you will get your  heat and you have a good start towards enlarging your bell system or putting in a bench with mass. For that day... when the sun does shine and you feel ready to enlarge your system.
 
errol hall
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Many thanks for your quick reply and help Thomas.  I am indeed from the land of the crazy (and wet) Englanders.

Just one question though.  Any system I build needs to have the ability to provide quick heating initially.  If I were to use brick instead of metal for the bell element would I not lose the ability to provide a quick charge of heat to the room off the hot metal?

 
thomas rubino
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Yes this true, although with a small bell it would not take very long.  A thicker metal plate could be utilized slightly away from the riser (would warp if directly above) it would heat sooner and be a nice place to sit your cup of tea on.
 
errol hall
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Tea!? Yuck! I'm all about the coffee my friend!  I may live with my English friends but I am not of them I must say.  

Anyway like your thinking and am defo keen on masonry bells for any future mass addition; but trying to source / transport metal plate and bricks would be very awkward at present; and I need to do something positive now.  

If I HAD to make a change I could possibly buy a steel oil drum and end my narrow gap woes - but money's real tight and I really need to save it for stove pipe.

Is there ANY way I could make the original proposition work, even if it was abandoning the idea of future mass and making a smaller bored (say 4 inch) stand-alone rocket stove?  (After all I am gonna need another one for the workshop eventually).

I don't mind the welding part btw.
 
thomas rubino
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Ahh my mistake then... coffee it is.
Well , sure you could do a small bore metal temporary unit.  
Metal does work in the short term .  Just don't burn it too long at a time. Move it to the workshop next summer.  
 
pollinator
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Location: Penticton, Canada
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Hi Errol,    Another option: If you can get your hands on an old/dead electric hot water heater, they have a metal core in them that can serve as a 'barrel'. Here in Canada, they are
18" - 20" in diameter (depending on the size of hot water tank). This is what I have been using in my shop for quite a while and works quite well. If you have a cut-off wheel on your grinder, your good to go. All it requires to purchase is black pipe caps to plug up the bung holes.
 
errol hall
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Aha!  Sounds cool but when you say:
 
'Metal does work in the short term...[Long enough to get to summer, June in the UK?]
 
Just don't burn it too long at a time...[Roughly how long is too long]?  

Move it to the workshop next summer... [Hooray! Progress...and Heat in that magnificent place.  Huzzah!]  
 
errol hall
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Gerry Parent wrote:Hi Errol,    Another option: If you can get your hands on an old/dead electric hot water heater, they have a metal core in them that can serve as a 'barrel'. Here in Canada, they are
18" - 20" in diameter (depending on the size of hot water tank). This is what I have been using in my shop for quite a while and works quite well. If you have a cut-off wheel on your grinder, your good to go. All it requires to purchase is black pipe caps to plug up the bung holes.



Yea,  good idea.  Unfortunately all the scrap metal get hoovered up by so many little outfits going round with vans picking it all up to make a living.  It's gettin'

kinda mad max up in here!
 
Gerry Parent
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errol hall wrote:Yea,  good idea.  Unfortunately all the scrap metal get hoovered up by so many little outfits going round with vans picking it all up to make a living.  It's gettin' kinda mad max up in here!


Those tanks mounted on the center truck/car.. thing would be perfect!  Now, just gotta get past those freaks and your downtown!
MadMaxFuryRoad-247-ft.jpg
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errol hall
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Gerry Parent wrote:

errol hall wrote:Yea,  good idea.  Unfortunately all the scrap metal get hoovered up by so many little outfits going round with vans picking it all up to make a living.  It's gettin' kinda mad max up in here!


Those tanks mounted on the center truck/car.. thing would be perfect!  Now, just gotta get past those freaks and your downtown!

 

'You distract 'em, I'll get the tanks'.
 
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2" of perlite-clay for riser insulation is standard, but in your situation I think it would work well enough to reduce it to 1" or 1 1/4", giving you the outer gap you need for downflow.

I would advise against trying a system smaller than 6" for your first build; that is the smallest size that tends to be reliable, and you want it to just work without adjustment and tweaking.

Can you get your hands on any sort of clay, and stones or brick pieces? If so, I would use that as the base for the manifold and set your "barrel" into it. This would let you make the transition space plenty large. How tall are the tanks you have, excluding domed top & bottom? You will want at least three feet from base of burn tunnel for riser clearance. I would make the duct transition almost as big in diameter as the (little) tanks for safety in flow.

And keep your eyes open for materials you can grab (legally ) as you enjoy the warmth from this first setup, to make up a bigger, better one for next winter.
 
errol hall
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Glenn Herbert wrote:2" of perlite-clay for riser insulation is standard, but in your situation I think it would work well enough to reduce it to 1" or 1 1/4", giving you the outer gap you need for downflow.

I would advise against trying a system smaller than 6" for your first build; that is the smallest size that tends to be reliable, and you want it to just work without adjustment and tweaking.

Can you get your hands on any sort of clay, and stones or brick pieces? If so, I would use that as the base for the manifold and set your "barrel" into it. This would let you make the transition space plenty large. How tall are the tanks you have, excluding domed top & bottom? You will want at least three feet from base of burn tunnel for riser clearance. I would make the duct transition almost as big in diameter as the (little) tanks for safety in flow.

And keep your eyes open for materials you can grab (legally ) as you enjoy the warmth from this first setup, to make up a bigger, better one for next winter.



Yea thanks that's great advice.  Ok 6 inch system with say One and one quarter inch wall thickness it is.  I only got about 27 inches total of straight sections in the gas bottles but I was thinking I could keep the on at the top of the riser for a little more height.

Do folks reckon I can fit a reliable 6 inch system in this space?  Cheers.
 
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Hi Glenn and Errol,

I had the thought to reduce the insulation thickness also before I got down to your post.  Running the math seems to indicate that reducing the insulation to 1.5" would be sufficient.  The CSA  of a 6" system is 28.3 si.  With 1.5" of insulation the I.D. of the outer gap is 9" and the O.D. is 12.4".  This gives a CSA of (12.4^2-9^2)*pi/4 = 57.1 si which is just over twice the CSA.  The gap is only 1.7", but if the surfaces are fairly smooth it should function just fine.  You might want to buy Erica and Ernies book just to read their design rules of thumb and their construction hints.
 
errol hall
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Ralph Kettell wrote:Hi Glenn and Errol,

I had the thought to reduce the insulation thickness also before I got down to your post.  Running the math seems to indicate that reducing the insulation to 1.5" would be sufficient.  The CSA  of a 6" system is 28.3 si.  With 1.5" of insulation the I.D. of the outer gap is 9" and the O.D. is 12.4".  This gives a CSA of (12.4^2-9^2)*pi/4 = 57.1 si which is just over twice the CSA.  The gap is only 1.7", but if the surfaces are fairly smooth it should function just fine.  You might want to buy Erica and Ernies book just to read their design rules of thumb and their construction hints.[/quote

Wow!  Thanks so much Ralph, truly impressed by your mathematicizing on this (yes I know that not a word - I just like making them up!).  Really appreciate your help on this inc the tip on the smooth sides.  I'll take a look at that book.

It's a sunny late autumn / winter's day in old Blighty right now; all pre-Crimbo.  A time for the first Robins of Christmas...and metal cutting.  

Wishing you all the peace and good will of the season.  

Hail Permies, We are well-met.

 
errol hall
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Gerry Parent wrote:

errol hall wrote:Yea,  good idea.  Unfortunately all the scrap metal get hoovered up by so many little outfits going round with vans picking it all up to make a living.  It's gettin' kinda mad max up in here!


Those tanks mounted on the center truck/car.. thing would be perfect!  Now, just gotta get past those freaks and your downtown!



Oh! and I should have added, 'LOL!'.
 
Glenn Herbert
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I agree, 1 1/2" perlite-clay with smooth surfaces and little slop or irregularity should work fine. I said 1 1/4" to be on the safe side, not knowing how precisely the unit would be built.

Using sheet metal (a piece of ductwork if the right one can be found) would be ideal for the outer form and skin of the riser.
 
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Cost is much higher than perlite in clay slip, but Morgan Superwool can be bought in 1" thickness, and using a 8" pipe with the thermal blanket on the inside would give you a 6"ID riser and a 2" gap between teh riser and the external bell. This would be the "5 minute riser" as used over at http://donkey32.proboards.com/ and a picture of it at https://www.facebook.com/photo.php?fbid=10154992096951974&l=3e4ec1844f
 
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errol hall
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Many thanks all for those links - I will check them out.

In the meantime I have been sketching up the lower, 'fire-box', structure to bring things together in a model - see attached image.

I intended to cast this fire-box element in clay/perlite/sand/small amount of portland cement - as I have seen done on you tube.  However given the complexity of the mould / form-work required and the fact that I have never used this material before I am wondering whether the structure would be easier to do in fire-brick instead.

Any thoughts on relative difficulty re casting or fire-brick construction would be appreciated.

Cheers.
House-Rocket-Heater_Front-End-v2_Complex-Castings-Detail.png
[Thumbnail for House-Rocket-Heater_Front-End-v2_Complex-Castings-Detail.png]
 
Glenn Herbert
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If you're not experienced in making molds for casting things, I think you would find it considerably easier to build with firebrick, and wrap insulation around it.

Trying to interpret your core dimensions, it looks like a 12" high feed tube, or maybe about 16" counting the thickness above it? Then 18" total floor length of burn tunnel, which is good for a 6" system, and about 34" total height of riser (from the burn tunnel floor), which is just about enough to fit with the other fundamental dimensions. If you are planning on a total feed depth of 16", the riser may not be tall enough; I would add a couple of courses of firebrick on edge to raise the barrel and the riser, in that case.
 
errol hall
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Glenn Herbert wrote:If you're not experienced in making molds for casting things, I think you would find it considerably easier to build with firebrick, and wrap insulation around it.

Trying to interpret your core dimensions, it looks like a 12" high feed tube, or maybe about 16" counting the thickness above it? Then 18" total floor length of burn tunnel, which is good for a 6" system, and about 34" total height of riser (from the burn tunnel floor), which is just about enough to fit with the other fundamental dimensions. If you are planning on a total feed depth of 16", the riser may not be tall enough; I would add a couple of courses of firebrick on edge to raise the barrel and the riser, in that case.



Cheers Glen.  The drawing is a little misleading.  The design dimension are actually Feed tube 12"; Burn Tube 18"; Heat Riser 36". Because I have limited bell height I need to accommodate more of the heat riser length in the lower half of the design.  This requires lowering the front of the feed tunnel to maintain the 12" height - please see new pic.  Do you think this approach would work?  Ther will be some kind of cover to seal the front clean-out hole.

Btw I have realised there is a mistake in the drawing dimensions.  I will correct and re-post.  

Must say I really value your sustained interest and input on this.  As we say in Ole' Blighty 'Your a diamond mate'.

House-Rocket-Heater_Front-End-v2_Front-View-1.png
[Thumbnail for House-Rocket-Heater_Front-End-v2_Front-View-1.png]
 
errol hall
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errol hall wrote:

Glenn Herbert wrote:If you're not experienced in making molds for casting things, I think you would find it considerably easier to build with firebrick, and wrap insulation around it.

Trying to interpret your core dimensions, it looks like a 12" high feed tube, or maybe about 16" counting the thickness above it? Then 18" total floor length of burn tunnel, which is good for a 6" system, and about 34" total height of riser (from the burn tunnel floor), which is just about enough to fit with the other fundamental dimensions. If you are planning on a total feed depth of 16", the riser may not be tall enough; I would add a couple of courses of firebrick on edge to raise the barrel and the riser, in that case.



Cheers Glen.  The drawing is a little misleading.  The design dimension are actually Feed tube 12"; Burn Tube 18"; Heat Riser 36". Because I have limited bell height I need to accommodate more of the heat riser length in the lower half of the design.  This requires lowering the front of the feed tunnel to maintain the 12" height - please see new pic.  Do you think this approach would work?  Ther will be some kind of cover to seal the front clean-out hole.

Btw I have realised there is a mistake in the drawing dimensions.  I will correct and re-post.  

Must say I really value your sustained interest and input on this.  As we say in Ole' Blighty 'Your a diamond mate'.



New, corrected drawing.  Any feedback gratefully received.

Ez.
House-Rocket-Heater_Front-End-v2.10_Corrected-Casting-Hgt.png
[Thumbnail for House-Rocket-Heater_Front-End-v2.10_Corrected-Casting-Hgt.png]
New Corrected Drawing
 
Posts: 22
Location: Western Montana
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My stove Roars great with barely 2 inch between the barrel and riser. my riser is made of dense heavy fire bricks along with the rest of my stove. Has a gap of 2 inch between top of riser and top of barrel. My burn tunnel is 4.5 inch wide 6.75 inch tall and i am running a small brick mass/bell after the barrel and 8 inch chimney. When burning pellets i get an awesome Blue/purple flame!
1220182142.jpg
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Screenshot_2018-12-20-21-45-10.png
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errol hall
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Roak Wolf wrote: My stove Roars great with barely 2 inch between the barrel and riser. my riser is made of dense heavy fire bricks along with the rest of my stove. Has a gap of 2 inch between top of riser and top of barrel. My burn tunnel is 4.5 inch wide 6.75 inch tall and i am running a small brick mass/bell after the barrel and 8 inch chimney. When burning pellets i get an awesome Blue/purple flame!



Sure would like to hear (and feel) that baby roar!  Flame colour is gorgeous - any particular type of wood pellet?  Lots of MASS!

:-)

Ez
 
Roak Wolf
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errol hall wrote:

Roak Wolf wrote: My stove Roars great with barely 2 inch between the barrel and riser. my riser is made of dense heavy fire bricks along with the rest of my stove. Has a gap of 2 inch between top of riser and top of barrel. My burn tunnel is 4.5 inch wide 6.75 inch tall and i am running a small brick mass/bell after the barrel and 8 inch chimney. When burning pellets i get an awesome Blue/purple flame!



Sure would like to hear (and feel) that baby roar!  Flame colour is gorgeous - any particular type of wood pellet?  Lots of MASS!

:-)

Ez



bag did not specify hard or soft wood, bought them at HomDepot currently running Fir Pellets atm. i do have more pics of the stove in the assembly process if you want.

 
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