• Post Reply Bookmark Topic Watch Topic
  • New Topic
permaculture forums growies critters building homesteading energy monies kitchen purity ungarbage community wilderness fiber arts art permaculture artisans regional education skip experiences global resources cider press projects digital market permies.com pie forums private forums all forums
this forum made possible by our volunteer staff, including ...
master stewards:
  • Anne Miller
  • jordan barton
  • Pearl Sutton
  • paul wheaton
  • Leigh Tate
stewards:
  • Nicole Alderman
  • Steve Thorn
  • r ranson
master gardeners:
  • John F Dean
  • Carla Burke
  • Nancy Reading
gardeners:
  • Jay Angler
  • Mike Barkley
  • Liv Smith

6" System Heat Riser - can I shorten the length without affecting performance?

 
pollinator
Posts: 132
9
  • Likes 1
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
I'm working on the core of my outdoor Batch Box RMH/heated bench project - it's a 6" (150mm) system.

On the Batchrocket resources site it lists the heat riser length for a 6" system as 43 3/16" (1080mm). I think this may be too high for my project if I allow enough clearance for the exhaust from the top of the riser to the ceiling of the bell. I'm proposing to build a '5 minute riser' using a 25mm  'Superwool' blanket inside an 8" ducting pipe.

I have two questions :-

1/ What is the minimum clearance between the top of the riser and the ceiling of the bell?

2/ Can I shorten the length of the riser and if so, by how much?

Many thanks as always for any help or advice.
 
Rocket Scientist
Posts: 4537
Location: latitude 47 N.W. montana zone 6A
1594
cat pig rocket stoves
  • Likes 2
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Hi John;
Well about 12 " is what is recomended as a safe clearance.
You might be able to reduce that to 8". However the top of the bell over the riser will need to be able to deal with  1800+ temps.
One way to deal with that would be to use ceramic board or blanket as a roof insulator.
Once that heat leaves the riser it is cooling rapidly.
With the roof insulated you can bring the riser within 6" or so if need be.

Yes , you could shorten your riser but draw will be affected.
How much can you shorten it?   Well Matt Walker builds and uses a "riser-less" core and he gets clean burning &  super hot temps from his builds.
 
John Harrison
pollinator
Posts: 132
9
  • Likes 1
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Many thanks for your reply Thomas.

I was going to roof the top of the bell with a 3" granite slab with a one inch (25mm) layer of Superwool between the riser and the granite.
The  'chimney' for the whole system is about  11 feet (3.5 metres) high of which over 7 feet of it is insulated. I'm hoping that this will provide a good draw so I could possibly compromise on the riser/bell clearance and the heat riser length without too much affect on performance.

Thanks again for your help.
 
gardener
Posts: 761
Location: +52° 1' 47.40", +4° 22' 57.80"
141
woodworking rocket stoves wood heat
  • Likes 1
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Minimum figures for the plain vanilla batch box rocket: riser length 8B or 8 times base number, top gap (between riser and top of bell) equal to riser diameter. So for that last point I second Thomas.
In your particular case: riser length 34.7" (86.8 cm), top gap 6" (15 cm) but keep an eye on that granite slab.
 
John Harrison
pollinator
Posts: 132
9
  • Likes 1
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Many thanks Peter for your detailed reply and figures for my application.

I'm factoring in easy access to the top of the bell so that I can monitor how the 'Superwool protected' granite slab performs and also to keep a check on the Superwool liner of the 5 minute riser. I'm planning to rely on a friction fit of the Superwool in the riser - would that work or do I need to fix it in place with some sort of adhesive or hardware fixing?

Thanks again.
 
thomas rubino
Rocket Scientist
Posts: 4537
Location: latitude 47 N.W. montana zone 6A
1594
cat pig rocket stoves
  • Likes 1
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Friction fit is all you should need on the riser.
 
John Harrison
pollinator
Posts: 132
9
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Great Thanks again Thomas for your swift reply.
 
Posts: 52
2
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
I may be wrong as I am far from experienced but I am sure that if you are following PVB rocket stove dimensions, it is not recommended to shorten the riser?
Also I think  granite  is prone to cracking and I would think you might need more than 25mm of fibre ?
 
gardener
Posts: 3609
Location: Upstate NY, zone 5
231
4
  • Likes 2
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Peter and others have experimented with shorter risers and found that a certain amount of reduction will still function fairly well as long as the rest of the system is optimal. Peter just gave his expert opinion on the minimum riser height.

I would be concerned about the granite slab - the more massive it is, the more risk I would expect of differential expansion cracking. I especially would not allow more cool edge extension than absolutely necessary.
 
John Harrison
pollinator
Posts: 132
9
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Thanks Glenn.

As regards the concerns with the granite slab, I have an option to use a larger, thinner slab as the bell ceiling. I could protect this with 25mm Superwool and cut out a section that would line up with top of the heat riser. I could then use, say a 10 inch square, thin or insulating firebrick to withstand the intense heat from the riser. Would that work better than the granite slab protected by a layer of Superwool?
 
Beauty is in the eye of the tiny ad.
Sepp Holzer's 3-in-1 Permaculture documentaries (Farming, Terraces, and Aquaculture) streaming video
https://permies.com/wiki/141614/videos/Sepp-Holzer-Permaculture-documentaries-Farming
reply
    Bookmark Topic Watch Topic
  • New Topic