Chimney volumes?

Ive just started learning about the beauty of the rocket stoves and have not built one YET..So please let me know if Im completly off track with any of these thoughts..

Has anyone experimented with multiple smaller chimney pipes to increase the hot surface area in contact with the thermal mass?  If I used a concave or conicle shaped baffel plate at the base of the riser with enough piping connected to equal the same volume of chimney space as a conventional chimey pipe, would it increase the efficiency of the system or would it just choke the fire?  What about attaching metal fins to the stove pipe to help radiate the heat faster? 

There is alot of talk about cob, are there any other materials that can be easlily used for the mass?

Any thoughts welcome.

Randall

one problem with smaller pipes is turbulent vs laminar flow smaller pipes flow gas less readily because of friction of the gas along the walls of the pipe also getting even flow among a tube bundle requires some pretty intense engineering
cob is the mass of choice due to being virtualy free, anything that will store and pass heat would work

Thanks for the response Brice.  it makes sense and the single pipe seems to work pretty well anyway..  Living in North Florida we have LOTS of sand and about 6 feet down I found a layer of clay.  I know theres hundreds of references on cob but could you narrow the search down to a site or person that you would recommend for a first timer?

Actually, cob is just the outer layer. You want higher mass materials for most of your bench. Rock, brick, dense mud (without straw), etc

brice Moss wrote:
one problem with smaller pipes is turbulent vs laminar flow smaller pipes flow gas less readily because of friction of the gas along the walls of the pipe also getting even flow among a tube bundle requires some pretty intense engineering
cob is the mass of choice due to being virtualy free, anything that will store and pass heat would work

this means that you would need more than equivalent CSA. but how much more? It would take more experimentation. Does it matter if the flow through mass has less back pressure than normal? for that matter does it matter if there is even flow among the pipes? So long as the rocket can push the exhaust out the other end, the mass should even itself out after a while heat wise.  These smaller pipes might take a lot more room though. A pipe 25% less diam is half the CSA... so if you dropped from 8 inch to 6 inch, it would now be 12 inches if you run two in parallel (for example). Then you are using more than the same CSA so your bench could be really wide or tall or both (and shorter in length).

There are some good points in there.  Id like to build two of these. the first one will be to heat the green house with a switchback chimney in a wall.  The smaller diamater pipes come into the bed.  The base would be a steel frame with a plywood deck.  I think if I then do a layer of heavy pearlite / cement mix, the base would be insulated and not loose too much heat out the bottom.  On top of the perlite would be the brick cob layer with the heat pipes running through it.  I was thinking of 5 2" I.D. conduits running from one end of the bed to exit out the wall at the side or foot of the bed.  This would be 10" of usable ducting and the whole bed frame base could be off the floor and the about 24" of heated mass the size of a queen sixe bed..

Would it be beneficial to put air flow fins in the base of the riser to create a vortex and more air mix for the burn at the top of the barrel?

HMMMM.... wrote:
I was thinking of 5 2" I.D. conduits running from one end of the bed to exit out the wall at the side or foot of the bed.  This would be 10" of usable ducting and the whole bed frame base could be off the floor and the about 24" of heated mass the size of a queen sixe bed..

You would need 10 2" pipes (3.14 sqin) just to get the same CSA as 1 6" pipe (28 sqin) But as already noted you would need even more to make up for the mess laminar flow causes. I would start with double right off on the thought that dropping the air speed by half might be enough so that the back pressure doesn't choke the rocket. You would compare the sound and visible draw at the feed with the output open. So 20 2" pipes or 10 3" pipes.

Would it be beneficial to put air flow fins in the base of the riser to create a vortex.

how would you do that? metal would probably deteriorate pretty quick, ceramic would cost the moon.... think space shuttle. Besides, it works well without. Could try kicking one of the bricks up on the bottom of the tunnel. If it was angled up on only one side it may start things spinning... try and tell us how it goes. The one problem I would see is that you would be trying to increase the gas speed next to the riser walls and laminar flow is trying to slow it down....maybe straight flow is better. try it and see is the general answer, thats what back yards are for

and more air mix for the burn at the top of the barrel?

I think I would leave that one alone, adding air at the bottom of the riser would make more sense. But what I hear over and over is that any leaks are bad. Again, try it and see.

Whoh back...  Im getting far to deep into the design with out any hands on yet.  I think Ill go with the existing design, but use a long 8" diamater sand bag for the "J" tunnel form and shop vac the sand out after the fire clay is formed around it.  Is there a formula for the feed bowl size in relation to the horizontal and rise from the base of the J to the top of the riser?  Or what are the relevant considerations. 

Thanks to all of you for your patience with a Newbie..

Rocket mass heat trroubles has the answers I was asking for..SO..  Ill be back when I get something put together. 



Randall

There is alot of talk about cob, are there any other materials that can be easlily used for the mass?

A volcanic stone like basalt, or perhaps (recylced?) ceramic blocks come to mind. I don't see any reason why salvaged brick or urbanite couldn't be used either.

Interesting to think about, however, the cross sectional area is not really a key element of the operation of the RMH.

The RMH has both flow patterns within, laminar and turbulent and this creates pressure differences within that cause some interesting things to happen.

I am not sure if a multi-pipe unit could create the same situations and fire off in the same manner and if it did it would be on such a smaller scale it would not increase the heat transfer.