lurker with a few questions

hello, great info here. thanks everyone! but: I have searched this and several other forums, watched every youtube i can find in RMH, and still have a few questions.

first, I see every heat vent/heat exchanger tube assembly advised to to be round to facilitate laminar air flow. In this publication http://bioenergylists.org/stovesdoc/apro/Heat/Heating%20Stoves%20LO-RES.pdf put out by Aprovecho they say a wide flat rectangle tube works better, as it forces the hot air to "scrub" against the surfaces thus transfering more heat. has this idea been modified since this publication came out? (I could not see a date on it anywhere)

As to heat transfer, would it make sense to add a bunch metal fins to the ouside of the heat tubes that radiate out into the mass matrix? this is common on radiators and such, and seems like it would be easy to do and add to the heat transfer into the mass

I am looking to build a RMH in a 200 sqft tiny house. since 6" pipe construction seems to be the smallest that anyone recommends, would this just blow me out of the house when it is burning? It seems to be a very big heater for a tiny house. Can anyone comment on how small of a space is reasonable with a RMH? (maybe I should just light a candle!

Also, since floor space is at a premium for the heated bench, how short of an exhaust run can be built and accomplish adequate heat transfer so the exhaust flue is as cool as it can be and still function? (I know if the exhaust gases are too hot, it means I am wasting fuel...) I see limits to long length mentioned, but I have not seen comments as to how short is too short...)

(If the answers to these Qs are in "the book", please know I am ordering it, but since these questions dont seem to be raised anywhere I thought I would toss them out here

thanks,
kyle

Utah Kyle :there are two good reasons for flat duct work, for the same square mm ,sq. cm ,sq. inch, or sq. ft. there is a larger surface area to radiate heat the skinny rectangular duct also will have a constant temp. as compaired to the very center of the round duct esp. as the 'air flow' speeds up, trade this off against the ease of working with round ductwork , its greater resistance to crushing ! and - - -I'd go with the round.

Probably the very best thing to pack around your ductwork is a finely devided iron ore body, or any dense stone. Cob should allow you to fill in between pieces of rock or stone, eliminating air spaces is key !

Build as small a bench as you want, you will always have the chance to change it with ether an extension, or by building a second Heat Riser/ barrel heat exchanger* ! before the chimney leaves your heated area .

If slightly over built the best way to deal with it is to tend your fire as needed, say 2-3 hrs in 24 to charge your heat exchanger/thermal mass to the weather conditions and slightly larger fires 3-4 hrs for much colder days

Hope this helps, ask any questions you want, for every one asked their are ten who want to know too ! pyro Al.


* this is usually called a 2nd bell and is not in the book but thats another story ! A.L.

hello, thanks, that all makes good sense. Is there a formula for predicting exhaust temperatures at a particular length of vent tube? I see many of the stoves vented out horizontaly (or mostly so)and the gas looks pretty cool (temperature wise ) and others go up higher up the wall or out a roof like a traditional chimney stack. I know that as the gases are cooled they will no longer rise out a vertical stack, so it seems like I need to predict that reaction before I build it in order to position a vent in the right place.

and did you have any ideas about adding metal radiator fins to the outside of vent tubes to enhance the heat transfer to the mass matrix?

thanks
kyle

Utah Kyle : Have you gone to www.rocketstoves.com and downloaded your copy $15,oo u.s., of Ianto Evans' Great Book "Rocket Mass Heaters" ? Nowhere else is there more- specific information on Rocket Stoves !

Fins work great if you are trying to radiate thru air, here we want total and immediate contact 360 degrees with the most dense material we can place against the pipe. Fins being metal also qualify !
There is a low(er) energy process called sintering which i understand to be that fine pelleted metals are placed under 'some pressure' to make close contact with each other and then fired, while this process does not hold up to machining it can replace simple machining.
The process also allows for the pore space between sintered pellets to be predetermined with some degree of accuracy.

Now imagine a dense material that could be pelleted and sintered with very small pore spaces, and then ground down to the shape of your ducting making a tight fit. This would be an ideal and there may even be a slag somewhere that as a waste material needs recycling up-cycling.
Even as I type this I can hear Greenies screaming 'SLAG' ='HEAVY METALS' OH MY GOD NOooooo!

This was supposed to be a mental exercise to allow everyone 1 idea of mating the internal horizontal ducting tightly down to the granular level with a high-density material, not my personal idea of a perfect goal !
Besides the trucking costs would mean you coundn't use this material to build on the cheap

Because the thermal mass is D.I.Y. out of local materials, with localized areas of human derived imperfections from an ideal - No, there is no formula, except in the mind of someone who has build several Rocket Stoves in the Same area, out of the Same Materials !

If we compare the rocket stove mass heater to any thing, it can only be a Nat. Gas or propane 'Monitor' type 'Space Heater'. These burn with high efficiency, pass the exhaust gases thru a heat exchanger that warms the pre-combustion gases, and the exhaust gases are cooled to the point that they are 'by code' allowed to be vented to the out side horizontally thru plastic pipe !
If we examine the exterior wall of the house where one of these 'space heaters' have been vented, we will find various deposits of carbon soot around the usually metal 'rain hoods'

Both these 'Space Heaters' and Rocket Stove "Space heaters" venting horizontally suffer from exposure to wind gusts and drafts.
The gas heater installation instructions recommend Installing on the leeward side of the house, which makes sense as the house does not move . Unfortunately the wind does,And changes direction !

This is why a vertical chimney is a recommendation, and often installed retro-fit on both of these types of 'space heaters' !

It is a unique feature of the Internal Chimney that we call the Heat Riser at the heart of our rocket stove that it provides the 'push to 'drive' exhaust gases out thru vertical chimneys right down very close to the exterior temperatures !

I really tired to be brief here and cover the basics, if I didn't cover it, send a question and someone will answer ! Pyro-maticly Yours , _ Allen L.