Small CSAs (less-than-6-inch systems)

There's been plenty of talk about systems with small cross-sectional-areas: most specifically, using ducting with diameters that are less than 6 inches-- specifically many questions about 4-inch-systems. Let's start with a quote:

Erica Wisner wrote:

paul wheaton wrote:
4 inch pipe: My impression is that all attempts have ended up with poor results. I have a theory ..... I suspect that a four inch system could work fine provided that the pipe is no longer than 12 feet with no more than two bends.

Smaller systems suffer from laminar flow constriction - in other words, the gas goes too slow because the pipe is too small, and it gets stuck along the sides. Especially at the bends. Shorter pipe, fewer bends - good place to start.

Also, it's hard to build and maintain a decent fire in a space with a 12.5" cross-sectional area - that's about 3 1/2" by 4". Can't hardly fit a stick in there. So fuel mix might be different: more bark, more twigs, quicker-burning kindling.

And the friction / bumps on the sides get more significant as the wood gets smaller and lighter, so it won't self-feed downward. More fiddling.

And... and...
To solve all the problems that crop up at 4", you're basically inventing a different animal.
If anyone makes a nice miniature Rocket Mass Heater at 4", please let us know.

The idea of small CSAs is intriguing because of, for example, portability and/or efficiency for heating small spaces. Erica has hinted at "all the problems that crop up," but really the most pressing problem seems to be the 'laminar-flow-constriction'. I have some technical questions about this:

If the smaller system is PROPORTIONAL to a larger one, it still won't work?

Is the laminar-flow-constriction occuring because there are tiny irregularities (divets and dimples) on the inner surfaces of the system, and when the CSA is smaller the ratio of the size of these irregularities to the CSP (cross-sectional-perimeter) is too large? (And therefore the system only appears proportional, but really has this tiny little disproportionality which causes a whole lot of turbulence?)

Does the size of air-molecules put a limit on how far 'proportionality' can really go?

Okay, I think those are my questions.

Does anyone else have questions about small-CSA-systems?

The problem is surface tension and friction are constant so they won't scale with the rest of the system.

R Scott wrote:The problem is surface tension and friction are constant so they won't scale with the rest of the system.

Yeah! Okay, that is a much more efficient and elegant way of articulating exactly what I was wondering, I think. Thank you.

Although, on the other hand, I'm still interested in hearing the details of those aerodynamics or thermodynamics or whatever dynamics are involved in the crucial details. That is, if anyone has the gumption, expertise and desire to explain or rant about such details. Anyone?

-Adam

ps. I just got Lanto and Leslie's RMH book. Looking forward to a thorough perusal of that. Woo!

I'm building a tiny cabin on a trailer frame. It's going to have about 120 square ft area inside, with the loft in the middle of the space. Given the size limitations and ESPECIALLY the severe weight limits, I'm not going to be able to build a traditional MRH. I have a nice 26 gallon air tank for the riser, and lots of 4" pipe, and some ideas about running that pipe horizontally for about 5 ft (might have to use 6" instead) buried in something lightweight and non-flammable, then running up about 8 ft and out a chimney. The horizontal section is over the strongest part of the trailer, so I want to use the best lightweight heat-retaining material possible. I realize that's a contradiction in terms to quite an extent, but there must be something I can use!

Feedback, ideas, help, etc is very much appreciated.

Adam Stjohn wrote:To solve all the problems that crop up at 4", you're basically inventing a different animal.
If anyone makes a nice miniature Rocket Mass Heater at 4", please let us know.

I've made such a RMH last year, wiithout the mass, admittently. The goal happend to be a near perfect combustion, so I've used two small barrels on top of each other to extract enough heat in order to make measurements possible. The thing could do better, efficiency-wise, with three of those drums as an extractor-tower. Or another two oil drums at the side, coupled with the first set at floor level as a crude double bell system.

This tiny stove do need an adequate chimney, and/or a longer heat riser. In order to cut back warm-up times it's very important to keep it as light as possible. The amount of mass inside the core of the stove has to be scaled down as well, so large and heavy fire bricks are a dead end. The same goes for mild steel because of the reducing fire which will eat the steel for breakfast.

For a full report including diagrams and numbers see this web page.

A local fireplace and spa dealer sells extremely lightweight and non-flammable sheets of material (I don't recall what it's called atm) for pretty cheap. It's normally used inside fireplaces and woodstoves, so does indeed handle very high temperatures. It is very flexible. This is the material I'm planning on using throughout my system whenever heat shielding is needed, like inside the riser. Next time I drive by the store I'll stop in again and ask what it's called. Any fireplace or wood stove dealer should be able to order it if it's not in fact already available in the store.

Parker Free wrote: I have a nice 26 gallon air tank for the riser,

Some 25 gallon drums are perfect for 6" systems, but the volume (in gallons) of a tank or drum doesn't guarantee any specific dimensions. What are the dimensions of your tank? (Diameter, height, etc ?)

Parker Free wrote: It's going to have about 120 square ft area inside, with the loft in the middle of the space. [...] I have [...] lots of 4" pipe, and some ideas about running that pipe horizontally for about 5 ft (might have to use 6" instead) buried in something lightweight and non-flammable, then running up about 8 ft and out a chimney.

This may be obvious, but warning: that vertical portion is still going to be very hot. 5 ft of mass-covered, horizontal pipe --even if its 6"-- isn't going to cool the exhaust a whole lot. I'm not sure how much, but the later, vertical pipe will still be very hot. So anyway, I mention this because of the tight size of your space: take appropriate steps to guard combustible materials from the heat. Anyway, surely you know about that already..

Parker Free wrote: The horizontal section is over the strongest part of the trailer, so I want to use the best lightweight heat-retaining material possible. I realize that's a contradiction in terms to quite an extent, but there must be something I can use!

I don't understand everything here: do you mean to say that regardless of how "The horizontal section is over the strongest part of the trailer" you nontheless "want to use the best lightweight heat-retaining material possible" ?

In any case, I understand what you mean by a contradiction in terms (lightweight vs heat-retaining). I'm not sure what to tell you, though. Anybody else have info on these matters?

Secondly,

Just to be clear: This quote:

Peter van den Berg wrote:

Adam Stjohn wrote:To solve all the problems that crop up at 4", you're basically inventing a different animal.
If anyone makes a nice miniature Rocket Mass Heater at 4", please let us know.

Should have read like this:

Peter van den Berg wrote:

Adam Stjohn wrote:

Erica Wisner wrote:To solve all the problems that crop up at 4", you're basically inventing a different animal. If anyone makes a nice miniature Rocket Mass Heater at 4", please let us know.

And Peter I'd like to learn more about this dynamic around the height of the heat riser, etc:

Peter van den Berg wrote:
[...] I've used two small barrels on top of each other to extract enough heat in order to make measurements possible. The thing could do better, efficiency-wise, with three of those drums as an extractor-tower. [...] This tiny stove do need an adequate chimney, and/or a longer heat riser.

Right now I have to run, but will be back with more comments soon enough. Thanks for the link, I look forward to delving into that more soon.

As soon as I'm finished building the last trusses for my roof and putting together a smoker, I'll see if I can get some measurements. They will be approximate but should help. In general, the tank is tall and slender rather than short and round.

The horizontal run is over the wheel base/axles, so is sturdy, but the axles overall can't tolerate too high a load, especially with all the added weight of the house itself. I am trying to figure out the best compromise of heat-retention vs light weight material for this section, as it's the only place where I can possibly put any mass.

I appreciate the comment on the high heat of the upright, "chimney" portion, as I strongly suspected it would be hot, and it's good to have that verified for safety's sake. Once again I'll be using non-flammable materials around this part of the pipe, and also plan to encircle it with something like a decorative steel plating that you can buy at the local box stores or at any welding shop. I think it will end up looking "steam-punk" ish, which is cool (no pun intended). This should keep unwary body parts and whatnot from contacting the pipe itself, while still allowing heat to flow out into the room.

Ah heck, now I'm really curious about the tank - off to measure now.

EDITED TO ADD: Height of tank is 30", circumference is 51". It is marked on the side as being as 26 gallon capacity - It's a broken air compressor machine's tank.

I'll highlight this again:

Parker Free wrote:
The horizontal run is over the wheel base/axles, so is sturdy, but the axles overall can't tolerate too high a load, especially with all the added weight of the house itself. I am trying to figure out the best compromise of heat-retention vs light weight material for this section, as it's the only place where I can possibly put any mass.

Because I'm really curious if anyone has any input on this question of heat-retention vs light-weight.

Secondly,

Parker Free wrote:Ah heck, now I'm really curious about the tank - off to measure now.

EDITED TO ADD: Height of tank is 30", circumference is 51". It is marked on the side as being as 26 gallon capacity - It's a broken air compressor machine's tank.

I should have been more specific earlier, anyway, so 30"H by 51"C-- are those the INTERIOR or EXTERIOR height and circumference? The interior is what we want to know about... And secondly, 51" circumference divided by Pi (3.14...) tells us that the diameter would be 16.23" -- does that seem about right? But yeah, more importantly, before we go any further, what are the dimensions of the interior?

Secondly,

Parker Free wrote:Ah heck, now I'm really curious about the tank - off to measure now.

EDITED TO ADD: Height of tank is 30", circumference is 51". It is marked on the side as being as 26 gallon capacity - It's a broken air compressor machine's tank.

I should have been more specific earlier, anyway, so 30"H by 51"C-- are those the INTERIOR or EXTERIOR height and circumference? The interior is what we want to know about... And secondly, 51" circumference divided by Pi (3.14...) tells us that the diameter would be 16.23" -- does that seem about right? But yeah, more importantly, before we go any further, what are the dimensions of the interior?





It is exterior measurements. I still have to take off the compressor, and figure out where to cut the tank. I have the tools to do so but have never cut metal (other than in building the house, cutting through nails - oops). Is it impossible to determine if this tank will work until I get interior dimensions?

I appreciate your input,

parker

The tank walls aren't that thick AS LONG AS there aren't fittings welded in the stick down on the inside. Those will have to be removed and welded shut. So that leaves you with 15 and 3/4 or a little more ID. Not a really thick riser, but probably OK if you use the right materials.

I was under the (perhaps mistaken) impression that a taller, thinner riser would work well for the mass-less rocket heaters. I am planning on using that heat-shielding fabric inside the tank to direct the air flow - it's thin but effective so should leave a good air space for the heated air to circle around and go out the exhaust. What do you think?

When you mention this "heat-shielding fabric":

Parker Free wrote: I am planning on using that heat-shielding fabric inside the tank to direct the air flow - it's thin but effective so should leave a good air space for the heated air to circle around and go out the exhaust. What do you think?

Is that the same stuff you were writing about earlier when you wrote,

Parker Free wrote:A local fireplace and spa dealer sells extremely lightweight and non-flammable sheets of material

?

Second of all,

Parker Free wrote:I was under the (perhaps mistaken) impression that a taller, thinner riser would work well for the mass-less rocket heaters.

I'm under the impression that ... oh wait.. I was thinking of something different from what you were just writing... Well, what I was going to say: I'm under the impression that a taller riser will help produce more draft-- stronger air flow-- which is what we need in a system with a small CSA -- as Peter mentioned earlier,

Peter Berg wrote: [...] I've used two small barrels on top of each other to extract enough heat in order to make measurements possible. The thing could do better, efficiency-wise, with three of those drums as an extractor-tower. [...] This tiny stove do need an adequate chimney, and/or a longer heat riser.

Also, Parker, not only will your RMH have a tall heat-riser, but also a hot vertical chimney, so it should be plenty drafty-- meaning that the small CSA (4" pipe) may be no problem at all.

Although, you questioned earlier "that a taller, thinner riser would work well for the mass-less rocket heaters" ... So now we're talking about how the vertical portions (heat-riser and chimney) may come into play with the 'masslessness' (rather than the small-CSA-ness). THAT I am less sure of. One one hand, the question of vertical portions in relation to small-CSA's has to do with draf and convection (in the vertical portions) and air-friction (in the exhaust ducts) ... On the other hand, the question of vertical portions in relation to 'masslessness' may have to do with draft and convection (in the vertical portions), radiation, conduction, and convection (around the massless exhaust ducts)... etc ?

Uhhhhhhhhhhh

Oh yeah, another idea about what you wrote,

Parker Free wrote: I am planning on using that heat-shielding fabric inside the tank to direct the air flow - it's thin but effective so should leave a good air space for the heated air to circle around and go out the exhaust. What do you think?

Firstly, I don't fully understand what you are intending to do with the fabric? How/where are you placing it?

Secondly,

Considering the high-temperatures inside the tank, are you sure that the heat-shielding fabric isn't going to be consumed in, as Peter Berg mentioned earlier,

Peter Berg wrote: [,..] the reducing fire which will eat the steel for breakfast.

?

Somewhere around these forums I've read that sometimes steel ducting (used for the exhaust ducts and used around the inside and outside of the heat-riser) will 'burn-off' after long-term use of the RMH; although, this is sometimes negligible because it the ducting is moreorless serving as a mold for the cob, clay, perlite, etc, so that even if the steel is 'eaten for breakfast,' the then well-formed cob, clay, perlite, etc will continue to stand. Eh?

Anybody else want to confirm that idea?

Ok. Took me a while to reply because I wanted to stop at the fireplace store and ask some questions, and then did some more research. The "fabric" I mentioned earlier is "thermal ceramic blanket insulation", not really a fabric at all. The local store here sells only the basic Kaowool, which has a heat tolerance of about 1200 degrees Celcius. It's pretty cheap at about $10 a lineal ft for the 1" by 24" stuff. The heat resistance safety factor seemed a little low to me, so I googled Kaowool and found out there are other versions (seen here: Ithttp://www.anvilfire.com/sales/pages/kaowool_index.htm). Now, doesn't the combustion chamber only get to about 1000 degrees C? If that's true, then there are several choices of this stuff that should hold up...please correct me if I'm wrong!

I've been pondering for months how to make a RMH work in my tiny home. After reading tons of stuff on what other people did, I might have come up with something workable yesterday regarding mass.....

So I'm thinking that I can use the 26gal air tank, create a combustion chamber inside using a stainless steel tube wrapped by a version of the ceramic blanket insulation. I don't know what the inner dimensions should be yet.

I move the stove location to where I can run the horizontal pipe close by the water tank (which is to lay horizontally across one wall over the axles), and use the water for my mass. Obviously there will have to be a lot a insulation between the pipe and the tank to keep the tank material from melting, but in theory, I should be able to achieve a safe (for the tank) transfer of heat to the water, where it will be stored to give me both warm water and warm air. Shouldn't that also draw off some of the heat from the exhaust before the pipe turns vertical? I'll still have to put up a perforated metal safety shield around this portion, but won't I harvest more long-term heat from the stove this way?

I'm also thinking that a slightly "massy" way to get even more heat is to pack smallish river rocks into the perforated shield around the vertical pipe. I'm afraid that negates the safety factor, though, as the rocks would get mighty hot. Or, form cob around the vertical pipe? Hold it there with the perforated metal? Gah.

All in all, I am hoping to gain SOME long-term heat storage from each burn. I already am planning on the water tank weight, so why not try to use that mass for it?

Oh, and naughty naughty, Adam! LOL.

just a thought, but if your water tank is higher than your bench, you could build in a thermosyphon style heat exchanger.   a simple check valve could stop the back syphoning.