Riser pipe theory

I live in a truck so due to space and weight thermal mass is out of the question. I have a stove design lined up which has multiple riser pipes of a smaller diameter in parallel. I'm assuming that the width of the pipe affects the speed of flow through the pipe due to resistance. Therefore smaller diameter pipes can be shorter and still allow complete combustion because they allow the required time for combustion. I then put multiple pipes in parallel to acheive the required draught. Yes, this would mean you wouldn't need a barrel in your house...

Has anyone done any experiments to find the relationship between pipe diameter and combustion? First stage of my build is set to do this experiment with observable pipes of different diameters but I was wondering if there's anybody else who's on it?

John Appleyard : Congratulations for thinking outside the box ! You need to find a non-flamible kind of piping that will survive the 1300ºƒ + Temperatures

that a Rocket Mass Heater RMH, burns at . Further, Due to Oxidation and ''High Temperature Hydrogen Attack'', also called ''Hydrogen Embrittlement'' ALL

metal is doomed within the combustion zone !

Just like a Tall Final Vertical Chimney draws better than a Short Chimney We need The Heat Riser to be 3 - 4 times the length of the Feed tube to provide the

strong draft we need without blowers. It is the increased length of the Heat Riser that provides the Exposure of Time, Temperature, and Turbulence needed

for the Freaky high combustion temperatures we need to get the efficiencies and clean burn We are looking for !


With no barrel and no thermal mass you will only be able to use avery small % of the Heat Energy your combustion creates, The Rest will just go up the

chimney!


The Nearest thing that exists and is close to what you are trying to create is the Pocket Rocket, You can certainly experiment with this outdoors or in another

safe place. This can be scaled down to the size of a Metal 5 gallon pail, or a lidded garbage can. Because of the amount of air drawn through the Pocket

Rocket to make provide a clean burn and also the lack of any thermal mass- as soon as the fire goes out you will be instantly cold again !

Hopefully this is both timely and Useful For the good of the Craft ! Big AL

Due to Oxidation and ''High Temperature Hydrogen Attack'', also called ''Hydrogen Embrittlement'' ALL

metal is doomed within the combustion zone !

So why is 6" pipe stated as an alternative in the 6" design? I will look into though, thanks.

Friction pressure loss http://www.engineeringtoolbox.com/duct-friction-pressure-loss-d_444.html also has a part to play in time in the pipes? very minor in larger pipes so it won't have been a factor in the standard designs. I found this after posting. As per my theory I think.

It will have a ~7' vertical flue creating draw. Draw gain No. 2 in the short riser model is the flue doesn't need a section going down, a major function of the barrel, getting the hot air down, to be used in the mass. No.3 It doesn't have a feed tube to service, the riser pipes sitting directly above a firebox.

I can't really compare a RMH to a stove for a truck, just nick the idea of the riser pipe in a closed stove. Yes I'm gna lose heat up the chimney, but hopefully a lot less energy as all the gases will be burned off, it should also be a lot cleaner for the environment. There is also the firebox which can be closed down overnight, so I get a box of hot embers for prolonged heat. I also need to say that in the UK a "truck" is generally a vehicle with max gross weight of 7.5T or more, I have a 14' x 7' insulated living space so I don't get instantly cold again, the difficulty is control so I don't get too hot!

"The main factors influencing HTHA are the hydrogen partial pressure, the temperature of the steel and the duration of the exposure. Damage usually occurs after an incubation period, which can vary from a few hours to many years depending on the severity of the environment. High temperatures and low hydrogen partial pressures favour surface decarburisation while the opposite conditions favour fissuring. In addition, the composition of the steel influences the resistance to HTHA; in particular elements that tie-up carbon stable precipitates such as Cr, Mo and V are very important. Increasing content of such elements increases the resistance to HTHA, so that chromium steels with more than 5% Cr, and austenitic stainless steels, are not susceptible to HTHA."

Also, Hydrogen Embrittlement is "not to be confused with HTHA".

John Appleyard ! I stand corrected ! High temperature Hydrogen attack is one of the causes of Hydrogen Embrittlement ! Hydrogen Embrittlement is the effect

on most steels when single atoms of Hydrogen enter the steels pore spaces and recombines with another single atom of Hydrogen or Carbon !

One is a cause the other is an effect !


I do not know which 6'' pipe you are referring too, often with a six inch system the pipe lining the inside of the Heat Riser is there to be a sacrificial form for the

heat riser which is made from Perlite and Clay slip, with or without using ''Water Glass''as a hardening agent !

Truck or Lorry I can not recommend any damper in the smoke pipe until you have proven your Wood Heater to be Air tight ! Good luck, Big AL

Ah, as a former it makes sense then.

Hydrogen embrittlement sounds like a different thing entirely though, http://www.twi-global.com/technical-knowledge/faqs/material-faqs/what-is-high-temperature-hydrogen-attack-htha-hot-hydrogen-attack/. But anyway.

Has anyone done any experiments to find the relationship between pipe diameter and combustion? First stage of my build is set to do this experiment with observable pipes of different diameters but I was wondering if there's anybody else who's on it?

I guess that's a no so far.

John.

The problem would be reproducibility ! I expect that a propane burner like used for Deep frying whole turkeys can be used to create a steady state test bed,

It costs tens of thousands of dollars to get a new appliance a Underwriters lab test rating and unless we are talking pellet stoves there is not much of a way

to maintain the change from solid fuel to gasification and combustion from moment to moment ! Big AL

Such accuracy is pretty pointless in this case, want I'm after is getting a few points on a graph so I get a curve out of it. For me a 15% tolerence would be enough to know I wouldn't be wasting performance or getting incomplete burn. I expect that'll be 1/2" of riser. John.

- !5% at say 1200ºƒ seems pointless, however I and many of your other Fellow Members are looking forward to your posts on the subject ! Big AL

15% on the length of the riser pipe, that is where all the gas is burnt, some of which will soak up changes in fuel and conditions.

John.

There is a severe surface friction effect at a certain range of reduction in pipe diameter. It has generally been found that 4" diameter is the smallest RMH that can be made to work well, by experience or luck. A rocket stove which has no downflow component or mass ducting friction to deal with can be smaller, I don't know how much smaller. I believe 3" systems have been successful, though I can't point to a specific source.

Part of what makes for complete combustion is an insulated burn chamber, which means these pipes need to be extremely well insulated or they will tend to cool the combustion too soon. It would probably be easier to have a single riser above your firebox with refractory masonry walls, possibly with irregularities which create turbulence to slow the flow without cutting it too much.

Your plans for a glowing coal bed overnight are precisely the polluting effect found from ordinary woodstoves used in the ordinary way.

I've seen a small stove with a 2" flue.

No space or weight capacity for single masonry risers.

Glenn Herbert wrote:
Your plans for a glowing coal bed overnight are precisely the polluting effect found from ordinary woodstoves used in the ordinary way.

Hardly, it is unburnt gases which is the pollution problem. Once you're down to the char and not providing it any air its effectively thermal mass. Perhaps my choice of the word "glowing" was a bit keen, but i'm looking at a 3/4 insulated firebox

Okay, if you mean just red hot and no longer combusting, that would obviously be non-polluting... although the amount of heat stored in that would be minuscule. Much more heat would be stored in the metal enclosure, for as long as it lasts.

A small system that would heat a 7' x 14' space without overpowering it would probably be 4" equivalent or less, which would not need much height in a single riser (possibly around 2') to get as complete as combustion will get. You could even make the riser of a heavy steel pipe say 4' tall, and insulate to varying heights until you find the best balance of combustion and radiation.

John Appleyard.
I am thinking along the same lines as you regarding a number of smaller diameter risers to achieve a greater draw.  My need for this invention comes from the fact that I wish to create a rather large cob/stone thermal mass in my 2000 square foot basement.  I have the advantage of immediate access to a 24 foot vertical chimney, but the down side is my ceiling is only 7 feet high and is made of wood joists topped with plywood.  I aspire to an 8" batch box design to minimize fussing with the fire too much, but I am also an older person and I do not wish to be on my knees tending the fire as an 8 inch batch box is said to require a riser that is 57" tall.  Simply not acceptable to my location.

These design constrictions limit my burn chamber, and top of my barrel, to a very narrow range.  To get the barrel as low as possible, I thought I could use 4-4" riser tubes as the cross sectional area is roughly equal to the cross sectional area of an 8" riser.   In theory, the 4 inch risers only need to be half as tall. If the risers were to be grouped in a hexagonal cluster, you could group 7 risers together and still have extra room in the barrel.  Of course, attention must be given to the junction between the risers and the burn chamber.  But I think this is doable.

I understand this doesn't really apply to your situation, but I think the subject of multiple risers as a means of lowering the overall height of the build needs to be explored.

Anyone care to chime in?

I am pretty certain that multiple small risers connected to an 8" batch box would not work properly. It has been found that a shorter riser than the specs call for can work as long as other factors are favorable. With a good internal chimney to connect to, this might work for you. A 48" riser with batch box floor at 2 feet above the floor gives a foot for riser top gap plus ceiling clearance (with metal shielding and ventilation). I would ask for Peter van den Berg's advice on this, as he is the prime researcher of batch boxes.

Glenn,
Thanks for chiming in.  I do have my doubts about multiple risers.  Though it would seem only 4 - 4" risers would equal the cross sectional area of an 8 inch riser, the reality may be very different.  Your suggestion would only leave me about a foot between the top of the barrel and the ceiling.  That makes me nervous, but I will do as you suggest and learn some more about shielding before I try to re-invent the wheel, or as it were, the rocket stove riser.

Given the vertical limitation, I would suggest something like a steel heat shield with a couple inches of rockwool insulation above it, and a couple inches of free air space above that to the wood ceiling/framing. It would be critical to have air freely flowing across that space to carry heat away from the rockwool. There are any number of ways you could arrange multiple layers of shielding, insulation and ventilation, as long as you have all three.

A point about cross sections: there is a surface effect of fluids flowing in a tube, where the fluid closest to the surface is nearly still, and moves faster as it is farther from the surface. In air, that layer of increasing speed is a half inch to an inch thick, so you would reduce the diameter of a duct by an inch or two to get the effective area. So an 8" (effectively 7") diameter duct would convert to more than five 4" (effectively 3") ducts... very roughly approximating.

Glenn,

Thank you for bringing the factor of surface effect to my attention.  I suspected such, but I didn't know how it played into the actual performance, therefore.....

I calculate a bundle of 7, 4" (effectively 3") risers to equal a cross section of 49.4 sq inches.  Even if each riser has a wall thickness of 1", 7 such risers honeycombed into a tight bundle equal a diameter of 18" and will fit into a 22.5 inner diameter barrel and leave a slightly greater cross sectional area for the downdraft.  I keep imagining what would happen as the first riser fills with gas and begins to swirl,  does the swirling create a restriction in and of itself that would permit the other chambers to fill with gas and begin the same process, until each riser was effectively "pulling" at the burn chamber equally, or would one or more risers perform, while others do not?

Would this kind of multiple riser be considered a type of manifold?  

Back to the heat shield.  I could see where I could hang a steel plate from the ceiling with bolts and chain, with a blanket of rock wool on the top as you have suggested.  Perhaps I could also add one of those stirling engine fans to it so it would induce an air flow over it when it is warm.  I think that would be a nice touch.

I have also given thought to my bench construction.  I have been looking over the Walker Stove information and I like the "bell" idea of using a hollow cavity channel in my thermal mass to conduct the hot gasses.  Since my stove will be only a few feet away from the old fireplace hearth, the heat will move though there first and preheat the chimney flue before it goes anywhere else and help ensure a good draft.

I have also considered a masonry bell type setup for the combustion area but I'm not certain I can, or wish, to build something that massive for various reasons.

I appreciate your thoughts on this,  Much obliged.

See Peter van den Berg's reply to your other post on the same subject: How to make your Rocket Stove more "rockety"

He has the actual experience with multiple risers and has found that they are unreliable. Also, getting seven small risers in a cluster to be fed from one batch box would be impossible. I could see as many as four being possible, all taking off directly from the back end/sides of the firebox with identical ports, but the issue with multiples would still be there.

I strongly advise going with the path that has been shown to work, and making your 8" riser as short as necessary down to 36" or so as Peter suggests (longer if you can). Note that the riser height is measured from the floor of the batch box, so this height is the total core height.