RMH mechanism, the drive behind the drive.

[Donkey]

Originally posted by ytksgmt, over in the Rocket bell project thread. It's a topic worth it's own thread.
--------------------------------------------------------------

I tried to analyze the mechanism of RMH.
I studied physics decades ago and didn't used it in my daily life.
So, I can't be confident.
I don't speak English, and never write English.
So, I'm afraid I can't explain properly.
But,I'll try.

RMH is consisted with Riser and Downer.
Heat Exchange Barrel is equivalent to Downer Pipe.
In Downer part, part of heat is emitted.
The height of heat release center depends on the system.
For example, in barrel system, large part of heat is emitted from top surface of barrel.
So, heat release height gets closer to the height of Downer.
If you insulate upper half of Downer, release height get lower.

I applied a very simple model.
(May be, too much simplified to appropriate.)
Downer part is mostly insulated.
In a very thin area at height of JE, all the heat which should be emitted from Downer would be emitted.

Set Tr,Td,Ta the temperatures(K) of air in Riser, in bottom of Downer, and open air, each.
Set Rr,Rd,Ta the densities of each air.

Now, set parameter u = (Rd-Rr)/(Ra-Rr)
While Rd varies from Rr to Ra, u varies 0 to 1.
Density is reverse equivalent of Temperature(K).
That is, if heat release from Downer is large,
then Td (Downer Bottom Air Temp) nears to Ta (Open Air Temp), Rd to Ra, and, u to 1.
If heat release at Downer is small, then Td nears to Tr (Riser Air Temp), Rd to Rr, and, u to 0.
So, u represents the efficiency of heat release of Downer.

After several calculations I reached the condition for Drafting.
u > (J-H)/JE

Say, Riser Length H = 1m, Downer Length J = 2m.
And, Downer is heavily insulated ,resulting JE < 1m.
Then, right part of the Inequality is larger than 1.
But u must be less than 1.
So, the Inequality can't be approved.
That means the RMT can't work.

In this case, Downer's upper part above the release height is larger than Riser.
And the temperature of the part is the same as that of Riser.
So, the stack effect in Downer is larger than that of Riser.
Downer is no more Downer.
It's better to be called RISER.

RMH's drive engine is NOT the stack effect in Riser alone.
It's the difference of the stack effects in Riser and Downer.
If you want more draft, you can get it by cooling Downer.
If you want exhaust heat reached deeper, you have to cool Downer
more effectively and/or cool it at higher position.



Attachment: SimpleModel.JPG (47.5 KB)

[Donkey]

My reply over at the other thread, transferred here.
----------------------------------------------------------------------
I no longer think that stack effect is the main driver in rocket stoves. The reality never seems to fit the numbers.

I've come to think that it may have something to do with pressure created by the heated air volume trying to expand against the constriction of the walls.

I have accidentally wrecked the flow of a stove by creating an overlarge chamber directly under the heat riser. If the stove were driven entirely by stack effect, I imagine that the volume below the heat riser would not matter much.

Also in other circumstances, I've repaired flow by stepping down pipe sizes downstream in the storage mass. Smaller pipes run hotter, one would think that this would provide a negative influence on the stack balance.. Though to be fair, it could also provide warmer air to the chimney, improving stack effect there.

[Donkey]

I wrote:

Are you willing to do this testing and post your results here?

to which, ytksgmt replied:
---------------------------------------
You mean practical experiment ?
I WAS willing to experiment it.
My purpose is to bring exhaust gas 50 to 100cm beneath the level of combustion chamber and warm the ground of my house.
But, after calculation, I'm thinking I have to reconsider the design completely.
I know I should experiment it.
Experiment of checking that my design can't work...

[ytksgmt]

Thank you, Donkey.

Apr 8, 2010 11:45:05 GMT -8 Donkey said:

I've come to think that it may have something to do with pressure created by the heated air volume trying to expand against the constriction of the walls.

Image of continuous small explosions ?
If so, I have to ask why not erupt from air-intake ?
Assuming that bottom of heat riser is the center of explosion,
the nearer to the point, the larger pressure.
Air-intake is much nearer than exhaust pit to the bottom of riser.

In reality, the pressure at the base of the riser is lower than the pressure of outer room air at the same level(height).
Air flows from high pressure point to lower pressure point.
That is DRAFT.

I have accidentally wrecked the flow of a stove by creating an overlarge chamber directly under the heat riser. If the stove were driven entirely by stack effect, I imagine that the volume below the heat riser would not matter much.

In a normal stack, top of the stack is open.
In that case,neutral-level is near the half height of the stack.
And the boa-up beneath the neutral-level won't affect the situation,probably.
But, in the case of RMH, top end of the stack is not open.
So, the neutral-level moves down from the half height.
In a normal RMH, the level may be 10 to 30cm above the base of the chamber.
Enlargements at the neutral-level do matter.

I tried to modify the simple model, just a little.
Cross-sectional area at neutral-level( = AN ) only differs from other positions ( = A ).
In most cases, with my model, the larger AN than A,the stronger draft is caused.

In some case, with increase of AN/N until some point, the draft increases, and,
when the ratio exceeds the point, the draft breaks.
This case mostly occur when Pressure Loss Coefficient of Downer's bottom-end is too small.
Namely,in the case when exhaust pit is fully opened without loads of long exhaust pipes.

Also in other circumstances, I've repaired flow by stepping down pipe sizes downstream in the storage mass. Smaller pipes run hotter, one would think that this would provide a negative influence on the stack balance.. Though to be fair, it could also provide warmer air to the chimney, improving stack effect there.

I think this is a different story from stack effect in RMH.
Differential and Integral may be needed, which I almost forgot.
But,one comment.
I am suspicious that smaller pipes run hotter.
When flow speed is kept constant by downsizing the cross-sectional area of exhaust pipe,
Heat transfer may also be kept.
Or rather say, heat transfer may not so much decrease as the case in which pipe sizes are not changed.
Heat transfer rate must be a function of the speed of fluid.
If the temperatures of gas decrease, the values of viscosity of gases also decrease.
Just guessing. I have no confidence on this explanation.


Neutral-level = the level where the pressure of gas in a stack is the same as that of outer air at the same height.
The height of the neutral-level from base of combustion must be positive.
In most cases , calculating the condition of draft-existence is calculating the condition of neural-level to be positive.
I imagine a new type rocket stove. I-style. Not J,nor L.
Straight vertical insulated tower with small hole at the height of neutral-level.
No exhale nor inhale from the hole, through which you throw small pieces of wood to the bottom of tower.
No need of expensive fireproof glass.
You can even touch the fire flow in the tower with your finger, with unpleasant result.
Another type of fire-showing rocket stove.
But lack of practicality.

[Donkey]

Apr 9, 2010 4:02:13 GMT -8 ytksgmt said:

Thank you, Donkey.

Apr 8, 2010 11:45:05 GMT -8 Donkey said:

I've come to think that it may have something to do with pressure created by the heated air volume trying to expand against the constriction of the walls.

Image of continuous small explosions ?
If so, I have to ask why not erupt from air-intake ?
Assuming that bottom of heat riser is the center of explosion,
the nearer to the point, the larger pressure.
Air-intake is much nearer than exhaust pit to the bottom of riser.

There is a law ( I forgot it's name) that says that once a fluid is induced to flow a direction, that it will continue flowing in that direction unless acted upon by a greater force. Also, warmed fluids always flow towards the highest chimney.

In reality, the pressure at the base of the riser is lower than the pressure of outer room air at the same level(height).
Air flows from high pressure point to lower pressure point.
That is DRAFT.


In a normal stack, top of the stack is open.
In that case,neutral-level is near the half height of the stack.
And the boa-up beneath the neutral-level won't affect the situation,probably.
But, in the case of RMH, top end of the stack is not open.
So, the neutral-level moves down from the half height.
In a normal RMH, the level may be 10 to 30cm above the base of the chamber.
Enlargements at the neutral-level do matter.

I tried to modify the simple model, just a little.
Cross-sectional area at neutral-level( = AN ) only differs from other positions ( = A ).
In most cases, with my model, the larger AN than A,the stronger draft is caused.

In some case, with increase of AN/N until some point, the draft increases, and,
when the ratio exceeds the point, the draft breaks.
This case mostly occur when Pressure Loss Coefficient of Downer's bottom-end is too small.
Namely,in the case when exhaust pit is fully opened without loads of long exhaust pipes.

Regardless, I have built a too large chamber underneath the heat riser and watched as the device failed to work properly. All I had to do was reduce the lower chamber and it worked again.

Also in other circumstances, I've repaired flow by stepping down pipe sizes downstream in the storage mass. Smaller pipes run hotter, one would think that this would provide a negative influence on the stack balance.. Though to be fair, it could also provide warmer air to the chimney, improving stack effect there.

I think this is a different story from stack effect in RMH.
Differential and Integral may be needed, which I almost forgot.
But,one comment.
I am suspicious that smaller pipes run hotter.
When flow speed is kept constant by downsizing the cross-sectional area of exhaust pipe,
Heat transfer may also be kept.
Or rather say, heat transfer may not so much decrease as the case in which pipe sizes are not changed.
Heat transfer rate must be a function of the speed of fluid.
If the temperatures of gas decrease, the values of viscosity of gases also decrease.
Just guessing. I have no confidence on this explanation.

For me, the "smaller pipe runs hotter" theory is a matter of a certain amount of faith. It was told me by someone I trust to get it right in these things.
Though I believe that the "Ideal Gas Law" covers it. Erica said it better than I can, so I posted her comments in the "good info" thread.

Neutral-level = the level where the pressure of gas in a stack is the same as that of outer air at the same height.
The height of the neutral-level from base of combustion must be positive.
In most cases , calculating the condition of draft-existence is calculating the condition of neural-level to be positive.
I imagine a new type rocket stove. I-style. Not J,nor L.
Straight vertical insulated tower with small hole at the height of neutral-level.
No exhale nor inhale from the hole, through which you throw small pieces of wood to the bottom of tower.
No need of expensive fireproof glass.
You can even touch the fire flow in the tower with your finger, with unpleasant result.
Another type of fire-showing rocket stove.
But lack of practicality.

Very interesting stuff here!!
I think that an experiment (actually a few of them) needs to be done to give REAL evidence either way.
And yes, you may indeed need to do a test to prove that your design won't work. On the other hand, you might get a surprise!
I can't tell you how many times reality has refused to align with my best ideas!

[ytksgmt]

I tried to modify the simple model, just a little.
Cross-sectional area at neutral-level( = AN ) only differs from other positions ( = A ).
In most cases, with my model, the larger AN than A,the stronger draft is caused.

First, I have to say Sorry. I miscalculated while modifying the model.
I have not fully analyzed the result, but the result is quite different from what I have described before.

Apr 11, 2010 12:36:02 GMT -8 Donkey said:

There is a law ( I forgot it's name) that says that once a fluid is induced to flow a direction, that it will continue flowing in that direction unless acted upon by a greater force. Also, warmed fluids always flow towards the highest chimney.

I used Continuity law, Bernoulli's theorem, Energy conservation law, Ideal gas law, D'Arcy-Weisbach Equation.
I myself think there can be other factors which derive directly from combustion mechanisms. But I hope those factors are not dominant, and can be neglected as first approximation.

For me, the "smaller pipe runs hotter" theory is a matter of a certain amount of faith. It was told me by someone I trust to get it right in these things.

Expression "smaller pipe runs hotter" has ambiguity.
Smaller pipe absorbs more heat from exhaust flue. As the result, the pipe gets hotter.
Or, in a smaller pipe, exhaust flue runs hotter.In this case, efficiency of heat transfer through pipe decreases. As the result, stack effect in an external chimney functions better.

He says he does this to produce an even heat across the breadth of the masonry column

For me, this Dale's explanation seems to point the former mechanism.
The verification may be rather easy.
If latter is the case, removing the external chimney will break the repairing effect of stepping down of pipe size, or worse.
If former is the case, removal of external chimney won't affect so much. In this case, stack effect in external chimney can hardly be selected as the mechanism of the repairing. So, the decline of viscosity which I described may be selected as one of candidates.

And yes, you may indeed need to do a test to prove that your design won't work. On the other hand, you might get a surprise!
I can't tell you how many times reality has refused to align with my best ideas!

Yea. I'll try.
We assume the world's phenomena are ruled by laws.
But FACTs determine the laws.

[ernie]

this is in the book. yes the density relations are the drive in a rocket stove. the cooling effect of the barrel coupled with the heat riser temps are what makes the system able to push down a slope or what have you. the other element is the secondary burn in the barrel. the heat spikes at the top of the riser and then the gas cools quickly on the barrel surface. if you dont have the reburn due to the void between the top of the riser and the barrel then all the chamber becomes is a gas containment.

another point is that yes if you peel the system like an onion the core driver is the hot stack. if the heat difference was the same between it and the barrel surface then there would be no hot stack effect. this same goes with if you have a hot stack in the open air. if the surrounding environment was the same temp as the top center of the stack it would not work at all.

your math is correct that in a system that the barrel is insulated to a very great degree the system will run in reverse. however; practice you would need to heat the barrel or get near total heat containment on the barrel surface. if you want to work a model up its pretty easy to do but the material s might cost a bit.

there are any number of experiments that need to be done and few people to do them. as a matter of course some of the more esoteric are going to point out flaws in the theories and i welcome this. these where not designed in a lab they where designed in the field from observed data. sometimes the physics is different then what our field observations tell us.

i guess what this is is me trying to understand what assumptions in the book are inaccurate and what leads folks to think that this is a perimeter that is negotiable? it is also an attempt to explain the relationship In text so i can get on with things myself.

thanks for taking the time to read my ramble.
Ernie

[rectifier]

My experiments have led towards some experimental proof of this, as there are a few factors in my stove that seem to accentuate these effects:
- constant fuel supply/fuel area - input power is affected by draft through the firebox, nothing else.
- cast iron 'barrel' which can soak up a ton of heat and responds slowly to temperature change, spreading out the effects of temperature changes and making them more visible.
- exhaust stack only a couple feet of rise so it doesn't contribute much draft
- riser and 'downer' heights identical

My stove goes through distinct 3 phases of operation:

1. Riser cold, barrel cold - low riser velocity, low exhaust velocity, almost no fuel consumption (just getting going)

2. Riser hot, barrel cold - extremely high riser velocity, clean burn. High exhaust velocity, but cool exhaust gas (~250F flue at barrel output). This stage is the maximum BTU input of the stove, it will chew through the fuel. Flames are slamming into the top of the barrel from the riser. Most of this heat is going in to heating up the big chunk of cast iron.

3. Riser hot, barrel hot - low riser velocity, still a clean burn as the riser is still very hot. Low exhaust velocity, warmer exhaust gas (~275F). The fuel consumption drops off dramatically to the point that the feed can jam due to the slow fuel consumption. The riser is still very hot and opening the top will result in flames shooting out within a few seconds, but without the cooling in the barrel, it will not pull a powerful draft. This is OK with me as the barrel is my main heat exchange surface - I want it to slow down once it is hot.

I believe that at longer runtimes it should oscillate between states 2 and 3 as the riser and barrel heat and cool with the varying power. The rocket effect seems very much an 'on or off' type effect, depending on whether the flame is pulled into the riser or not.

This supports ytksgmt's model where the cooling and contraction of the hot gas forms the main drive. Whereas the stack effect was the driver of the original 'rocket' stoves designed for cooking, in the case of the RMH, the cooling effect in the barrel is necessary to drive the gases down and out.

I also have experienced a reversal of the drive in an initial test with an uninsulated heat riser and no external stack. It ran great for 30 minutes, slowed, and then backed up and completely reversed, blowing a huge flame out of the feed hopper (lots of preheated incoming air). It makes sense that this occurred at the point where the riser and barrel were the same height and temperature (0 draft), and the hopper was warm from sitting above the firebox, causing it to pull backwards.

[mintcake]

Nov 2, 2011 20:34:56 GMT -8 rectifier said:

I also have experienced a reversal of the drive in an initial test with an uninsulated heat riser and no external stack. It ran great for 30 minutes, slowed, and then backed up and completely reversed, blowing a huge flame out of the feed hopper (lots of preheated incoming air). It makes sense that this occurred at the point where the riser and barrel were the same height and temperature (0 draft), and the hopper was warm from sitting above the firebox, causing it to pull backwards.

Eeek... Just what you don't want in your living room!

I imagine that if you have a sufficiently big mass that you're heating then the same effect could occur even if you have an outside stack, especially on a warmish morning after a cold night. If the stack is cooler than the outside air then the draw from that is backwards.
I've a relative with a (non-rocket) tiled/masonry stove and some mornings they have to light a small fire in their chimney clean-out to get it to draw in the right direction.

[Donkey]

Nov 9, 2011 11:14:29 GMT -8 mintcake said:

Nov 2, 2011 20:34:56 GMT -8 rectifier said:

I also have experienced a reversal of the drive in an initial test with an uninsulated heat riser and no external stack. It ran great for 30 minutes, slowed, and then backed up and completely reversed, blowing a huge flame out of the feed hopper (lots of preheated incoming air). It makes sense that this occurred at the point where the riser and barrel were the same height and temperature (0 draft), and the hopper was warm from sitting above the firebox, causing it to pull backwards.

Eeek... Just what you don't want in your living room!

I imagine that if you have a sufficiently big mass that you're heating then the same effect could occur even if you have an outside stack, especially on a warmish morning after a cold night. If the stack is cooler than the outside air then the draw from that is backwards.
I've a relative with a (non-rocket) tiled/masonry stove and some mornings they have to light a small fire in their chimney clean-out to get it to draw in the right direction.

Right.
It's why internal stacks are a pretty good idea.
Also, rocket stoves should have a "primer box" for doing EXACTLY as your relative has. 'Course, it's best if you locate the primer box somewhere convenient, where you'll ACTUALLY use it instead of blowing futilely into an obstinate stove.

[rural]

I read through this thread a while ago. It made me question the wisdom of my plans to encase the whole barrel (except the top) in cob. It was also running through my head as I mocked up a RMH in my children's sandbox last night. This is a fairly standard RMH mock-up. Seven-by-seven, slightly over-size old bricks and everything dry stacked with some fairly large gaps between the bricks in a couple of places. At this point, I have no insulation around the riser. It's just sand underneath, bricks, and a barrel.

As I fired it up last night, I was expecting more draw than I'd seen without the barrel. After all, the cooling affect of the barrel should contribute to the draw, right? Maybe I misinterpreted much of this thread, but increased draw was my expectation. I was disappointed.

The stove was much harder to light and get flowing in the right direction. The draft was weak, lots of smoke, and there was no "rocket sound". It reversed on me a couple of times when I added fuel, even when it was well warmed up. It occurred to me that I could take the barrel off and eliminate some other changes I had made as contributing factors. (The earlier mock-up had a 6"x3.5" throat. The current mock-up added an ash-pit.) Unfortunately, the barrel was too hot to handle by that point.

I did rearrange the bricks at the fuel feed to eliminate the vertical opening in favour of a horizontal feed (ie. the J became an L). The stove became much less stubborn at that point. Satisfied, I turned in for the evening.

This morning, I removed the barrel, checked that it wasn't choking the flow at the riser (4" gap, so I don't think so), fired up the stove, and then alternated between the barrel being off and on while observing the results. My observation was that adding the barrel did not positively affect air flow, quite the opposite. The difference was fairly profound, and my bias was pro-barrel, so I trust my observation.

Caveats: No insulation. Dry-stacked bricks with gaps.

My interpretation: Although the cooling of the gasses as they pass down the barrel must contribute, it doesn't seem like the contribution is larger than the reduction in the chimney affect or the resistance that the barrel introduces. Perhaps in a complete stove, where there barrel's resistance is dwarfed by a long run of exhaust pipe, the situation would be different.

I would also suggest the stack effect not be ignored. If your exhaust is lower than your air intake, bad things will probably happen when the right conditions occur. I only say this, because my expectation, after reading the RMH book and various stuff on the Internet (including this forum), was that the riser and the cooling affect of the barrel would provide enough drive to do crazy things like push the exhaust well below the air intake. My mind has been changed. I'd now be very cautious about any downward flows after the barrel. In my case, I'll likely be exploring ways to lower the air intake and raise the bottom of the barrel.

In all honesty, I just about jumped back to a more traditional masonry wood-stove for the house we're building. Just about, but not quite. That heated bench/bed is really attractive to me.

[rectifier]

I did a little test today while welding up a new piece for the pellet rocket. The new design is a lot more robust so it can be handled more while in operation, allowing me to look into rocket theory a bit more.

I fired the stove up with the top off the 'barrel'. Draft was established and the flames were shooting out horizontally and hitting the back wall of the riser, with no turning upward. It reached a steady state, and I slapped on the lid.
The stack attached to the bottom of the 'barrel' is 3" x 3' high - this puts the final exhaust roughly 1' above the barrel lid.
With the lid on, an immediate increase in draft occurred, visible via turbulence seen through the gasifier port.
Note that any additional stack effect seems negligible as I have added 3' of stack and 2' of anti-stack. However, this may not be true - let's analyze it.

The theory around rocket stoves seems very hard to put into words. Here's my whack at it from my personal experiments.

- The riser has an enhanced stack effect due to high temperature. Let's say that it's double, for the sake of conversation. My 1' riser becomes a +2' equivalent normal-temperature stack.
- The open top rocket stove has +2' of stack. Now let's put the lid on.

- The barrel also has stack effect. Heat rises - period. If the barrel was the same temperature as the riser, it would have a net stack effect of -2', as the gas has to flow downwards in it, against the stack effect. This comes out to 0' of stack and choking of the fire.

- However, the barrel has a much lower temperature than the riser. The riser rapidly hits above 800F. The barrel at that time is still below 100F. Lets be tough though, and say that the gas in the barrel is the same temperature as the rest of the exhaust gas will be. The barrel thus contributes -1' of stack.
- The total stack is now +1' at the exit of the barrel, which is enough to keep the flame lit at least while the gas moves on to the exhaust system.
- The exit of the barrel turns upwards into the 3' stack. The exhaust is now quite cool after the downdraft section, but it will still rise at the same speed it tried to oppose the flow inside the barrel. So we get +3' of rise through this stack.
- Total stack is +4', double the draft without the barrel.

Note that at no point did the barrel actively push gas down. What it did do, was allow the gas to return to the level of the firebox with a net positive draft. Assuming the firebox is on the floor, this is how your bench gets heated. After that initial downwards push to the bottom of the barrel, the gas then is able to rise gently throughout the rest of the system until the final stack.

The downdraft section could be considered a 'counter-flow' heat exchanger. The gas travels slowly through it as it wants to rise. This long dwell time causes massive losses of heat into the barrel, translating into an efficient heat exchanger - in small stoves like my pellet rocket, this means there is no need for further heat removal - it is already almost entirely extracted within the barrel!

Anyways, as you see, you need two mods to your setup.
1. The heat riser MUST be hotter than the barrel. It needs to be insulated in some way. The bricks should do, but the gaps won't! Stick a piece of pipe inside that riser! Any leakage, and you lose your internal stack effect.
2. There must be an external stack to burn at any power level at all. Otherwise, you will end up with only a foot or two of stack equivalent after the barrel - only enough to barely keep a fire lit.

[Donkey]

OK, rectifier..
The NEXT thing to do is to take the end of the stack and move it down to a position BELOW the firebox, far enough to make the theoretical sum at this point 0 or less. If you find that the machine still works, then what is needed is a NEW EXPLANATION, otherwise not.
Try it and see what happens.

What I've seen over the years makes me doubt the classic explanation. Though my methods are sometimes sloppy and leave room for second-guessing.

[rural]

I'm skeptical. (It's in my nature. I'm a scientist. Apologies.) Not saying I disagree, just that I'm going to need more evidence to push me off the fence. Unfortunately, most of what I read here is anecdotal, just the nature of the forum medium. Video would help. (And I'm pointing that last sentence at myself as well. I took some videos but haven't bothered to upload them.)

In my test, the outside of the bricks were fairly cool as the stove hadn't been running in "pure rocket mode" for long enough to heat them clear through. The barrel was cool too, as it wasn't on the stove at all until the test.

But I'll add another caveat to my experimental method: There was no casing for the heat riser. From the outside, the dry stacked bricks were far from aerodynamically arranged.

So I've got to cut some bricks to size, assemble everything with mortar, get some insulation for the heat riser, and put a casing around the heat riser. Then I'll be able to test the performance with and without the barrel in something closer to the final configuration.

Now I'm going to stray from the topic a bit. Feel free to PM me rather than reply here.

I'm doing this outside in the family sandbox. And the weather is here in mid-Alberta is getting cool fast. The project is going to have to move inside pretty soon. At that point, comparison of barrel/barrel-less will become much harder. In all honesty, I'm not that interested in comparing the two configurations, and just want to confirm the safety of the stove for myself (and wife).

Anything I do is going to have to come apart again before being assembled in its final location. So I have to ask: Are clay-sand mortared bricks difficult to disassemble? Is clay-sand mortar easier to clean off bricks than very old lime mortar?

[Donkey]

Nov 13, 2011 6:03:18 GMT -8 rural said:

I'm skeptical. (It's in my nature. I'm a scientist. Apologies.) Not saying I disagree, just that I'm going to need more evidence to push me off the fence. Unfortunately, most of what I read here is anecdotal, just the nature of the forum medium. Video would help. (And I'm pointing that last sentence at myself as well. I took some videos but haven't bothered to upload them.)

GOOD! We need more open minded skeptics.
'Specially ones willing to do the work and report.

In my test, the outside of the bricks were fairly cool as the stove hadn't been running in "pure rocket mode" for long enough to heat them clear through. The barrel was cool too, as it wasn't on the stove at all until the test.

But I'll add another caveat to my experimental method: There was no casing for the heat riser. From the outside, the dry stacked bricks were far from aerodynamically arranged.

So I've got to cut some bricks to size, assemble everything with mortar, get some insulation for the heat riser, and put a casing around the heat riser. Then I'll be able to test the performance with and without the barrel in something closer to the final configuration.

Umm, the thing that jumped out at me with your test, more than ANYTHING else is that you didn't seal it up well. It should work (not the best, but) with only the brick riser, assuming no hidden bottlenecks . Each part of the stove needs to be well isolated from the rest, if there are leaks it'll kill the action better than most other things.

Now I'm going to stray from the topic a bit. Feel free to PM me rather than reply here.

I'm doing this outside in the family sandbox. And the weather is here in mid-Alberta is getting cool fast. The project is going to have to move inside pretty soon. At that point, comparison of barrel/barrel-less will become much harder. In all honesty, I'm not that interested in comparing the two configurations, and just want to confirm the safety of the stove for myself (and wife).

Anything I do is going to have to come apart again before being assembled in its final location. So I have to ask: Are clay-sand mortared bricks difficult to disassemble? Is clay-sand mortar easier to clean off bricks than very old lime mortar?

It's easier to take off than lime. Just scrape em with your mortaring trowel or wash em off in a bucket (I wouldn't do that in freezing weather, at least bring em in and dry em before use.)
Remember, mortar is for holding the bricks apart, NOT for sticking them together. Use just enough to fill in the differences between the brick, no more. I usually just dip the bricks in clay slip and stick em down. I'll then try to wiggle them, if they do I'll slick a little bit of mortar in the low spot and squish into place. Lately I've been mixing slip with screened wood-ash to make the mortar.