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pebble style rocket mass heater in the library

 
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A bit about the bypass in this particular rocket mass heater:

 
paul wheaton
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new video about this build.  This is about an hour from the dvds cut down to 8 minutes.

 
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Chad Johnson wrote:Has someone tried an adjustable bell (barrel) height to control the amount of rocket?



If you have a bell the exhaust to the chimney is at the bottom. Say that the system is 6x6 inches and exhaust to the chimney is square, too, you can try to make exhaust rectangular say 6in width and 12inch height. The upper 6inch height of exhaust can be closed with guillotine style doors ( so you can choose the exhaust to the chimney will be 6x12 or 6x6 or anything in between. The more it is open, the sooner this bell will "spill" gasses to the flue. How much difference this makes also depends on the bell horizontal surface size... But it should work. Imo this can work as a bypass and as a regulation of speed of glasses. Obviously the lower parts of the bell would receive less heat if the rectangle exhaust would be full open (6*12).
This option is not possible with flue-only systems...

If it doesn't make sense, I can draw
 
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Well, the principle of a rocket burner of any kind. Is not to be able to to regulate the fire. To avoid unburned gases, soot, creosote etc. So adding a regulation of any kind, you get back onto a normal wood stove, which is not as efficient, and more dangerous, because of the creosote and soot, which are prone to chimney fire.

The usual way to deal with the flat out burning. Is to recover the otherwise wasted heat, from the exhaust gases.
 
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We just did a major overhaul of this rmh.  Hopefully there will be pics later - maybe even a youtube video ...

The library rmh has always been ...  okay.  Lots of experiments.  Lots of testing.  Not a super strong draw.  So the mission was to take out all of the experiments and make it a super-star performing rocket mass heater.  

In the end - we did that.  And so much more.  

There is a very twisted thing happening in my brain.  We can heat a home with one tenth the wood.  And "marketing" says I should say "heat your home with half the wood" but my engineering brain says "there is still heaps of room for optimization - I think we can do this with one twentieth of the wood ..."



replace double loop-de-loop with huge barrel stratification chamber: Hot gasses, about 300 degrees enter the barrel and go to the top of the barrel.  The vertical exhaust is at the bottom of the barrel pulling up the cooler gasses at the bottom - about 110 to 115 degrees.  

add some big rocks and cob to the top of the barrels to grab more heat to give it off later:  making this a sort of cob/pebble hybrid.  The best of all worlds.  Cob and rock placed along the top edge of the barrel will harvest the heat in the barrel through conductive heat - the most efficient form of heat transfer.  



six inch vertical exhaust: Arguably too small for an 8-inch system.  But for this experiment, we want that.  With an 8 inch vertical exhaust, a small bit of air is constantly pulled through the system when there is not a fire - pulling the heat out of the mass and taking it outside.  

kiss the barrel: having the vertical exhaust super close to the barrel makes the vertical exhaust be a tertiary thermosiphon when there is a fire.  This helps create a strong draw when there is a fire, and allows the system to plug a bit better when there is not a fire. To accomplish this we moved the whole core closer to where the vertical exhaust would be - which was where the hole in the roof was.  So the whole rocket mass heater became about 14 inches shorter.

two more inches above the riser: the system was still running .... okay.  We want magnificent.  Time to do some "lawn chair design" (where you patiently sit and try to figure it out).  When a fire is running, we are seeing the top of the barrel get quite red.
While that is great for boiling water, it suggests that there might not be enough gap between the top of the riser and the barrel.
We decided to pop off the barrel and measure the gap between the top of the riser and the barrel.  If it is 2 inches or less, let's change that to 3 or 4 inches.  ...   2 inches.  So we added more than two inches below the barrel.  Test:  the system is now performing MUCH better.  But we want it to be even better still.

two more inches below the juice box straw:  Where the vertical exhaust comes to the bottom of the stratification chamber (juice box style), we started with a 3 inch gap.  Let's adjust that to be 4 or more inches.  The first test was for a 7 inch gap.  Now the whole system had an incredibly excellent draw!  Magnificence achieved!  After a bit of lawn chair design, we decided that the whole system perform about 10% better if we dropped it to 5 inches.  Done.  Magnificent performance and the coming winter will give us some ideas about overall efficency.

There were loads and loads of other improvements - but they were a bit more about patching holes in the wall from previous experiments, adding steel supports across the box to prevent bulging, and stuff like that.


Here is the grand summary of what I think is accomplished here:  The whole system will burn cleaner and faster.  Less heat goes out the roof during the burn and the space is heated faster when there is a fire.  And the biggest improvement, I think, is the efficiency between burns.  The mass will hold a bigger charge and less of that heat will go outside between burns. Rather than burning a fire every other day, the people in that space might go an extra day or two until the next fire.    

The general design is to have a magnificent and efficient burn during the fire, and to PASSIVELY plug the system when it is not burning.

(pics from stephen's brk thread)
 
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a pic from caleb

 
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paul wheaton wrote:two more inches above the riser: the system was still running .... okay.  We want magnificent.  Time to do some "lawn chair design" (where you patiently sit and try to figure it out).  When a fire is running, we are seeing the top of the barrel get quite red.
While that is great for boiling water, it suggests that there might not be enough gap between the top of the riser and the barrel.
We decided to pop off the barrel and measure the gap between the top of the riser and the barrel.  If it is 2 inches or less, let's change that to 3 or 4 inches.  ...   2 inches.  So we added more than two inches below the barrel.  Test:  the system is now performing MUCH better.  But we want it to be even better still.


Well done Paul! Here's an explanation and how to calculate it.
Gases tend to pose friction while forced to change direction. In order to avoid this, one could choose to have a wider top gap. This gap above the riser could be viewed as an imaginary ring. When this ring is high enough, most of the friction isn't there anymore. It's possible to calculate the ring by taking the circumference of the riser and multiplying it by the gap. In order to get an adequate gap the resulting figure should be twice as large as the cross section area of the riser.
Now what you've done here is enlarging the gap to 4 inches. This multiplied by the circumference of the 8" riser will give just over 100 sq. in. Which is exactly twice the cross section area of the riser, left-over friction would be neglectable. Two inch as a top gap is far too cramped, in my humble opinion of course.

paul wheaton wrote:two more inches below the juice box straw:  Where the vertical exhaust comes to the bottom of the stratification chamber (juice box style), we started with a 3 inch gap.  Let's adjust that to be 4 or more inches.  The first test was for a 7 inch gap.  Now the whole system had an incredibly excellent draw!  Magnificence achieved!  After a bit of lawn chair design, we decided that the whole system perform about 10% better if we dropped it to 5 inches.  Done.  Magnificent performance and the coming winter will give us some ideas about overall efficency.


The same as above, direction change of the gas stream isn't 180 degrees in this case although a 3 inch gap is too cramped again. Especially when the plunger pipe or juice box straw is close to one of the walls of the bell. Actually, the wider gap under the exhaust pipe is better because in this case this pipe is a 6 inch chimney on an 8 inch heater. One could get away with that if the chimney is straight up and insulated.  Plus by eliminating most if not all friction points of course. It'll help enormously that the bench is now a low-friction system instead of the former convoluted piped bench.
Calculation for an 8" pipe would result in a 94 sq. inch figure which is nearly twice the cross section of the pipe. Very good. But it's a 6" pipe, in this case the resulting figure is 3.3333 times the csa of the pipe. Lots of space then, no problem.

It's nice to see that a J-tube works admirably together with a bell system (sorry, stratification chamber). A similar calculation as the above was used 8 years ago for the 8" batch box rocket in the larger space of the shop. https://permies.com/t/40/40007#313900
 
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Peter,

I think the key is that the rmh in the library was built 9 years ago.  At that time (if memory serves) Ianto advocated that the gap be 1.5 inches to boil water, or 2 inches.  And a lot of the rocket mass heaters built then tended to be a bit shy of magnificent.  Now, I tend to think of 3 inches as the minimum for an 8 inch j-tube.  I don't want people to even try to boil water.

The important lesson is that the system was not magnificent.  So we went into sleuthing mode. There was a lot of certainty that the gap was 3 inches or maybe even 3.5 inches.  But ...   the red spot ...  So we decided to be sure - and pop the barrel off and measure.  Shore nuff - 2 inches.  

----

The important thing here, while you are reading this thread:  do you agree with my analysis about the reduction in wood burning with this system due to:

    - the additional p-trap in the stratification chamber

    - the reduced vertical exhaust diameter combined with the "kiss the barrel"

When the fire is out, the system is "more plugged" so less heat from the mass goes outside and less cold air from outside sneaks in.  Therefore the mass gives off more heat for longer - and the operator waits longer between burns.


 
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peter, you will probably like this:  https://permies.com/t/193821/permaculture-projects/inch-batch-box-rocket-mass
 
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paul wheaton wrote:The important thing here, while you are reading this thread:  do you agree with my analysis about the reduction in wood burning with this system due to:

    - the additional p-trap in the stratification chamber

    - the reduced vertical exhaust diameter combined with the "kiss the barrel"

When the fire is out, the system is "more plugged" so less heat from the mass goes outside and less cold air from outside sneaks in.  Therefore the mass gives off more heat for longer - and the operator waits longer between burns.


I'm in the dark what you mean by p-trap in the stratification chamber. Unless... you mean the plunger pipe of the exhaust. This would work partly as a passive chimney shut-off, provided all of the core is well above the lowest point of the plunger pipe (or juice box straw). For that to happen, the chimney needs to be cold.

Your second point is working in the plus as well as minus. The reduced exhaust diameter could limit the heat that's escaping through the chimney stack. Kissing the barrel works well for priming the chimney as soon as the barrel starts to heat up. Both are positive points, potentially adding to the system's efficiency.
Kissing the barrel is a negative point at the end of the burn when the fire is out. Both barrel and chimney pipe needs to be cold for the passive shut-off to be effective. The barrel being the highest point in the system means it would take quite some time to cool off sufficiently. Therefor, the chimney pipe will be warm for a longer period since it's hugging the barrel.

In practise, some points will be positive in one stage of the burn and negative in another. Your theory might or might not hold in real life, for example because of a point's positive aspect is much larger than the negative aspect. It's possible to be the other way around as well. The only thing one can do is measuring and watching the thingy for a whole season. It's plausible that the outside temperature is also one of the factors in play. The higher the temp difference, the more loss to name just one.
Hard to tell, an experiment cannot be a failure, they are for to learn.
 
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paul wheaton wrote:peter, you will probably like this:  https://permies.com/t/193821/permaculture-projects/inch-batch-box-rocket-mass


Yes I do! It seems to be unclear what size of system this is. Sometimes it's called a 7" system and sometimes a 6". The 3 barrels together is regarded as the top limit for a 6" system so maybe it's a 7 incher? I'll follow this one closely to see how it'll work out.
 
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