If rmh was vertical, would the barrel still be needed?

Hi everyone.

This is a purely theoretical question to help be better understand the true nature of the barrel used in rocket mass heaters.

If the mass was above the burn tube, so the insulated burn tube transitions seamlessly into an un-insulated thermal mass and the tube was vertical from beginning to end, would the efficiency of an rmh be affected?

To rephrase my question, is the barrel just 1) a radiator into the room, and 2) a way to get the gasses down to floor (bench) level? Is there some other magic that the barrel does I'm not aware of? Perhaps it is performing critical gas mixing that cannot happen any other way.

Thanks in advance.

I've also heard that the barrel (by way of radiating heat) cools the gasses which gives an extra kick to the flow of gas through the mass. If so, how critical is this?

In a typical J-tube RMH with a steel barrel, the barrel is involved in a few things:

1) Hot flue gasses flow up the riser, hit the top of the barrel, then flow outwards towards the edges and down.  Because of the large surface area, and because steel is a good thermal conductor, heat is conducted away from the flue gasses rapidly.

2) Most of the cooling of flue gasses occurs near the edges and along the inside wall of the barrel.  Since there is room for the cooled gasses to sink thanks to reduced buoyancy, they do.

3) The updraft in the riser, the downdraft along the inside wall of the barrel, and any vertical flue sections, work in the same direction and create enough combined draft to push/suck the flue gasses all the way through the (lengthy) flue within the thermal mass (which, being horizontal, is slowing the gasses down due to friction).  So, the barrel helps the heater to actually exhaust flue gasses (particularly when the system is cold).

4) The volume occupied by gasses is related to their temperature.  Hot gasses take up a lot of space.  Cold gasses take up much less space.  When the barrel bleeds off heat and suddenly cools the flue gasses, they shrink in volume.  This created a low-pressure zone that sucks in air (preferentially) via the feeder tube.

5) A strong airflow through the feeder > burn chamber > riser effectively acts like bellows in a forge, and is what is primarily responsible for rapid burning of the fuel, high burn temperatures, and complete combustion of flue gasses.  So, the barrel is also instrumental in making a rocket mass heater more 'rockety' and delivering hot and clean burns (which translates to high heat production and low emissions).

The barrel should not have much to do with mixing — that should occur in the burn chamber and, most critically, in the riser.  By the time the flue gasses have hit the barrel everything that could be burnt already should have been.  If you are still burning gasses in the barrel, it means your riser isn't long/convoluted enough, and you're going to burn out the top of your barrel in no time.

A large thermal mass, by definition, takes a long time to heat up.  It is thus of little use in providing the rapid cooling function that a (highly thermally conductive) object like a steel barrel (radiating into a relatively cool room) provides.  Take out the sudden cooling, and you generate less draft.  This is especially important when the fire is first being lit and you want to quickly create a draft so that minimal smoke escapes back into the room.

I don't think I'm understanding. If the rmh was vertical, it would be a chimney or stove pipe with some mass glued to it. Most of the heat would go straight up and out the chimney.

I might be wrong, but I was under the impression that the barrel or bell was critical to the settling out of heavier, cooler, partially combusted gasses that circle back to be completely combusted before being pushed on to the rest of the system, increasing system efficiency by ensuring a complete burn and reducing soot and ash buildup by that same token.

Plus, I think the idea of having a spot to put your kettle/stew pot/cauldron/open system hot water tank is a terrific way to stack functions, as those places where you really need an rmh seasonally would also be great places for the enjoyment of tea, hot soup, stews, and potions or spells (because, why not, right?). Also, hot water is great for washing dishes and showers or baths.

Yes, my answer got a bit flippant, but seriously, your purely theoretical question is answered by looking at an old woodstove. If an rmh were vertical, it wouldn't be an rmh. If an rmh didn't have a barrel/bell, it wouldn't be an rmh. You'd simply have an rmh-style burn tunnel that exhausted directly from the burn tunnel, kind of like if you assembled a quick rocketstove from cinder blocks and put a stovepipe covered in cob vertically over the exhaust. Sure, the mass would get heated, but only by the heat transferring through the stovepipe in contact with the combusted gasses, and only those gasses that touched the metal. The bulk of the heat would be shot skyward.

I hope this answers your questions about the need for a barrel/bell.

-CK

The barrel is not critical to cool the gases.

It has no sucking down effect. It might juste impede less with the downdraft flow forming in the barrel, to reach the mass or Bell.

My backup on this. If the barrel was sucking down. You wouldn't need to put a chimney on, when doing an outdoor test with the barrel.

http://batchrocket.eu/en/applications#redbell

Thank you all for your replies so far.

Tim Bermaw:

Your first and second points explain the effect the barrel has, but does not, in my mind, explain why it absolutely must be there.

Your third point corroborates what I mentioned in my second post, but I'm just not convinced the rmh would fail completely if you removed this extra push from the contracting gasses. After all, the gasses are contracting all the way through the thermal mass, correct? What difference does it make which precise location the contraction happens?

Your fourth point has led me to completely reevaluate my understanding of the rmh as follows:

Hot air is less dense, which causes it to become more buoyant and convect upwards. The lower density means hot air creates a zone of low pressure, not high (as your comment suggests). Put a different way - hot air means the molecules are rarefied = low pressure. Increasingly hot air causes high pressure only in sealed containers (because it desperately seeks to become low pressure). The rmh is not a sealed container. So the cooler gasses after the barrel means that this air is more dense, higher pressure, and therefore harder to push through the mass. So cooling the gasses in the barrel actually opposes air flow.

Is that correct or have I got my wires crossed? I need to hear from somebody with a PhD in fluid dynamics

EDIT: I've just realised that although the rmh is not a sealed container, calling it open is also incorrect - the mass of air moving through the cob bench is more massive than the air in the burn tube, so there actually is back-pressure on the expanding air in the burn tube. Further more, the air in the burn tube is confined at the sides. These factors combined is what causes the air to accelerate. The air has a force opposing its expansion, so it must accelerate - like air over a wing. But air over a wing is low pressure, which brings me back to the start. I need to go away and think about this some more. Now I REALLY need somebody with a PhD in fluid dynamics

I do completely agree with your reasoning that no combustion should be happening in the barrel.

Chris Kott:

I understand your comment that most heat will go straight up the "chimney", but try to think of this as purely theoretical. What if the chimney (and mass) was of infinite height? The heat would eventually be transferred by either conduction or infra-red radiation until the gas and mass were in equilibrium. Of course, an infinitely long chimney would also mean an infinite air pressure at the base, unless you took gravity away, which would then make convection impossible.

I could modify my original scenario by asking, what if the burn tube turned 90 degrees at the top and then went into a traditional cob bench? So imagine a scenario where your burn tube is in the basement, but the cob bench is on the next floor up. Would it still work? I don't see why not. Therefore the barrel is not performing some critical "magic" that I'm not seeing, and could be removed in this scenario.

Your second point is saying that combustion is still happening in the barrel, which I now believe with confidence it is not.

Agree with your third comment about the hot barrel top being a "nice to have". But it's not mandatory.

In your final comment, you're forgetting conduction is not the only way that the heat is transferred from the air to the mass. It will radiate infra-red as well - which is uninfluenced by gravity. EDIT: Infra-red is influenced by gravity but only in the Einsteinian sense, not in the Newtonian sense

To wrap up:

I'm trying to distill the principles of the rmh down to its absolute purest form. As this point, I still feel the barrel does not contribute to the overall efficiency of the system. However it does contribute to the overall practicality of the system.

An exposed  barrel is  not needed for a rocket to work.
Many documented systems use no barrels at all.
If you want to run a rocket without a barrel, you can use an actual chimney to create draft.
Many of the top rocketers  advocate  a good chimney for any system.
The original rocket mass heaters were intended to be used surreptitiously, so avoiding a chimney was a key design element.
A way to exhaust a rocket strait up yet still collect the heat is to use a bell rather than a bench.

The barrel is just a radiator.

If somebody can show me a rocket, with a barrel on top, as an outside test. With no chimney, flue or anything but the barrel on top of the heat riser, and a gap at the bottom of the barrel, to let the gases out. And this setup works without smoking or works as well as the same rocket without barrel. I want to see.

Except in a few and far between cases, where there is no chimney. And in these cases, the heat riser acts as a chimney anyway.  A chimney is needed to run a Rocket. There is no sucking from the barrel.

Matt, you have heard that this is the case. But, it's all gibberish. Try the test i mention above. If the barrel was beneficial to the draft, a J tube rocket; feed tube, burn tunnel; and heat riser. Well made and properly insulated; would run better when we put a barrel over it.  It's not!

Again, people will disagree. Guys, do the test.

If you want no barrel, it's perfectly possible.

Look at this one

 

How strange! No barrel? How can it work?  And it's a J tube, not a Batch.

Matt Coston wrote:

If the mass was above the burn tube, so the insulated burn tube transitions seamlessly into an un-insulated thermal mass and the tube was vertical from beginning to end, would the efficiency of an rmh be affected?
.

To answer this one. Yes it would be affected.  Just think. What you are describing is a chimney.

Taller chimney, with no hindrance from a bench, bell or anything. Means bigger draft. Bigger draft equals to more excess air in the fire, so, fire gets cooler. Equals to a combustion of a lesser quality. Ok, it might self adjust up to a point, burning the wood faster. But in the end, the excess air will be too much. So combustion will be worse.

This is just for the fire part of it.

The other part of a rmh efficiency is heat exchange. In your tall "mass", heat makes the hot air rise (well not exactly, it's pushed up buy colder denser gases)  The hotter the gases, the faster they travel up. So your mass can't extract the heat much, since there is no time to do so. Again, the efficiency is worsened.

I have played with something along this line. On a small rocket, about 5 inches diameter. I used 35kg gas bottles. To make the "barrel" and also, wanting to make a bell, with a plunger tube. In order to extract more heat.  I used another of those big gas bottles, with an inlet at the bottom, and an outlet on top, with a tube plunging in the tank,  to near floor level.  Well, at the time, not knowing the bell's rules. It didn't work too well. Gases were bypassing to the plunger tube directly. And gases around in the tank were insulating it from the outside.

So i removed the plunger tube. And, the heat extraction went way up. It was still a "chimney" an up pipe, or whatever you would want to call it. But the gases had room to spread, 5 or 6 time the CSA of the 5 inch pipe. So these were slowing down, and  had more internal surface area to shed their heat.

Being stubborn, as i had been told a bell was better, i later turned this "expansion chamber" to a bell  With no good results. My heat exchange dropped down again. The gas bottle was about right at 4 times the CSA of the intake and exhaust, but it was bypassing.

So what you are describing is possible. Only, i think, with a far bigger section than the heat riser's. And a restriction at the top, of the same size of the heat riser.

Tho, if you go this route. Even if you block the rocket's feed tube. Hot gases will still escape on top of this "chimney" mass. You would need to block the top too. To keep the heat in.

So, , if you were to follow this path. It would be beneficial to use a bell instead.

In france, in old houses, we have those big chimneys. May be 6 sq/ft in section. That i would love to turn into a bell. Usually with a monstrous fireplace bellow.

This would go as

Make a rocket in the fireplace, with the feed tube sticking out of the footprint of the fireplace. Send a plunger tube from the chimney cap to near the bottom of the fireplace. Cast a slab on top of the chimney, around the plunger tube. Close the front of the fireplace around the rocket, so heat riser is in the fireplace, and feed tube out.

I have drawn this before, with a batch. https://permies.com/t/40/31382/Rocket-Stove-cast-riser#246585

HTH.

Max.

William Bronson wrote: The original rocket mass heaters were intended to be used surreptitiously, so avoiding a chimney was a key design element.

Thank you for reminding me about this.

Satamax Antone wrote:If somebody can show me a rocket, with a barrel on top, as an outside test. With no chimney, flue or anything but the barrel on top of the heat riser, and a gap at the bottom of the barrel, to let the gases out. And this setup works without smoking or works as well as the same rocket without barrel. I want to see.

I like your thought-experiment a lot. I agree that a rocket stove would not work better with a barrel over it.

Thank you also for the video of the masonry alternative to the barrel. That is the first time I have seen that. I think that is actually exactly what I was trying to find.

Satamax Antone wrote:The other part of a rmh efficiency is heat exchange. In your tall "mass", heat makes the hot air rise (well not exactly, it's pushed up buy colder denser gases)  The hotter the gases, the faster they travel up. So your mass can't extract the heat much, since there is no time to do so. Again, the efficiency is worsened......So, , if you were to follow this path. It would be beneficial to use a bell instead.

I seem to have given you the impression that I actually want to build this. I do not. The first sentence in my OP was "This is a purely theoretical question". I'm trying to reduce the rmh down to its absolute purest form. Please see the modified example I gave in an earlier post:

Matt Coston wrote:I could modify my original scenario by asking, what if the burn tube turned 90 degrees at the top and then went into a traditional cob bench? So imagine a scenario where your burn tube is in the basement, but the cob bench is on the next floor up. Would it still work? I don't see why not. Therefore the barrel is not performing some critical "magic" that I'm not seeing, and could be removed in this scenario.

At this point I am now convinced the only thing the barrel does is 1) release heat into the room quickly (because the cob bench is slow-release only), and 2) direct the gasses down to floor level so they're at an appropriate height to enter a cob bench. Also to a lesser extent, the low-level of the pipe in the bench means a floor-level exhaust port can be used which means no chimney to indicate somebody is living there.

Matt Coston wrote:Your first and second points explain the effect the barrel has, but does not, in my mind, explain why it absolutely must be there.

It doesn't absolutely need to be there.  In fact, once the riser tube is up to temperature, the draw created by it (the riser) dwarfs everything else.  The biggest contribution that the barrel has is when the system is cold and you are lighting it up.  A few minutes into a fire and the barrel's contribution to draw is minimal (not zero — minimal).

Having said that, some people really, really don't like smoke in their house, so they will use every trick in the book to maximise draw during the lighting phase — which is when most smoke is released due to insufficient/reduced draw from a cold system.

Your third point corroborates what I mentioned in my second post, but I'm just not convinced the rmh would fail completely if you removed this extra push from the contracting gasses. After all, the gasses are contracting all the way through the thermal mass, correct? What difference does it make which precise location the contraction happens?

I never said a barrel was essential, and I never said a RMH would fail completely without one...  A barrel is not essential (as others have mentioned), but it does help increase draw and that is particularly important when lighting a fire.  Is it possible to design a different type of RMH without a barrel which has sufficient draw (in a certain location, under certain conditions)?  Sure.  But that wasn't the part of your original question that I was answering.

All other things being equal, if you contract gasses in the middle of a piping system, air will be drawn equally from both ends.  If you contract air closer to one end, then the resistance to flow will be lowest towards that end, and air will preferentially be sucked in through/from that end, rather than the other end (through the longer/more complicated/more friction-generating pipe section).  Contracting gases within ~1.5m of the feeder tube and ~8m from the chimney cap will result in air being preferentially drawn through the feeder.  Thus having the primary contraction zone close to the feeder helps the air move in the right direction.

The amount of contraction that occurs is directly proportional to the amount of cooling that occurs (the temperature differential between incoming and outgoing gasses).  A short (large diameter) section of thermally conductive steel (freely radiating into a cool room) will strip much more heat from extremely hot incoming gasses than a long section of (narrow diameter) pipe through a large thermal mass with just hot gasses entering it.  Thus the barrel is responsible for more contraction than the flue through the thermal mass, and that's why the draw is preferentially from the front of the system (i.e. via the feeder tube).

It is possible to build a RMH which will work fine (in certain conditions) without something like a barrel, and incorrectly extrapolate that to think that barrels perform no function and should/need not be used anywhere.  However, you can also build a RMH in a marginal environment, where the barrel makes a noticeable difference because it 'tips the balance'.  Since your post gave me the impression that you appreciate subtle nuance, I chose to offer the nuance for your consideration.  Whether your specific location/conditions/design allow you to generate sufficient draw with a cold system or not I don't know.  But at least you now know one trick that might help you keep your ceiling from turning black.

Your fourth point has led me to completely reevaluate my understanding of the rmh as follows:

Hot air is less dense, which causes it to become more buoyant and convect upwards. The lower density means hot air creates a zone of low pressure, not high (as your comment suggests). Put a different way - hot air means the molecules are rarefied = low pressure. Increasingly hot air causes high pressure only in sealed containers (because it desperately seeks to become low pressure). The rmh is not a sealed container. So the cooler gasses after the barrel means that this air is more dense, higher pressure, and therefore harder to push through the mass. So cooling the gasses in the barrel actually opposes air flow.

Is that correct or have I got my wires crossed? I need to hear from somebody with a PhD in fluid dynamics

How about a retired physics teacher?  

The actual physical volume of a gas decreases as it cools upon collision with (and transference of energy to) a lower-temperature steel barrel wall.  That means that — momentarily — a vacuum is created in the space formerly occupied by the gas.  Thus wherever contraction occurs, vacuums are formed, and low-pressure zones are temporarily created.  Since hot gasses are free-flowing fluids, adjacent gasses will be sucked into these vacuum locations.  Upon hitting the barrel wall, they too will lose energy, contract, and create momentary vacuums.  The cooled gasses, now more dense, move down (and out of the way) due to buoyancy — clearing the way for more hot gasses to collide with the barrel wall and repeat the process.  This effectively creates a thin 'layer' (call it a 'zone' if you like) along the inside edge of the barrel wall that is always at low pressure (compared to anything else nearby).

That also happens to be exactly the same thing that happens on the inside pane of a glass window when it's cold outside, and why drafts are always felt on the floor next to such windows.  The failure to connect these two phenomena makes it amusing to listen to people who vehemently insist that barrels don't contribute to draft.  My cats understand the physics quite well — which is why they don't sleep under windows in winter.

EDIT: I've just realised that although the rmh is not a sealed container, calling it open is also incorrect - the mass of air moving through the cob bench is more massive than the air in the burn tube, so there actually is back-pressure on the expanding air in the burn tube. Further more, the air in the burn tube is confined at the sides. These factors combined is what causes the air to accelerate. The air has a force opposing its expansion, so it must accelerate - like air over a wing. But air over a wing is low pressure, which brings me back to the start. I need to go away and think about this some more. Now I REALLY need somebody with a PhD in fluid dynamics

Nah, it'll 'click' for you soon.  It's not that complicated.

If you want to understand RMH airflow, don't worry about what happens in the horizontal burn chamber of a typical J-tube RMH.  The action is in the riser, the barrel, and in any vertical piece of flue leading outside.  Everything else just predominantly creates annoying levels of friction (but sometimes useful levels of turbulence).  Buoyancy is the engine that drives rocket mass heaters.  If you create a path for hot gasses to go up, you increase draft.  If you create a path for cool gasses to go down, you increase draft.  If you try to force hot gasses to go down, you decrease draft.  If you try to force cold gasses to go up, you decrease draft.  If you design a system with slopes, only the vertical component of those slopes will effect draft substantially.  Horizontal components decrease draft slightly.  Bends decrease draft slightly (more).  Your system needs enough draft to function at your location and under your local conditions.  Any less and you'll end up with backdraft and a black ceiling.  Any more and you lose efficiency because the flue gasses will move through the system too fast, and will thus have insufficient time to transfer their heat to the barrel and the thermal mass.

No need to over-complicate things.

PS:  Nearly forgot.  A small vertical piece of flue/chimney at the end of the horizontal run of pipe through the thermal mass is all that you need to offset the frictional forces enountered within the thermal mass.  If you balance the thermal mass/chimney then it becomes neutral as far as pressures and airflow are concerned.  That simplifies the equation greatly, and lets you focus on the riser/barrel section independently.  Divide and conquer, as they say.

A data point on the barrel topic: It has been reported from multiple sources that people who have added cob around the barrel of a well-functioning RMH have sometimes seen reduced draft, even to the point of malfunction. I would say this indicates that a barrel as part of a complete system has value. A well-built bell can serve the same function, and has very little friction, as the long duct is eliminated.

Tim Bermaw wrote:The biggest contribution that the barrel has is when the system is cold and you are lighting it up.

Yes, that feels right to me.

Tim Bermaw wrote:Thus the barrel is responsible for more contraction than the flue through the thermal mass, and that's why the draw is preferentially from the front of the system (i.e. via the feeder tube).

I think that's an important piece of new information to me. I hadn't considered where most of the heat transfer is happening. The thermal mass really is just to soak up the low-grade left-over heat from the main heat transfer through the barrel. If you tried to dump all the heat into the thermal mass, it would have to be gigantic, and cause extreme resistance to air flow.

Tim Bermaw wrote:That means that — momentarily — a vacuum is created in the space formerly occupied by the gas.

That's another great insight. As the hot gas cools, it creates a boundary zone of even lower pressure around it. This also reinforces your idea that the barrel has a greater effect at the start, when it is cold. If you extrapolated the scenario out into the realm of total insanity - imagine a barrel that transfers heat so fast that it liquefies the gas at the top of riser. You'd get a near perfect vacuum in the barrel, pulling additional air up the riser. And as you say, there is less resistance down the short leg to the burn tube than through the cob mass bench, so air is preferentially drawn through the burn tube.

Tim Bermaw wrote:If you balance the thermal mass/chimney then it becomes neutral as far as pressures and airflow are concerned.  That simplifies the equation greatly, and lets you focus on the riser/barrel section independently.

Another great insight.

Glenn Herbert wrote:A data point on the barrel topic: It has been reported from multiple sources that people who have added cob around the barrel of a well-functioning RMH have sometimes seen reduced draft, even to the point of malfunction.

This corroborates what Tim said - the barrel is doing most of the work to extract heat from the gas. If you cover the barrel up, that small vacuum being generated by the barrel disappears and stalls the entire system.

This is all really excellent stuff. I feel like I'm finally getting a grip on what's really happening inside a RMH.

At this point I just want to re-run a thought-experiment I did in a earlier post:
If you just took a rocket stove and put a barrel three-quarters over it, would the performance improve? The answer is no, but this is only because the small vacuum caused by the cooling air in the barrel would preferentially suck air in from outside the barrel (path of least resistance), not through the burn tube. If you were to somehow restrict the exhaust of the barrel, the path of least resistance becomes the burn tube. The idea of restricting the exhaust to increase air intake feels at first completely illogical. But the resistance only needs to be just enough to tip the balance of path of least resistance. This perhaps hints at why RMHs are so easy to get wrong. They are an incredibly fine balance.

Well, I'm convinced - the barrel improves efficiency. Thank you again to everyone who has contributed, especially Tim Bermaw. I really feel like I learned something here.

You could remove the barrel, but its essence must remain.

What I have heard from experts is that around half of the heat is dispersed instantly by the barrel, and half into the mass. this would obviously vary based on the balance of sizes. An important feature of the barrel or equivalent is instant heat while the mass is warming up and still cold to the touch.

Glenn Herbert wrote:An important feature of the barrel or equivalent is instant heat while the mass is warming up and still cold to the touch.

This 'feature' can be very important, depending on the climate, orientation and design of your house.

A RMH that is 'all mass, no barrel' is unresponsive.  It takes a substantial amount of time to get the mass up to temperature and, once it's there, it takes a substantial amount of time for it to cool down.  Whilst this is ideal for folks that live in cool-temperate, alpine or Arctic climates where you go into winter-mode for a solid 3+ months at a time, it's far, far less useful for those living in temperate, warm-temperate or Mediterranean climates with passive solar designs (or for those that work long hours and always come home to a cold house).

In warmer climates, the temperature is more varied.  Even in the middle of winter you can get runs of nice, sunny days that pour light through your equator-facing windows and, thanks to high SHGC glazing and passive thermal mass (e.g. an exposed concrete slab), the inside of the house can get quite warm without any additional heating whatsoever.  So you might start off with a cold morning, but by ~1100 the rooms are comfortable and by ~1400 the rooms are quite toasty.  In that sort of situation you really want a RMH that can output the majority of its heat up-front (i.e. during the cold morning), and only bank a relatively small amount of heat for the rest of the day.  In such a situation, 'less mass, more barrel' makes for a more responsive RMH and one which is better-tailored to your conditions and needs.

It seems as though a very large fraction of RMH enthusiasts live in cool-temperate (and colder) climates, so a lot of the information/opinions out there are biased towards banking as much of the heat as you can, and minimising the barrel where possible (or eliminating it completely).  That's fine — for them, where they live, in their house — but not everyone lives where they live, in their house.

Ultimately, whether or not a barrel (or equivalent) is needed primarily depends on the thermal profile of the house.  Specifically, how responsive the heater needs to be in order to provide immediate heating of the house when other forms of heating (e.g. passive solar) are unable to.  If you have at least some need for immediate heating, then your RMH design has to incorporate some type of 'feature' to allow said heat to be released without lag.  A steel barrel is one way to implement that 'immediate heat' feature.

It would perhaps be wise for anyone interested in a RMH to decide what level of responsiveness they need prior to progressing far down the design path.  A desired responsiveness of 0% (i.e. 0% immediate heating, 100% banked) will demand/suggest a very different RMH design than a desired responsiveness of 70% (i.e. 70% immediate, 30% banked).

Matt Coston wrote:As the hot gas cools, it creates a boundary zone of even lower pressure around it. This also reinforces your idea that the barrel has a greater effect at the start, when it is cold. If you extrapolated the scenario out into the realm of total insanity - imagine a barrel that transfers heat so fast that it liquefies the gas at the top of riser. You'd get a near perfect vacuum in the barrel, pulling additional air up the riser. And as you say, there is less resistance down the short leg to the burn tube than through the cob mass bench, so air is preferentially drawn through the burn tube.

Yep.  You got it.  The location of contraction zones is very important.

Glenn Herbert wrote:A data point on the barrel topic: It has been reported from multiple sources that people who have added cob around the barrel of a well-functioning RMH have sometimes seen reduced draft, even to the point of malfunction.

This corroborates what Tim said - the barrel is doing most of the work to extract heat from the gas. If you cover the barrel up, that small vacuum being generated by the barrel disappears and stalls the entire system.

This is all really excellent stuff. I feel like I'm finally getting a grip on what's really happening inside a RMH.

Excellent!  Sounds like things are starting to 'click'.

At this point I just want to re-run a thought-experiment I did in a earlier post:
If you just took a rocket stove and put a barrel three-quarters over it, would the performance improve? The answer is no, but this is only because the small vacuum caused by the cooling air in the barrel would preferentially suck air in from outside the barrel (path of least resistance), not through the burn tube.

Bingo!  Trying to compare RMHs with barrels to RMHs without barrels by first removing the flue, is like trying to compare sedans to coupes by first removing the wheels.

If you were to somehow restrict the exhaust of the barrel, the path of least resistance becomes the burn tube. The idea of restricting the exhaust to increase air intake feels at first completely illogical. But the resistance only needs to be just enough to tip the balance of path of least resistance. This perhaps hints at why RMHs are so easy to get wrong. They are an incredibly fine balance.

Well, I'm convinced - the barrel improves efficiency. Thank you again to everyone who has contributed, especially Tim Bermaw. I really feel like I learned something here.

You could remove the barrel, but its essence must remain.

Cheers.  Note that none of my posts were intended to suggest that you design your RMH in any particular way.  I was just trying to help you understand why the stereotypical RMH actually works by exploring some of the nuance so you can use that to your advantage.  Mission accomplished, methinks.

Just one last point though:  I would be careful with the "its essence must remain" generalisation.  If you don't need immediate heating, then you don't need the primary function that a barrel performs and hence can design a RMH that does not have a barrel.  Since I covered that in my response to Glenn (above) I refer you simply to that.

Satamax Antone wrote:The barrel is just a radiator.

If somebody can show me a rocket, with a barrel on top, as an outside test. With no chimney, flue or anything but the barrel on top of the heat riser, and a gap at the bottom of the barrel, to let the gases out. And this setup works without smoking or works as well as the same rocket without barrel. I want to see.

Except in a few and far between cases, where there is no chimney. And in these cases, the heat riser acts as a chimney anyway.  A chimney is needed to run a Rocket. There is no sucking from the barrel.

Matt, you have heard that this is the case. But, it's all gibberish. Try the test i mention above. If the barrel was beneficial to the draft, a J tube rocket; feed tube, burn tunnel; and heat riser. Well made and properly insulated; would run better when we put a barrel over it.  It's not!

Again, people will disagree. Guys, do the test.

If you want no barrel, it's perfectly possible.

Look at this one

 

How strange! No barrel? How can it work?  And it's a J tube, not a Batch.

Hi, I thought the chimney needed to be next to the combustion chamber in order to heat up and create more thermal siphoning....in the video, his chimney is on the other side of the wall!! ?

The chimney doesn't necessarily need to be in any particular place; if it is close to the barrel or riser, then it gets the boost from that. If you have a good chimney that doesn't require the boost, no problem.

Glenn, thanks.  I guess that's where the art (or maybe the science?) comes in....to know how to build all of the components correctly so that you don't need that extra boost.  i'm guessing those components include
-having the right air intake, primary and/or secondary
-having the riser tall enough
-having a manifold of the right size in relation to other part sizes

it seems like there are formulas for all this with the J tubes, but I haven't yet read Erica and Ernie's book.  i wonder if all the formulas stay true when one is using a stratification chamber/bell(s) instead of a flu system.  i also wonder if the formulas ring true when building batch boxes.  There appears to be so much innovation going on between Peter and others with the batch boxes right now....very exciting!!  Sometimes when I think about what Peter and others have done...it just blows my mind, as I can't even fathom the smarts required.  My goal is to make it to Wheaton's place or an ATC someday and finally get some hands on experience with building a RMH.

The formulas all still apply; though there is no duct length to worry about, the bell internal surface area has to be considered for full heat extraction while not extracting so much that the exhaust is too cool for draft. There is not a "manifold" to account for with bells, but entrances and exits from bells call for similar attention to avoiding flow constriction. It appears that for bell sizing, an 8" J-tube is fairly similar in capacity to a 6" batch box designed according to Peter van den Berg's formulas at batchrocket.eu.