Batchrocket.eu, information site

[Deleted]

There is a small error in the text about the energy being given to the pool :

"In the end, this resulted in the build I would like to show you all in this thread. It's proven to be capable of heating my 16000 liter (565 cu ft) pool from 20 to 30 °C (68 to 86 °F) in 24 hours of wood burning. I did the math, and this would mean that on average, the heater is giving off 10 kWh to the pool, which pleases me a great deal."

It takes 1.16 kWh to add 1°C to 1 m3 of water so for the pool, so the batchbox is rather giving off 1.16*16*10 = 186 kWh in 24 hours instead of 10 kWh.

It means that the 150 mm batchbox fired this way has a power of 7.7 kW (without taking in account the deperditions in the pool) which is that of a big mass heater. Not bad !

[terry]

thanks yasintoda.

'Step six, insulating the rocket stove with vermiculite and creating an oven floor with t-bars and 4 cm thick concrete slabs. T bars were given expansion room. All in all, maybe not the best technical option, but wanted to keep footprint as small as possible. The black soot on the backplate is because I had played with fire on that in the past already.'              possibly 'room for expansion'   and    'from previous firings' or similar.


(I hope Tom is not offended by me 'correcting his english'! I know you are fine with it peter, fingers crossed he is the same)   'Step seven, insulating the concrete slabs with vermiculite /portland cement mix, and laying refractory tiles. Not much space between the walls and the start of the dome, I know. (small footprint remember) I "insulated" that with 5 layers of aluminum foil, which in the end proved sort of satisfactory. The wall heats up to the point you can't keep your hand to it for more than a few seconds, but the good thing is that sitting beside the wall in the evening is rather cosy and comfortable.'   'point where you'


'Step ten, bricklaying on a second mould, restarting over and over. Until in the end I had an arch at the front and a closable window in the back. Of the same surface area as the riser, which is 150 mm diameter. The top of the rear window is flush with the oven ceiling, to minimize flue gas obstruction.'  Maybe that should be 'opening'? A window is kinda, well, a window.  Maybe get rid of the full stop so it reads more fluently, ...''closable opening in the back which has the same cross section as the flue, 150 mm diameter.'



'Step eleven, making a lid for the rear window with a brick and testing the oven. (beer in the butt chicken)
The oven stood in this stage for quite some time, until I could figure out how I would insulate the walls of the boiler compartment at a reasonable cost, and with good efficiency.'   if you decide to change window to opening, then here possibly 'step eleven, closing the opening...' etc. Personal question, I presume at this stage the front served as the exit for the flue gases? That would explain the smoke/soot markings around the front arch opening.



'Step twelve, bought a roll of superwool, and created a skeleton I could tie the insulation to and put around the boiler.
I also created a condensation fluid collecting tray from the bottom of the outer shell of the boiler, that I cut off. The outer shell was no stainless steel, so I hesitated to use it for this purpose, but the metalworker who welded the piping to it assured me that I would not see it rust to shreds any time soon. (of course stainless would have been the better option anyway... time will tell)'      presumably 'not'.



'Step seventeen, I covered both the top of the dome and boiler compartment with vermiculite / portland cement mix, to insulate and make it gas tight. Then I bought and installed a second hand insulated stainless steel chimney, and bended some sheet metal and used some leftover insulation to produce two front covers for the oven and the batch box. Stainless steel handles were bought from IKEA.'    'bent'




'(In order to avoid rantings about the dangers of heating water with fire, and possible pressure buildup and so on, I will mention that I consider this design as safe, since the boiler is connected to the pool and pump unit without any valves or obstructions. My pool filter pump is programmed to pump filtered water through the boiler for 15 minutes, every other 15 minutes. In case the power fails, I can always open up the pizza oven door, and close off the back window, to stop heating the water, and avoid melting my hoses with boiling hot water.)'

Not a correction as such, but I note that 'in the event of failure' which he is at pains to say won't happen, at least the boiler is outside and not in a dangerous position as it would be in a basement (say).



'I should mention that, due to the heat, one crack formed in the outside brickwork during one of the first firings. The crack is slightly larger when the oven is hot, and reduces again when the oven is cold. It doesn't seem to get any worse, so I guess it just means it has created its own expansion joint. Better design on my part might have avoided that, If I would redo this, I would not rest the T bars on the outer wall, but on an inner wall, not touching the outer skin.'  If the comma stays, the capital letter is not used. If the capital letter stays, then the comma is replaced by a full stop. Either way (comma or full stop) works, so this needs adjusting.



'Around the batch rocket the insulation is loose vermiculite, at hindsight it would be better to stabilize this with a little bit of portland cement or clay.'    'in'


not much needed to be done with this one. I am however, not quite sure how it works!! I think the hot gases exit the cook chamber by that 'window' at the back. It looks like one of the photos had it's 'head chopped off' and it is hard to see. From that window it enters the chamber with the ss tank. But there is so much superwool everywhere it is hard to imagine it being heated by anything. So really unsure on the gas path, it is hard to tell from the pics.

Of course, I could be especially dense, don't discount that one.

[peterberg]

Bell sizing is an issue, yes. Some bells are built over and over again so there's expertise about how large a 150 mm could be. And I've built a scaled up system myself with commendable results.

Comparing results to commercial stoves isn't easy because manufaturers are unwilling to show gas analizer diagrams like I am used to. The only thing they mention is their best numbers but that is very misleading.

[peterberg]

May 23, 2016 23:07:33 GMT -8 terry said:

(I hope Tom is not offended by me 'correcting his english'! I know you are fine with it peter, fingers crossed he is the same)

I think he is grateful, at least to somebody who's doing the proof reading of something which is shown to the world.

May 23, 2016 23:07:33 GMT -8 terry said:

Personal question, I presume at this stage the front served as the exit for the flue gases? That would explain the smoke/soot markings around the front arch opening.

I would say the heater is warmed up with the opening at the back closed.

May 23, 2016 23:07:33 GMT -8 terry said:

not much needed to be done with this one. I am however, not quite sure how it works!! I think the hot gases exit the cook chamber by that 'window' at the back. It looks like one of the photos had it's 'head chopped off' and it is hard to see. From that window it enters the chamber with the ss tank. But there is so much superwool everywhere it is hard to imagine it being heated by anything. So really unsure on the gas path, it is hard to tell from the pics.

In fact, when the oven door is closed the gases exiting from the riser will go through the opening at the back and down between the boiler vessel and the insulated external wall. I think it would be better to have more space around the tank, this is probably a bit cramped. So it is a contraflow heater heated by a batch box and somewhere there's a pizza oven in the gas path.

[terry]

May 24, 2016 1:26:34 GMT -8 peterberg said:

Bell sizing is an issue, yes. Some bells are built over and over again so there's expertise about how large a 150 mm could be. And I've built a scaled up system myself with commendable results.

Comparing results to commercial stoves isn't easy because manufaturers are unwilling to show gas analizer diagrams like I am used to. The only thing they mention is their best numbers but that is very misleading.

'manufacturers' and 'analyzer' (terry is giggling at his old man daddy joke over his first cup of coffee in the morning)

Yeah, see the problem with bell sizing. I think it needs a bit of attention put on it at some stage. I recall stating somewhere that 'later data will be given how to size bells for the heater size' tho not in those words.

I was not particularly hinting at comparing stated manufacturer results, more just 'in europe the maximum CO limits are ZZZZ and as seen from the testo the batch does YYYYY. In the USA blah blah'.  Just some sort of framework around or in which we can see how these heaters fare.

However, a critique as it were of how the manufacturers obtain their results might be interesting? I have seen plenty on permies along these lines. 'In the lab, using controlled wood sizes and water content etc etc etc'. In other words, an understandable explanation of why these burn far less wood, and seemingly 'violate the laws of physics' when we use a quarter of the wood against a heater supposedly '80% efficient'. Sure, 80% efficient...but under what conditions? Certainly not real world real user conditions. (unless people are lying) our-admittedly anecdotal-reports are real world real people conditions.

[terry]

may as well  make a start at cleaning up the first few chapters.

'workings'



'What actually is a wood fire?

During the combustion of wood the organic molecules are decomposing into smaller pieces. Eventually those become gases, which will form flames when it burns. The results of complete combustion are: heat, carbondioxide (CO2) and water.'    not needed, 'sounds a bit odd'   perhaps ' and are the source of flames as they burn'.



'Mixing

Woodgas is extremely combustible, explosive even, when it's hot enough and thoroughly mingled with fresh air. This mingling doesn't occur spontaneously, the gases and air need to be shuffled quite a bit in order to achieve that.'      I'd keep it in line with the title, 'mixed' and 'mixing'.   shuffled does not work that well, but there is no 'single word' quick fix.  How about '...occur spontaneously, we achieve it because of the highly turbulent conditions in the port and riser.' (maybe add a 'see below' as these get explained later?)

A quick scan ahead shows that, to date, there is no 'overall picture' of the core (which you define as the riser together with the combustion chamber). You mention things like 'combustion chamber, heat riser, port' etc, to us that is all understood but in the mind of a 'new person' there is little or no concept of what they are, where they are and how they fit together. There are many suitable sketch up pics on the later pages, perhaps bring one in here with labels on each part???


here is a perfect example....'Turbulence in the Batch Box

In the world of Rocket Heater builders the name of that short insulated internal chimney is called a "heat riser". The high and narrow opening at the bottom side is called the "port". The combination of riser and combustion chamber is called the "core". The way turbulence is invoked is different from a normal wood stove.'

A sketch up, or picture here would be helpful in getting a mental grasp of what was just written.


Hmm, I think the first time through-whilst 'getting to know you'-I only corrected the obvious spelling mistakes etc. I think it needs a bit of a re-write, that I don't have time for right now. I will have a go some time soon, put it here for your consideration and you can take or leave it (or parts thereof) as you see fit. Just like always really!

I am pretty sure I will get all of the points you make in it, hopefully it will read a little better tho.

[peterberg]

Hi Terry,
The descriptions at the start of each chapter are meant to function as an index. I don't want any pictures there, these belong in the article itself. Honestly, I like this structure, but it's possibly not very clear at first glance. But hey, the index of a book isn't either, the difference being here is a short description of what is coming instead of just a title.

Housework is done.
Heaters in Europe are regulated by means of the CO levels but fine particles regulation is on its way. In the US, regulation is done by PM (particulate matter) and there are new rules coming as well. I find this too complicated so I'll leave this ongoing battle to the specialists.

[terry]

hi peter. Pity about not wanting to wade into the pool of european and US regs, tho I fully understand. If I have your permission, I might ask on permies for links to any previous posts that 'explain why RMHs seemingly violate physics by being many times more efficient than standard heaters that are rated as 80% efficient', or something along those lines. I know I have seen discussions like that, just have to find them. One being a facebook page??? Anyway, if some good links turn up I will have a look and see what I can turn it into, and present it here and you can rule it in or out. (that's if you agree on me asking) Obviously, a standard heater MAY be 80% efficient in the actual combustion, but as sure as my name is john (which it ain't) if the flue gases are a couple of hundred degrees the heating side is not that efficient.

Anyways, that is a bit of a longer term assignment, and won't be done without your go-ahead.


I also get your thoughts on the layout of the site, and that in the opening descriptions you do not want diagrams/pictures. That is fine. My suggestion of an overarching clarifying picture of what all the different parts are does not have to be within the opening descriptions at all. As an example, the picture could simply be inserted after the verbal 'headlines' you already have, and just before the first enlarged description (wood fire).  There is a half cutaway of a core down the page, even with that (and also that it is not a full view) the different parts are not show or labelled. I just had a quick look at the next page (building??) and even on that page there is no overall picture that shows what is what and how they go together.

It's a bit like discussing how to build a tractor, 'we have an engine, wheels etc etc' yet at no point is there an overview that show where each part is and how they relate. Ok, this is not as complex as a tractor, but you get the drift. All the mental effort required to 'put it all together' here is done by the person who has no idea of what it is!

It is simply a (unnecessary in my view) barrier to understanding.



Anyway, last I'll say on it. I am sorry, got out of bed late this morning so I'll have to hold off a little longer on starting the re-write.

[terry]

Wood is able to react with oxygen under specific circumstances. The result being the production of heat plus CO2 and water. Along the lines what natural gas is doing in a proper natural gas burner. Wood combustion isn't difficult at first sight. Some small dry branches, some paper, a match put to it and the fire is started.

To keep that fire going well becomes more difficult but not much. Higher temperatures than the few hundred degrees Celsius of a small fire without letting the fire grow excessively calls for a little more effort. It becomes necessary to build something around in order to shield it from the surroundings. By doing that, something like a housing will emerge which will keep part of the heat inside.

Next step should be a smokeless fire although this will take quite a lot more effort to achieve. Smoke is just fuel that hasn't reacted with oxygen and dissipates in the air. Disappearing from sight, because a stove which combust its fuel badly can be smelled over a wide area in the outside environment.

Quite literally the entire content of wood can be converted into heat, and as such smoke is an indicator of incomplete conversion of fuel into heat. Provided the temperature of the smoke is quite high and is thoroughly mixed with fresh, preferably pre-heated air. As soon as the smoke cools down there's no chance of easy combustion anymore. A plume of smoke rises, the sign of wasted fuel. Even the carbonmonoxide which is produced by every fire which is rightfully feared as very dangerous is fuel. The town gas which was produced in Britain and the USA which was produced well into the twentiest century consisted for 8% of CO. In order to reduce the risk of poisoning a foul-smelling substance (mercaptan) was added to the gas to indicate to the user that there was a leak or an unlit burner, the gas having no odour of its own.




Wood combusts completely with oxygen under specific circumstances (heat and ignition source), resulting in heat, CO2 and water. Even kiln dried wood contains some moisture, and the chemical reactions in the fire have water as a product and so water is naturally present in the flue from both sources. These same end products are found in natural gas burners. On the surface, burning wood does not seem too difficult, some small dry branches with paper, put a match to it and the fire is started.

To keep that fire going well becomes more difficult but not much. Higher temperatures than the few hundred degrees Celsius of a small fire without letting the fire grow excessively calls for a little more effort. It becomes necessary to build something around in order to shield it from the surroundings. By doing that, something like a housing will emerge which will keep part of the heat inside. (did you mean something like 'to keep it burning cleanly'?? I am assuming that in my rewrite, correct as needed)

Once we have the fire, to have it burn cleanly takes just a bit more thought and effort. Firstly, we need higher temperatures than the few hundred degrees of a small fire and we need to 'keep it under control', not let it grow excessively. By insulating the fire itself we keep the heat from the fire 'within the fire' which aids complete combustion and retains most of the heat from the fire within the combustion chamber.

The result of these 'new conditions' (very high heat, 'Goldilocks air supply'-not too much, not too little, juuust right) will be a smokeless fire. Whilst being simple aims, a lot of effort was required to find the parameters presented here that achieve this smokeless fire. To understand why there is no smoke, it must be realised that smoke is simply unburnt fuel, nothing more, nothing less. In fact, up to 60% of the available energy of wood is found in these combustible gases. [source...http://www.heartheat.com/flameworksOLD/index_WoodComb.htm only added here so you can check the source, not really intended to be part of your site. Tho it can be of course] So smoke is not 'just a nuisance' which affects a large area in the outside environment, it is factually 'money up the chimney' in a very real sense.

Quite literally, apart from around one percent ash, the entire content of wood can be converted into heat, and as such smoke is only an indicator of incomplete conversion of fuel into heat. These heaters (as described earlier) have very high temperatures of combustion, so the smoke is very hot. We combine pre heated air with that smoke which results in spontaneous combustion of the smoke. Even the (rightly) feared carbon monoxide-a deadly poison-is in fact simply unburnt fuel (recall above that in complete combustion CO is not a product of wood burning). CO has no odour of it's own, so when town gas (up to 8% CO) was used for heat well into the twentieth century mercaptan (a foul smelling substance) was always added to the gas, simply to alert people to any leaks or unlit burners. The point remains though, CO is a fuel (and used as such in town gas) and all of the woods energy content can be extracted if burnt correctly.




Change, modify or ignore as you see fit. If I got something 'wrong', let me know how and where and we'll work it out.

[peterberg]

May 26, 2016 13:41:59 GMT -8 terry said:

If I have your permission, I might ask on permies for links to any previous posts that 'explain why RMHs seemingly violate physics by being many times more efficient than standard heaters that are rated as 80% efficient', or something along those lines. I know I have seen discussions like that, just have to find them. One being a facebook page??? Anyway, if some good links turn up I will have a look and see what I can turn it into, and present it here and you can rule it in or out.

Hi Terry,
Do you mean writing a full-blown article or just a small introduction to a couple of links? As such I am not against links when they are functional.

May 26, 2016 13:41:59 GMT -8 terry said:

I also get your thoughts on the layout of the site, and that in the opening descriptions you do not want diagrams/pictures. That is fine. My suggestion of an overarching clarifying picture of what all the different parts are does not have to be within the opening descriptions at all. As an example, the picture could simply be inserted after the verbal 'headlines' you already have, and just before the first enlarged description (wood fire).  There is a half cutaway of a core down the page, even with that (and also that it is not a full view) the different parts are not show or labelled. I just had a quick look at the next page (building??) and even on that page there is no overall picture that shows what is what and how they go together.

I see what you mean, you've got a point, I'll think it over.
I changed the text of 'wood fire' for yours, looked pretty good.
Maybe I should add that link too, but it's asking for a password. It could be clicked away, I ain't sure it is meant to be publicly visable.

[terry]

'Wood combusts completely with oxygen under specific circumstances (heat and ignition source), resulting in heat, CO2 and water. Even kiln dried wood contains some moisture, and the chemical reactions in the fire have water as a product and so water is naturally present in the flue from both sources. These same end products are found in natural gas burners. On the surface, burning wood does not seem too difficult, some small dry branches with paper, put a match to it and the fire is started.'   comma prob not needed.


'Once we have the fire, to have it burn cleanly takes just a bit more thought and effort. Firstly, we need higher temperatures than the few hundred degrees of a small fire and we need to 'keep it under control', not let it grow excessively. By insulating the fire itself we keep the heat from the fire 'within the fire' which aids complete combustion and retains most of the heat from the fire within the combustion chamber.'   Well, there does not appear to be a 'secondly' so firstly not needed. duh.


'Quite literally, apart from around one percent ash, the entire content of wood can be converted into heat, and as such smoke is only an indicator of incomplete conversion of fuel into heat. These heaters (as described earlier) have very high temperatures of combustion, so the smoke is very hot. We combine pre heated air with that smoke which results in spontaneous combustion of the smoke. Even the (rightly) feared carbon monoxide -a deadly poison- is in fact simply unburnt fuel (recall above that in complete combustion CO is not a product of wood burning). CO has no odour of it's own, so when town gas (up to 8% CO) was used for heat well into the twentieth century mercaptan (a foul smelling substance) was always added to the gas, simply to alert people to any leaks or unlit burners. The point remains though, CO is a fuel (and used as such in town gas) and all of the woods energy content can be extracted if burnt correctly.'          Just a comment. Feel free to alter anything I write, in this section (for example) when I re-read it I seem to use a lot of brackets (which might not be how you would like it to be) so if you changed it to get rid of excessive brackets (if you felt that way) it would not offend me (at all). Yeah, I went over the top there to make the point clear, it is just that sometimes you get into a habit. Anyway, think we've flogged that horse to death, what I write are simply suggestions that can be used as written or as a guide.

Have not started the next section yet, BUT with all this talk of smoke being unburnt fuel..which can be a surprise to a lot of people-"Oh, I thought it was what you always get when you burn things", ie the concept of no smoke is something beyond their ken-maybe a quick video like the following would be helpful?

[terry]

'Actually, wood isn't burning by itself but is being cooked when heat is applied to it. The gaseous components which are released are highly flammable and are visible as flames. A pure woodgas flame should be blue, hard to see in sunlight. Because there's also glowing carbon dust carried with the flame the colour is red, orange or yellow, the higher the temperature the lighter the colour. When there's a lot of gas in the flame and a tiny bit of carbon it could appear as yellow-purple.

There's also the problem of mixing, that doesn't happen just like that. A cloud of unburned gases is rising above the fire where the mingling of the components doesn't take place by itself. It is more like a column of smoke which burns at best on the outside under the right conditions but inside the cloud or column there's no oxygen so it won't oxydize at all. Mixing is in this respect the most important and at the same time the most troublesome to achieve.

The common system which is used to induce enough turbulence is injecting fresh air at a number of locations at the same time. A lot of air is needed for this system, which is a serious disadvantage because it will lower the temperature of the fire. For most of the metal stoves this isn't a problem, a real wood fire is able to reach such high temperatures that the steel, even stainless steel, will be destroyed in a surprisingly short time frame. In combination with an oxygen rich and carbon frugal environment the steel will corrode very rapidly. As a result, during every burn at the hottest spots heavy spalling will occur.'




Strangely enough, wood itself does not burn. It is similar to petrol, petrol itself does not burn. If you are quick enough you can douse a match in liquid petrol. It is not recommended that you try it, as most likely the match will ignite the mixture of petrol vapours and oxygen just above the liquid petrol. Once that is understood, it is clear then that it is wood 'vapours' mixed with oxygen that are the substances that burn. Heat causes the chemical components in the wood to break down into smaller, flammable components which then combine with oxygen (in the presence of heat), in turn releasing more heat which causes the cycle to continue. A pure woodgas flame should be blue, hard to see in sunlight. Because there's also glowing carbon dust carried with the flame the colour is red, orange or yellow, the higher the temperature the lighter the colour. When there's a lot of gas in the flame and a tiny bit of carbon it could appear as yellow-purple.

Once it is understood that it is the substances released from the wood by heat that combine with oxygen, we can see that complete and thorough mixing of these substances with oxygen is needed for complete combustion. In practice this is the most important objective to achieve and the most difficult. A column of smoke rising from a fire will most likely burn on its 'outer surface', the interface between the smoke (fuel) and the oxygen rich air. Inside the column of smoke there is very little oxygen hence no combustion. It finally leaves as smoke (unburnt fuel) because even though it eventually meets enough oxygen it has cooled enough that combustion does not occur. Recall that there are  three conditions necessary for combustion, fuel, oxygen and heat.

The common method used in combustion heaters to induce this oxygen and fuel mixing is to inject fresh air (usually not pre heated) into the fire from multiple locations. A lot of air is needed for this system, which is a serious disadvantage because it will lower the temperature of the fire, one of the three conditions needed for complete combustion. In metal stoves this cooling of the fire is not regarded as a disadvantage but rather an advantage, as the temperatures that can be reached from a wood fire are great enough that the steel, even stainless steel, will be destroyed in a surprisingly short time frame. The combination of an oxygen rich and carbon poor environment  coupled with high temperatures will corrode the steel very rapidly. As a result, during every burn heavy spalling will occur at the hottest spots.

The heaters described here are designed to maintain the hottest burn temperatures possible-far greater than metal heaters can withstand-and ensure proper mixing of the fuel gas and oxygen by methods described in the following sections.


As always, keep reject or modify as seen fit.

[peterberg]

I read that last part thoroughly and at the end I felt the urge to read along "the following sections".
You are doing well, to my mind, the whole of the text is very readable now. There are a lot of people like me out there, to whom English is their second or even third language. So it is very important the message is understood by non-native readers as well as native readers.

In one word, thanks! Please carry on, your help is appreciated by many.

[terry]

Turbulence in the batch box

In the batch box design a different system to induce turbulence is employed. To be precise, due to the shape of the core and the positioning of the air intakes. Behind the firebox which is deeper than wide the riser is situated. This is a round or octagon tube, placed vertical and the top end is open. The diameter of this tube is equal to or smaller than the chimney diameter.

Between the firebox and the tube there's a high and narrow opening which is called the "port". The cross section area of the port is 70% of the cross section area of the riser. The air intakes are both situated at the front of the firebox. The primary inlet in the door, close to the firebox floor. The secondary inlet is a narrow rectangular steel tube which is on top of the firebox and reaches down by means of a sharp 90 degree corner to end directly at the top of the port inside the firebox.

Due to the fact that turbulence isn't induced primarily by incoming air the total cross section area of the combined air intakes is smaller than would be expected in a normal box stove. Especially when the speed of fuel consumption is considered.


The mixing of wood gas and oxygen in the batch box heater is achieved by the carefully designed geometry of the combustion core and the correct positioning of both the primary and secondary air intakes. These critical dimensions will be given later. The arrangement of these building components is quite simple, the combustion chamber which is longer than it is wide vents to the rear, where a vertical insulated 'chimney' is placed. This is the heat riser referred to above. The function of the combustion chamber is naturally to combust the wood, the function of the insulated heat riser is to allow the final (and complete) combustion of any wood gas produced from the wood fire. From the preceding sections we now understand what is needed to have that complete combustion, fuel (the wood gas), heat (from the fire and from the combustion of wood gas itself, and note again that it is heavily insulated, ensuring as little heat as possible is lost from any combustion process) and oxygen.

Note particularly the connection between these two elements of combustion chamber and heat riser, a high and narrow opening between the two. This is known as the 'port'. It is actually 70% cross sectional area of the cross section of the heat riser (the cross sectional area of the heat riser is one of the 'standard dimensions' in these heaters, namely a dimension from which other dimensions can be scaled). This sudden narrowing in the path of the gas stream has a very important function, as the *same* amount of gas is flowing through the system at all times, when it comes across (or passes through) a narrower opening, it must then speed up at that point (remember, the same amount of gas is passing through at all times and all places). The result of that speeding up at that exact point is that the pressure at that point lowers.

Now referring to the diagram, a hollow tube (shown in black at the top of the combustion chamber) connects the external air directly to the point of lowest air pressure in the system, the opening of the port. This then causes air (or more properly oxygen) to be introduced into the gas stream as it enters the port. As can be imagined, we now have the three conditions needed for combustion, fuel heat and oxygen. The wood gas is already extremely hot, there is an awful lot of heat in this location, all we need now is the final complete mixing of the oxygen and wood gas. One last point to be noted now, as the secondary air being introduced into the port has to pass through the steel pipe located on top of the fire, it has been pre-heated on it's journey to the port.

The thorough and complete mixing of the wood gas and oxygen occurs as the mixture passes through the narrow port and into the heat riser behind. As the gas flow passes through it speeds up, when it suddenly hits the *larger space behind* it suddenly slows down again (remember, we have the same amount of gas at each point flowing through the system, so it speeds up and slows down as required in order to maintain the same flow rate). This sudden slowing down creates a highly turbulent environment in an extremely hot space.

We have just created the perfect set of conditions for complete combustion. Thorough mixing of fuel and oxygen in the presence of extraordinary amounts of heat.

The rather unique way these heaters create the turbulent conditions required for complete mixing of oxygen and fuel has other favorable results for combustion efficiency. The metal box heaters blunt sledgehammer approach means lots of air was introduced. This extends the life of these heaters but as noted reduces efficiency. Because the preheated secondary air in the batch box heater is introduced into the exact point for complete mixing to occur, we do not need anywhere near the same amount of introduced air as metal heaters. So the total cross section area of the combined air intakes is smaller than would be expected in a normal box stove. These smaller than expected air inlets are perhaps even more surprising when it is seen how quickly the fuel is consumed in these heaters.

One last explanation of why 'excess amounts of cool air' is contrary to good efficiency. The essential component in air for combustion is oxygen. Any other component in air is only a passenger, it contributes nothing to the combustion but only serves to cool the fire (they are known as ballast gases. Just as on a ship, ballast is only excess weight and not cargo). As you can see these heaters, by careful use of geometry and taking advantage of natural laws, introduce the right amount of oxygen to the exact spot required for complete mixing and combustion.

Maybe we can extend this idea of 'goldilocks air'. Not only is it not too much or too little air, juuust right, it is also in juuust the right place.




Keep, modify or reject as fit.

[peterberg]

Terry,
Before I add the admittedly very readable text above, maybe it is best you read through the sub-part of this article first, named 'Mixing in the batch box'. This is explaining an important part of what you covered already although not completely, and it feels very double now.

There's a saying in stage playing circles in my country which is called "double opened French porte-prisé doors". Which as you probably can see means French doors, triple over. I very much like to avoid such an effect, it is considered bad practise in stage acting and in writing also.

By the way: what does 'Goldilocks air' means?