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Post by bulbius on Mar 20, 2017 12:48:19 GMT -8
Hi guys! After spending many hours on this forum and on batchrocket.eu I finally decided to convert my rocket mass heater to a batch box system. First of all I want to thank Peterberg and all people who contributed to the project. So this is the starting point: 7" L-channel system featuring a bench crossed by the flue duct 3 times venting to a 13 foot tall chimney.  I used this configuration for about 3 years and I have to say that it worked very well. Very seldom I've experienced smoke problems only during cold starts. I decided to switch to an RBB because I wanted more power (also to heat a fair quantity of water) and because it was quite annoying to feed the stove almost every hour. After destroying the burn chamber and the heat riser this is what remains: a bench crossed by a flue made of brick scraps. The opening is about 20 cm x 26 cm 7.9" x 10 "
I spread some high insulative material over the floor (aircrete actually) and I started to lay down some refractory bricks to build the burn chamber:
 I surrounded the burn chamber with an expanded clay, water and clay mixture contained by a wall made of normal bricks  I made the heat riser out of a mold using a pvc tube of the right diameter. I put a welded wire mesh around the tube to contain a mixture of water, clay and sand to form the inner part of the heat riser. After I used an outer mesh to contain the insulative mixture with expanded clay.  A piece of the riser surrounded by the insulative mixture and featuring the port  P-channel:  Here you can see the smoke passage from the barrel to the bench. The water boiler will be placed on the right next to the wall. Subsequently I made the barrel opening bigger to about 50 cm x 20 cm 19.6" x 7.9"  The water boiler in place: to insulate I used rock wool and aircrete blocks. Practically the boiler is contained inside a 50 cm x 53 cm 19.7" x 20.8" bell 100 cm 39.4" tall  The top of the barrel is in place and the bell containing the water boiler is now closed.  I made an high insulation burn chamber door where I put inside the aircrete block. I made all airtight using fiberglass gaskets.   Inside the boiler I placed a 10m 32.8 foot copper serpentine to heat sanitary water.  |
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Post by bulbius on Mar 20, 2017 14:09:57 GMT -8
Some figures about my RBB:
heat riser:
diameter = 15 cm 5.9"
h = 123 cm 48.4"
box:
width= 22 cm 8.6"
height= 33 cm 13"
depth= 66 cm 26"
port:
width= 6,2 cm 2.4"
height= 25 cm 9.8"
depth= 6 cm 2.3"
p-channel:
width= 6 cm 2.3"
height= 2 cm 0.8"
overhang= 1,8 cm 0.7"
thickness= 2 mm 0.07"
primary air:
width= 6 cm 2.3"
height= 6 cm 2.3"
area= 36 cm 14.17"
gap between heat riser and barrel: 23 cm 9"
water boiler:
diameter: 40 cm 15.7"
height: 82 cm 32.3"
volume: 100 l 26.4 gallons
For several reasons I couldn't follow exactly the sizes from the guidelines. I hope that it will not affect the performance too much.
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Post by bulbius on Mar 20, 2017 14:31:55 GMT -8
I hope you enjoyed the work I shared! After some days of testing and tweaking I can draw some conclusions:
- The stove is much more powerful
- the water heating system is very satisfying
and some side effects:
- From a cold start I almost always experience smoke problems
- Often when I open the door some smoke comes inside the room especially after a new full load when a big quantity of wood gas is extracted.
All this leads me to some doubts:
1) From the readings I understood that this kind of stove runs better with the door closed but sincerely I didn't get whether it is normal that when I open the door or I let the stove work with the door opened some smoke escapes.
2) Does the sloped bricks inside the burn chamber and the heat riser make a big difference on performance?
3) I read somewhere that it's not a good idea to put a grid in the combustion chamber to raise the fuel and the embers. Can you explain me why?
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Post by drooster on Mar 21, 2017 11:19:55 GMT -8
Why are you opening the door?
(Its a batch-box, let it run to completion)
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Post by pinhead on Mar 21, 2017 11:52:09 GMT -8
A better-insulated heat riser and a heat exchange bypass will undoubtedly eliminate the smoke-back.
You've got two effects at work here, due to the relatively poorly-insulated riser:
1) Relatively incomplete combustion within the riser, causing
2) Reduced temperature ratio between the riser and the firebox.
With a super-insulated riser, combustion temperatures are elevated within the riser, producing more draft.
You need the riser to be as HOT as possible to maintain most of the burn IN the riser. This is due to the expansive burn-box when compared to the csa of the riser. As the temperature in the burn box approaches the temperature in the riser, the draft is reduced.
Failing a super-insulated riser, you'll need to re-gain the draft elsewhere. This is where a bypass and/or higher temperature at the exhaust/chimney comes into play.
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Post by peterberg on Mar 22, 2017 3:23:03 GMT -8
Hi Bulbius, welcome to the boards.
Your conversion is done with the convoluted flue route in the bench intact. A 6" system is able to serve a maximum amount of internal surface area in a bell system. What you have now is a barrel around the riser, a bell-like void nearly completely filled with a water tank and a flue bench which is switching back and forth 3 times. As far as ISA concerns, you are in uncharted territory, nobody knows what the heat extraction capacity of this combination could be.
I didn't try to calculate the ISA because there are some things missing: the bench is left out in the list and the water tank is given but the bell around it isn't. Also, I don't see any figures of the exhaust temperature.
Possible restrictions or construction deviations in such a complicated setup which could cause smokeback, in no particular order:
The top and side gaps of the riser and the transition opening between the barrel and the bell.
The space between the brick bell and the water tank.
The transition opening between the brick bell and the bench flue.
The sloped sides of the firebox should be there, now the height/width proportions are different from the recommendations, making the firebox larger than specifications.
The sloped backsweep in the riser should be there, it is part of the design and essentially to ensure good combustion is early stages of the burn.
The depth of the firebox isn't that tight, but extending it over 50% above specifications is almost certainly too much.
If you do want to avoid smokeback in mid-burn, don't open the door. When you want to have a peek now and then, replace the aircrete block by glass.
And last but not least: a grid would convert some of the over air to under air. The net result would be a higher off-gassing rate, killing the clean burning properties. It has been tried and tested, been there, done that.
This is a deceitfully simple looking but very tight design. Everything needs to be there, all the unnecessary bells and whistles are left out already. Even making it more slick looking, like rounding the edges of the port will kill the clean combustion.
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Post by bulbius on Mar 22, 2017 23:21:00 GMT -8
Why are you opening the door? (Its a batch-box, let it run to completion) I just wanted to check what's going on inside  . So I guess the right way is to full load the firebox, close the lid and wait until all fuel has reached the ember phase and then open the door and load it again right? |
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Post by bulbius on Mar 22, 2017 23:38:40 GMT -8
A better-insulated heat riser and a heat exchange bypass will undoubtedly eliminate the smoke-back. You've got two effects at work here, due to the relatively poorly-insulated riser: 1) Relatively incomplete combustion within the riser, causing 2) Reduced temperature ratio between the riser and the firebox. With a super-insulated riser, combustion temperatures are elevated within the riser, producing more draft. You need the riser to be as HOT as possible to maintain most of the burn IN the riser. This is due to the expansive burn-box when compared to the csa of the riser. As the temperature in the burn box approaches the temperature in the riser, the draft is reduced. Failing a super-insulated riser, you'll need to re-gain the draft elsewhere. This is where a bypass and/or higher temperature at the exhaust/chimney comes into play. Thank you for the excellent explanation. I already knew the importance of a highly insulated riser to gain a better draft but I never considered the relation between the riser temperature and the firebox. Usually I build heat risers with refractory bricks surrounded with expanded clay but this time I wanted to make a cylindrical shape with a smaller thickness. I live in Italy and I couldn't find insulating bricks anywhere. I think this would be ideal www.firebrickengineers.com/vacForm.asp but I cannot find even that in Italy. By the way the heat riser should not have a minimal mass before insulation? or is it better if it's made only of insulating material? |
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Post by bulbius on Mar 23, 2017 2:00:07 GMT -8
Hi Bulbius, welcome to the boards. Your conversion is done with the convoluted flue route in the bench intact. A 6" system is able to serve a maximum amount of internal surface area in a bell system. What you have now is a barrel around the riser, a bell-like void nearly completely filled with a water tank and a flue bench which is switching back and forth 3 times. As far as ISA concerns, you are in uncharted territory, nobody knows what the heat extraction capacity of this combination could be. I didn't try to calculate the ISA because there are some things missing: the bench is left out in the list and the water tank is given but the bell around it isn't. Also, I don't see any figures of the exhaust temperature. Possible restrictions or construction deviations in such a complicated setup which could cause smokeback, in no particular order: The top and side gaps of the riser and the transition opening between the barrel and the bell. The space between the brick bell and the water tank. The transition opening between the brick bell and the bench flue. The sloped sides of the firebox should be there, now the height/width proportions are different from the recommendations, making the firebox larger than specifications. The sloped backsweep in the riser should be there, it is part of the design and essentially to ensure good combustion is early stages of the burn. The depth of the firebox isn't that tight, but extending it over 50% above specifications is almost certainly too much. If you do want to avoid smokeback in mid-burn, don't open the door. When you want to have a peek now and then, replace the aircrete block by glass. And last but not least: a grid would convert some of the over air to under air. The net result would be a higher off-gassing rate, killing the clean burning properties. It has been tried and tested, been there, done that. This is a deceitfully simple looking but very tight design. Everything needs to be there, all the unnecessary bells and whistles are left out already. Even making it more slick looking, like rounding the edges of the port will kill the clean combustion. Hi Peter, I know I ventured too much for my first setup expecting it to work . But it's a conversion and you know it's always more problematic than a new setup especially because I was bonded with the position of the bench entry flue which led me to work in a confined space. Anyway apologies for underestimating some parts of your design. If it can helps the bench hosts a flue of about 500 cm2 77.5 square inches CSA. The length is 7,5 m 24.6 feet. The water heater bell is 50 cm x 53 cm 19.7" x 20.8" , the height is 100 cm 39.4". Since to calculate a bell ISA I know that need consider only the surfaces capable of absorbing heat do I need to include also the walls made with insulation material (aircrete and rock wool) or only the surface of the water boiler exposed to the heat? The surface of the water boiler exposed to the heat is about 1 m2 10.7 ft2 Considering also the walls of the bell: (50*100*2 + 53*100*2 + 50*53) - 10000 (barrel opening CSA) + 10000 (water boiler surface) = 23200 cm2 = 2,32 m2 25 ft2 Considering that the CSA of the bell should be at least 5 times the CSA of the entry flue how to calculate the CSA of the bell in this case where there is a water boiler inside? The CSA of the barrel opening to the bell is 1 m2 10.7 ft2 Regarding the firebox door I think I can live without looking inside all the times and I think that insulating the door that normally shed a lot of heat out of the firebox it's a good thing. isn't it ? You stated that rounding the edges of the port will kill the clean combustion, I posted one picture where you can see the port from inside the riser point of view. Do you think is correct as I did or I would have to continue the port with a right angle before connecting with the round heat riser? Thank you Peter |
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Post by pinhead on Mar 23, 2017 7:10:38 GMT -8
A better-insulated heat riser and a heat exchange bypass will undoubtedly eliminate the smoke-back. You've got two effects at work here, due to the relatively poorly-insulated riser: 1) Relatively incomplete combustion within the riser, causing 2) Reduced temperature ratio between the riser and the firebox. With a super-insulated riser, combustion temperatures are elevated within the riser, producing more draft. You need the riser to be as HOT as possible to maintain most of the burn IN the riser. This is due to the expansive burn-box when compared to the csa of the riser. As the temperature in the burn box approaches the temperature in the riser, the draft is reduced. Failing a super-insulated riser, you'll need to re-gain the draft elsewhere. This is where a bypass and/or higher temperature at the exhaust/chimney comes into play. Thank you for the excellent explanation. I already knew the importance of a highly insulated riser to gain a better draft but I never considered the relation between the riser temperature and the firebox. Usually I build heat risers with refractory bricks surrounded with expanded clay but this time I wanted to make a cylindrical shape with a smaller thickness. I live in Italy and I couldn't find insulating bricks anywhere. I think this would be ideal www.firebrickengineers.com/vacForm.asp but I cannot find even that in Italy. By the way the heat riser should not have a minimal mass before insulation? or is it better if it's made only of insulating material? Whether or not the riser should have "some mass" has been debated and I'm not sure there's a consensus. There are basically two arguments: A riser with some mass, while increasing "start-up" time, adds a slight heat flywheel that can assist draft later in the burn. On the other hand, a riser with as little mass as possible (as in the case of a ceramif-fiber blanket riser), start-up is drastically decreased, and the height of the riser can be slightly reduced. Both arguments presuppose, however, that the riser will still be made of very light and insulating materials such as insulating firebrick, ceramic-fiber board, or ceramic fiber blanket. Dense anything is not welcome in the heat riser. IFB would be on the more "massive" end of the scale while ceramic-fiber blanket would be on the opposite end. I can compare my current build heater - with a ceramic-fiber riser - to my previous build with a clay-perlite mix (still fairly insulating). The new heater with the ceramic-fiber riser starts much more quickly and draft is much stronger - even with lower exhaust temperatures when compared to the previous build. Adding a bell bypass increases draft even further. Note, however, that even with the ceramic-fiber blanket heat riser, I still can't open the front door without smoke escaping unless the bell bypass is open. Even in an optimized system, the chimney provides a large portion of the draft once the heater is up and running. |
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Post by satamax on Mar 23, 2017 7:48:51 GMT -8
Bulbius, where do you live? Anywhere near turin?
I'm on the other side of the border, about 100km from turin.
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Post by satamax on Mar 23, 2017 7:54:54 GMT -8
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Post by bulbius on Mar 23, 2017 11:01:56 GMT -8
Bulbius, where do you live? Anywhere near turin? I'm on the other side of the border, about 100km from turin. No, I live in Marche area |
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Post by drooster on Mar 23, 2017 11:18:04 GMT -8
Why are you opening the door? (Its a batch-box, let it run to completion) I just wanted to check what's going on inside . So I guess the right way is to full load the firebox, close the lid and wait until all fuel has reached the ember phase and then open the door and load it again right? At that point you could add a nice foil packet of salmon and herbs... |
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Post by bulbius on Mar 23, 2017 11:19:39 GMT -8
I express myself badly, I meant that I couldn't find nothing on local hardware stores. Indeed unfortunately most of the people I asked didn't even know what I was talking about Of course I searched for it online but I was discouraged by the fact that most of the companies don't have an online shop and usually they sell only big quantities. Furthermore I couldn't wait too much so I decided to try the diy way with the expanded clay mold. |
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. So I guess the right way is to full load the firebox, close the lid and wait until all fuel has reached the ember phase and then open the door and load it again right?
