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Post by peterberg on Mar 24, 2017 8:20:20 GMT -8
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? Distract the cross section area of the boiler from the cross section area of the bell which contains it. When the resulting figure is less than 4 times the riser's csa, you almost certainly have a problem built in. Lots of people try to do things which are recommended against in hopes it won't work anyway, I am not surprised. But you asked why the thing showed unwanted behaviour and I told you the possible faults. You built your heater off specs, when it doesn't work as expected you are the guy to rebuild it. 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 ? That might be true in a conservative box stove, but in a batchrocket it's a little bit different. The highest temperatures are obtained in the riser instead of the firebox, the whole system is developed with incomplete combustion at first as an accepted fact of life. Moreover, when the temperature in the firebox is increased too much there will be a combustibles overload and black smoke from the chimney. The capacity of the riser as an afterburner isn't unlimited so it's wise to keep the initial fire from running out of hand. Heat that's shed into the living space isn't wasted, rather the contrary I'd say. 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? Hard to see whether the sides of the port are rounded off, your picture shows the riser side, what I meant was the firebox side. Edit: Insulating the riser is mandatory, as Pinhead already stated. Best riser: highly insulative, high heat resistant, no mass to speak of. No bulbs and lumps inside, straight up. |
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Post by bulbius on Mar 25, 2017 1:49:35 GMT -8
Distract the cross section area of the boiler from the cross section area of the bell which contains it. When the resulting figure is less than 4 times the riser's csa, you almost certainly have a problem built in. CSA water boiler = 20*20*3.14 = 1256 cm2 CSA bell + boiler = 50 x 53 = 2650 cm2 CSA bell = 2650 - 1256 = 1394 cm2 CSA heat riser = 7,5*7,5*3.14 = 176,6 cm2 1394 / 176,6 = 7,9 so the bell CSA is about 8 times the riser CSA. Is this correct? That might be true in a conservative box stove, but in a batchrocket it's a little bit different. The highest temperatures are obtained in the riser instead of the firebox, the whole system is developed with incomplete combustion at first as an accepted fact of life. Moreover, when the temperature in the firebox is increased too much there will be a combustibles overload and black smoke from the chimney. The capacity of the riser as an afterburner isn't unlimited so it's wise to keep the initial fire from running out of hand. Heat that's shed into the living space isn't wasted, rather the contrary I'd say. Yes, in fact I saw this black smoke sometimes on the chimney. Being said that what do you think about putting a little water boiler over the firebox? I mean in the first part near the lid. Will it lower the temperature too much or do you think is a good compromise? Naturally having a super insulated riser. |
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Post by bulbius on Mar 25, 2017 4:18:26 GMT -8
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. Does this happens only during a cold start or also when the stove is hot and running? No doubt that the chimney has its importance, I'm working to improve it by adding 1 m of stainless steel pipe with windproof cap and insulating the existing chimney. |
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Post by bulbius on Mar 25, 2017 4:59:48 GMT -8
Hard to see whether the sides of the port are rounded off, your picture shows the riser side, what I meant was the firebox side. ok from the firebox side, they are not rounded |
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Post by peterberg on Mar 25, 2017 6:47:07 GMT -8
CSA water boiler = 20*20*3.14 = 1256 cm2 CSA bell + boiler = 50 x 53 = 2650 cm2 CSA bell = 2650 - 1256 = 1394 cm2 CSA heat riser = 7,5*7,5*3.14 = 176,6 cm2 1394 / 176,6 = 7,9 so the bell CSA is about 8 times the riser CSA. Is this correct? Yes, it looks like it. So there's no restriction there. Your firebox is still way too large though, adding the side slopes would help. The sloped backsweep in the riser and using shorter fuel as not to fill the full depth of the box would help as well. I don't trust the bench either, lots of friction in that convoluted gas path. Better to have that implemented as a blind ended bell. It works like a charm, you know. Here's a recent build by Donkey, sporting two blind benches. www.facebook.com/sundogbuilders/Maybe a major rebuild is on the horizon? |
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Post by bulbius on Mar 27, 2017 0:06:14 GMT -8
Your firebox is still way too large though, adding the side slopes would help. The sloped backsweep in the riser and using shorter fuel as not to fill the full depth of the box would help as well. My firebox is large 22 cm 8.6" not so far from the guidelines: 21,6 cm 8.5". Anyway I will add the slopes in the firebox and in the riser. I don't trust the bench either, lots of friction in that convoluted gas path. Better to have that implemented as a blind ended bell. It works like a charm, you know. Here's a recent build by Donkey, sporting two blind benches. www.facebook.com/sundogbuilders/Maybe a major rebuild is on the horizon? Sure! I very like how the bell works and undoubtedly in the future I'll build it |
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Post by pinhead on Mar 27, 2017 5:28:38 GMT -8
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. Does this happens only during a cold start or also when the stove is hot and running? No doubt that the chimney has its importance, I'm working to improve it by adding 1 m of stainless steel pipe with windproof cap and insulating the existing chimney. With the bypass closed, there are only two times in which I can open the door without smoke escaping into the room. The first is a few minutes after start-up. At that point, there is enough heat inside the well-insulated heat riser to support secondary combustion. With the firebox fully loaded with wood, the velocity moving around the fuel is high enough to prevent any gasses from moving towards the door. At this point, the batch box is performing very similarly to a J-tube, with the only tips of the wood burning just in front of the throat of the stove. The second time is at the very end of the burn and at the beginning and through the coaling stage. At this point, there are no more volatile elements burning and, thus, only CO and CO2 is produced. However, even at this stage I suspect CO and CO2 will escape the room but, being colorless and odorless, my sniffer can't detect them in the air. The easiest way to tell if gasses are escaping is whether or not heat can be felt escaping the top of the firebox with the door open. If there is heat escaping, there is smoke escaping (even if you can't smell it). In my stove, I can open the bypass for about 10 seconds - enough to get the chimney temperature elevated - and open the door during any part of the burn without fear of smoke escaping. This presupposes, however, that the stove is burning "normally" (meaning secondary combustion has started and thus the firefox isn't full of smoke). |
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Post by bulbius on Mar 27, 2017 8:21:31 GMT -8
With the bypass closed, there are only two times in which I can open the door without smoke escaping into the room. The first is a few minutes after start-up. At that point, there is enough heat inside the well-insulated heat riser to support secondary combustion. With the firebox fully loaded with wood, the velocity moving around the fuel is high enough to prevent any gasses from moving towards the door. At this point, the batch box is performing very similarly to a J-tube, with the only tips of the wood burning just in front of the throat of the stove. The second time is at the very end of the burn and at the beginning and through the coaling stage. At this point, there are no more volatile elements burning and, thus, only CO and CO2 is produced. However, even at this stage I suspect CO and CO2 will escape the room but, being colorless and odorless, my sniffer can't detect them in the air. The easiest way to tell if gasses are escaping is whether or not heat can be felt escaping the top of the firebox with the door open. If there is heat escaping, there is smoke escaping (even if you can't smell it). In my stove, I can open the bypass for about 10 seconds - enough to get the chimney temperature elevated - and open the door during any part of the burn without fear of smoke escaping. This presupposes, however, that the stove is burning "normally" (meaning secondary combustion has started and thus the firefox isn't full of smoke). Very clear, thank you for sharing. I experienced similar situations but I have to wait more in the start, probably because of a not so well insulated heat riser. The same during the coaling stage: no visible smoke, no smell but I can feel some heat escaping from the top of the firebox (although the CO detector doesn't alert me). |
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Post by bulbius on May 19, 2017 4:29:13 GMT -8
Hi! I want to update you on the last changes that involved this stove. Every modifications brought a little improvement but the new heat riser made the difference. First of all I extended the chimney a meter and added a wind proof chimney cap. I also changed a corrugated tube near the stove outlet with one smooth in stainless steel. As a result the draft was quite stronger and the stove produced a nice roaring sound. Then I added the sloped bricks inside the firebox to respect the proportions of the design and it seems to me that the embers consume faster this way. Let's start with some serious changes: the fall of the high mass and pretty not insulating heat riser... 
This is the water boiler located behind the heat riser 
I fixed some little imperfections about the port and p-channel sizes  To build an high insulation heat riser in a short time I used aircrete blocks, I know these are doomed at least as metal but however I planned a major rebuild for the next winter that will involve also the bench. This work only served me to appreciate the improvements that a better heat riser can brings. 
I also added the sloped brick at the bottom of the heat riser and I covered its lower part with clay hoping it will protect a little the aircrete blocks.  
Some little tests...  
Heat riser to the final height: 
lower part of the barrel mounted 
I increased a lot the space where the gases pass from the barrel into the bell   The result was pretty good and beyond my expectations: the stove starts to burn clean (from what I can see from the chimney) after few minutes, almost no smoke back. If I open the door in the middle of the combustion no smoke comes out except in some cases where there is a big production of gases. Unfortunately during very cold starts some smoke continues to leaks out but I think this is due to the convoluted gas path inside the bench. Moreover the exit temperature after two burns is still quite low, around 55° C   this was taken before the change, you can see the flames are escaping from the firebox  |
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Post by bulbius on May 19, 2017 4:37:54 GMT -8
This is overloading right?  |
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Post by peterberg on May 19, 2017 6:30:15 GMT -8
Yes, definitely. Maybe the primary air inlet is bigger than 20% of the riser's csa?
When the firebox is really hot, the inlet should be smaller to begin with and/or the pieces of fuel bigger. Also, loading fuel criss-cross instead of lengthwise only is a common mistake.
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Post by bulbius on May 19, 2017 18:00:30 GMT -8
Yes, definitely. Maybe the primary air inlet is bigger than 20% of the riser's csa? When the firebox is really hot, the inlet should be smaller to begin with and/or the pieces of fuel bigger. Also, loading fuel criss-cross instead of lengthwise only is a common mistake. The primary air inlet is right. The thing occurs when the firebox is very hot and full of embers, usually after the first fire. Filling up the firebox again with wood leads to a big and fast production of wood gas. I put the wood lengthwise, a bit far from the port and until 6 cm from the firebox ceiling. For the first load I use little pieces of wood, for the next loads usually I put medium or big pieces. I regulate the inlet by starting from 20% of the riser's csa and narrowing until I hear that the stove "limps", then I open again a little. Maybe the firebox is too much insulating, remember that I used aircrete for the door and also for the firebox ceiling. Can the shape of the heat riser have a role in this? |
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Post by peterberg on May 20, 2017 0:29:02 GMT -8
You seem to do almost everything right, except when adding the second load. What happens is that the capacity of the riser as an afterburner has its limits. It's doing quite well when the process is stabilized but is very picky about overfuelling. This design is specifically tuned for a cold or medium warm start, like you would expect in a high mass heater. The riser/firebox/air supply/port proportions can't be easily changed during a multiple-staged burn so one has to be careful with refilling. When designed and optimized for a hot start I would expect the cold starts to be miserably dirty, like most bog standard box stoves. One way to get around this problem is waiting a while longer until the coals has been down a bit. Another way is to add two pieces of fuel at first and when the double vortex is firmly going to add more, the biggest pieces you might have. Or a combination of the above. By the way, this combustion core which I like to call a batchrocket is in fact a sideways, atmospheric pressure operated gasifier.  |
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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 ?
