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Post by Orange on Oct 27, 2019 12:37:50 GMT -8
So, we're primarily building rockets because they have higher combustion efficiency compared to box stoves from the shops.
So I've randomly selected a box stove to compare it:
price: ~1000€
system size / chimney size: 120mm power: 6 kW (max 7,3 kW) (total?) efficeincy: 82 % mean CO emission (on 13% oxigen): 750 mg/m³ (also max?)
mean flue gas temperature: 242 C
price: ~2000€
system size / chimney size: 150mm
power: 1,9 kW (2 fires a day)
total efficeincy: 92,6 %
mean CO emission (on 12,4% oxigen): 480 ppm (max 1400 ppm)
mean flue gas temperature: 92,8 C
so first thing that stands out is box stoves have very high power output to system size ratio. And efficiency doesn't looks bad considering the high flue gas temperature which can be harvested. What are your thoughts, what's the math here?
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Post by Karl L on Oct 27, 2019 13:18:36 GMT -8
Orange,
Is the 2KW for the DSR2 the *average* output from the bell? It's not very informative to compare this to the *rated* power output of the box stove, which it only produces when it is burning wood.
In order to compare the two stoves meaningfully you need to take account of the amount of wood being burnt in each to produce the same average power output.
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Post by Orange on Oct 28, 2019 4:43:14 GMT -8
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Post by esbjornaneer on Nov 6, 2019 3:43:11 GMT -8
It is interesting to compare thing but helps if the units are the same: 750mg/m3= 654.68ppm CO (according to www.aresok.org/npg/nioshdbs/calc.htm)The thing is that you can't extract more heat from the box stove (i.e. lower the chimney temps) without slowing down the speed of gas in the system which affects the combustion (lowers efficiency?) and changes (increases?) the CO output. It would be interesting to hear where you got the price for the DSR2? On the point of wood use, the DSR2 would use approx 6kg/load i.e. 12kg total in 24 hours, while the box stove would have to be burning about 1kg of wood per 34 minutes to reach its rated 6kW. I am basing this on 14.9MJ of energy per kg of wood at 82% efficiency gives 12.22MJ=3.39kWh, and using the kWh>kW calculator found here (https://www.inchcalculator.com/kwh-to-kw-calculator/) So to me it looks like the box stove use 1.77kg wood/hr at its rating, while the DSR2 uses .5kg/hr in a 24hr period. And to compare apples with apples instead of pears I have to say that if it were possible to run the box stove at 1.9kW output with the rated 82% efficiency it would use .56kg wood per hour. It would need to be fed more frequently than twice a day. The 24hr consumption difference is only 1.44kg (12% more for the box stove), in a 150 heating day location it would come to 216kg consumption difference, over 10 years 2160kg wood. But there was also a mention at some point on this forum or from Erica Wiseman that the efficiency rating is calculated differently for a box stove than for a mass heater due to regulations regarding the minimum flue temps required for a box heater, so the efficiency rating on a box stove looks better than it actually is, if I remember correctly?!? (please correct me) What conclusions do you draw from the comparison? (If I am reading you original post correctly you are saying that there is no point in building a rocket stove... not sure you meant that?) |
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Post by Orange on Nov 20, 2019 9:05:21 GMT -8
I don't know. Slowing donwn the gasses gives them more time to burn. And you can increase gas velocity it with good chimney. I think they've intentionaly left high exit temps to fit all possible scenarios in people's homes.
I've roughly caclulated price of materials and multiplied by 2. It can be less or more of course. For metal stove it's probably average power during the burn (not in 24h). Hopefully someone has the definition. I think for BB and metal box stove ther is only data for total efficeny (combustion efficiency x heat extraction). I just wanted to know true efficiency of the box stove to compare it with BB. Peter's and Yasin's testing showed that geometry and secondary air are crucial for high efficiency and insulation is not that important. It would make sense that engineers in stove productions know that too and that've designed box stoves to go for the maximum efficiency too, especialy pressured with the eco standards. |
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Post by Orange on Dec 4, 2019 12:50:01 GMT -8
here's BB max power generated in 1h to compare with other stoves:
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Post by Vortex on Dec 4, 2019 15:45:08 GMT -8
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Post by Orange on Dec 7, 2019 11:40:57 GMT -8
thanks, there is some info like they fill only 20% of combustion chamber with wood. So then a 150mm BB has an official power output of 4,4kW (instead of 22,2kw).
but the 2 stoves are incomparable with different methods, we need a Testo run of the store box stove. Is anyone interested?
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Post by Vortex on Dec 7, 2019 15:00:59 GMT -8
When I read up on it all a while ago, I found a site then that described the methods used to test wood stoves. I cant seem to find it now, but it said they set the stove up on a weighing scale connected to the chimney by a flexible flue-pipe. Then burned the stove until it was up to normal operating temps, and when the fire had burned down to a hot bed of coals they would take note of the stove weight, then load 5KG of wood with a 20% moisture content. They then started the test and ran it until the weight of the stove was back to the original weight, then the test was stopped. If that was correct it would mean they avoided the startup and coaling phase CO peaks.
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Post by gadget on Dec 10, 2019 21:08:10 GMT -8
So, we're primarily building rockets because they have higher combustion efficiency compared to box stoves from the shops.
So I've randomly selected a box stove to compare it:
price: ~1000€
system size / chimney size: 120mm power: 6 kW (max 7,3 kW) (total?) efficeincy: 82 % mean CO emission (on 13% oxigen): 750 mg/m³ (also max?)
mean flue gas temperature: 242 C
price: ~2000€
system size / chimney size: 150mm
power: 1,9 kW (2 fires a day)
total efficeincy: 92,6 %
mean CO emission (on 12,4% oxigen): 480 ppm (max 1400 ppm)
mean flue gas temperature: 92,8 C
so first thing that stands out is box stoves have very high power output to system size ratio. And efficiency doesn't looks bad considering the high flue gas temperature which can be harvested. What are your thoughts, what's the math here?
This is a great topic Orange. The efficiency measurements for wood stoves are not a good reference in my opinion. The difference in the amount of wood needed comparatively is proof enough that something is not right with the way they come up with their typical numbers. How could a heater that uses at least 4 times as much wood only be 10% less efficient. It doesn't add up. Its not a fair comparison anyway since wood stoves don't get hot enough for compete decomposition of all the wood gases. Acetic acid alone needs to be at 1400F for 2 seconds to break down so it can be oxidized. You need to have your burn temps up very high to get that residence time down to fractions of a second. Some of the tars and turpins are even worse. I'm preaching to the choir here.. This is a pic of my heater where a piece of insulation broke off in the burn chamber and bare metal was exposed. It was literally approaching white hot. Definitely at the bright yellow stage. Death to metal temps. Way to hot for the metal box stoves but not for a RMH, DSR2 or other masonry heater. These temps would destroy a metal wood heater in a matter of hours if you could even get it that hot. I actually have to replace my lower burn barrel now since it has gotten damaged where the insulation was lost.  I don't hate metal box stoves personally, I just would never own one. who would want car that sent raw fuel out the tail pipe? Maybe if fuel was only 50 cents a gallon? Why own a stove the send raw fuel out the chimney(and sticks to the inside) and burns through tons of wood? They are easy to install and cheaper to buy, thats why I guess. I like to call them.. wood guzzlers (Trademark by gadget) |
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