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How many BTU's/hr can a 5" or 6" Batch Box Rocket produce?  RSS feed

 
John Harrison
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As per the title really - Approximately how many BTU's per hour can a 5" or 6" Batch Box Rocket produce?

I'm planning to burn a mixture of sub 20% mc Ash, Oak, Sycamore and Larch.

Thanks for any help.
 
Anthony Donner
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it all depends on your fuel source... I see you are going to use wood... if you check on the internet there are many sources that tell you there btu capabilitys, with that being said , moisture content will lower your btus avaible, and your efficiency of your stove will also determine your btu's avaible for heat, after you figure that out, do a test see how much fuell ( lbs ) a hour you are burning and your eficency of your stove ,then you should be able to roughly determine your btu output to your home, example let say you used 5 Lbs of wood pellets a hour, well there is roughly 8,000 btu hour in a pound of pellets, so lets say your stove is 80% efficient 80% of 8,000 = 6,400 btus per hour times 5 = 32,000 btus per hour.... I stand to be corrected... but this is my take on figuring this out.
 
John Harrison
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Thanks for the reply Anthony - your response makes sense to me and gives me a good indication on how much I can expect from my contraption.
 
Peter van den Berg
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A 6" batch box rocket is a very powerful combustion core. I have a good idea of what it is capable of but I need to convert it into btu/hour. Start with the ISO figures first: one kg of wood fuel at 15% moisture contains 4kW. The 6" version can burn only 6 kg at a time and it will turn that into ashes in almost an hour at a rate of 90% efficiency in a well-built heater.

Now in imperial figures, on the conservative side: 13 pounds of woody fuel can be burned in an hour with an efficiency of 90%, provided the moisture content is 15% or less, the dryer the better. One pound contains 7510 btu, 13 pounds could yield 97630 btu. Lower heating value efficiency is 90%, that would be 87867 btu.
Burn that in one hour, account for dry gas losses and convert to higher heating value and a well-built batch box rocket heater will yield about 77323 btu/hr. Quite a lot when I look at it.

I used the Cleave Books converters, these will convert nearly everything.
 
John Harrison
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Thanks very much Peter - crikey, that is a lot of BTU's! Can the BTU output easily be scaled for a 5 inch, 4 inch, 7 inch, 8 inch, etc size riser?

Thanks again.
 
Peter van den Berg
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Yes, it can be scaled, because the scaling method is quite straightforward and tied to the riser size. Provided the fuel type and moisture content stays the same it will burn a batch in under an hour whatever size the core is, like clockwork. So an 8" version is much, much more powerful, the firebox is able to hold double the amount of fuel, easily. By the way, the specific burn time is the same for half a batch too, the delivered energy is halved also.

Note: the burn time varies a bit, depending on the specific air inlets, construction of the core and the size of the fuel. But once you've established the burn time for that specific heater and fuel you'll find out it will be very consistent.
 
John Harrison
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Thanks again Peter.

How is the scaling method worked out? Is it based on a percentage of the figure you previously quoted for the 6 inch riser version in conjunction with the firebox size - say for every extra/smaller 1 inch riser diameter add or take away 10%, or is it more complex than that?
 
Peter van den Berg
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The scaling method isn't complicated although a bit different. It is tied to the diameter and/or the cross section area of the riser. See http://batchrocket.eu/en/building#dimension for the dimensions and how to calculate it. When you are about to build your first batch box rocket it is recommended you read all of the English version of the resources site and try a mock-up of the core outside to get the hang of it.
 
John Harrison
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Hi Peter and thanks again for the quick reply.

I'm not brilliant with calculations... Please bear with me. If our 6 inch riser produces 77323 btu/hr which numbers in Dougs table do we use to work out what our 5 inch riser would output?
For all of us 'mathematically challenged' Rocketeers, would it be possible to add an extra row in the table that gave the 'real life' btu/hr output for each of the tabulated heat riser/firebox combinations?

Thanks again for your help.
 
Peter van den Berg
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John Harrison wrote:If our 6 inch riser produces 77323 btu/hr which numbers in Dougs table do we use to work out what our 5 inch riser would output?

John, it's about time you do some work yourself. Calculate the volumes of both the fireboxes, divide the largest by the smallest and you see what the ratio is between those. The same ratio goes for the weight of the fuel and also for the btu's or kWh's or whatever you like to use. Simple multiply and divide, no differential calculus or something like that.

And no, there won't be an extra row in the spreasheet simply because hard or soft fuels makes a world of difference, as is the moisture content. A whole new spreadsheet would be needed to cater for a plethora of different fuels and before you ask, I ain't the right person to provide such a spreadsheet. But maybe there's someone out there who likes challenges?
 
John Harrison
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Hi Peter, thanks for the reply.

I'll work out the various volumes of the fireboxes and arrive at the ratios from that - now I understand the principle I'm happy to do some work for myself

I really appreciate all your work and help with developing the batch box.

Thanks again.
 
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