http://firewoodhoardersclub.com/for...ms-out-with-the-old-and-in-with-the-new.6196/ Did the third all out batch burn today. Burned 111.5 lbs of dry old growth Douglas Fir 7 % moisture content or less. What would be the BTU value per lb of this wood? 560,000 BTUs ended up in the storage tank. I'm trying to calculate the percentage of those BTUs ended up in the storage.
There's 6203 BTU/lbm for doug fir - this is at 20% moisture content 2805lb/cord 17.4MBtu So.... 2805 x .20 = 561lbs of water in a 20% cord. 561/20 = 28lbs/1% drop in moisture content. 28 x 7 = 196lbs/cord 561-196 = 365lb reduction in weight of a cord, but you've also lost some BTU's from the water - water actually has a BTU content by my calculations if your wood is close to 70F you have close to 20btu/lbm losing 365x20= 7307BTU's out of 17,400,000Btus is negligible....and your wood burns gooder, but it has altered the btu/lbm 2805-365 = 2440lb/cord @7%MC 17,392,693/2440lb = 7128 btu/lbm 7128 x 111.5lbs = 794,787 btu's burned 560,000/794,787 = 70.45%
Using Sweep's BTU charts, I calculated 6203 BTU/lb for Doug Fir. If yours is "old growth", it may be higher. Given that figure, the wood should have yielded 691657 BTU. So approx 131657 BTUs lost - or about 19% loss. Doesn't sound too shabby to me! Dang - Basod beat me to it! And his calculation looks more accurate!
I actually screwed up above and used the specific heat vice the actual Enthalpy which is closer to 20btu/lbm for water @68F it's edited, but didn't change the efficiency .02%
Basod, are you sure the BTU's would start dropping under 20% MC? Water doesn't "burn", it turns into a gas, and I thought it consumes BTUs to change phases. Yes, I looked it up. In fact, 1 BTU is actually the amount of energy needed to raise 1 lb of water by 1 degree F. If the wood is at 70F, 142 BTUs would be consumed to boil off 1lb of water. So if the wood is at 7%, the 115lb of wood should yield approx 27832 BTUs *more* than if it was at 20% - the energy it didn't need to consume to boil off the water. Oh well - at least we can agree that campinspecter's OWB was operating somewhere between 70-80% efficiency.
Yes water has a heat content in Btu's - known as Enthalpy, shoot even a pound of air has energy content You are correct it takes some of the Btu's to gas off the water - and that's why everyone wants to burn the driest wood they can. I'm subtracting the enthalpy of the water not in the wood and calculating an all in heat rate - we can't do a combustion efficiency calculation without flue temps, air flows etc. The wood can't yield anymore btus than what is available, it can put more useful heat into heating the boiler vs. steaming off water in the form of moisture content
No doubt there is some heat returned to the system from the steam; it just looks like you forgot to account for the BTU gain from having less moisture. 2805lb/cord at 20%MC, at 70F, approx 79662 BTUs consumed to boil 561lb water; I'm thinking those get added to the 17.4 for the "all in" theoretical 0%MC figure. May lose the 7307/cord, but you're gaining 51830/cord not having to boil water inside the wood. Obviously there is a lot of variables needed to do a proper efficiency calculation, but in terms of BTUs stored in the tank vs lost to the atmosphere, I think we can agree that he ended up with 70-80% of the total. Sounds pretty darn efficient to me!
No they don't get added, they are part of the 17.4Mbtu/cord and I'm not adding anything from the steam that is created during the combustion process. Just think a about a gallon jug of water just sitting on a counter- there's 8.3 lbs/gallon if that water is sitting at 68F each pound of water has 20 btu's There is 166 btu's in the gallon of water - if you poured the gallon of water onto your woodpile you would have added 166btus to it(yes it would require more of the wood burning to remove the water 1195btu's to be exact) Same thing if you removed the gallon of water or in this case(from 20%-7%) ~44 gallons of water from a cord.
Thanks to Basod & Shawn Curry. Your information is a great help. I used to use the number of 6,900 BTUs per lb of wood. Gasification Boilers can be very efficient but burning very dry wood can be a double edge sword so to speak. Really dry wood because it burns with more intensity than say wood of 17% moisture content, can generate more smoke than there is oxygen available for complete combustion so it's very easy to lose that extra efficiency you had hoped to gain through the really dry wood. With the old boiler in perfect conditions, it would see as high as 86%. With the new boiler, there are several small differences such as the fire tubes are a 1/8 of an inch larger in diameter and the burn chamber is about 1" larger in circumference. I was getting a little blue smoke so this indicated the burn was not complete, so I will have to experiment with reducing the fuel supply to 3 pieces of firewood instead of 4. It is possible to increase the oxygen for combustion but this also raises the combustion chamber temperatures and I'm trying to keep those temperatures below 2,000 degrees or a 1,000 degrees below the maximum working temperature of the refractory. Because combustion air is taken from directly outside the house, yesterday was a very wet day with 60mm of rain in 24 hours so on these days efficiency will be lower. The blower for the forced draft and draft inducer produces 99 cubic feet per minute at 85 inches of water column lift. Again, thank you for your help !
From storage, temperature rise of 56 degrees for 1044 imperial gallons of storage. BTU draw storage was about 5,000 BTUs per hour for a 6 hour burn.
I guess you may be underestimating storage transfer? 56F rise x 1044IGx 1.2 US gal x 8.3 lb/gal = 582kBtu The draw storage 0f 30k is what you are using to heat the house, hot water etc? Sorry not familiar with OWB terminology
Heat loss form storage is , heat ,hot water , and storage heat loss that was based on the temperature drop of the storage tank for the past 24 hours .
So for our heat storage tank a 10 degree heat rise represents how many BTU'S ? I very much appreciate the calculations !
1044 Imperial gal x 1.2 USg/IG = 1252.8 US gal 1252.8gal x 8.3 lbm/gal = 10398 lbm (this the tank weight) So for any temperature rise just use 10,400lbs 10F rise x 10,400 = 104,ooo Btu If you want to skip the conversions an Imperial gallon weighs 10lbs. I was surprised when you used it as the measurement, but it makes sense in this application... 1044 IG x 10lb/IG = 10440 lbm 10F rise x 10440 = 104400 Btu You see what happens when you eliminate significant digits
