... Delivered efficiency is a simple calculation
(1 - (mass exhaust x temperature)/(kg wood x energy content/kg)) x100 to give a percent.
... Basically you are measuring how much heat escapes vs how much stays in the house. ....
I was interested because of the difficulty in getting insurance if you build your own stove.
Thanks for the lengthy reply, the teacher reference was rather appropriate as I teach high school science and math though my trains would have collided in Saskatoon in -30 C weather.
I have heard that round pipe is not the most efficient thing for transferring heat to mass. Something skinny and flat with the same cross area being better. Also easier to crush.
Erica Wisner wrote:
careful of the assumption that structure in flow has little result. the exhaust stream is a chaotic system one little deviation can cause a whole lot of results. Test test test every time make sure the system dont do something you dont expect.
My God! What is it with you people and colliding trains!?
dale hodgins wrote:
I have a much simpler method that you might want to try when determining rocket stove efficiency. Bagging and weighing exhaust gases seems like something that your average homesteader would get terribly wrong. It seems that the important matter is not the exact burn efficiency, exhaust temperature or exhaust component measurements but rather you need to answer the question of how much heat is delivered to the home and thermal storage for a given weight of wood consumed. If you have a cob bench and are able to calculate how many cubic feet of cob there are, from that you should be able to calculate a total weight of your structure. I'm sure there's a table somewhere that will tell you the heat capacity of cob, that is the number of BTUs per pound, per degree of heat rise for a typical mixture. If the entire structure is covered with blankets and possibly straw bales or some other good insulator you could run a fire using say 5 pounds of bark free firewood of known species and moisture content. Once this wood is consumed close all baffles and prevent airflow through your system. You might want to hold off on taking temperature measurements until the inner heat has time to reach an equilibrium with the outer surface of the cob. A system with a cobbed over heat riser would be best for this test since you want to store the heat and not let it radiate to the room. By taking many temperature measurements of the bench in different places you will be able to come up with a fairly accurate average temperature rise. So suppose it turns out your bench weighs 2 tons. It's now a simple matter of figuring out how many BTUs it takes to raise the temperature by whatever rise you get. BTU tables are available for most species of wood.
So in summary what does it weigh? What was its temperature before the fire? What is the temperature after the fire? How much wood did it take to attain this temperature gain? This will provide a relatively accurate measure of your systems efficiency. The biggest of variable would be any screwup in calculating bench weight. But after we have all of these measurements that is the weight, temperature gain, weight of wood consumed and heat capacity of cob it comes down to grade 5 math skills. No fancy formulas just multiplication and division. Feel free to call me if you need clarification on any of this. I live in Victoria British Columbia and would gladly conduct this test for free if there is someone local who has such a system and is willing to participate.
I'm very wary of high-tech answers to simple questions. This is the simplest, non-laboratory method I can think of. Thank you: Dale 250-588-3366
dale hodgins wrote:
Hi Erika, the calculations concern only the weight of the material. Density variations and volume reductions when you mix don't matter at all. And since clay and sand have almost identical heat capacities it's not terribly important to track the components of various mixes.
As long as your stated weight of 95 pounds per cubic foot for finished dry cob is accurate then my example will be accurate.