One more quick propane question regarding the dickinson stoves.
1 gallon of Propane ~= 4.23 lbs ~= 91690 Btus
1 lbs of Propane ~=21,676 Btus
20 lb tank of propane holds approx 4.73 gallons of propane (433,694 BTUs)
Then for the dickinson 9000model if you run it at 5500btu/hr you should only get 78.9 hours of usage (433,694/5500 ~= 78.9) yet the dickinson website claims 140hrs at this rate.
For the high setting of 7500btu they claim 100hrs though 433,694/7500 yields 57.8hrs by my math.
What gives? Surely a pound of propane can't produce more than 22000 btu. So does the stove not run as long as they say, does it not put out quite as many btu as they claim, or am I missing something else? I'm hoping that I'm missing something as I'm actually sold on the idea of a dickenson 9000 for a forepeak heater on my boat if their numbers are indeed correct.
That's actually pretty interesting -- I'm a bit intrigued. I've never picked apart those numbers like you did. It might be interesting to do the same analysis on their diesel heaters too.
Hopefully some of our physics and engineer types will chime in here and explain this to us. But until they do, and at the risk of being completely wrong in my assumptions, I'll take a common sense stab at it.
As I see this, the figures you cite above are for the BTU output of the propane alone when it combusts. But, there is also an appliance in the equation -- the heater -- which is designed to maximize the return from the heat output of the fuel source. The heater -- made from heavy gauge metal, containing baffles and chambers -- in a sense "stores" and accumulates the heat from the fuel source. This allows a smaller flame, and therefore a lesser amount of fuel, to achieve a higher radiated BTU output from the appliance.
Consider the extreme case where the flame has been extinguished and yet the appliance continues to radiate heat long afterwards. When it's doing that, there is zero propane consumption, and therefore zero BTUs from the flame, yet the appliance is still a viable heat source. In other words, with the flame extinguished, the radiated heat output from the appliance is greater than the amount being generated by instantaneous propane combustion. The same would be true, but to a lesser extent, when the flame is burning.
Not sure if that explanation makes sense, or if it would withstand scrutiny from our scientific community. But another way to think about it might be to question the need for or benefit of the appliance at all. Why bother putting all that effort into designing and constructing the complicated appliance, if the only factor that matters is BTU output of propane when it burns? If according to some immutable law of physics BTU output is BTU output and it makes no matter how BTUs are captured, stored, manipulated, reflected, amplified, why would we bother innovating new heating devices? Why not just have a simple burner with a flame and a chimney to vent it?
Anyway, I'll be interested to hear from folks with a more scientific grounding
in this subject matter.