Stove Power

Dean Still September 2003

Dear Peter,

Congratulations on your successes with brick making in Uganda. I hope that you can fill ETHOS in, one of these daysI The fastest way to get some math done, I think, is to appeal to the ETHOS net for assistance... You need to know the approximate power delivered from different sized Rocket feed magazines and combustion chambers? Can the engineers in ETHOS check the following, please?

I just talked to Damon and he looked at the tables in Baldwin. I will go through our reasoning and hope that the ballpark numbers are enough math to befuddle and satisfy...What follows is based on very rough approximations! Maybe tomorrow I'll see how much fuel is used per minute in the 6" by 6" Rocket combustion chamber in the bread oven...Brian is making bread with the new interns...

The 4" by 4" Rocket combustion chamber uses 30 grams/minute at high power which is 4 pounds per hour. 4 times 8,000 btu's (8,000 btu per pound wood) is 32,000 btu's/hour.

FIREPOWER IS:

1.) kg of wood times 18 MJ divided by 60 times number of minutes equals kilowatts and 1 KW equals.09478btu's/second

2.) 3.4 btu's/hour equals watts

Both ways of looking at the problem seem to result in about 9 KW power in the 4" by 4" Rocket combustion chamber given that the sticks are small. The 6" by 6" has 2.25 the cross sectional area of the 4" by 4".

The sticks get bigger as the feed magazine grows. The draft increases as the Rocket chimney gets taller (wood burns faster) so we figure at the whole increase in cross sectional area although we realize that this may result in bigger than life increases. So, here is our guess:

4" by 4" uses .5" square sticks equals 9KW

6" by 6" uses .75" square sticks equals 20 KW

8" by 8" uses 1.0" square sticks equals 32 KW

10" by 10" uses 1.25" square sticks equals 56 KW

We throw these guesses out to give Peter some numbers and to stimulate better estimates....I'll try to give a real life high power estimate of the 6" by 6" tomorrow.

Steinbeck in the "Log of the Sea of Cortez" said that their skipper believed speculation to be utterly worthless...I agree but only if the speculation is wrong!

All Best,

Dean