[Stoves] Water heating fuel efficiency formula

[ajheggie at gmail.com]

[Default] On Sun, 29 Sep 2013 14:41:32 -0400,"Lanny Henson"
<lannych at bellsouth.net> wrote:

>I need the formula or spread sheet, for fuel efficiency, for heating water, 
>considering the moisture content of wood,  and using red oak as fuel. Not a 
>water boiling test with simmering,  just a water heating test.

Lanny 

What form do you want this to take?

My guess is red oak will have a calorific content of around the 18.6
MJ per kilo average suggested for most biomass. When you burn it
completely it gives off CO2 and water. All the water, both the
original moisture content and the water formed from oxidising the
hydrogen atoms in the fuel, in the exhaust contains 2.3MJ of latent
heat and the sensible heat from the exhaust temperature. This heat in
water vapour and the sensible heat of the other exhaust gasses, mostly
nitrogen, any remaining oxygen from the excess air and CO2, is waste
heat, the other waste heat is the heat conducted, radiated and
convected from the stove body. The remainder, the usable heat is what
gets into the pot. 

The simplest conversion of the potential heat in the wood to usable
heat in the pot is to weigh the wood used, calculate its heat value
and measure the rise in temperature of the pot contents and calculate
the heat input. Divide the first by the second value and you have an
overall wood to pot efficiency as a first approximation.

Beyond that you get into the realms of interminable arguments about
whether to allow for residual energy in any char left and how many
fairies can dance on a pin head.

AJH