[Stoves] Error in the recommended Shell Foundation HEH Project Water Boiling Test http://ehs.sph.berkeley.edu/hem/hem/protocols/WBT_data-calculation_sheet.xls

[AJH]

On Tue, 18 Sep 2007 11:59:58 +0100, AJH wrote:

>All this heat has to be provided by the wood burning but as the oven
>dry proportion of wood varies as moisture content increase the dry
>portion of wood is given by 1-mc divided giving a heating value
>corresponding to mc of:
>
>mc   dry wood   heat value  less steam available heat
>10%  90%        16572.6     257.5      16315.1
>20%  80%        14731.2     515        14216.2
>30%  70%        12889.8     772.5      12117.3
>40%  60%        11048.4     1030       10018.4
>50%  50%        9207        1287.5     7919.5

Just a small comment because I was thinking faster than typing (and my
maths still bears checking out):

We should not get too carried away with the significance of moisture
in use because most cooks don't weigh or buy would by weight. The cook
simply chooses the volume of sticks perceived necessary to cook the
food. Now it we look at my calculation again and assume the volume of
wood doesn't change noticeably as it dries (OK it does by about 10% in
2 dimensions and next to none in the other as it moves from 25%-0% but
let's ignore that) then the mass of oven dry wood in the stick doesn't
change. Hence if we add a column to the table to show the effect of
burning the volume of wood in the 50% moisture content sample as it
dries to 10%mc wwb then:

mc    available heat

10%   8949.5
20%   8692
30%   8434.5
40%   8177
50%   7919.5

So  the wet stick yields 88% of that given by the dry stick, again
given my caveat about heat of rejection of flue gas. IMO this is only
just about significant, the best reason for sticking with dry wood is
that it burns more efficiently and cleanly in the absence of
sophisticated stoves.

AJH