[Stoves] Water heating fuel efficiency formula

ajheggie at gmail.com ajheggie at gmail.com
Sat Oct 5 07:11:08 MDT 2013

[Default] On Fri, 4 Oct 2013 18:14:31 -0400,"Lanny Henson"
<lannych at bellsouth.net> wrote:

>2- A few points I am still trying to understand is how to calculate the pot 
>mass since metal has a different specific heat than water. I am using a 316 
>stainless pot so I am using .5 as a factor. Is that about right?

Specific heat of water is 4.187kJ/kg, a quick google suggests 316 ss
is 0.5kJ/kg so it's less than water by a factor of ~10 but of course
it weighs 8 times as much so they are similar on a volume basis. 

>3- Also I still need the formula for calculating the loss from woods 
>moisture content. I an drying a batch of wood as I type. Before the moisture 
>here in NW Georgia USA has been but we have had a wet season and the last 
>batch was 14.5%. Wood sure does burn well at 14.5% in my stove.
>4- Another confusing point is the difference between net and gross calorific 

I addressed these two points in my earlier post. Weigh the wood first,
then dry it at 110-125C until it loses no further weight. Reweigh it
and subtract the last figure from the first to get the water content.

As a rule of thumb the useable heat (Lower Heating Value), net of
losses due to latent heat of vaporisation, is the dry weight times the
calorific value for dry wood (18.6MJ/kg its LHV) minus about 2.7MJ/kg
for the water content calculated from the above.


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