[Stoves] Heat loss in gases

[AJH]

On Mon, 05 Nov 2007 10:15:00 -0600, Paul S. Anderson wrote:

>I was also thinking of capturing some heat from the flue gases by passing them
>through a chimney pipe inside a water jacket.  But it seems there would be
>little heating of the water.  Would you agree?

It's too broad a question Paul. What you have to consider is that if
you remove too much heat from the discarded flue gases then you risk
condensation in the chimney, as this moisture will also contain acidic
compounds from the combustion you will attract corrosion. This will
even attack stainless steel flues, so, depending on the initial
moisture content of your fuel, it's best to exhaust the gas from a
chimney at >160C. One way of keeping the temperature this high is to
insulate the chimney.
>
>Your comments also suggest to me that lateral insulation to prevent 
>lateral loss
>of heat from the stove structure would be even more important than I had
>realized.

All losses from the stove should be minimised because what leaves the
stoves sides not only lowers the combustion temperature but reduces
the temperature of the gases passing the pot, of course this is not so
much an issue if the stove is also providing space heating.

>  If the radiant heat cannot escape laterally, will it 
>actually end up
>going upward to the pot?  How? in the form of gases that are hotter, or some
>other reason?

The radiant heat is either reflected or absorbed, depending on the
surface it hits. If the heat cannot get out of the sides because of
insulation it can only leave with the flue gases or be absorbed by the
pot bottom. 

Radiation is an ideal means of heat transfer, that's how we feel the
heat of the sun, and there are ways of maximising it but the simple
fact is that given an insulated stove and no sight of the combustion
chamber then all heat losses are in the sensible heat of the flue
gases or losses from the pot.

Most of the stoves we consider have high excess air values, the
updraught rocket type have more difficulty controlling this than a
tlud with secondary air mixing. Now this has a profound effect on any
heat transfer by radiation alone. 

This is because the radiated power varies with the 4th power of the
absolute temperature ( deg K) of the body radiating the heat. This
means a 150mm stove combustion chamber diameter has a maximum possible
radiated power of 13kW(t) if it were operating at the maximum flame
temperature of wood in air (which I think is about 1160C or 1870K).
Half this because the wood has a high moisture content and needs a lot
of excess air and it falls to 1/10 of the above maximum.

A few simple experiments should be able to show the contribution to
the pot by radiation and that by conduction  but I suggest the
available heat for further processes downstream is the difference in
temperature of the flue gas after the pot and about 150C times the
specific heat of the gases.

Would someone measure the temperature of the flue gas after it has
passed by a skirted pot at boiling point?

AJH