[Stoves] Heat loss in gases

[IPC]

Dear Paul

If you look at the heat transfer in a stove, much is transferred from the
gas to the pot by radiant heat - comparatively little by sensible heat once
the gas has cooled a little (because of the limited sensible heat in the
gas).  

To answer your 2-pot question, therefore, if you stack the two pots one
above another in a chimney sort of arrangement, the upper one will not see
much radiant heat, merely the residual sensible heat in the gas, whereas if
you put them in a horizontal arrangement with the gas flowing under them,
the second pot will still be heated by radiant heat and, as you say, by some
conduction from the support plate.  

I did a top-lit updraft stove like this for Botswana with three pots in a
row between the fire and the flue, and yes, Nos 2 and 3 were progressively
slower in cooking but still cooked reasonably rapidly. It had the advantage
that you could be bringing to the boil in the first pot and simmering in the
second and third. 

Hope that helps

Philip

-----Original Message-----
From: stoves-bounces at listserv.repp.org
[mailto:stoves-bounces at listserv.repp.org] On Behalf Of Paul S. Anderson
Sent: 04 November 2007 04:18
To: Discussion of biomass cooking stoves; IPC
Cc: 'Discussion of biomass cooking stoves'
Subject: [Stoves] Heat loss in gases

Dear Philip and all other Stovers,

I found very interesting what Dr. Philip Lloyd wrote Quoting IPC
<ipcipc at mweb.co.za>:

> Yes, there is some heat loss in the hot gases, but it is very small, 
> because the heat content of gases is very small.  I cannot stress that 
> enough - it is very counterintuitive, and I got caught by it as a 
> young scientist.  I needed to heat a fluidised bed.  A long pipe 
> carrying the gas had an electric element down its centre and was 
> insulated on the outside.  The heater was set so that the exit 
> temperature of the gas was 420deg C. The gas then passed through 5cm 
> of un-insulated pipe before entering the fluidised bed. The bed never 
> got even faintly warm - all the heat in the gas was lost over that 
> uninsulated 5cm, even when the gas velocity was set high enough to blow
the fluidised bed out of the pipe!
>
> The heat lost in the combustion stream at 100 deg or 150 deg C is 
> measured in terms of a few joules, not kilojoules, and can be neglected.

I neither dispute nor confirm this, but please help me comprehend this so I
can apply it to TWO-pot cooking.

The volume (or mass?) of the hot gases would seem to be important.  If the
long pipe is only 1 cm in diameter or is 10 cm diameter would be a factor.
And the speed of the exiting gases is higher if the same amount of gases
goes through the smaller or larger diameter, thereby influencing also the
blowing of the fluidized bed out of the pipe.

When trying to heat two pots in sequence (either laterally or horizontally
arranged, but not with both sharing the same flame simultaneously), is there
much chance of getting serious heat to the second pot only by the gases.
(This question implies basically not relying on conduction of heat in a
plancha top and not having the flames extend past the first pot to reach the
second pot.)

I am trying to understand the implications of what Philip wrote.

Paul


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