[Stoves] RE Temperatures of pot-skirts?

[Boll, Martin Dr.]

Dear Crispin,
Thanks for your answer which reports as well about the "side"-effects of the
skirts. -And we often must make us aware of those things.

I know that most heat is transferred in the part under the pot and the sides
do not take very much heat. 
I think, at least in the beginning of the burn (When skirt and pot have the
same temperature) the heat-transfer from the burning gasses to the pot is
nearly the same. With time, the pot gets more heat from that, because it
heats up slower, and temperature-difference is larger. By that, a radiation
will occur from the skirt to the pot. -I assume, the pot-skirt is insulated
at the outside-
And so the difference is somehow diminished.
Now the quantitative question:
How much heat can be used within the range: Temperature-difference of the
skirt-mass-temperature (260°C) to say 80°C multiplied by the
specific-warmth-factor, to use further in a (somehow by that) activated RHC
(Retained heat cooker).
Is the gathered heat (from an outside good insulated skirt) worth to take
all into a RHC; pot plus mass-pot-skirt?
To discuss how, is after that a __second step__ question, I am thinking of
since a while.
By my thumb-thoughts, I think it is realistic, because:
In the old German cooking-book, mentioned in the bioenergy-list, the old
hay-box was told to accomplish with heat by some hot bricks. I think they
were heated in the oven of the cooking-stove; or on the plate?
A mass-pot-skirt had at least the same mass or even more.
But my thumb-calculation's result is only 46Wh per 1kg brick.
What do you think?

Regards

Martin


> -----Ursprüngliche Nachricht-----
> Von: stoves-bounces at listserv.repp.org [mailto:stoves-
> bounces at listserv.repp.org] Im Auftrag von Crispin Pemberton-Pigott
> Gesendet: Donnerstag, 22. Februar 2007 14:36
> An: Stoves
> Betreff: [Stoves] RE Temperatures of pot-skirts?
> 
> Dear Martin
> 
> I have measured a maximum external temperature of 265 and inside
> temperature
> of 400 with charcoal burning inside the ceramic.
> 
> For a skirt, the maximum temperature on the inside is probably not going
> to
> higher than 400 C at any time if there is water in the pot.
> 
> It is unusual to get temperatures as high as 300 unless there are flames
> getting all the way out from under the pot and over to the skirt (which
> should not happen at any time).
> 
> The reason the skirt should not get very hot is that if there is so much
> heat still in the gases as they exit the pot bottom, something is wrong
> with
> the stove design.
> 
> The gases should have transferred their heat to the pot by that point.
> 
> With the Lion Stoves in Swaziland, the skirt is made of face bricks, so it
> qualifies as a ceramic skirt, and the maximum temperature of the gases is
> 400 at the side of a globe-shaped pot, and that is with the fire at about
> 15
> KW.
> 
> The brick is unlikely to go over 250.
> 
> A caution about assuming things caused by testing skirts:
> 
> Most tests of the pot skirt do not reproduce the burn rate for the stove
> with and without the skirt, so in fact they are tests of two different
> stoves.  Adding a skirt that fits to the stove well (at the joint)
> increases
> the draft, usually significantly.  For example, if you have a stove with a
> fire-to-pot bottom distance of 180mm and you add a 180mm skirt, you have
> effectively doubled the draft.
> 
> With increased draft, the burn rate changes.  This gives different gas
> flow
> patterns, different mixing and a more rapid boiling of the pot because in
> fact there is more wood burning per second.
> 
> Dale Andreatta found that some skirts' main addition to efficiency was to
> limit excess air, in stoves with high excess air ratios, so again, the two
> stoves are not really comparable and it is not a test of the skirt alone,
> it
> is a test of two different excess air ratios AND a skirt on one of them.
> 
> If you want to test the true effect and you want to work out the heat
> gained
> by a skirt, you should constrain the fuel burn rate so that the input
> power
> is the same.
> 
> Note that increasing the draft can be counteracted by decreasing the gap
> to
> the point where the total gas flow is the same as in the original
> configuration.  This should limit the fuel burn rate to the original
> value,
> however when interpreting the result, be careful not to attribute to the
> skirt what is in fact the effect of increasing the draft and burning more
> fuel.   It can be compared with testing cars, one with and one without a
> turbocharger.  It is incorrect to attribute increased power to the
> turbocharger when the real effect is to burn more fuel per second which
> could be accomplished by several methods.
> 
> Other than Dale Andreatta's work 2 years ago, I have never seen a set of
> test results which were performed in a way that revealed the actual
> addition
> of heat transfer from the skirt alone.  He showed, in particular, that the
> gap between the pot and the skirt was not very sensitive at all.  Only
> when
> it affected gas flow did it significantly increase the efficiency, most of
> which could have been achieved by limiting the excess air flow and leaving
> off the skirt.
> 
> In the field I have noticed that the main effect of skirts is to limit the
> wafting of hot gases by the wind out from under the pot pushing them to
> one
> side.  As 'wind
> shields' they are very effective even if they are only 6 cm high.
> 
> In short, skirts are effective, but not as effective as claimed and not
> for
> the reasons most frequently stated.
> 
> Regards
> Crispin
> 
> ----- Original Message -----
> From: "Boll, Martin Dr." <boll.bn at t-online.de>
> To: <stoves at listserv.repp.org>
> Sent: Wednesday, February 21, 2007 11:14 PM
> Subject: [Stoves] Temperatures of pot-skirts?
> 
> 
> Dear all,
> 
> 
> 
> I have a simple question: What temperature reaches a ceramic pot-skirt
> within a normal time, a pot needs to boil?
> 
> Or are there no ceramic pot-skirts?
> 
> Has someone measured the temperatures (or even temperatures by time) of
> the
> skirt-surface or of the inside of the skirt?
> 
> 
> 
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