[Stoves] RE Temperatures of pot-skirts?

Thu Feb 22 16:12:10 CST 2007

Martin,
I recommend you do not assume that the pot skirts are insulated on the
outside.  I think that might fall under a case by case basis.

cheers

On 2/22/07, Boll, Martin Dr. <boll.bn at t-online.de> wrote:
>
> 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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>
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-- 
David Whitfield V.
Executive Director
CEDESOL Foundation