[Stoves] RE Temperatures of pot-skirts?

[Crispin Pemberton-Pigott]

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?