[Stoves] Subject: Re: insulated pots - Tony Chan

[tony chan]

Tony's reply:

Dear Crispin and all:

I follow this list regularly and am interested in biomass stove
dissemination work being done in the Dominican Republic/Haiti.  I'm still in
the design/learning stages, so not much practical experience.  But I have
cooked for a living.  I'm interested in controlling/generating the heat
source (stove) to facilitate the cooking of the food.

These are my personal opinion/observations as I've no references for my
statement about 'reasons'.

0. other then the most dedicated applications, most pots are multipurpose so
the lids need to multi task as well.  Sometimes applied fully tight, semi
tight, ; sometimes inspected often during the cooking period, sometimes
rarely.   An insulated lid would hinder many of these options.

1. Why lids are not insulated?  [a] Hygiene would be my guess.  Porous
insulation would need to be sealed, a complicated manufacturing process.  (I
have tri-ply ss,aluminum,ss pots that have leaks between the plies, washing
them introduces water into the seams, making them virtually useless except
in special applications when I can work around the problem) Simple lids are
easy to clean.  ... Cooks use towels placed properly on lids, both for
handles and insulation. [b]Light weight, simple convenient storage, - ready
and close to stove.- easily to remove//shift for food inspection.

2. A heat transfer analysis would study delta T from gas to metal, metal to
metal ( from heat source around sides), metal to content.  The question of
where the inflection point of heat gain vs. heat loss would have some
academic value, but in the kitchen, a pot that is optimized with insulation
would have limited its versatility.  A water heater would simplify the
analysis of heat flow, (our water boiling test criteria) but in my opinion
could lead us to approve a design that saves on fuel in a special instance,
but lead to many unhappy cooks and abandoned projects after the introduction
and follow up studies are terminated.  Client adoption of the technology has
many factors that we don't see/appreciate outside their shoes. (? bad pun)
Our improved stoves should fit the end users diet/ their sense of
improvement, rather those of the stove makers.

3. The pot skirt is a practical solution where pots are squared tubes with
flat bottoms.  (Western pots?)Tradional pots (your lion stoves match such a
cauldron,
http://www.repp.org/discussiongroups/resources/stoves/Crispin/Libhubesi/libh
ubesi.html) with a concave bottom prolong flame contact as the gas rises
along the curved bottom/sides. Woks are cut down versions of the ball shaped
'witches cauldrons'.  These cauldrons with its concave tops/lips, reflect
heat back into the top of the content when the pot is less then half full,
( dried broiler effect ) between 50-100 percent, the evaporation area is
reduced as the throat size decreases.  The round bottoms allow for easy
stirring (gravity will help rotate the food as they fall against the curved
sides, bringing a different surface to maximizing metal to food heat
transfer.), reduced hot spots, easy cleaning.
3.1  You would notice the concave lid.  Even though it is easier to make a
flat metal lid.  Cooks know a concave lid works better.  The steam condenses
and drips back as a stream back down the sides, some lids have a convex
centre where the drips are toward the middle of the pot, where most burning
would occur.
3.1.1 Some lids have a high dome, with lots of 'dead air space'.  The
entrapped steam has a longer dwell time to condense back into the pot,
reducing the pressure gradient against the edges.  The lost in thermal
efficiency is countered by the greater moisture retention as the steam
condenses against the cooler upper surface of the tall dome.  Close
tolerance lids may not need a high dome to trap moisture, but loose
tolerance lids would trap more moisture if they are tall.
3.3  I have seen 50-75 litre aluminium flat bottom pots similar to the one
in the apovecho barrel stove
http://www.aprovecho.org/web-content/media/inst_stove/p1.htm
buckle where starch has burn in. (rice, soups, porridges)  Ruining the pot
for most applications.  The food sticks inside the ridges, making
stirring/scraping the bottom in the cooking process impossible.
Round bottom cauldrons/woks are more forgiving, as they tend to expand and
contract without these disastrous folding.
3.4 a lion stove set-up would make me a happier cook then a barrel stove.
But having a barrel stove sure beats not having one ! ! !

4. The cooks have a choice between hauling fuel, food, water (for cleaning),
depending on the availability of these three, what the cook is willing to
haul is a matter of experience and personal bias.
4.1	Cooks need to store pots, so transportable (camp to camp ,or sink to
stove  to storage) stackabilty, versatility, maintenance, Stability, Ease of
use, are all extra factors about pots that stovers generally do not study.
4.1.1 heat retention vs. rapid heating is another factor. This list has
talked about metal vs. ceramic bodies for stoves. Material selection for
pots are just as important for the cook.  An insulated pot would be sometime
akin to a new pot making material (composite).  Probably better to let the
cooks decide.  A thin metal wok is great for stir fry, but maybe not so
great a crock pot.

4.1.1 If I were a cook trekking on foot in Sudan.  I have a round solar
cooker that can be split to use as a wind screen for cooking at night.  2
thin body cauldrons that stack one inside the other.  ( I believe old
spittoon design would work well for me); 2 woks that can dual purpose as
lids for the cauldrons; 2 lids for the woks that can be used mixing bowls,
water transport, cleaning pot. (no handles on the bottom please)

I believe Paul had asked about anyone using these wood stoves daily.  I
would like to answer affirmative one day. (Electricity is so convenient.)

I appreciate all the effort and input from so many people on this list. From
all the cooks that use your improved stoves.  May your meals be seasoned by
joy and blessings.

Tony in response to ===


Date: Wed, 5 Sep 2007 09:14:15 +0200
From: "Crispin Pemberton-Pigott" <crispinpigott at gmail.com>

To: "'Discussion of biomass cooking stoves'"
	<stoves at listserv.repp.org>
Message-ID: <00ba01c7ef8c$7c7b8c40$7572a4c0$@com>
Content-Type: text/plain;	charset="us-ascii"

Dear Tony

This is getting a little old but just in case ....

There is an idea in the below (in your message) that deserves following.

Friends:  [snip]   My question is, has anyone
quantified what the influence of a pot insulator would be, based on material
and pot size? It is a good 'thermo' question.

I think the answer would be a general formula not a single number.  For
example if you insulated the lid of a pot there is only gain.  If you
started placing an insulating blanket around a pot starting at the top and
working down, there would be a heat retention gain up to the point where
heat arriving in from the sides would be kept out.  Where is that point?
Does it exist if there is no side skirt?

Then, developing Tony's point, somewhere there is enough gain from
insulation to justify the loss during boiling.  It may turn out that the
heat retained during most of the cooking more than offsets the loss during
the initial high power boil, or in fact that there is a net gain all the
time even with a pot that is insulated on the sides all the way to the
bottom corner.

I have not yet seen such a pot but it would be pretty simple to make one.
Why aren't all pots insulated (especially on top)?

Regards
Crispin


-----Original Message-----
From: stoves-bounces at listserv.repp.org
[mailto:stoves-bounces at listserv.repp.org] On Behalf Of tony chan
Sent: January 12, 2007 6:29 PM
To: stoves at listserv.repp.org
Subject: [Stoves] insulated pots

Hello everyone:

This is my first post, so please tell me if this is properly done.

Insulating a cooking pot is great on top, but around the edges, would reduce
the heat transfer from flame to content, so we can lose more then we gain.
(insulated a simmering pot would work, but hard boiling we could lose lots
of heat transfer area).  So pot design (tall, squat; concave/convex/round
bottoms; etc) is another rabbit trail, just as ingredient preparation ; )
Martin

[snip]

Tony Chan
Winnipeg MB