[Stoves] Chimneys
Choppalli Venkata Krishna
krishnacreat1 at rediffmail.com
Tue Nov 2 02:35:35 CDT 2010
I agree with Crispin. The damage of Improved stoves in India was, one main, was the drudgery the women faced in cleaning. And they removed it. The purpose thus failed.
In Nepal, a chimney has been innovated which has a provision to clean it easily than the AC pipes provided in Indian Improved stoves when the National programme was on. Can any body/HEDON provide it?
-Krishna
On Mon, 01 Nov 2010 08:56:08 +0530 wrote
>
Dear Crispin
Excellent presentation and analysis!
Your point about over-firing, and not knowing when
to turn down the damper is a good one.
Would a simple bimetal magnetic stack thermometer
be able to tell you the temperature of stack gases, so that the Operator could
turn down the damper tomaintain the stack temperature to the temperature
observed at minute 35, where maximum efficiency occurs?
Best wishes,
Kevin
----- Original Message -----
From:
Crispin
Pemberton-Pigott
To: 'Discussion of biomass cooking
stoves'
Sent: Monday, November 01, 2010 12:04
AM
Subject: Re: [Stoves] Chimneys
>
Dear
Joyce
There are two answers to your
question. The first is that CO is not all that big a problem for most people.
Yes it is a problem in certain places, Johannesburg and the col burning
Highveld regions for example, but smoke exposure is a much larger concern in
my experience.
The second is that chimneys
are relative expensive. If you put a chimney on a stove that is not very clean
burning, it quickly gets clogged and is a maintenance problem. An
example of this is the stoves made from clay and sand in Kenya. In the high
regions (tea estates especially) there are ‘fuel efficient stoves’ promoted by
the tea estate corporations as a beneficial idea. They have chimneys but are
pretty dreadful is terms of combustion efficiency. In as little as 3 months a
3 inch diameter chimney gets clogged with condensed, boiled biomass vapours.
The stove have chimneys but don’t really save much fuel and waste a great deal
of it by simply not burning the gases.
So chimney are not as easy to
work with as one would hope. Cleaning up the combustion is actually the most
important if there is nearly zero money in the community.
Chimney stoves, in answer to
your question about the effect of putting on a chimney, have to have pretty
good air control or they are not very efficient.
Attached is a chart of a coal
stove with a chimney attached, and no flue damper to control the draft. There
is really no way for anyone to know how and when to close or partially close a
damper for optimum efficiency. This is the result of an open chimney attached
to a fairly large fire. The peak burning rate can be seen by looking for the
steepest portion of the brown line. That is the mass burned during the
operation.
As you can see the initial
burn rate is low so the line is nearly horizontal, then it gets going like
crazy to about 16 kW. Then the coal runs out and the burn rate slows. Then it
is refuelled with a sharp jump up which tapers off in the end after about 200
minutes.
The thermal efficiency is the
green lines, The darker one that moves up and down is the instantaneous
efficiency calculated from the temperature of the gases in the chimney and the
excess air at the time. The smoother green line is the cumulative efficiency,
meaning how things have gone so far, all things considered. Two features are
noticeable. The first is that it is pretty constant at about 65% efficient
when the fire is large and burning at a high rate. The second is that as the
fire dies down, the thermal efficiency drops to zero and in fact goes
negative. Because it is negative (the fire is actually cooling the room by
throwing more heat up the chimney than it is generating) the average for the
whole burn drops from 60% at minute 100 to 33% at minute 200. That is amazing,
eh?
So putting on a chimney does
not guarantee overall success. The main reason for the poor performance is
excessive draft – there is simply too much air getting into the stove,
allowing it to operate at a high power level – too high to be useful actually.
This is followed by a period when the stove cools the home drawing, as it
does, about 50 cubic metres of -35 degree C air into the house to feed the
fire.
So, chimneys make things a lot
more complicated providing expected results and additional expense. The
expense is not just for the chimney which might cost $5, but also for a stove
that is air tight enough to control the combustion reasonably and now waste
fuel.
Best
regards
Crispin
++++++++
Why is no one talking about chimneys that
get rid of the CO safely? And doesn’t the addition of a chimney change the
dynamics of any stove?
Joyce M
Lockard
rj.lockard at frontier.com
503-533-4190
Home
503-201-9548
Cell
503-533-4209
Fax
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