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