[Stoves] Chimneys

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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