[Stoves] Chimneys

Sun Oct 31 22:25:53 CDT 2010

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 to maintain 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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