[Stoves] Skirt on institutional stoves

Paul Anderson psanders at ilstu.edu
Sun Dec 24 15:38:57 CST 2017


Crispin,

You made some excellent points about pot skirts (that also can apply to 
chimneys).  Too much increase in draft can be negative..

Thanks,

Paul

Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD
Email:  psanders at ilstu.edu
Skype:   paultlud    Phone: +1-309-452-7072
Website:  www.drtlud.com

On 12/24/2017 10:56 AM, Crispin Pemberton-Pigott wrote:
> Dear Paul and All
>
> This investigation by Dale Andreatta was quite valuable in once respect that is not normally mentioned when discussion Rocket Stoves: excess air.
>
> Rocket stoves, if made in the 'usual manner' to 'usual dimension ratios' have very high excess air. This cools the fire and produces excess CO. It also lowers the heat transfer efficiency.
>
> " Dale has done work on skirts on regular-size pots, with (I believe) results showing some but not so much impact as we would like."
>
> What he showed was that while the skirt does not itself produce a big advantage, it does two other things that were not the 'intended effect'. One is that it increases the draft on the fire which can be a big negative. Two, Dale showed that when the skirt was tight enough, it reduced the free flow of air and decreased the excess air ratio.
>
> There are multiple advantages to this. Reducing the unneeded air flowing into the stove raises the combustion temperature, increases the heat transfer efficiency, reduced CO and PM, and makes the fire power more controllable.
>
> If you can re-read his presentation to ETHOS on the testing he mentions this effect. Extensive testing by Aprovecho showed them that there was a particular pot-skirt gap that seemed to optimise the heat transfer efficiency. It is quite a small gap. It is not the gap that is 'magic', it is that for any given architecture, there is a way to use the skirt as a choke on the total gas flow that increase the performance on all metrics.
>
> This concept was put to use in the development in 2012 of the SAE stove in Java, and the Keren Super Nova, both of which have a greatly reduced pot-stove body gap. It was traditionally 25 mm. We found the best performance was achieved by reducing it to 7mm.  The experiment was conducted using 1m steps from 3 to 20mm. It only took a few hours. The Keren stove, the traditional clay product very much like a thin Mandaleo stove, is widely used and employs something like 300,000 people in the production and distribution chain (Ref Cecil Cook).
>
> The advantages of a skirt are optimised when checking the O2 content of the exhaust stream to see that it is in the range of 90-120% EA (about 11% O2 concentration). Even a primitive stove will give reasonable results of that target can be achieved.
>
> If the skirt mates to the stove (seals pretty well all round) the heat transfer efficiency is best and it is resistant to the negative influence of cross-drafts.  N small stoves the skirt can double the draft so be careful that make the skirt does not make the efficiency or combustion of heating speed worse.
>
> Regards
> Crispin
>
>
> Stovers, especially Fred, Damon, Adam, Dale, Sam and Dean, and Jim Jetter.
>
> First, Merry Christmas to all of you, everywhere!!
>
> Now my stove topic:   What is known about using a skirt or double skirt with institutional size stoves.   The heat source is not the issue (Rocket, TLUD, LPG, other).   The issue is heat transfer and reduction of fuel used, and not about emissions.
>
> Dale has done work on skirts on regular-size pots, with (I believe) results showing some but not so much impact as we would like.
>
> The InStove folks (Fred, Damon and Adam) are (I believe) the front runners on putting skirts onto large pots.   And they do it with a DOUBLE skirt (heat up one side of the skirt next to the pot, then downward on the other side of that skirt, withing an outer barrier
> (cylinder) that even has some insulation.
>
> Certainly Rocket and probably other institutional stoves with "imbedded pots" or "sunken pots" are made and used.   I believe that they have been measured by Aprovecho and EPA and testing centers, but maybe not with comparative data without the skirts / brick surrounds.
>
> So, the questions are:  are there results (we hope for numeric, carefully collected data) that compares with some "baseline" or "un-skirted" pot, about
>
> 1.  the impact of a single skirt.
>
> 2.  Impact of the double skirt
>
> 3.  with and without insulation.
>
> I hope some data already exist and can be shared.
>
> Objective:  is the extra expense of the skiirt (whatever configuration, even the brick enclosure structure)  suficiently offset by the improved heat capture?
>
> Thanks in advance!!   And Season's Greetings to all!!!!!!!!!!!!!!!!
>
> Paul
>
> --
> Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD
> Email:  psanders at ilstu.edu
> Skype:   paultlud    Phone: +1-309-452-7072
> Website:  https://eur01.safelinks.protection.outlook.com/?url=www.drtlud.com&data=02%7C01





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