[Stoves] Improving Thermal Efficiency (TARP-VE)

[Dean Still]

Hi Frank,

My answer would be:

1.) Air is very light and by volume does not hold much heat. So a lot of
hot air needs to contact a surface to get it hot. Slowing down the air
generally decreases the heat transfer for this reason.
2.) The boundary layer of still air is punctured more effectively by high
velocity hot air that heats the molecules near the pot surface and replaces
them as they cool with new hot molecules.
3.) Extending the time that hot gases flow next to the pot is good. Make
the skirt length longer. Don't slow the flow.
4.) The most effective heat transfer technique is to decrease the channel
gap until velocity of very hot gases starts to diminish.
5.) A smaller fire creates less hot gasses that can flow successfully
through a quite narrow channel (5mm-6mm) resulting in generally higher
theoretical thermal efficiency.
6.) Larger diameter pots have an advantage because narrow channel gaps add
up to bigger constant cross sectional areas. Larry's rule of thumb, when
beginning a design, is to maintain equal cross sectional area throughout
the entire stove/pot. Sam Baldwin has a chart on page 48 of his book
(Biomass Stoves:) showing firepower/channel gap/length of channel
gap/thermal efficiency.
7.) Use radiation, too!

Best,

Dean

On Fri, Jun 1, 2012 at 12:29 PM, Frank Shields <frank at compostlab.com> wrote:

> Hi Dean,****
>
> ** **
>
> Do know why hot gasses moving faster transfers heat better to the pot than
> slow movement? Just seems the longer the heat (whatever that is) would go
> into the pot with the most contact time. Perhaps ‘they’ need be pushed up
> against the pot so they don’t bounce off.  ****
>
> ** **
>
> Thanks****
>
> ** **
>
> Frank****
>
> ** **
>
> ** **
>
> *From:* stoves-bounces at lists.bioenergylists.org [mailto:
> stoves-bounces at lists.bioenergylists.org] *On Behalf Of *Dean Still
> *Sent:* Wednesday, May 30, 2012 4:55 PM
> *To:* Discussion of biomass cooking stoves
> *Subject:* [Stoves] Improving Thermal Efficiency (TARP-VE)****
>
> ** **
>
>  Hi All:****
>
> ** **
>
> This winter the new International Working Agreement (IWA) was ratified in
> The Hague. Four health based tiers divide stove performance. A top rated
> Tier 4 stove (PM and CO) without a chimney is estimated to protect human
> health in a small kitchen with 15 air exchanges per hour. Carbon Monoxide
> isn’t as bad as Particulate matter when inhaled so it’s a lot easier to get
> a 4 (an “A”) for CO but only super clean stoves can make Tier 4 for high
> power PM. (Low power PM is easier.)****
>
>  ****
>
> Most improved stoves that we have tested recently get 2’s and 3’s for PM
> (a little bit of PM causes serious respiratory problems) but even
> relatively ‘normal improved' stoves can get into the 4 category for CO.
> It’s also possible for stoves to get a “4” for thermal efficiency. The
> stove has to score above 45%. We used to think that this kind of score was
> almost impossible to achieve. But now the understanding of heat transfer
> efficiency has been ratcheted up a notch.****
>
>  ****
>
>  Here’s how we try get a “4” on the IWA report card for thermal efficiency:
> ****
>
>  ****
>
> We use an acronym (TARP-VE) to remind us of the most important variables
> when designing a stove that uses the least fuel to boil and simmer water.
> ****
>
>  ****
>
> 1.)    *Temperature*: increase as much as possible the temperature of the
> gases/flames touching the pot. ****
>
> 2.)    *Area*: increase as much as possible the area of the pot touched
> by the gases/flames.****
>
> 3.)    *Radiation*: Don’t forget how powerful radiation is! While Rocket
> type stoves depend on optimizing convection, if the pot can be exposed to a
> hot radiating surface, water will boil a lot faster.****
>
> 4.)    *Proximity*: Force the hot gases into narrow channels as close to
> the pot or griddle without decreasing the velocity of the flow.****
>
>        5.) *Velocity*: Slowing the hot gases decreases the heat transfer
> efficiency. Keep the hot gases flowing as fast as possible without
> decreasing the temperature of the gases. Insulate the heat flow path.****
>
>        6.) *Evaporation*: We add this reminder that, while a very large
> pot has the advantage that more of the pot is exposed to the heat, a larger
> surface of water is evaporating which can make it difficult to achieve full
> boil. That’s the reason that traditional pots have large bottoms and
> smaller tops.  ****
>
>     ****
>
>  OK, dear friends, that’s our newest check list for making stoves more
> thermally efficient. ****
>
> ** **
>
> Best,****
>
> ** **
>
> Dean****
>
>  ****
>
>  ****
>
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