[Stoves] Improving Thermal Efficiency (TARP-VE)

Mon Jun 4 12:07:23 CDT 2012

Thanks for all the response and I realize we have gone over this many times
before. The movement of heat is mysterious to me. 

Frank

Frank Shields

Control Laboratories, Inc.

42 Hangar Way

Watsonville, CA  95076

(831) 724-5422 tel

(831) 724-3188 fax

 <http://www.biocharlab> www.biocharlab.com

From: stoves-bounces at lists.bioenergylists.org
[mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Dean Still
Sent: Friday, June 01, 2012 10:59 PM
To: Discussion of biomass cooking stoves
Subject: Re: [Stoves] Improving Thermal Efficiency (TARP-VE)

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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