Re: Blower parameters (for updraft biomass cookers)?

["Ron Larson" <ronallarson@qwest.net>]

Jan:   In addition to Andrew's kind and full response,  four more points:

a)  Make sure you are lighting on the top
b)  The forced convection approach is used by Tom to achieve mixing via a
clever technique not possible with natural convection (or at least no one
has reported success I believe) so as to achieve a blue flame.  Some on the
list have said they preferred a yellow flame to get better radiative
transfer to the pot.
c)  The combustion of the charcoal after pyrolysis is complete is occurring
well away from the pot - so not much is added to the useful heat output.even
as the necessary increased air is supplied.
d)  On the "solar cooking" list about a month ago there was mention of a
spring-powered fan/blower used to move air around - much as in a convection
oven (reported I think at about $15).  I still haven't heard the name of the
manufacturer and hope someone on this list can provide more data on this
strictly mechanical design.

Ron


----- Original Message -----
From: AJH <andrew.heggie@dtn.ntl.com>
To: <stoves@crest.org>
Sent: Saturday, March 09, 2002 2:21 PM
Subject: Re: Blower parameters (for updraft biomass cookers)?


On Sat, 2 Mar 2002 11:51:41 +0100, Jan Cáp <capjan@vol.cz> wrote:

>I very interested in cooking devices based on "updraft gasifier" principle
>with forced konvection (as is cooker by Thomas Reed).

In the absence of a reply from a more learned source:

Tom's turbo stove or the reed-larson idd stove are special cases of an
updraught burner. The "normal" updraught burner has the products of
combustion rising through the fuel, both drying and pyrolising it.
Natural convection will achieve this. Forced convection has a place
when sufficient chimney effect is not available (height or cooking
constraints). to aid mixing of combustion gases produced and to gasify
char in the bottom of the burner to CO rather than convert it to CO2
in the lower part of the stove, the CO then joining with other off
gases to produce a secondary flame in the "working" part of the stove
under the pot.
>
>Which blower type/construction is good for this purpose (membraned or
>other)?

Small electric fans from PCs seem popular, there are many other ways.
>
>How mouch air input (blower power) is needed /optimal for small burner (1-5
>kW thermal output)?

This is entirely dependent on your stove configuration, in general you
need to pass in excess of 7kg of air over each kg of dry biomass you
use as fuel. The pressure difference between the air input and the
flue outlet will determine this. The product of the volume of air
(related to its mass and absolute temperature) times this pressure
difference is the total power required. Some of this power will be
provided by the chimney effect. In addition to this is the power
needed to promote turbulence in the secondary flame, again very much
down to burner configuration.

I believe Tom Reed has powered stoves of 2-3kW(t) output with 3W and
10W fans to augment natural draught. As I believe fans only achieve
50% efficiency you can calculate the required electrical power for any
given air movement against a back pressure.

--
AJH




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