[Stoves] Fan-blaster Combustors, Stove Camp 2006, and the Dell-Point Pellet Stove

[John Davies]

Greetings,

Crispin describes the burner very well, but replacing the suction draft, 
which is created by a 2.7 m chimney, by a fan driven secondary air blast 
could totally destroy the flow characteristics through the burner and the 
fuel bed. Remembering that the design was for burning volatile coal, reduced 
draft through the fuel bed could be beneficial for biomass combustion.

The system works on the following principles:
1. suction from the chimney sucks air through the fuel bed and through the 
secondary air heating system, via the "venturi burner"

2. The burner creates a velocity increase with reduced pressure in the cone 
section as well as a vortex flow. the air jetting into the vortex on a 
downward slope causes turbulent mixing and almost complete combustion of the 
gasses, with the very hot surface of the fuel bed and hot gasses, acting as 
a continual ignition source.

3 The gas /air mixture is controlled with a damper on the secondary air, 
which also acts as a turn down device by admitting additional secondary air 
which reduces the suction through the fuel bed, and the inverse.  Control of 
the primary air is mainly achieved by the fuel size. and dense packing, with 
the particle size reducing as the bed becomes shallower. ( easily achieved 
with coal )

To my thinking, blowing in secondary air with a fan would have the effect of 
raising the pressure in the combustion system where it is designed to be the 
lowest.

Worth thinking about, or maybe putting to the test.
John Davies.


----- Original Message ----- 
From: "Crispin" <crispin at newdawnengineering.com>
To: "Stoves" <stoves at listserv.repp.org>
Sent: Monday, September 25, 2006 10:24 AM
Subject: Re: [Stoves] Fan-blaster Combustors, Stove Camp 2006,and the 
Dell-Point Pellet Stove


> I think John Davies uses the skin around the fuel bed to pick up heat and
> then a perforated cone to blow that hot air downwards onto the top of the
> fire.  When the draft is correct, it is completely smokeless burning black
> (i.e. not hot) coal from the top downwards.  The centre of the cone is 
> open
> for the upward exit of gases to the chimney.
>
> I believe it works because the draft is sufficient to burn well.  A
> downward-blowing fan would create the same condition without the need for 
> a
> chimney.  The amount of fuel in the chamber is not, I feel, relevant.  It
> could be a cup or a ton depending on how long the burn needed to be 
> between
> fuelings.