[Stoves] Auto Regulation of Secondary Air Re: Natural draft TLUD turn-down

[Julien Winter]

Hello Crispin, and list;

In response to the excellent description of Natural draft TLUD turn-down
provided by Kirk Harris, you propose the following:

"I can suggest one additional effect to try which is to put a shell around
the outside that is closed to the body at the bottom and open at the top.
This is a down-drafting secondary air preheater. The height of the upper
lip strongly affects the performance so you need to find where to put it.
The effect of this is to create a negative draft on the secondary air
supply that balances the heat inside in such a way that you get the exact
airflow level you want at different power levels. It can take a couple of
minutes to stabilize when you make a change in the heating power, but it
will settle when the metal changes temperature."



I am trying to get a better understanding of your system for controlling
secondary air.  If I understand correctly, the mechanism looks something
like the diagram below:


    |                       |
    |                       | Secondary air entry
    |                       |
|   |                       |   |
|   |                       |   |
|   |                       |   |
|   |      Gas Fire         |   |
|                               |
|                               |
|___                         ___|
    |                       |
    |                       |
    |                       |
    |                       |
    |                       |
    |///////////////////////|
    |//// Fuel-charcoal ////|
    |///////////////////////|


A) When the rate of flaming pyrolysis is high, the gas fire is strong, and
there is a strong buoyancy effect drawing secondary air in.  Any effect of
preheating the secondary air is minor in comparison to the strength of the
draw in the chimney.

B) When the rate of flaming pyrolysis is low, the gas fire is small, and
there is a weak buoyancy effect drawing secondary air in, plus the warm
metal preheats the secondary air, and reduces its rate of entry by
counteracting buoyancy.

Do I understand the principle correctly?

Cheers,
Julien.



-- 
Julien Winter
Cobourg, ON, CANADA
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