[Stoves] The upside of Down feed

[Crispin Pemberton-Pigott]

Dear Alex

The draft is about 25 Pascals. That is pretty strong. According to the
calculator, guessing at a few things like a 7% moisture content for the
pellets, it is about 8.5 kW.

If the chimney is 125 mm in diameter the gas velocity is about 0.45 m/sec
(average)

At 100% EA it is drawing in 2 litres of air per second and producing almost
three times that as combustion products (calculated at 0 Deg C). That info
might be helpful.

>>> I use a loose lid/follower to ride down on the pellets.
>> Was that to limit the air flow or more to make it feed well?
>If there was any chance of gasses convecting up the space the lid 
follower would create bit of a choke point  around the edge where the 
air would be uniformly moving down.

I worry about the pellets bridging and failing to flow. 

>The hopper gets hot and could otherwise generate some convecting fumes . 
>My current thinking is that the straight portion of the hopper should be 
>double walled. The inner cylinder would not be thermally connected to 
>the  outer. The inner should have a lid. 

My first thought was to isolate the hopper from the Tee that gets so hot.
What we have done with the BLDD 5 (SeTAR Centre stove) is to use a stainless
steel Tee and connection to limit heat transfer upwards. See our paper
dealing with this approach coming out in March.

>> The difference between this and a downdraft burner is that the fire is
not
>> at the bottom of the hopper, it is in the tube so technically it is a
>> hopper-fed cross-draft fire.
>It can be that or a combination with some air from every where.  Nothing 
>about the burning bed part of this is settled.
I will try this though I will not be able to easily get a 6 m chimney. In
principle it should work with 3 which is adequate for generating substantial
draft if the chimney is a major part of the heat exchanger, or hot with low
gas velocity (insulated). Believe it not I have seen +700 in a chimney not
long ago. Your heat capture is really good. If you CO was 1% of CO2 in the
stack, and the EA 100%, the thermal efficiency is 88%.

>> Do those corn stoves have a wiggling wire worm because of ash issues? Or
>> just or move the fuel?
>I think its the ash issue.

OK.

>What temperatures do you see in the coal hoppers?

In the BLDD 5 the hopper runs red hot. See attached photo. The temperature
inside is really high. Again, we have a development of interest in that
regard which will be reported in March.

We try to keep the stack temperature in the 180-220 range to get good draft.
Remember that the temperature in the chimney is not a measure of the heat
loss which is actually a function of gas composition, temperature, dilution
by air and velocity.

Regards
Crispin

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