Primary & secondary C F M Question

[waynecub]

I have been building my wood down burn gasification system this summer and have read a lot on the forum about primary and secondary air. I am still not sure how many C F M's it takes. I hope that my system will put out 100 thou. B T U's I would like to have a draft fan for the primary, and a draft fan for the secondary air. This way I Think I could control the burn better. Or should I have one fan and control it like most of you? In a lot of the pictures that I see on your systems the air intake is closed down a lot.
This is a great forum to read and learn Thank You.
Marcus

 

[Nofossil]

I have been building my wood down burn gasification system this summer and have read a lot on the forum about primary and secondary air. I am still not sure how many C F M's it takes. I hope that my system will put out 100 thou. B T U's I would like to have a draft fan for the primary, and a draft fan for the secondary air. This way I Think I could control the burn better. Or should I have one fan and control it like most of you? In a lot of the pictures that I see on your systems the air intake is closed down a lot.
This is a great forum to read and learn Thank You.
Marcus

Been a while since I crunched the numbers, and I've only had one cup of coffee, but here goes:

As well as I can remember, you need something like 25 CFM of air to get 80,000 BTU/hr at an oxygen ratio of 1.6 x stoichiometric (considered a good goal to ensure enough excess oxygen to get complete combustion).

The ratio between primary and secondary is more of a challenge. My personal suspicion is that the ideal ratio changes over the course of a burn, with more secondary air needed early and less towards the end as you start burning charcoal. At this point, I don't know any answer except experimentation. I have a lambda sensor that I intend to use to get more definitive answers.

 

[waynecub]

Thanks Nofossil, that gives me a number to look for. I will use a CFM fan that is larger and choke down the air to primary and secondary to try and get a good burn.
I will give you a call when it's in my basement and plumbed. I still am trying to find a storeage tank.
If you are ever looking for a elec. hot water tank there is a man in Bristol that removes them for CV & GMP. The tank that I picked up is a 75gl. fiberglass $40. and it look good. He also has 80gl. that are I think stone lined. My 75gl. will have a side arm to heat the water.
thank Nofossil
Marcus

 

[Nofossil]

Thanks Nofossil, that gives me a number to look for. I will use a CFM fan that is larger and choke down the air to primary and secondary to try and get a good burn.
I will give you a call when it's in my basement and plumbed. I still am trying to find a storeage tank.
If you are ever looking for a elec. hot water tank there is a man in Bristol that removes them for CV & GMP. The tank that I picked up is a 75gl. fiberglass $40. and it look good. He also has 80gl. that are I think stone lined. My 75gl. will have a side arm to heat the water.
thank Nofossil
Marcus

So where are you in Vermont? I'm not far from Bristol, and I have a couple of friends who are looking for cheap tanks for solar DHW storage.

 

[waynecub]

Nofossil, I'm in South Starksboro. I'f you remember I went to your house last winter and looked at your system.
I will e-mail you with more info on the hot water heaters.
Marcus