Air inlet locations for gasification furnace

[Wade]

Need a little help on this topic. I'm building a downdraft gasification furnace, everything is going good but the layout of the primary and secondary air inlets and the ratio that they should be set at.The firebox is 30 wide by 36 deep with the gas nozzle (hole in refractory) 2/3 back in the bottom of the firebox. The fan is set up like the garn unit, where it sucks the gases through the system. Where would the best place for both be to get maximum burn and gasification? Any help would be appreciated.
Thanks

 

[deerefanatic]

Well, on a downdraft, conventional wisdom says that the primary air should come in low on the top chamber and the secondary air should be routed from outside, through the refractory and to the nozzle's sides.....

I eagerly await the results of your negative-draft system. That was originally my plan, but ran into problems finding fans that could take chimney duty for my system. What did you use for a fan?

 

[Wade]

Thanks Deerefanatic. The fan im using is from a gas fireplace insert. It blows the hot air out of the unit. The counter guy said it was good for 250 F. Its a squirrel cage design rated at 550 cfm free air. My heat exchanger is a cleaver brooks boiler, so i hope the temp has dropped low enough by the time it hits the end of the run! Thanks again.

 

[deerefanatic]

I'd worry about that 250F rating... Right now I have too much air going through my unit and my stack temps hit close to 700 sometimes.......

The thing with wood is, at 250F, that's about as low as you want to go or you risk having condensate problems which will eat right through regular steel......

 

[Wade]

I thought i was pushing the limit, but my heat exchanger has 250 sq ft of heat exchange area and the exhaust hopefully will have cooled off. Like you mentioned, im worried about condensation in the boiler even before the fan due the temp drop.
I ran a smaller version of these fans on my prototype experiments, and most of the time it had the flame shooting out of it due to being only 5 feet from secondary chamber. No paint on it but still works good. Hope the larger model is just as tough!

 

[deerefanatic]

How hot did the motor get? If the wheel and housing held up, then the motor is your weak link.........

 

[Wade]

The motor held up too. It was a good unit. As for temperature im not sure.But i do know when i put a rag over the fans exhaust it took about 2 seconds to burn a hole right through it, but the motor itself could be handled ,but not for long.

 

[SnaykeByte]

You will need an axial draft inducer fan. That centrifical blower will NEVER stand up to what you are trying to do.

 

[Wade]

Why wont it last?

 

[SnaykeByte]

Because they are designed to move ambient air, not heat, or at least not very much heat.

 

[Duetech]

With my old wood furnace I used a tjernlund draft booster fan that was built directly in to the exhaust pipe about 4-5' from the furnace exhaust outlet and sometimes the pipe would get a dull glow going. Do a google for tjernlund and you might come up with something. The exhaust fan I had was variable speed and last time I priced them they were going for around $110.00. I had a 12' horizontal run before I got to the wall for my chimney exit. The draft booster ran 24-7 and the biggest problem I had was creosote build up on the fan blades that impeded roatation. The exhaust stack was 6" but the booster fan is advertised for 6-8". It's simple design and a simple hook-up. I don't know if it would work for you but it will take the heat. Cave2k

 

[deerefanatic]

I too looked at the Tjernlund units..... I'm not sure if they'd do the trick for what we want... Although, I guess there's only one way to find out!

 

[Wade]

Because its a gasification furnace i hope there is no creosote buildup. I have moved the blower to the very end of the exhaust run, right after it leaves the 3200 gallon water tank. It will be easily accessible and open to exterior air. I cant see heat being a problem there. I could be wrong and it might be a very cold winter!

 

[deerefanatic]

Hey, you can't know until you try!

 

[winton]

How about a fan from a coal forge . They are cast iron or cast steel .

 

[winton]

would it be advantage to have multiple smaller air inlets along the length of the gasification chamber? My gas forge has three air mixers along the top.

 

[deerefanatic]

Multiple air inlets are a necessary item in the nozzle.... As for the primary air, not really important.....

 

[Wade]

What sizes of holes works best in the nozzle, and which is the best direction to point them? More of a cross stream or down with the flow of gases?

 

[deerefanatic]

I'm assuming cross stream would be best. As for me, I used 3/4 inch diameter holes......