under grate blower

[steve J]

Bill mason is making me a 2x4 evaporator with a blower. He says its similar to the wood saver blower unit. Apparently this blower blows from under the grates. Do you believe this will use more wood then not using the blower? or is there a potential savings on the wood pile. My intent is to use it I just getting a little short of time on cutting more wood and would love to know I could quit cutting soon.

[maple flats]

IT will use less wood and burn hotter. A lot of evaporator inefficiency comes from the air being used up and unburnt gasses going up the stack. More air blown in helps a lot. In another thread I talk about my project of adding high pressure air over and under the fire. Doing both I saved about 1/3 of my wood. I don't have as good figures from when I just added under fire but it was definately noticable. One way to notice ineffiecieny when your evaporator is burning full tilt is look at the top of the stack after dark. You will see a ball of flame. That is not fire going the full length but rather hot gasses re-lighting when they hit the air. Add enough forced air and you will no longer see the ball of fire on the stack and you will get more bang for your wood.

[steve J]

Thanks that is good information Dave

[Daryl]

I added air over on my 18 X63 sportsman. I think it helped to increase the efficiency over just air thru the grates. I started out with a 2 x 3 Mason and burned close to 3 large bundles of slabs to make 12 gal. With the sportsman, it and only used less than 1 1/2 cord of slabs to make 16 gal. The rig also has 45' of copper in the hood for a preheater. We got close to 20 gph. No smoke out the stack and hardly any sparks.

[twofer]

In my opinion I think you'll save some wood because the rig is sealed up and the force air is only pushing air to where it is needed for primary combustion. I also think that you'll be missing out on larger efficiency gains from adding over fire air, where the majority of your air should be to create secondary combustion.

[maple flats]

I agree with that. After the under fire air is installed, and before you brick in the firebox read this : http://www.uvm.edu/~pmrc/Combustion.pdf You will do well to add both. If high pressure does not fit the budget, add it with the std squirrell cage blower and then change to high pressure when you can swing it. Low pressure will be better than nothing and then the air manifold will be there for when you go to HP. The main advantage of high pressure over the fire is the turbulance it creates which mixes and burns most if not all of the gasses available. Realize, wood does not burn, the gasses burn. To see this light a wooden match and look closely, the flame is above the match, the heat is gasifying the wood, the gas rises and mixes with air and then burns. When you add over fire air you burn the gasses that are already there from the initial burn. Without air blown in much of the unburnt gasses go up the stack and burn at the top (where it does you no good.

[Ausable]

Hi Dave - Is there a point when You have too much forced air or too much air pressure in the arch -- or is your stack able to maintain enough draft to exhaust the burnt gasses outside and not in the sugar shack. It would seem like a stack damper of any kind would be useless with the arch and stack under pressure. Have You ever heard of anyone using induced draft on an arch? - like is used in a commercial boiler - or is that getting way out there? Then I wonder - what kind of temps an arch could take without a meltdown - interesting though ----