Need input on air

[eddy6963]

So last year I built an arch from a 275 gallon fuel oil tank. I put in a blower and I believe it is considered under fire, but I may be wrong. I have a 2'x4' flat pan, I'm looking for people who run a similar setup on what you run for a blower, and if you can send pictures of your blower setup and piping inside the arch. Thanks much.

[BAP]

Post some pictures of your setup and maybe people can offer some suggestions on how to improve it.

[maple flats]

Where does the air enter the arch? If under the grate it is under fire, if above the grate but under the pan it is over fire (AOF). For AOF to work it needs a high pressure blower, identified by paddles the throw the air, as opposed to a squirrel cage blower that flings the air. Those are high volume but very little pressure unless it is spinning very fast.
My air blast sprayer (used to spray my blueberries out to 20' from the sprayer) approaches high pressure but is really just low pressure.
The high pressure blowers used for AOF are low volume but high pressure so is makes the fire roar and mixes the air for more complete burning.

[Rockfallsmaple]

I have a barrel arch and a 18 x 24 flat pan so somewhat smaller than your setup, but still similar.
I use a blower from a Hot Water heater. It can be controlled either of 2 ways.
The easiest is to use a light dimmer, when doing it this way the fan is running at about 50%.
I can also run it using a Temperature controller with a temperature probe in the stack. The setpoint is usually at about 1000 DegF. This seems pretty hot but the stack is never red hot so its not being overheated. As can be seen in the photos there is a sap preheater on the stack this takes 35 DegF sap to about 90DegF.
The air is forced into the fire from the back of the arch thru a 2 inch iron pipe. There is a metal strap on the end of the pipe to force the air up and down, I not sure this is really needed. I don't have a grate, the fire burns right on top of the fire brick. After each days boil the ashes need to be cleaned out.
I batch boil 40 to 60 gallons at a time and finish on an electric stove in the house. Hope some of this helps.

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[eddy6963]

So my setup would be air under fire, one question I have now is do I have to big of pipe? It is 1" pipe with holes drilled in them. Should I go smaller? My blower is a 75cfm blower, but it doesn't appear that I can adjust the power. I am going to rebrick the inside in the coming weeks, as it is crumbling from last year.

[RileySugarbush]

For air under the fire all you need is to get the air into a closed volume under the grate. If you already have a blower just make the pipe/duct to the bottom the fire box the same size as the blower outlet. No need for small nozzles, in fact you probably have a low pressure fan or blower and if you restrict the flow at all you will get very little air. Just let it run up through your grating and into the fire.

You do have at get a big enough blower since all the air for your fire will be coming through it.

If you make a little sketch of what you are planning, it woill be easier to offer appropriate advise.

[Super Sapper]

The blower sounds a little small, I believe a 2 x 6 needs about 250 cfm. If you have restricted the discharge at all it will reduce the cfm and you may starve your fire. I agree with just getting the air under the grates without nozzles. You need to make sure that all of the grate is covered with wood or coals. Any open areas with let the air short circuit around the wood.