Air Under Fire in a Barrel Evap - Pipe Size?

I'm in the process of rigging up some air under fire in my barrel evaporator. Would like to plumb the air in from the back of the barrel via some 4" flex duct and a variable speed duct fan, and then with some 2" black iron pipe (1.5" ID, about 3.5 feet or so) into the barrel, and that pipe will have a bunch of holes drilled into it. Any thoughts or concerns whether that 1.5" line will be able to deliver enough air to run as hot as I'd like? Could probably go to 2.5" if I needed to, but 3" feels like overkill.

I would think the 1.5 would be fine. due you already have the 1.5 pipe on hand? if you have to buy it how much more would the 2.5 cost?

I'm not a expert at all and hoping to next year put AUF on my 2 barrel stoves as well. for this year I have AUF on my home basement stove which is just 3 computer fans set in the ash tray blowing air into the fire box.

I am doing the same thing, adding a blower to my evaporator. I think I am going to use 2 inch exhaust pipe.

I'd suggest you use some 24 ga stove pipe in 3" or even 4". Back when I added AUF I used 4", then I had 3 rows of 5/16" holes spaced about every 3", one row top enter, and one each facing the outer edges of the grates. Even though mine was high pressure, I suggest you also drill 5/16" holes, put a cap on the end and come in thru the ack and stop 6" short of the front. If your blower won't blow effectively, try 3/8" holes. You can lay the 4" pipe on the bottom of the barrel, and you might need to brush ashes off the holes before you fire up each day. With high pressure air I only needed to brush to clear the rows of holes about every 7-10 days, but a squirrel cage type fan won't have the force to keep the holes pen as well.

i did similar to MapleFlats when i had a little home made 4'x5' evaporator. 3" stove pipe, plugged at the end. with 5/16" holes drilled in 3 rows the length of the fire box. now i have a 2 1/2 x 8' and just have a 4" stove pipe that goes to a 4" to 5" elbow that points up into the fire box... with no holes drilled. that works well also. i advise blocking off the front of the grates near your door so the ash doesn't blow up the front and out the door.

I do it a little different. I have a barrel stove with a 24" x 18" flat pan. This makes a 24" long fire box. The fire box is lined with fire brick, and there is not any grate. I build the fire right on top of the brick. I have a 2" iron pipe that comes in on the brick from the back of the barrel, it extends into the fire box a couple of inches and blows directly into the fire. I call it air into fire, it works great. I use a squirrel cage blower from a gas water heater. I do modulate the blower speed based on stack temperature.

I use a bathroom fan with a 3" outlet. I use 3" galvanized pipe straight from the bathroom fan, through the back of the barrel, at the bottom of the barrel, and shoved against the front of the barrel to seal it. Then brick goes up the sides of the barrel and a grate over the pipe. I have a bunch of 3/8" holes drilled in the galvanized pipe under the grate. I have a fan speed control to control it.

Thanks all for the encouragement and affirmation. Good to know I'm in the ballpark. Here's what I rigged up over the weekend, a section of 1.5" nominal black iron with 5/8 holes drilled under the grate. 4" duct on the outside connected to a variable speed fan. I was reluctant to go bigger with the black iron and reduce the size of my firebox. Note the 1" ceramic insulation is also new (firebrick was always there). Hope that the hardware cloth holds up. Pretty excited to see what a difference this makes compared to last year when it was uninsulated and natural draw, and I topped out at about 4.5 gph.

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I doubt that the hardware cloth will last. What did you make the grate out of? It really looks heavy duty.

You can always drill more holes if you do not get enough air. The wire should be gone on the first boil if you have enough air. It looks like you used angle for your grate. Make sure that the angle is like V and not with the point going up. This will fill with ash and protect the angle, otherwise it will burn up also.

Yeah the grate has definitely burned up. I will flip it and try to get more life out of it, thank you for the tip. As for the cloth, oh well...

Wow what a difference! Went from 4.5 gph last year to almost 8 last weekend! What took me so long to do that?!? Appreciate all the help!