For air under fire it is usually just 1 pipe, powered by a squirrel cage blower. To control the air it's easiest to just use a flat piece of tin as a sliding door over the air inlet to the blower.
For Air over fire (AOF), you need a high pressure blower. Likely not in the budget or practical on a oil tank arch, but something to definately worth studying for future improvements if you get into this in a bigger way. However, if your 275 gal tank made arch is going to get all the efficiency it deserves to get AOF. With that you add air from the high pressure blower every 6" thru a nozzle all around the firebox at 6" below the pan(s). Do some reading for that. It greatly improves the efficiency of the burn, increases the boil without burning more wood, by mixing the high pressure air with the wood gases for more complete combustion.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.