On mine, I have a 4" stove pipe entering the back, under the grates, extending to within 6" of the front and capped. I then have 3 rows of 5/16 holes, spaced every 6", 1 row top center and 1 row each facing each outer edge of the grates. It works very well, but I run it off my high pressure blower for my Air Over Fire too, with a valve to regulate the flow. With the HP blower ash doesn't seem to cover the pipe, with a lesser blower you might need to watch the ash more so it doesn't block the air flow.
Dave Klish, I recently bought 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.