Sorry for a late response, here’s some thougts on double wall condensing hoppers.
This is about how i imagine the steam/moisture mowing in the hopper, i also base this on my experiences with the old volvo gasifier, and some research from SMP, and “Svenska Gengas” and like.
This is how i built it on the old Volvo gasifier, no holes for “circulation” in the lower part, condensed a lot of water, most after shut-down, possible to run it on “green” wood, don’t ran very well though, but could be done.
This is how i believe it would work with holes in the lower part, when low load, or just after start, like a “heavy”, saturated, “pillow” of moisture in the upper part, stops the flow up, over the edge, between the shells. Instead moisture pushes out the lower holes, to be condensed.
And here it is in full power, moisture travels up by heat, condenses between the walls, some moisture goes out the lower holes to be condensed, due to air draft around the outer wall, the condensing decreases the volume between the walls, which supports the “circulation”, mowing of the “steam”.
All this could be just wrong, but my earlier attempts, and some research reports seems to support my “fantasies”