And in addition to effective swirling, let it happen in the lower end of vertical tubes, so the "center string" lands in a long "bottom bucket" instead of laying and covering the tube-surface on the lower side in horizontal tubes!
That asks for two rectangular horizontal tubes; the upper one for collecting cooled gas, the lower one with an intermediate bottom for distributing hot, sooty gas and the lower part as a common "soot box", rakeable and rinsable from the ends.
This makes in fact each vertical cooler tube into a small cyclone... remember, that small cyclones work well in the Netherlands!
Here layes also the possibility to change the count of active and passive tubes for the day; turning the tubes makes it possible to activate or close their inlets... for freezing or warmer days.
To stop the common up-flow from the soot-box through all tubes, this up-flow has to be stopped individually in the upper end, aswell as the inflows at the lower end.