In article nl24c.5465$C51.50125@attbi_s52,
"Mike Hide" wrote:

Considering the above seems to me the heavier dust particleswill just fall
to the bottom of the duct and stay there as the velocity gradient there is
theoretically zero, that is until enough of them are deposited to reduce the
effective area and in turn increase the duct velocity. To me it's kind of
like which came first the chicken or the egg, I suppose at some point the
whole thing stabilizes ???mjh

I would imagine the extensive turbulence cause by joins, etc. would kick
up anything lying on the bottom; i.e. actually not give it a chance to
rest. Laminar airflow is actual quite difficult to achieve (ask airplane
designers; it is one of the holy grails).