Smitty Two writes:

The part I'm having trouble with is the completely unjustified and
unsubstantiated claim that doubling surface area doubles (or improves by
any factor) air flow, given the same cross-sectional area of duct.

I haven't seen anyone claim the airflow would double. They have claimed
that doubling the filter area reduces pressure drop across the filter;
do you disagree with this?

Now, with a typical fan, reducing the pressure drop across the filter
slightly increases the pressure at the fan inlet, which will slightly
increase airflow. Not double, certainly, but more than zero.

Dave