The calculation provided above will only tell you how much airflow you need to achieve in order to dissipate a given amount of heat.
System impedance will always be a factor when it comes to fan selection, and to work it out in detail you need to know the impedance curve of the system in question and then relate that to the static-pressure:flow-rate curve of the fan.
This graph shows some (imaginary) system-impedance/flow-rate/pressure curves. Find the desired flow rate on the bottom, go up until you hit the relevant impedance curve, and then go left to determine the static pressure required to achieve that flow rate. (This is for illustration purposes and so doesn't have any actual values attached.)
From this, it's obvious that a higher impedance system will need a higher pressure to achieve a given flow rate.
This next graph shows the pressure:flow-rate curve for a typical (but imaginary) fan. (Accurate data for specific fans should be available from the manufacturers of those fans.)
This graph shows the throughput a given (imaginary) fan will achieve through systems of differing impedance. The intersections between the fan curve and the impedance curves will give the flow rate and pressure for this particular fan in those systems.
Although it's possible to calculate the impedance (losses) caused by ductwork it is fairly complicated. It's definitely not impossibly difficulty though, but I haven't spent much time on it yet. If (when?) I do then I'll post a short how-to, or at least some rough correction factors.
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