There are multiple different things that can affect the performance of an air flow system, a good place to start would be by ignoring most of them as they aren't significant enough. There are a handful of things that really make the big differences...
1. If you have the redundant vents closed off with blast gates then you can ignore everything except the direct path between the open gate and the dust extractor vent to atmosphere. If there's no flow down them, there's no pressure drop and no interactions. Make sure this is true, or the rest doesn't work
2. It is important to remember that you can only suck air in at the rate it can leave the extractor, if your bags/filters are full or blocked then that's the first problem, what goes in, must come out (or is stored)... I learned that at university!
3. As any fluid flows down a pipe it has to rub against the walls which slows it down, therefore big pipes have faster flows (same as deep rivers etc...). Pressure drop and flow are proportional to each other, so a super slick large diameter pipe will loose very little energy through friction down its length, so have a small pressure drop and small decrease in flow rate. A huge fan connected to a tiny pipe half a mile long will loose all its energy in friction and have very little flow at the far end and a big pressure drop.
4. So why not have huge pipes all the way? If a certain volume of air is being pushed down the pipes in a certain time (for your extractor 3,500 cubic metres per hour) then the cross sectional area dictates the flow speed, if I had a 10 square metre pipe it would need to flow at 350 metres per hour (=0.35kmph), if I had a 0.1 square metre pipe if would need to flow at 35,000 metres per hour (35kmph = much faster). Back to rivers again... tiny streams (flowing slowly becasue of paragraph 3) can carry soil particles and maybe a small bit of gravel, a tsunami travelling much faster can carry cars and houses. So the mass of particles a flow can support (and hence extract) depends on the speed it's moving at. Big pipes with slow speeds may be energy efficient but will struggle to carry large chips from a planer, but will be great for dust from your sander.
So large pipes all the way won't carry the material, and small pipes will loose all your power in friction, as your chippy says the diameter should reduce away from the extractor. How much by is a matter for experimentation or calculation, and life's too short to get the calculator out.
I would recommend that at least a 6" main tube would help depending if it's really long, if it's over a few feet then you probably need to upgrade. Having seen Bob '9fingers' extractor yesterday with a 3hp/2.2kw fan and miles of ducting there's no shortage of power in your system, I hope he hasn't got any pets because that system could pull cat of the ground and send it into the vortex separator!
My extractor with a 4" hose is pretty weak too, but it has a flow 1/6th of yours. Thanks my tuppence worth
Aidan
