Help with fitting a new extractor hood motor

I think I have found my performance problems. The section to the ceiling and the hoods exit hole is a round 100mm but then it changes to a flat channel 100mm x 50 mm. I held my hand over the exit on the hood without any pipe connected and there is what I would of thought plenty of blow there to extract well.
It seems the problem is the pinch point which I need to swap the 2 90 degree bends and 1.2m of flat ducting to 100mm round all the way.
 
7850mm² is the surface area of the exit.

5000mm² is the area of the ducting. Bends will impact flow- I have no idea how to calculate that.

The charcoal filters, if on the side of the motor, might be to stop the cooking fats from sticking to the blades.

I have a friend that purchased a wide but noisy extractor hood. He glued plates of car engine insulation in it. It made a massive difference to the noise levels- BTW, he checked the fire/temperature ratings first.
 
Thanks for the spec, I read somewhere that there is a max run length and for every bend you take have to off a certain amount from the length.
Yes it came with a carbon filter wach side but pretty sure that were for the hood to be used on recirculate so I have always used it with them off.
Interesting idea about the car engine stuff - the hood is quite noisy but from reading up that may be because of the reduction in air space at my round to circle bend.
Don't think the engine thing is a avenue i will go down I think it would just impeed the air flow within the hood space and be a greases trap fire hazard unless changed frequently.
 
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