If your air velocity is too low in your ductwork you could get dust dropping out in straight runs. And dust collecting in ductwork is not a very good idea.
ALC - Aerodyne
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Posts about Pressure in Dust Collection System (2):
Dust Collection Systems: Instrumentation 101 – Differential Pressure Gages
A dust collection system is much more than just the dust collector (ex. Cyclone, baghouse, cartridge collector, wet scrubber, etc.). It constitutes the exhaust fan, hoods, instrumentation and interconnecting ductwork. If any of these components are improperly sized or operating, the whole system can fail in its primary objective, dust collection.