A fan is bought by two numbers, and static pressure is the second one: the resistance the system throws back against the flow, which the machine must develop to deliver its rated volume — a fan curve point reads like “1,000 m³/h at 500 Pa.” The system side of that number is a sum of three parts: straight-run friction from the Darcy-Weisbach equation (with the Swamee-Jain friction factor in turbulent flow and 64/Re in laminar), fitting or “minor” losses at every elbow, tee, and transition, and any static head. Checking that total against the available fan or pump head is what a pipe and duct sizing calculation is for.
Duct design typically works the problem in reverse through the equal-friction method: pick a target friction rate — pressure drop per unit length — and select each duct size to hold it, which yields a round size and an equivalent rectangular alternative while keeping velocities inside the application limits set by ASHRAE, SMACNA, and CIBSE Guide C guidance.
When the operating point moves, static pressure moves violently: the affinity laws make flow proportional to the speed ratio but pressure proportional to its square and shaft power to its cube. Doubling a fan’s speed from 1,000 to 2,000 rpm takes 500 Pa to 2,000 Pa and 10 kW to 80 kW — and, run the other way, a 20% speed cut nearly halves the power, which is the whole economics of variable-speed drives.