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Centrifugal Fan Airflow & Pressure Calculator engineering
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Centrifugal Fan Airflow & Pressure Calculator

Compute centrifugal fan total pressure (FTP), velocity pressure (FVP), brake horsepower (BHP), and motor sizing per AMCA Standard 210 and ASHRAE.

Airflow & System Resistance

Fan Efficiency & Drive Transmission

Fan Power & Pressure Results

Required Brake Horsepower (BHP)
-- BHP
-- HP NEMA motor recommended
Fan Total Pressure (FTP)
-- in. w.g.
-- in. w.g. velocity pressure
Outlet Duct Velocity
-- FPM
Fan Discharge Speed
Air Horsepower (AHP): -- HP
Air Density Ratio (d): --
Actual Air Density: -- lb/ft³
Accurate AMCA 210 air density correction prevents motor overload during cold winter startups.

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Frequently Asked Questions

What is the difference between Fan Static Pressure (FSP) and Fan Total Pressure (FTP)?

Fan Total Pressure (FTP) is the algebraic difference between total pressure at fan outlet and total pressure at fan inlet. Fan Static Pressure (FSP) equals FTP minus fan outlet velocity pressure (FVP). FSP represents the net pressure available to overcome downstream friction and resistance.

Why must fan motor sizing account for cold startup air density?

Fans are constant-volume machines: CFM stays constant regardless of air density, but mass flow, pressure, and required horsepower vary directly with density. If a fan operates on 300°F air (density ratio 0.69), cold startup at 50°F will demand 45% more shaft horsepower, tripping undersized motor overloads.

What is the 6356 constant in the air horsepower formula?

Air Horsepower = (CFM × Pressure in in. w.g.) / 6356. The constant 6356 derives from converting 1 inch of water column (5.197 lb/ft²) multiplied by CFM (ft³/min) divided by 33,000 ft-lb/min per horsepower (33,000 / 5.197 ≈ 6356).