Fan velocity pressure is the kinetic energy component of airflow, representing the dynamic pressure created by air movement speed. Measured in inches of water column (in. wc) or Pascals (Pa), velocity pressure depends on air velocity and density according to the equation VP = (V/4005)² for air at standard conditions. Typical residential fan discharge velocity produces 0.1-0.4 inches of water column.
Pressure Component Relationship
Total pressure equals static pressure plus velocity pressure. As air velocity increases, velocity pressure increases proportionally to the square of velocity. A fan producing 1500 feet per minute discharge velocity generates approximately 0.14 in. wc velocity pressure. This kinetic energy converts back to static pressure through ductwork expansion and bends.
System Performance Impact
Excessive velocity pressure indicates oversized fan discharge relative to ductwork size, creating turbulence and noise. Undersized ducts force higher velocities, increasing friction losses by 10-15 percent. Proper ductwork sizing maintains velocities below 1500 FPM in supply ducts and 1000 FPM in return ducts to minimize velocity pressure losses.
Energy Considerations
Converting velocity pressure to useful static pressure requires proper duct design with smooth transitions and appropriate sizing. Pressure recovery devices and well-designed plenums can convert 40-60 percent of discharge velocity pressure into usable system pressure. Poorly designed ductwork wastes this energy as heat and noise.