Table of Contents
Fan Basics
Fans move air by converting rotational energy into airflow. Performance is characterized by airflow rate (CFM or m3/s), static pressure rise, and power consumption. Selecting the right fan requires matching its performance curve to the system resistance curve from ductwork, filters, and other flow restrictions.
Fan affinity laws relate speed, flow, pressure, and power. Doubling speed doubles flow but requires 8x power. Increasing diameter has even greater effect since power scales with D^5.
Fan Laws
Types
| Type | Flow | Pressure | Use |
|---|---|---|---|
| Axial | High | Low | Ventilation |
| Centrifugal | Med | High | HVAC ducts |
| Mixed flow | High | Med | Inline duct |
| Cross flow | Low | Low | Air curtains |
FAQ
How much CFM do I need?
Room ventilation: 1 CFM/sq ft. Bathroom: 50-100 CFM. Kitchen hood: 100-400 CFM. Server room: 200+ CFM per rack. These are guidelines; actual requirements depend on heat loads and air change rate requirements.
Why does power scale cubically?
Flow scales linearly with speed (Q~N) and pressure quadratically (dP~N^2). Power = flow x pressure, so P~N^3. Running a fan 20% faster increases power by 73%. This cubic law makes speed reduction the most effective energy-saving strategy.
What limits fan efficiency?
Typical fan efficiency is 50-85%. Losses include aerodynamic inefficiency (turbulence, separation), motor losses, bearing friction, and tip clearance losses. Backward-curved centrifugal fans achieve the highest efficiency (80-85%).