Fans

A fan is a turbomachine that moves air or gas by adding mechanical energy to the fluid, producing a pressure rise that overcomes the resistance of a duct system. Fans are classified by the direction of airflow through the impeller: centrifugal (radial) fans draw air axially into the impeller and discharge it radially, producing high pressure at moderate flow; axial fans move air parallel to the shaft and are used for high flow at low pressure; and mixed-flow fans combine both characteristics. Fan performance is described by a set of curves that relate volumetric flow rate Q, total pressure rise ΔP_t, shaft power P, and efficiency eta at a given rotational speed. The fan operates at the intersection of its pressure-flow curve and the system resistance curve, which for a duct system follows ΔP_sys = K · Q² (turbulent flow dominates). The operating point is found where the fan curve and the system curve intersect. Fan laws allow performance to be scaled with speed and impeller diameter: for a given fan and constant air density, Q ∝ N, ΔP ∝ N², and P ∝ N³. These relationships are used to predict performance at a new speed or to estimate the effect of impeller trim. Fan total efficiency is eta_t = Q · ΔP_t / P_shaft, and the air power delivered to the airstream is P_air = Q · ΔP_t. Static efficiency uses static pressure instead of total pressure. Key selection issues include avoiding operation to the left of the peak pressure point (surge or stall region), matching the fan to the system, controlling flow with dampers, inlet vanes, or variable-speed drives, and considering sound power level, since fan noise rises sharply with tip speed. In HVAC systems, fan energy is often a significant fraction of building energy use, so the system effect, duct leakage, and part-load control strategy strongly influence operating cost.