Fan & Blower Calculator

Fan Laws (Affinity Laws)

The fan laws describe how fan performance changes with speed, diameter, and air density. These fundamental relationships govern all fan engineering:

• Law 1 - Flow: Q2/Q1 = (N2/N1) x (D2/D1)^3 - Flow varies linearly with speed, and with the cube of diameter
• Law 2 - Pressure: P2/P1 = (N2/N1)^2 x (D2/D1)^2 - Pressure varies with the square of speed and diameter
• Law 3 - Power: W2/W1 = (N2/N1)^3 x (D2/D1)^5 - Power varies with the cube of speed and fifth power of diameter

Key insight: Reducing fan speed by 20% reduces power consumption by approximately 50% (0.8^3 = 0.51).

Static Pressure vs Total Pressure

• Static Pressure (SP): The pressure exerted perpendicular to airflow - the "push" against duct walls. This is what overcomes system resistance.
• Velocity Pressure (VP): The kinetic energy component: VP = 0.5 x rho x V^2. Converts to/from static pressure.
• Total Pressure (TP): TP = SP + VP. The sum of static and velocity pressures. Always decreases in the direction of flow.
• Fan Total Pressure (FTP): The total pressure rise across the fan = TP_outlet - TP_inlet
• Fan Static Pressure (FSP): FSP = FTP - VP_outlet. Used when fan discharges to atmosphere.

System Curve

The system curve represents the pressure required to move air through a duct system at various flow rates:

• P_system = K x Q^2 where K is the system resistance constant
• The system constant K depends on duct geometry, fittings, filters, coils, and other components
• Adding filters/coils increases K (steeper curve); larger ducts decrease K (flatter curve)
• The operating point is where the system curve intersects the fan curve
• System effect: Additional losses from non-ideal inlet/outlet conditions not in standard ratings

Specific Speed (Ns)

Specific speed is a dimensionless parameter that characterizes fan design and optimal application:

Ns = N x sqrt(Q) / P^0.75

Where N = RPM, Q = flow (CFM or m3/s), P = pressure (in.wg or Pa)

• Ns 500-2,500: Radial/backward-curved centrifugal - high pressure, lower flow
• Ns 2,000-4,000: Backward-curved centrifugal - medium-high pressure applications
• Ns 3,000-6,000: Forward-curved centrifugal - low-medium pressure, compact design
• Ns 4,000-8,000: Mixed flow - medium pressure, higher flow
• Ns 6,000-15,000: Axial (vaneaxial/tubeaxial) - low pressure, high flow
• Ns 10,000+: Propeller fans - very low pressure, very high flow

Fan Types and Applications

• Centrifugal (Radial): Air enters axially, exits radially. High pressure capability. Types: backward-curved (most efficient), forward-curved (compact), radial blade (material handling)
• Axial: Air flows parallel to shaft. High flow, lower pressure. Types: propeller (low pressure), tubeaxial (medium), vaneaxial (highest pressure axial)
• Mixed Flow: Combines axial inlet with centrifugal outlet. Good balance of pressure and flow in compact design.

Efficiency Considerations

• Total Efficiency: eta_t = (Q x FTP) / (Power_input)
• Static Efficiency: eta_s = (Q x FSP) / (Power_input)
• Backward-curved centrifugal fans: 75-85% peak efficiency
• Forward-curved centrifugal: 60-70% peak efficiency
• Vaneaxial fans: 70-80% peak efficiency
• Operating away from BEP (Best Efficiency Point) reduces efficiency significantly