A centrifugal fan transfers energy to air through a rotating impeller. Air enters near the centre, passes outward through the blades and leaves through the casing outlet. Use the calculator to compare ideal flow and pressure changes when the same fan runs at a different speed, with unchanged air density.
Fan Blower Interactive Calculator
Vary blower baseline flow, baseline static pressure, and speed ratio to see fan-law changes in flow and pressure on an animated centrifugal blower diagram.
Equation Used
The speed ratio r = N2/N1 scales blower volume flow directly. For the same fan operating in the same air, the common fan law estimates static pressure as proportional to the square of speed.
- Same fan diameter and geometry.
- Same air density and similar duct system behavior.
- Static pressure follows the standard same-fan speed law.
- Results compare corresponding points under fan-law similarity; a changed duct system requires the actual fan and system curves.
Impeller, inlet and scroll casing
The rotating impeller adds energy to the air. In a centrifugal arrangement, air enters through the inlet eye and moves outward through the blade passages. The scroll casing collects the discharge around the wheel and directs it into the outlet.
The inlet, blade shape, casing and connected ductwork all affect performance. The drawing illustrates these flow paths; it does not specify wheel clearances, blade profiles or manufacturing tolerances.
Fan and system operating point
A fan has a pressure-versus-flow characteristic. The connected system also requires pressure to move air through its ducts, filters and fittings. Their intersection sets the operating point. Changing a filter, damper or duct can change that point even when speed stays fixed.
Comparing a speed change
Fan-law comparisons help assess a proposed speed change from a known operating point. Enter baseline volumetric flow, baseline static pressure and the new-to-original speed ratio.
The result is useful for understanding the direction and size of a change under the stated similarity assumptions. It does not select a fan for a dust, fume, combustible-gas or other process duty. Those applications require equipment and system data specific to the service.
Same-fan speed relationships
With the same fan geometry and air density, let r = N₂/N₁. At corresponding operating points, volume flow scales as Q₂ = Q₁r and pressure as Δp₂ = Δp₁r². These are the two fan-law relationships used by this calculator.
The flow-change output is 100(r − 1) percent, and the pressure-ratio output is r². Under the additional assumption of unchanged efficiency, shaft power scales approximately as r³; power is not an output of this calculator.
Similarity does not establish a permitted speed or guarantee that a modified duct system will follow the calculated point. Use the actual fan curve and system resistance when those conditions change.
Example: 20% lower or higher speed
Take a baseline of 3,200 CFM and 5.0 inches of water static pressure. At a speed ratio of 0.80, the calculator gives 2,560 CFM and 3.2 inches of water. Flow changes by −20% and the pressure ratio is 0.64.
At a speed ratio of 1.20, the results are 3,840 CFM and 7.2 inches of water. Flow changes by +20% and pressure ratio becomes 1.44.
The numbers describe a same-fan similarity comparison. They do not predict a particular plant's measured flow, noise, motor temperature or particle capture.
Before using a higher speed
A higher speed can raise pressure much faster than flow, and power demand can rise faster still. The calculation does not check the impeller speed rating, motor load, bearings, vibration or duct pressure limits.
Different impeller families have different curves and operating ranges. Their efficiency, sound and material suitability must be taken from the selected equipment data rather than a universal percentage or clearance rule. This tool does not diagnose surge, wear or a blocked filter.
Frequently asked questions
Why does pressure change faster than flow?
At corresponding points for the same fan and density, flow scales with speed while pressure scales with speed squared. A 20% speed increase gives a pressure ratio of 1.44.
Does the calculator check motor size?
No. It has no motor or shaft-power input. Under unchanged efficiency, power varies approximately with speed cubed, so motor loading needs a separate check.
Will the result remain valid after changing the ductwork?
Not necessarily. A changed system resistance moves the operating point. Compare the actual fan curve with the new system curve.
Does it calculate mass flow?
No. The flow input and output are volumetric. Air density is assumed unchanged.
Can this establish a safe maximum speed?
No. The fan, drive, bearings and connected system have limits that the calculator does not evaluate.
References
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