A centrifugal fan is a machine that moves air by spinning an impeller inside a spiral housing (volute): air enters the center axially, the impeller throws it outward radially, and the volute converts that velocity into static pressure. It is the workhorse of industrial ventilation wherever air must be pushed through resistance — ducts, filters, coils, silencers.
Working Principle in Four Steps
- Air enters axially through the inlet cone into the impeller eye, usually at 8–15 m/s inlet velocity.
- The rotating impeller accelerates air radially outward — centrifugal force does the work, adding kinetic energy.
- Air exits the impeller at high velocity, tangentially, typically at 20–45 m/s blade-tip speed depending on wheel size.
- The volute casing slows the air down, converting velocity pressure into static pressure with 70–85% recovery efficiency.
That pressure conversion is the whole point: a fan rated 10,000 m³/h at 1,200 Pa is doing 3× the "work per unit air" of an axial fan at the same airflow. This is why axial and centrifugal fans are not interchangeable once duct resistance climbs past roughly 500 Pa.
Why the Volute Matters
The volute is not decoration. Without it, the high-velocity air leaving the impeller would dissipate as noise and turbulence. A well-designed spiral volute recovers that energy into usable static pressure. The volutes are sized per wheel so the cut-off gap and expansion angle keep system effect low; a poorly matched volute can lose 10–20% of the theoretical pressure. Inlet and outlet connections follow AMCA 210 layout guidelines to avoid flow distortion.
The Four Blade Families and Their Numbers
| Blade type | Blades | Typical efficiency | Pressure class | Best for |
|---|---|---|---|---|
| Forward curved | 40–60 small | 55–65% | Low | Compact AHU, low-noise HVAC |
| Backward curved | 8–16 large | 70–85% | Medium–high | Industrial duty, energy-sensitive |
| Airfoil (BC wing) | 8–12 hollow | 75–85% | Medium–high | Large AHU, highest efficiency |
| Radial | 6–10 flat | 60–70% | Medium | Dust, chips, material conveyance |
Five Numbers That Define a Centrifugal Fan
- Airflow (Q): 500–380,000 m³/h across the's range — from workshop ventilation to power-plant duty
- Static pressure (Ps): 250–15,000 Pa — the 9-19 / 9-26 high-pressure series top the range
- Efficiency: 70–85% for modern BC wheels at design point; below 60% means wrong selection
- Outlet velocity: 8–25 m/s typical; above 30 m/s noise and system effect grow fast
- Service life: 10–15 years for well-selected units; bearings are the first wear item at 20,000–40,000 h
How to Read a Centrifugal Fan Performance Curve
A fan curve plots static pressure (Y) against airflow (X) at a fixed speed. Three things matter: the peak pressure point, the knee where efficiency drops, and the stable operating region. Read your duty point (your required m³/h and Pa) and confirm it lands on the curve above 60% efficiency. Move 10% off the design point on a forward-curved wheel and power can swing 20–30%; backward-curved wheels stay flat and non-overloading. Always check the curve at your dirty-filter condition, not clean — resistance rises 15–40% as filters load.
Sizing Checklist Before You Order
- Total system resistance: duct friction + fittings + filter + coil + silencer, summed in Pa
- Required airflow at dirty-filter condition, in m³/h
- Confirm duty point sits above 60% efficiency on the curve
- Select blade type by duty: BC/airfoil for energy, radial for dust
- Motor class IE3 minimum; verify starting current against your supply
Installation and Maintenance Notes
Mount the fan on a rigid base with isolation to keep vibration below 4.5 mm/s RMS. Align the inlet cone within 2 mm of the duct centerline to avoid system effect. Grease standard ball bearings every 2,000–3,000 running hours with the specified quantity — over-greasing causes overheating. Sealed-for-life bearings need no schedule but monitor temperature; a 10°C rise above baseline usually signals a failing bearing. Vibration trending catches imbalance 1–3 months before failure.
When a Centrifugal Fan Is the Wrong Answer
Straight-through airflow with almost no resistance — wall fans, roof exhaust, tunnel jet ventilation. Forcing a centrifugal fan into these duties costs 2–3× more for the same airflow and adds a 90° turn you do not need.
Centrifugal Fan by Application
| Application | Series | Pressure class | Note |
|---|---|---|---|
| General HVAC, clean air | 4-72 / 4-79 | 250–3,000 Pa | BC wheel, 70–85% efficient |
| High-pressure process, boiler draft | 9-19 / 9-26 | 3,000–15,000 Pa | Forged impeller, up to 15 kPa |
| Dusty / material conveyance | Radial-blade | 1,000–5,000 Pa | Wear plates, easy access |
| Compact AHU, low noise | Forward-curved | 200–800 Pa | Small footprint, 55–65% |
| Cleanroom behind HEPA | 4-79 airfoil | 500–2,500 Pa | Leak-tight casing option |
| Corrosive exhaust | FRP / PP casing | 200–2,000 Pa | Resists acid / alkali |
Common Failure Modes and What They Tell You
- Vibration rising 1–3 mm/s above baseline → impeller imbalance from dust or wear; rebalance to G2.5.
- Bearing temperature 10°C above norm → over-greasing or wrong grease; relube to spec at 2,000–3,000 h.
- Airflow 15–20% below curve → system effect from elbows at inlet; fix duct connection.
- Motor overload at start → overloading wheel (forward-curved/radial); add inlet damper or VFD.
- Casing erosion after 20,000–40,000 h → abrasive duty; switch to hardened blades.
Construction and Material Choices
Casing gauge scales with pressure: 2–3 mm steel to 3,000 Pa, 4–6 mm above that, with reinforced cut-off for high-speed wheels. Inlet cones are spun for smooth flow; outlet flanges match duct class. For wet or corrosive duty, hot-dip galvanize, stainless 304/316, or an FRP/PP casing extends life from 5–8 years to 10–15. Impellers are welded then balanced in two planes to G2.5; airfoil wheels are sealed hollow to avoid moisture ingress. Drive options are direct (compact, no belt loss) or belt (speed tuning, easier motor swap).
Verifying a Supplier's Test Data
Ask for a test report tied to your unit's serial number, not a generic curve. A credible report states the test standard (AMCA 210 / ISO 5801), the measured points across the operating range, and the fan's actual efficiency at your duty. Cross-check that the stated airflow and pressure sit on the curve, and that brake horsepower matches the motor you were quoted. Field verification is simpler: measure static pressure at the inlet and outlet with a manometer (expect 250–15,000 Pa by class) and compare delivered airflow to the curve within 5–10%. Large gaps usually mean system effect or a mis-selected wheel, not a bad fan.
Keruite builds all four blade families in Dezhou, China, on an 80,000 m² factory site with an in-house AMCA-210-style test lab. Every impeller is balanced to G2.5 and every unit tested before shipment — the test report ships with your order.
FAQ
Why does my centrifugal fan draw more current than nameplate at startup?
Forward-curved and radial wheels are overloading types: power rises as resistance falls. Damper the inlet at start or use a VFD; backward-curved wheels are non-overloading, so current stays bounded across the curve.
What does "system effect" mean for my fan?
Non-ideal duct connections (elbows at the inlet, sudden area changes) distort flow into the impeller, shifting real performance below the lab curve — typically 5–20% airflow loss. Follow AMCA inlet/outlet connection guidelines to recover it.
How often should bearings be greased?
Standard ball bearings every 2,000–3,000 running hours with the specified grease quantity — over-greasing causes overheating. Sealed-for-life bearings need no schedule but monitor temperature for early failure warning.
Which blade type saves the most energy?
Airfoil and backward-curved wheels reach 75–85% at design point. Over 10–15 years of continuous duty that 5–10 point efficiency gap usually outweighs the higher purchase price of the impeller within 2–3 years.