A centrifugal fan is the right choice above roughly 500 Pa static pressure; an axial fan wins below that line when you need maximum airflow per dollar. That single pressure threshold resolves most fan selection decisions, and this article explains why, with the numbers to back it.
The Core Difference in One Paragraph
An axial fan moves air parallel to its shaft — a propeller in a ring or short casing. A centrifugal fan pulls air into the center of a spinning impeller and throws it outward at 90 degrees through a spiral volute. The volute converts velocity into static pressure. Everything else in the comparison follows from this geometry: axial fans move large volumes against little resistance; centrifugal fans overcome significant resistance without collapsing.
Comparison Table
| Parameter | Axial Fan | Centrifugal Fan |
|---|---|---|
| Typical static pressure | 50–500 Pa | 250–15,000 Pa |
| Airflow range | 1,000–200,000 m³/h | 500–380,000 m³/h |
| Duct direction | Inline (straight-through) | 90° turn required |
| Noise at equal duty | Higher broadband noise | Lower, but tone at blade-pass frequency |
| Efficiency at design point | 65–75% (tube-axial) | 70–85% (airfoil BC wheel) |
| Handles dust / grease | Poor (blade fouling) | Good with radial / BC blades |
| Typical price (same airflow) | Lower | 1.5–3× higher |
| Overload behavior | Power rises with low resistance | Non-overloading (BC wheels) |
Efficiency and Life-Cycle Cost
At equal airflow below 500 Pa, an axial fan is 30–60% cheaper to buy and 5–10 points less efficient at the duty point. That efficiency gap matters only in continuous duty: over 15 years an axial fan running 6,000 h/year at 5 kW can waste 30,000–60,000 kWh versus a well-selected centrifugal. Above 500 Pa the centrifugal's ability to hold efficiency under rising resistance flips the math — energy cost over 15 years typically exceeds purchase price 5–8×, so the 1.5–3× higher fan price pays back in 2–4 years.
Real-World Noise
Catalog noise is measured at free discharge. In a real duct, elbows within one diameter of the fan, high tip speed (above 60 m/s), and operating left of the peak pressure point raise axial-fan noise by 3–8 dB. Centrifugal fans shift that energy into a tone at the blade-pass frequency, which is easier to silence with a short inlet duct silencer. For occupied spaces near the fan, plan 10–20 dB of attenuation either way.
When to Choose an Axial Fan
- Wall or roof mounting with free discharge — no duct resistance to overcome
- Parking garages, warehouses, greenhouses — high volume, low pressure
- Reversible smoke control in car parks (jet fans)
- Budget-constrained general ventilation
When to Choose a Centrifugal Fan
- Ducted systems with filters, coils, silencers — resistance adds up past 500 Pa
- Kitchen exhaust with grease (radial or backward-curved blades)
- High-pressure process air, boiler draft (9-19, 9-26 series reach 15 kPa class)
- Cleanrooms and hospitals where the fan sits behind HEPA banks
A Worked Sizing Example
Take a supply system needing 20,000 m³/h. With only a 150 Pa free-discharge louvre, an axial T35-11 at 3 kW does the job. Add a 300 Pa filter bank, a 200 Pa coil and 150 Pa of duct — total 650 Pa. The axial now needs a 7.5 kW motor and sits left of its peak; a 4-72 centrifugal at 5.5 kW lands at 78% efficiency. Same airflow, 27% less power, and it holds pressure as the filter loads.
The Checklist Before You Order
- Add up system resistance: duct friction + fittings + filters + coils + silencers, in Pa
- If total exceeds 500 Pa → shortlist centrifugal
- If under 500 Pa and straight-through layout → axial saves 30–60% cost
- Check the fan curve at your duty point: efficiency should land above 60%
- Confirm motor class (IE3 minimum for continuous duty in most tenders now)
Quick Application Map
| Duty | Pick | Why |
|---|---|---|
| Free-discharge wall / roof exhaust | Axial | Straight-through, lowest cost under 500 Pa |
| Ducted supply with filter + coil | Centrifugal 4-72 | Holds pressure past 500 Pa, 78% efficient |
| Kitchen grease exhaust | Centrifugal radial | Blades shed grease, resists fouling |
| Boiler / kiln draft | Centrifugal 9-19/9-26 | Up to 15,000 Pa, forged impeller |
| Car park smoke control | Axial jet fan | Reversible, high volume, free discharge |
| Cleanroom HEPA bank | Centrifugal airfoil | Stable, low-noise, leak-tight option |
Footprint, Enclosure and Noise
A centrifugal needs a 90° outlet and more floor area — roughly 1.5–3× the footprint of an equivalent axial at the same airflow. In occupied plant rooms, a double-skin acoustic enclosure adds 100–200 mm per side but cuts radiated noise 10–20 dB. Axial fans fit tight ceiling or wall grids but push more tonal noise into the space unlessisolated. Plan the motor and bearing access: belt-drive units need 600–800 mm clearance for pulley swaps; direct-drive need only side access.
Maintenance and Spare Parts
- Belts: inspect every 1,000–2,000 h; replace at 3–5% stretch or cracking.
- Bearings: grease every 2,000–3,000 h; keep one set of matched spares on site.
- Impeller: wash annually in dusty duty to recover 2–5% efficiency.
- Vibration: trend monthly; a 2× rise in velocity warns 1–3 months before failure.
- Alignment: re-check belt tension after first 200 h of run-in.
Beyond the Two: Mixed-Flow Fans
When duty lands between the two — say 500–1,500 Pa with moderate volume and a straight-ish run — a mixed-flow (cross-flow / inline centrifugal) fan bridges the gap. It keeps an axial-style inline layout while adding a shallow impeller for 1.5–2.5× the pressure of a pure axial at similar footprint, at 65–80% efficiency. Use it for medium-pressure duct boosts, inline supply in tight ceilings, and pressurization shafts where a 90° centrifugal outlet will not fit. Treat it as the middle option, not a replacement: above 1,500 Pa, step up to a true centrifugal; below 300 Pa, an axial costs less for the same air.
Sizing for Redundancy and Future Load
If the system may grow, size the fan for 10–20% airflow headroom and pick a wheel that still sits above 60% efficiency at the future point; a VFD then covers the gap without a second unit. For critical duty (smoke control, cleanroom, process), plan N+1: two fans at 60–70% each so one can carry the load during service. Remember the dirty-filter condition — design for 30–40% resistance rise, or the second fan becomes mandatory sooner than planned. Redundancy adds 30–60% capital but avoids a full shutdown on a single bearing failure.
Keruite manufactures both families — T35-11 axial and 4-72 / 4-79 / 9-19 / 9-26 centrifugal series — and every unit is performance-tested in our own laboratory before shipment. If you share your airflow and system pressure, our engineers will return a fan selection with the test-matched curve within 24 hours.
FAQ
Can I use an axial fan in a ducted system with filters?
Only if total resistance stays under ~500 Pa. Filters load up over time, so size the fan for the dirty-filter condition, otherwise airflow drops as the filter clogs.
Why is my axial fan louder than the catalog value?
Catalog noise is measured at free discharge. Duct elbows near the fan, high tip speed and operating left of the peak pressure point all raise real-world noise by 3–8 dB.
Which type suits a dusty exhaust?
Centrifugal with radial or backward-curved blades. Axial blades foul with dust and lose balance, causing vibration and premature bearing failure.
Does a higher-price centrifugal fan always save money?
In continuous duty above 500 Pa, usually yes — energy over 15 years typically exceeds purchase price 5–8×, and efficiency differences of 10 points compound significantly.