Centrifugal, axial and mixed-flow fans, and what a smoke-extract rating means

Choose the fan type for the system first, then check whether any fire duty is actually certified.

Chinese fan catalogues often list dozens of families with codes such as 4-72, T35, HTFC, SWF or DKT. The code identifies a design lineage, not a duty. Before comparing models, decide which fan type suits the system, and treat any fire rating as a separate claim that needs its own certificate.

Three ways to move air

The DOE and AMCA fan sourcebook groups industrial fans into centrifugal and axial types, with mixed-flow designs between them.

Centrifugal fans turn the air through ninety degrees and build pressure in a scroll housing. Forward-curved impellers are compact and quiet at low pressure and are common in fan coils and cabinet units. Backward-inclined and airfoil impellers run faster, reach higher efficiency and usually have a non-overloading power curve. Radial blades tolerate dust and material handling.

Axial fans move air parallel to the shaft. The sourcebook notes they are more compact and have less rotating mass than centrifugal fans of comparable capacity. Propeller and tube-axial fans suit low pressure; vane-axial designs with guide vanes reach higher pressure.

Mixed-flow fans sit between the two, with a diagonal flow path that gives higher pressure than a plain axial fan in an in-line duct housing.

Pressure basis decides the comparison

A fan curve plots airflow against pressure. Total pressure is static pressure plus velocity pressure, and AMCA's guidance warns against comparing a fan rated on one basis with a system calculated on the other. The fan operates where its curve meets the system curve, and selections close to peak pressure are unstable. Our duty-point guide turns this into enquiry fields.

Examples on this site

Centrifugal families: the Gerui 4-72 centrifugal fans and the Pengxiang 4-73 family are general ventilation designs; the Gerui 9-19 high-pressure fans serve higher-pressure process duties. Cabinet centrifugal fans package the impeller and motor in an insulated box, as in the Qili cabinet centrifugal fans, the Laienke BKB cabinet fans and the HTFC families from Yuhang and Zhongkai. Double-inlet forward-curved fans for air-conditioning units appear as the Laienke LKB fans and the Sihe DF-E fans.

Axial families: the Laienke T35 axial fans and the Gerui T35 axial fans share a family name but come from different factories, so compare published curves rather than the code.

Mixed-flow families: the Laienke HL mixed-flow fans and the Qinye SWF mixed-flow range are both in-line designs with published pressure ranges.

What a smoke-extract rating means

A fan that runs during a fire must keep working while hot smoke passes through it. In Europe, powered smoke and heat exhaust ventilators are tested to EN 12101-3 and given a temperature-time class. Manufacturers' declarations show classes such as F200 (200 °C for 120 minutes), F300 (300 °C for 60 minutes) and F400 (400 °C for 120 minutes), and the 2015 edition also lists other durations. The class applies to the tested product and mounting: a declaration of performance records the arrangement, motor position and insulation that were tested.

Many Chinese catalogues label HTF or HTFC fans as smoke-exhaust fans, with temperature claims tied to Chinese standards. Treat the label as a starting point. The Dashun HTF smoke-exhaust fans page shows why the variant and the two-speed arrangement must be fixed before a rating can be checked, and the Zhongkai HTFC page sets out what its smoke-duty evidence does and does not cover. Fire dampers and smoke-control dampers have their own tests, as the Qinye dampers page shows.

Questions to ask

Which fan type and impeller form is proposed, and why for this system? Is the published curve on static or total pressure, and at what air density? What is the selected operating point, efficiency and motor power? For any fire duty: which standard, which temperature-time class, which certificate number, and does it cover this size, motor position and mounting? Which efficiency rules apply in the destination market? Our page on conformity documents explains the EU fan regulation.

Common questions

Is a centrifugal fan always quieter than an axial fan? No. Sound depends on the selection and operating point; compare sound power at the duty point, as our note on sound data explains.

Can one fan handle both daily ventilation and smoke extraction? Some do, as dual-duty fans, but the fire rating must cover the fan as installed and the control sequence must be designed for both modes.

Does the same family code mean the same fan? No. Codes such as T35 or 4-72 are shared across many factories.

By HVACDeck Editorial · Updated 2026-11-04