The illustration shows the recirculation fans of a conveyor oven: two housed centrifugal fans built into the oven casing, each belt-driven from a motor mounted outside on its own base, with belt guards and insulated ducting above. A fan like this is selected from tables made for air at 21 °C, then spends its working life in air two or three hundred degrees hotter, and starts every morning cold. Both conditions have to be designed for.
Covers recirculation and circulation fans for industrial ovens, dryers and curing chambers: fan laws with air density, temperature correction factors, motor sizing for cold start, start-up dampers, maximum safe wheel speed against temperature, impeller materials, and high-temperature construction (shaft coolers, bearing grease, pedestals, bearing temperature monitoring). Oven chambers, heating and temperature uniformity are on Industrial hot air oven and the oven-type pages; airflow layout in food dehydrators is on Air circulation system for industrial food dehydrators; burner combustion air blowers are on Combustion air blower; greenhouse and hydroponic circulation fans are outside this page.
CNFX editorial research · Sources checked October 13, 2026 · 3 sources
Aerovent's engineering note FE-1600 states the rule: at constant speed a fan moves the same volume whatever the air density, while pressure and power vary directly with density.
Ratings are published at standard air, 1.2 kg/m³ at 21 °C and 101.32 kPa. A correction factor, standard density divided by actual density, converts them: divide catalogue pressure and power by the factor to get hot performance. Aerovent's example fan delivers 7.08 m³/s against 0.75 kPa at 21 °C taking 9.13 kW at 1,621 rpm; at 176.7 °C (factor 1.53) the same fan and speed give only 0.49 kPa and take 5.97 kW. If the oven needs 0.75 kPa at temperature, the fan must be selected for 0.75 × 1.53 = 1.15 kPa at standard air; it then runs at 1,742 rpm and needs 12.07 kW cold and 7.89 kW hot.
Aerovent also warns that density correction is not enough on its own: temperature can make an alloy too pliable or brittle, and speed changes can exceed the limits of wheel, shaft and bearings.
Air temperature (Aerovent Table 1)
Factor (1.2 kg/m³ ÷ actual density)
21.1 °C
1.00
93.3 °C
1.25
148.9 °C
1.43
176.7 °C
1.53
204.4 °C
1.62
260.0 °C
1.81
315.6 °C
2.00
371.1 °C
2.19
426.7 °C
2.38
At 315.6 °C the air is half as dense as at 21 °C, so the same fan gives half the pressure and takes half the power. Give the fan supplier the duty at operating temperature and say so explicitly.
02
Size the motor for the cold start
Because cold air is denser, a fan selected for a hot duty draws more power when it starts with the oven cold. Aerovent's first example could run hot on a 5.5 or 7.5 kW motor, but starting before the air warms needs the full 9.13 kW, so it suggests an 11.5 kW motor; the alternative is a shut-off damper that stays closed until the air is up to temperature, where that fan needs 4.47 kW at standard conditions.
Greenheck's QEID supplement makes the same point: a fan selected for high temperature must have a motor able to take the load at any lower temperature where the air is denser. Its 30-QEID needs 31 bhp at standard air and 26.3 bhp at the operating density (factor 0.848), which a 30 hp motor covers; but starting in 0 °F (−18 °C) air at 3,000 ft (factor 1.033) needs 32 bhp, so a 40 hp motor is specified.
Case
Hot running
Cold start
Motor chosen
Aerovent example 1 (176.7 °C)
5.97 kW
9.13 kW
11.5 kW (or damper closed until warm)
Greenheck 30-QEID
26.3 bhp
32 bhp
40 hp
Ask whether the fan will ever start or run with the oven cold, whether a start-up damper or variable speed drive is fitted, and what motor power covers the cold case.
03
Wheel speed, materials and the bearings
Twin City Fan's HIB backward-curved fan catalogue shows what high-temperature duty changes in the fan itself.
Standard steel construction is suitable to 425 °C, aluminium impellers only to 120 °C, and above 20 °C the maximum allowable speed must be reduced by the de-rating factor; stainless steel impellers must be de-rated even at ambient temperature. A wheel limited to 2,000 rpm cold may run at only 1,860 rpm at 260 °C (Editorial calculation, example limit).
TCF's high-temperature construction is stepwise: 150 to 260 °C adds a shaft cooler and high-temperature grease bearings; 261 to 315 °C also adds high-temperature aluminium paint; 316 to 425 °C also modifies the pedestal. Thermocouples or RTDs can be fitted on the bearings, and vibration switches are available. In the illustration the motors and belts sit outside the hot casing for the same reason.
Gas temperature (TCF)
Max-rpm de-rating factor, steel wheel
20 °C
1.000
95 °C
0.990
150 °C
0.975
205 °C
0.955
260 °C
0.930
315 °C
0.904
370 °C
0.880
425 °C
0.837
Ask for the wheel material and its maximum speed at operating temperature, the high-temperature package (shaft cooler, grease, paint, pedestal), and bearing temperature and vibration monitoring.
04
What to give a supplier for an oven circulation fan
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
State the airflow and the static pressure at the operating temperature, the maximum and minimum air temperatures including cold start, altitude, the gas composition if not air, and whether the fan will be started hot or cold. Ask for the fan selection at standard air and at temperature, motor power for cold start, damper or variable-speed provision, wheel material and safe speed at temperature, shaft cooler and bearing grease, and bearing temperature monitoring.
Explore company information and manufacturing capabilities, then discuss the scope of your project with a relevant supplier.
Share this page
Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
Does a fan move less air when the air is hot?
No. Aerovent states flow is unchanged at constant speed; pressure and power fall with density, to half at about 316 °C. [1]
Why is an oven fan motor bigger than its hot running power?
It must start the fan in cold, denser air: Greenheck's example needs 26.3 bhp hot but 32 bhp at a 0 °F start, so a 40 hp motor is used. [2]
Can a fan run at its catalogue speed in hot air?
Not always: Twin City Fan de-rates maximum speed above 20 °C, to 0.930 at 260 °C and 0.837 at 425 °C for steel wheels. [3]
When does a fan need a shaft cooler?
Twin City Fan adds a shaft cooler and high-temperature grease bearings from 150 to 260 °C, with further changes above that. [3]
Sources and what still needs a direct check
Sources checked 2026-10-13. Two fan makers' engineering data and a third fan maker's catalogue engineering pages.
