High-Temperature Centrifugal Blowers: Materials, Bearing Cooling and Expansion

Heat attacks a blower in four ways at once. It weakens the metal of the wheel, it overheats the bearings, it makes every part grow, and it thins the gas so the blower develops less pressure. A blower that handles 500, 650, or 800 °C gas is built differently from a standard machine in each of these respects, and the differences are what separate a blower that runs for years from one that fails in weeks. This guide sets out the design points, based on published engineering guidance from fan manufacturers, for the temperature ranges we cover: ovens up to 500 °C, kilns up to 650 °C, and furnaces up to 800 °C.

Temperature Bands and What Changes

Gas temperature	What changes in the blower
Up to about 150 °C	Standard construction with the bearings outside the airstream is usually acceptable; check motor and lubricant
About 150 to 480 °C	Bearing cooling, high-temperature lubricant, shaft seals, and insulation become necessary; carbon steel wheels still viable
About 480 to 540 °C	Upper edge of carbon and low-alloy steel practice, depending on stress; wheel stress and creep decide
Above about 540 °C	Heat-resistant alloys for the wheel and hot parts, with speed limited by strength at temperature

Published guidance says standard fans with the bearings outside the airstream are usually limited to around 149 °C without special provision, and bearings inside the airstream are limited to what the bearing can withstand, typically about 54 °C. So any duty hotter than a warm room needs a design that keeps the bearings out of the gas and cools them.

Materials

Carbon steel, low-alloy high-strength steel, and chrome-moly steel are satisfactory up to roughly 480 to 540 °C, depending on stress levels. Above that, heat-resistant alloys are used: stainless steels such as 304, 316, 321, and 330, and nickel-based alloys for still higher temperatures. One manufacturer builds furnace fans in 316 stainless for temperatures up to about 815 °C and uses higher alloys above that. Our oven range of 500 °C is at the upper edge of carbon-steel practice, and our kiln (650 °C) and furnace (800 °C) ranges sit in heat-resistant alloy territory. Material choice always depends on the stress in the wheel as well as on the temperature, so it is confirmed for each duty.

Why Speed Must Be Derated

Every rotating wheel is stressed by centrifugal force, and that stress rises with the square of tip speed. Metals get weaker when hot, so the maximum safe tip speed falls with temperature. If the allowable stress at temperature is half its room-temperature value, the permissible tip speed falls to about 71% of the room-temperature limit. At still higher temperatures, creep, the slow permanent stretching of metal under stress, must be considered as well.

This matters for pressure, because a blower’s pressure is proportional to gas density times tip speed squared. So heat reduces the pressure a blower can develop in two ways: the gas is thinner, and the wheel is not allowed to spin as fast.

Illustration. In 800 °C gas, density is about 27% of standard. If the alloy’s allowable stress at that temperature is half its room-temperature value, permissible tip speed squared is halved. The pressure the blower can develop is then about 0.27 × 0.5, roughly 14% of its room-temperature standard-air figure, so a wheel that develops 500 mm WC at room temperature would develop only about 68 mm WC. The strength ratio here is an assumption to show the effect, and real alloys differ, but the direction and scale are typical. This is why hot-gas systems must be designed with low resistance. See high-temperature blowers for furnaces, kilns and ovens.

Bearings and Shaft Cooling

The bearings must stay cool even when the gas is hot. Published practice includes:

• Keeping the bearings outside the airstream, on a pedestal beyond the housing.
• A shaft cooler, a finned or air-cooled section of shaft between the housing and the bearing.
• Heat slingers, small fan blades on the shaft that blow air over the bearing, which work but can be noisy.
• Water cooling of the shaft, usually paired with belt drive so that the cooling arrangement can be fitted.
• High-temperature lubricants, and sometimes circulating oil.
• Shaft seals, such as packing glands, to stop hot gas escaping along the shaft.

Insulation and Thermal Expansion

Hot alloy parts are insulated from cold mild-steel parts, so that heat does not travel into structure and bearings. Everything grows when hot: the wheel, the shaft, the housing, and the ducting. A wheel sized for room temperature has different clearances at 800 °C, so the gap between wheel and housing is set differently, and wheels may use compression joints or special mounting so they can grow without binding or loosening. The housing and ducting connections should be flexible, or arranged to expand, so the blower is not loaded by expanding ducts.

Motor and Drive Position

Centrifugal blowers suit the hottest gas because the motor and bearings sit outside the airstream. The motor should be kept away from radiant heat and hot ducts, and belts and couplings near the hot end need heat-stable compounds and materials. Where the drive passes close to the hot zone, provide shielding and ventilation.

Start-Up, Shutdown, and Thermal Shock

• Start-up in cold gas means high density and the highest motor power, so size the motor for the coldest, densest condition.
• Warm-up and cool-down should be gradual, because thermal shock stresses the wheel and housing.
• After a hot shutdown, an unevenly cooling shaft can bow, so many installations keep the blower turning slowly until it has cooled, with a plan for power failure.

Common Failure Modes

Failure	Cause	Prevention
Bearing failure	Heat travelling along the shaft, lubricant breakdown	Shaft cooling, high-temperature lubricant, seals
Wheel cracking or deformation	Stress at temperature, creep, thermal shock	Correct alloy, derated speed, gradual heating
Rubbing at the housing	Uneven expansion, distortion	Correct clearances, flexible mounting
Shaft bowing	Uneven cooling after shutdown	Slow rotation during cool-down
Gas leakage at the shaft	Seal wear	Suitable seal, inspection
Loss of pressure	Lower density and derated speed	Selection at temperature

Where Our Range Fits

NextAir Systems manufactures high-temperature centrifugal blowers for ovens up to 500 °C, kilns up to 650 °C, and furnaces up to 800 °C, up to 800 HP (about 597 kW), with backward-curved and forward-curved wheels and single-inlet and double-inlet designs. Materials, bearing cooling, speed limit, and motor arrangement are engineered for each duty, and the pressure a blower can develop at temperature is confirmed for each selection. Gas containing flammable vapour or combustible dust is a hazardous-area duty, and we do not currently manufacture ATEX-certified or spark-resistant blowers.

What to Specify

1. Gas temperature, normal and peak, and the equipment.
2. Flow, stated as mass flow or actual volume at temperature.
3. System resistance at operating temperature.
4. Gas composition, dust, and corrosive content.
5. Start-up conditions and cool-down provisions.
6. Motor position, drive arrangement, and cooling preference.

Tell Us Your Gas Temperature and Duty

Send your temperature, flow, and system resistance, and we’ll work out the selection at temperature. Contact us for a free quote.

Frequently Asked Questions