High-Temperature Fan Motor: Shaft Heat and Motor Protection in Furnace and Process Fans

A high-temperature fan motor is a standard three-phase induction motor that runs outside the hot gas, not in it; what protects it from heat is not the motor itself but the arrangement on the fan side: the motor sits outside the gas stream, a long shaft separates impeller and motor, a heat slinger (cooling disc) is fitted on the shaft and a heat shield is placed in front of the motor flange. Motor power, however, is checked against the first start with a cold oven, not against hot operation: a fan sized for 300 °C needs about 1.96 times the power in 20 °C air at the same speed.

This article explains where the heat reaching the motor comes from in oven recirculation, heat-treatment, drying and process gas fans, how the motor is protected, and how the cold-start load changes motor selection. Motor connection, overload and inverter settings must be done by a qualified electrician. You can find suitable 4- and 6-pole cast iron models among the 120 models on the Buy Electric Motors Online page; general fan motor information is on our Fan Motors page.

What is a high-temperature fan and how does it differ from a smoke extraction fan?

"High-temperature fan" is not an operating principle but a service condition. The fan inside is usually a centrifugal (radial) fan; large ovens also use axial or mixed-flow recirculation fans. What they share is that they move hot gas all the time: oven air recirculation, heat-treatment and annealing furnaces, paint drying booths, hot process gas extraction, hot air generators.

A smoke extraction fan is a different subject. That fan normally runs in cold air; in a fire it must survive a given temperature for a given time, and the fan and motor set must be certified to EN 12101-3. DRG standard motors are not sold with this certification; for smoke mode the certified set is bought from the fan manufacturer. Details are in our article Smoke Extraction Electric Motors. This article covers only process fans that carry hot gas during normal operation.

The division of responsibility should be clear from the start: fan selection, the fan curve, impeller material, shaft and bearing arrangement and casing insulation are the job of the fan manufacturer or the process engineer. The motor supplier's job is to provide a motor that suits the given shaft power, speed, mounting and the ambient conditions where the motor sits.

Where does the heat reach the motor from? Shaft, radiation and ambient

In a correctly built high-temperature fan the motor never touches the hot gas. Heat reaches it in three ways:

• Conduction along the shaft: The impeller is in the hot gas; the shaft carries heat out of the casing to the bearings and to the coupling or motor shaft. The part most affected is the motor's front (drive-end) bearing and its grease.
• Casing radiation: An uninsulated fan casing or oven wall heats the motor frame facing it.
• The motor's cooling air: The motor draws surrounding air with its own fan. If the motor sits in a hot compartment above the oven, it is cooled with hot air and the winding runs hotter at the same load.

A fan manufacturer's engineering note states that bearings should never run in an air stream hotter than 54 °C (130 °F) [1]. That is why an arrangement with the impeller directly on the motor shaft and the motor close to the gas stream is used only at low temperatures. According to the same source a standard axial fan is suitable up to 93 °C; adding a cooling wheel to the bearing housing raises this limit to 149 °C [1].

Which devices protect the motor from heat?

The protection lies between the fan and the motor, not inside the motor. One manufacturer's catalogue describes the heat slinger as an aluminium cooling disc mounted on the shaft between the inboard bearing and the fan housing to dissipate heat conducted along the shaft [2]. Another makes the heat slinger and its guard standard in the 149–204 °C range [3]; a third recommends a shaft cooler for every application above 149 °C [1].

Making room for a shaft cooler increases the distance between impeller and bearing (the overhang); the larger this distance, the lower the safe shaft speed, and it must never be increased during maintenance [1]. In practice this matters for motor replacement: the new motor must sit in place of the old one with the same shaft end and the same shaft height; do not try to "make it fit" by shifting the coupling or pulley.

Thermal expansion is a separate issue. In hot operation the fan shaft lengthens and the casing expands; one bearing should be fixed and the other of the expansion type [1]. In a coupled arrangement, alignment set cold can drift when warm; check alignment after the first hot run.

Click to see current electric motor prices and order nowSEE PRICES →

Why is the motor overloaded at a cold start? Calculation with the density ratio

At a given speed, the power a fan draws is proportional to the density of the gas it moves, and gas density is inversely proportional to absolute temperature. Whatever temperature the fan maker's power figure refers to, it is scaled for cold air as: P_cold = P_hot × (273.15 + T_hot) / (273.15 + T_cold).

Example (assumptions stated): a heat-treatment furnace with 10,800 m³/h (3 m³/s) recirculation and 1,200 Pa total pressure in 300 °C gas; with an assumed fan efficiency of 0.70 the shaft power is P_shaft = Q·Δp / η_fan ≈ 5.1 kW. With a 0.95 allowance for a belt drive the motor shaft needs about 5.4 kW. With the furnace at 20 °C the ratio is (273.15 + 300) / (273.15 + 20) ≈ 1.96, so the same fan needs about 10.6 kW. At 1465 rpm this means a torque of about 35 Nm hot and about 69 Nm cold.

The 7.5 kW motor is loaded above rated until the gas warms to about 141 °C; during that time the damper must be throttled or the speed limited by an inverter. This limitation depends on a process setting, and if it is forgotten the motor trips. Choosing 11 kW directly needs no control logic, but in hot operation the motor runs at roughly half load. The choice depends on how often the furnace is started cold and how reliable the automation is. You can repeat the calculation with your own values in the Pump & Fan Motor Power calculator; for the speed–power relationship see our article on affinity laws.

How many kW is a high-temperature fan motor?

A reliable general power range cannot be given for these fans; power is determined from application data. What sets it: impeller diameter and blade type, air flow, static pressure at gas temperature, speed, system resistance (load inside the furnace, heater coil, grilles), gas density and temperature, belt or coupling losses and the cold-start allowance. Ask the fan maker for the shaft power at design temperature, the shaft power of the same fan at 20 °C, and the maximum permissible impeller speed.

DRG range: aluminium frame 0.25–11 kW, cast iron 0.55–355 kW; 1000, 1500 and 3000 rpm; 3-phase, 230/400 V or 400/690 V; mounting B3, B5, B14 and B35 (with an order note). Cast iron frames are preferred in hot compartments. The price table below covers 4- and 6-pole models from 3 to 18.5 kW, a range often seen in recirculation and process fans.

Speed and poles: why a slower motor for a hot impeller?

Impeller material strength falls with temperature, so manufacturers reduce the maximum permissible impeller speed at high temperature [1]. The number of motor poles is chosen against this limit. Example: an 800 mm impeller (assumption) runs at a tip speed of about 61 m/s with a 4-pole motor (1465 rpm) and about 41 m/s with a 6-pole motor (970 rpm). Large impellers with little strength margin when hot use a 6-pole motor, for example DRG-160L-6 (11 kW, 970 rpm, 22.0 A at 400 V), or a belt drive that reduces speed. A 2-pole motor should only be considered on small-diameter high-temperature impellers and with the maker's explicit approval.

Do not change the pole count because the kW is "the same": fitting a 6-pole motor to a 4-pole fan cuts the flow roughly in proportion to speed and the power with the cube of the speed ratio, and the furnace cannot distribute its heat; the reverse overspeeds the impeller.

Mounting, connection and motor selection checklist

Mounting the fan on the furnace (roof, side wall, rear wall) is separate from the motor's IEC mounting type. On recirculation fans hanging vertically from the furnace roof the motor is usually vertical flange-mounted (V1); in arrangements with a separate bearing housing or a belt the motor is foot-mounted on a base (B3). The exact type is taken from the fan maker's drawing. The 11 kW DRG-160M-4 has a 42 × 110 mm shaft and 6309-C3 bearings; it draws 20.4 A at 400 V and 21.5 A at 380 V.

• Which gas temperature does the power on the fan plate refer to? Was the 20 °C value requested separately?
• Maximum permissible and design impeller speed.
• Drive: impeller on the motor shaft, separate bearing shaft with coupling, or belt?
• Are the heat slinger, heat shield and shaft seal in place and sound?
• Has the air temperature of the motor compartment been measured; does the motor fan draw clean air?
• Frame, shaft D × E, mounting type and shaft height of the old motor.
• Cold-start method: larger motor, damper or inverter.
• Is the overload relay set to the nameplate current (overload, fuse and contactor selection)?
• When the furnace shuts down, does the fan keep running until impeller and shaft cool? Stopping the fan while hot heats the shaft and bearing.

For drying-oven specific practice and inverter settings, see our article Electric Motor Selection for Drying Oven Recirculation Fans.

DRG motors for high temperature fans

• DRG-132S-4 – 5.5 kW – 1460 rpm – IEC 132S cast iron
• DRG-132M-4 – 7.5 kW – 1460 rpm – IEC 132M cast iron
• DRG-160M-4 – 11 kW – 1465 rpm – IEC 160M cast iron
• DRG-160L-6 – 11 kW – 970 rpm – IEC 160L cast iron
• DRG-160L-4 – 15 kW – 1465 rpm – IEC 160L cast iron
• DRG-180L-6 – 15 kW – 978 rpm – IEC 180L cast iron

Click to see current prices and order these motors nowSEE PRICES →

High temperature fan motor: FAQ

Does a high-temperature fan need a special high-temperature motor?

Usually not. The motor sits outside the gas stream and the heat is stopped by the heat slinger, long shaft and heat shield. The real check is the air temperature in the motor compartment.

Why does the oven fan motor trip at the first start in the morning?

Cold air is denser than hot gas; a fan sized for 300 °C draws about 1.96 times the power at 20 °C. A damper, an inverter or a larger motor is needed; raising the overload setting is not a solution.

What speed should an oven fan motor have?

Whatever the impeller maker allows; recirculation fans mostly use 4-pole motors, and large hot impellers use 6-pole motors or a belt-reduced speed.

How many amps does an 11 kW 1500 rpm motor draw?

DRG-160M-4 draws 20.4 A at 400 V and 21.5 A at 380 V; its rated speed is 1465 rpm and rated torque 71.7 Nm.

Send us the fan label, the design temperature and the old motor's nameplate, and we will confirm the right DRG model together: phone +90 (542) 666 11 11, e-mail [email protected], contact page. For all models and current prices see Buy Electric Motors Online.

Sources

1. Clarage – Fan Engineering FE-3200: High Temperature Fans (PDF) – shaft cooler recommended above 149 °C (300 °F); bearings must not run in air hotter than 54 °C (130 °F); standard axial 93 °C, 149 °C with cooling wheel
2. Greenheck – Industrial Process Fans, Model IP (katalog, PDF) – heat slinger: aluminium cooling disc on the shaft between inboard bearing and fan housing; 260 / 427 / 537 °C packages by arrangement
3. Cincinnati Fan – PB Series (teknik katalog, PDF) – heat slinger and guard standard in the 149–204 °C (301–400 °F) range