High-Temperature Centrifugal Blowers for Furnaces, Kilns and Ovens

A blower handling hot gas is a different machine from one handling ambient air. At 500, 650, or 800 °C, steel loses strength, bearings cannot sit in the gas, metal grows, the gas thins out, and the pressure the blower can develop drops sharply. NextAir Systems manufactures high-temperature centrifugal blowers for ovens up to 500 °C, kilns up to 650 °C, and furnaces up to 800 °C. This page explains what those numbers mean in practice, how hot-gas blowers are built, and the design trap that catches many hot-gas systems: the system usually asks for more pressure just as the blower can supply less.

Our Temperature Range

Equipment	Maximum gas temperature	Typical blower duty
Ovens	500 °C	Recirculation, exhaust, cooling zones
Kilns	650 °C	Recirculation, exhaust, cooling air
Furnaces	800 °C	Exhaust, recirculation, hot-gas handling

Combustion air blowers handle cold ambient air even when they feed a furnace running at 800 °C, so they are ordinary blowers and are covered under combustion air. This page is about the blowers that move the hot gas itself.

What Hot Gas Does to Blower Performance

Hot gas is much less dense, and a fan’s pressure and power fall in proportion to density. These are the values at sea level.

Gas temperature	Density (kg/m³)	Ratio to standard	Volume of the same mass of gas	A 500 mm WC (standard) blower develops
20 °C	1.204	1.00	1.00	500 mm WC
500 °C (oven limit)	0.457	0.38	2.64 times	about 190 mm WC
650 °C (kiln limit)	0.382	0.32	3.15 times	about 159 mm WC
800 °C (furnace limit)	0.329	0.27	3.66 times	about 137 mm WC

The last column assumes the blower is rated at 500 mm WC at standard air density. The blower’s rated pressure is a standard-air number, and in hot gas the pressure it actually develops falls to a fraction of it.

The Trap: Hot Gas Systems Ask for More Pressure

Here is the point that surprises people. For the same mass of gas flowing through the same ducts, the volume rises with temperature, so the velocity rises, and the pressure loss rises with it. At constant mass flow, system pressure loss varies as the inverse of density. So as the gas gets hotter, the system needs more pressure, while the blower can develop less.

Example. A duct system carries a fixed mass of gas, and at 20 °C its resistance is 1,000 Pa (about 102 mm WC). The table shows what happens at the same mass flow when the gas is hot.

Gas temperature	Volume vs 20 °C	System resistance	Blower rating needed at standard density
20 °C	1.00	1,000 Pa	102 mm WC
300 °C	1.96	1,955 Pa	390 mm WC
500 °C	2.64	2,637 Pa	709 mm WC
650 °C	3.15	3,149 Pa	1,011 mm WC
800 °C	3.66	3,661 Pa	1,366 mm WC

A system that needs only 102 mm WC at room temperature becomes a 709 mm WC duty at 500 °C, beyond our 500 mm WC standard maximum. To keep a hot-gas duty within 500 mm WC (4,903 Pa) at standard-density rating, the system’s resistance at ambient conditions must not exceed about 705 Pa (72 mm WC) at 500 °C, about 494 Pa (50 mm WC) at 650 °C, and about 366 Pa (37 mm WC) at 800 °C.

The practical lesson is that hot-gas systems have to be designed for low resistance: generous duct sizes, gentle bends, and low velocities. It also means the blower selection must be worked out at temperature, not at ambient. These figures are an illustration of the physics. Real limits are tighter still, because heat lowers the speed at which the wheel can safely run, as explained below.

How High-Temperature Blowers Are Built

The following points reflect published engineering guidance from fan manufacturers, and each is confirmed for our blowers on the specific duty.

Materials. Carbon steel, low-alloy, and chrome-moly steels are satisfactory up to about 900 to 1,000 °F (roughly 480 to 540 °C), depending on stress levels. Above that, heat-resistant alloys are used, including stainless steels such as 304, 316, 321, and 330, and nickel alloys. One manufacturer builds furnace fans in 316 stainless for temperatures up to about 815 °C, and uses higher alloys still above that. Our oven limit of 500 °C sits at the upper edge of carbon steel practice, and our kiln (650 °C) and furnace (800 °C) limits are in heat-resistant alloy territory.

Speed derating. Metals get weaker when hot, so temperature lowers the maximum speed at which a wheel can run. At higher temperatures, creep, the slow deformation of metal under stress, must be considered as well. Since blower pressure depends on wheel speed, this limits the pressure a hot blower can develop.

Bearings and shaft. Standard fans with the bearings outside the airstream are usually limited to around 149 °C (300 °F) without special provision, and bearings inside the airstream are limited to whatever the bearing can withstand, typically about 54 °C (130 °F). High-temperature blowers therefore keep bearings out of the gas and cool the shaft, with a shaft cooler, heat slingers, or water cooling, use high-temperature lubricants, and add shaft seals. Water-cooled shafts are usually paired with belt drive so the cooling arrangement can be fitted.

Insulation and thermal growth. Hot alloy parts are insulated from cold mild-steel parts, and the housing and wheel are built to grow when hot without binding. Clearances between wheel and housing that are right at ambient temperature are wrong at 800 °C.

Motor position. Centrifugal blowers suit the hottest exhaust because the motor and bearings sit outside the airstream.

Starting, Running, and Stopping

Start-up in cold gas means high density and the highest motor power, so the motor is sized 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, a stopped shaft can bow as it cools unevenly, so many installations keep the blower turning slowly until it has cooled. Plan for a power failure with a hot blower.

Wheels: Forward-Curved or Backward-Curved

Both wheel types are used at high temperature. Forward-curved wheels are common in oven and dryer recirculation, where the pressure is modest and the flow is large, and they are compact. Backward-curved wheels suit higher pressure and better efficiency, and their non-overloading power curve protects the motor. The choice follows the pressure and flow, and the hot-gas density correction applies to both. See backward-curved versus forward-curved blower wheels.

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 hot blower can develop is confirmed at temperature. 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: oven, kiln, or furnace.
2. Mass flow or actual volume flow at temperature, and which you are quoting.
3. System resistance at operating temperature, and at ambient.
4. Gas composition, dust, moisture, and any corrosive content.
5. Start-up conditions, cold or hot, and cool-down arrangement.
6. Motor position, drive arrangement, and bearing cooling preference.
7. Duty: continuous or intermittent, and control approach.

Get a Blower Selection for Your Furnace, Kiln or Oven

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

Frequently Asked Questions