Centrifugal Blowers for Combustion Air: Burners, Heaters and Furnaces

A combustion air blower has a demanding job for a simple-looking machine. It has to deliver a steady flow of air at a steady pressure to a burner, in the right ratio to the fuel, across a wide firing range, for years without pulsation. Get it right and the flame is stable, emissions are low, and fuel is not wasted. Get it wrong and the burner surges, hunts, or runs rich. This page explains how much air a burner needs, what pressure it takes, where our range fits, and how to control the blower.

How Much Air Does a Burner Need?

Air requirement follows from the heat input and the fuel. A widely used combustion handbook gives about 1.18 Nm³ of air for each 1,000 kcal of lower heating value for gas firing, and burners run with excess air above that stoichiometric figure.

Example. A burner has a heat input of 5,000,000 kcal/h, about 5.8 MW. Stoichiometric air is 5,000 × 1.18 = 5,900 Nm³/h. With 15% excess air, the blower must supply 6,785 Nm³/h.

Nm³ is measured at 0 °C and standard pressure. At 20 °C, the actual volume is about 7,280 m³/h, or 2.02 m³/s. At a hot, high site the actual volume is larger still, which is why the site conditions belong in the specification. If the burner needs 350 mm WC (3,432 Pa), the air power is 2.02 × 3,432 ≈ 6.9 kW, and at 65% total efficiency the blower needs about 10.7 kW at the shaft.

What Pressure Does a Burner Take?

Standard industrial combustion systems need air pressures from roughly 100 to 1,400 mm WC, depending on the burner design, and some controlled-atmosphere systems need much more. The pressure has to cover the burner’s own resistance and any pressure in the combustion chamber, because chamber pressure changes the air the burner really receives.

Our blowers reach 500 mm WC static pressure. That covers a large share of industrial burners, in the high-pressure class, but not the burners that need more than 500 mm WC. Check the burner’s required air pressure at nominal capacity and at maximum firing, and include the chamber pressure. Where a burner needs more than 500 mm WC, we will say so instead of forcing a selection.

Backward-Curved Wheels for Combustion Air

Combustion air is normally clean, ambient air, so backward-curved wheels are the natural choice. They deliver high efficiency, a stable curve with no stall region, and a non-overloading power characteristic that protects the motor. Combustion applications need stable airflow and constant pressure, with aerodynamic design that avoids pulsation. See backward-curved versus forward-curved blower wheels.

Local Conditions Change the Selection

Burner pressure is quoted as the pressure the burner needs, at the actual air density. A blower catalogue is rated at standard air, so a hot or high site needs a higher standard-air rating to deliver the same actual pressure.

Example. A burner needs 350 mm WC of actual pressure. At a hot inland site with air at 0.94 kg/m³, the blower must be rated at 350 × 1.204 ÷ 0.942 ≈ 447 mm WC at standard air. At about 2,355 m altitude, where density is 0.889 kg/m³, the rating rises to about 474 mm WC, close to our 500 mm WC standard maximum. Site altitude can therefore decide whether a burner blower is within range. See gas temperature, altitude and density correction.

Controlling Combustion Air

Burners modulate over a firing range, so combustion air must follow. There are three common approaches.

• Damper or inlet vane control. Simple, but it wastes energy at low fire because the blower keeps running near full speed.
• Speed control. A variable-speed drive on the blower follows the firing rate, uses far less electrical energy, and gives faster, more accurate modulation than a damper.
• Fixed speed with burner-side control. Suits steady loads.

Where a burner’s air and fuel are regulated by measuring the differential pressure between the burner’s air chamber and the furnace, speed control of the combustion blower gives tighter regulation. See flow control for blowers.

Keeping the Wheel Clean

Fouling on the blades changes the wheel’s balance and performance. The air should be filtered where dust, oil mist, or lint is present, and the blower checked and cleaned during shutdowns. See maintaining industrial blowers.

Furnace and Kiln Applications

Combustion air blowers handle cold ambient air, so the blower itself does not need high-temperature construction, even when it feeds a furnace or kiln that runs at 800 °C or 650 °C. Where combustion air is preheated, or where the same furnace also needs hot recirculation or exhaust fans, those machines are a separate high-temperature duty. See high-temperature blowers for furnaces, kilns and ovens.

Drive Arrangement and Noise

A combustion blower runs for long hours close to the burner. Direct drive keeps things compact and simple, and belt drive lets the speed be set after commissioning. Inlet silencers and acoustic enclosures reduce noise where the blower sits near people, at the price of extra inlet resistance that must be included in the selection.

Where Our Range Fits

NextAir Systems manufactures centrifugal blowers up to 800 HP (about 597 kW) and up to 500 mm WC (about 4.9 kPa) static pressure, with backward-curved and forward-curved wheels and single-inlet and double-inlet designs. Combustion air for burners needing up to 500 mm WC of actual pressure is within that range, and airflow and drive arrangement are confirmed for each selection. Burners needing more than 500 mm WC, and controlled-atmosphere systems needing very high air pressure, are outside our standard range.

What to Specify

1. Burner heat input and fuel, and required air at minimum and maximum firing.
2. Air pressure the burner needs, and the combustion chamber pressure.
3. Site altitude and air temperature.
4. Control approach: damper, inlet vanes, or speed control.
5. Air cleanliness and filtration.
6. Drive arrangement and noise limits.

Get a Blower Selection for Your Burner

Send your heat input, fuel, required air pressure, and site conditions, and we’ll size the blower and confirm it is within range. Contact us for a free quote.

Frequently Asked Questions