Centrifugal Blowers for Boiler Forced-Draft and Induced-Draft Duty

Every fired boiler moves two streams of gas: combustion air in, and flue gas out. A forced-draft blower pushes combustion air through the burner and boiler, and an induced-draft fan pulls the flue gas out through the economiser, dust collector, and stack. Centrifugal machines do both jobs because both paths carry real resistance. This page covers what each blower does, how the gas conditions differ, and how the duty maps onto our range: small and medium boilers and heaters, not large utility units.

Forced Draft and Induced Draft

Forced-draft (FD) blowers handle clean ambient air. They push it through the air heater, burner, and combustion chamber. Because the gas is clean and near ambient temperature, backward-curved wheels with high efficiency and a stable, non-overloading curve are the natural choice.

Induced-draft (ID) fans handle flue gas. It is hot, may carry ash or soot, and can be corrosive from sulfur compounds, so the fan needs materials, wear protection, and a motor and bearings outside the gas stream. In a balanced-draft boiler, the ID fan holds the furnace slightly below atmospheric pressure so flame and gas do not leak into the boiler house.

Some boilers use only a forced-draft blower, running the furnace under positive pressure, and others use both. Balanced draft is common where leakage of hot gas into the plant room would be unsafe.

Worked Example: Sizing FD Air for a Small Boiler

Example. A small industrial boiler has a fuel input of 7 MW. That is 7,000 kW × 860 = 6.02 million kcal/h.

Stoichiometric air is 6,020 × 1.18 ≈ 7,100 Nm³/h. With 15% excess air, about 8,170 Nm³/h. At 20 °C that is about 8,770 m³/h, or 2.44 m³/s.

If the burner, air heater, and ductwork need 400 mm WC (3,923 Pa), air power is 2.44 × 3,923 ≈ 9.6 kW. At 65% total efficiency the blower needs about 14.7 kW at the shaft. A duty of this size is well within our range, and the pressure sits inside our 500 mm WC standard maximum.

The Flue Gas Side Is Hotter and Thicker

The same combustion produces more gas volume on the flue side, at higher temperature. Flue gas of about 9,000 Nm³/h at 180 °C is around 14,900 m³/h in actual terms, about 1.66 times the volume at 0 °C, and the density is about 0.78 kg/m³, roughly 65% of standard. Two consequences follow.

1. The ID fan moves a larger actual volume than the FD blower.
2. At the same speed, the ID fan develops only about 65% of its standard-air pressure, so it must be selected from a density-corrected curve.

Flue gas often carries fly ash or soot, which builds up on blades and wears them. Where there is a dust collector ahead of the ID fan, the gas is cleaner, but wear and build-up still need attention. See gas temperature, altitude and density correction and maintaining industrial blowers.

Controlling Draft

Boiler load changes constantly, so draft must follow. Damper and inlet-vane control are traditional and simple. Speed control with a variable-speed drive saves large amounts of energy at part load, since power varies with the cube of speed, and it modulates smoothly. On a balanced-draft boiler, the FD and ID controls must work together, so that furnace pressure stays steady while the firing rate changes. See flow control for blowers.

Standby and Redundancy

Boiler auxiliaries are critical, so many plants fit two 50% or 100% blowers, or a spare, so that a single failure does not trip the boiler. Plan isolation dampers and start-up interlocks. Boiler safety systems will trip the burner on loss of combustion air, so the interlocks and time delays are part of the system design, not an afterthought.

Materials and Corrosion

Boiler flue gas can be acidic when the fuel contains sulfur and the gas cools below its acid dew point. Where the ID fan sees cold or wet gas, casing and wheel materials, coatings, and drains need selecting for it. On the FD side, clean dry air needs only standard construction.

Vibration and Balance

Ash and soot deposit unevenly on ID fan blades and cause unbalance, and rising vibration is the first sign. Vibration monitoring and scheduled cleaning are standard practice. For blowers above 300 kW, ISO 20816-3 governs vibration criteria, and for smaller machines ISO 14694 applies. See bearings, balancing and vibration.

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. That covers forced-draft blowers and induced-draft fans on small and medium boilers, heaters, and thermal-oil systems. Large utility boiler fans, which move far more air at far higher power than 800 HP, are outside our range, and we will say so early. Flue-gas fans handling hot, dusty, or corrosive gas need the temperature, wear, and material details confirmed for each duty.

What to Specify

1. Boiler fuel input, fuel type, and excess air.
2. FD pressure: burner, air heater, and duct resistance.
3. ID conditions: flue gas temperature, dust load, acid dew point.
4. Site altitude and ambient temperature.
5. Draft arrangement: forced only or balanced.
6. Control approach, redundancy, and interlocks.
7. Motor position, drive arrangement, and noise limits.

Get a Blower Selection for Your Boiler

Send your fuel input, draft arrangement, and gas conditions, and we’ll size the blower and confirm it is within range. Contact us for a free quote.

Frequently Asked Questions