Backward-curved centrifugal
Efficient and stable on clean to lightly dusty gas. The usual choice behind bag filters and ESPs.
Typical use Boilers, dryers, bag-filter exhaust
The ID fan guide
ID Fan – Working, Types & Selection
What an induced draft fan does, how it works, the types and how to size one – from the engineers who build them.
- ISO 9001:2015 · CE
- 1500+ projects since 1997
- Built to your duty point
What is an ID fan?
The fan that pulls gas out of your process
An induced draft (ID) fan sits at the outlet side of a boiler, furnace, dryer or filter system. It pulls the flue gas or exhaust through the equipment and sends it up the chimney. Because it creates suction, the system runs slightly below atmospheric pressure – so hot gas and dust stay inside the duct.
ID fans are usually centrifugal: a spinning impeller throws the gas outwards into a scroll casing that turns speed into pressure. The blade shape, materials and drive are chosen for the gas, the dust and the temperature.
- Keeps furnaces and process equipment at negative pressure
- Handles hot, dusty and moist gas
- Sized to your flow, pressure and temperature
How it works
Suction in. Pressure out.
- 1Pulls
The impeller spins and throws gas outwards, so the pressure at the fan inlet drops below the surrounding air.
- 2Drafts
That suction – the induced draft – draws flue gas or process exhaust out of the furnace, dryer or hood.
- 3Pushes
The scroll casing turns the speed into pressure and sends the gas through the outlet duct to the stack.
- 4Keeps negative
The system stays slightly below atmospheric pressure, so hot gas and dust do not leak out into the plant.
Fan types
Pick the blade for the gas
Clean gas, dusty gas or abrasive gas each need a different impeller. Here are the four designs we are asked about most.
Radial / paddle-blade centrifugal
Self-cleaning, wear-resistant blades for heavy dust, sticky or abrasive gas.
Typical use Cement, coal, biomass, sinter, wood dust
Airfoil centrifugal
Highest efficiency and lowest noise on clean gas – the power-saving option.
Typical use Power plants, large clean-gas duties
Axial-flow
Very large volumes at low pressure in short duct runs.
Typical use Cooling, tunnel and plant ventilation
Inside the fan
The parts that decide how long it runs
Tap the dots on the 3D fan at the top of the home page, or read them here.
1Inlet bell & flange
Guides gas smoothly into the eye of the impeller. A good bell cuts turbulence and noise.
2Impeller
Backward-curved blades between a back plate and a shroud throw the gas outwards. Balanced to cut vibration.
3Scroll casing
The spiral casing turns the speed of the gas into pressure. Liners protect it from dust wear.
4Shaft & bearings
A stiff shaft on cooled bearing housings carries the impeller. It must stay cool with hot gas.
5Motor & coupling
Drives the fan directly or through a coupling or belts. A variable frequency drive saves power.
6Outlet duct & joint
Sends gas to the filter or chimney. A flexible joint keeps duct loads off the casing.
7Base frame & plinth
A rigid skid on concrete plinths keeps the fan aligned and carries the vibration away.
ID fan vs FD fan
Two fans, two jobs
In a balanced-draft system you need both. The ID fan pulls, the FD fan pushes.
| FD fan (forced draft) | ID fan (induced draft) | |
|---|---|---|
| Where it sits | Before the boiler or furnace | After the boiler, filter or process – near the chimney |
| What it does | Pushes fresh air in | Pulls flue gas or exhaust out |
| Pressure in the furnace | Positive – gas can leak out | Negative – gas stays in the duct |
| What it handles | Clean, cool air | Hot, dusty, sometimes corrosive gas |
| Typical wear | Low | Higher – needs liners and robust blades |
| Used together | Paired with an ID fan in balanced-draft boilers | Paired with an FD fan in balanced-draft boilers |
Duty-point calculator
How much power will your fan need?
Move the sliders – see shaft power, a suggested motor size and the yearly power cost. 20 seconds.
0 kWshaft power needed
- Gas density at this temperature
- 0 kg/m³
- Same duty on standard (20 °C) air
- 0 mmWC
- Suggested motor
- 0 kW
- Power cost per year*
- ₹0
*Indicative only: shaft power = flow × pressure ÷ (3600 × 1000 × fan efficiency). The motor is rounded up to the next standard size with a 10% margin. Final selection depends on the fan curve, the system curve and the dust and gas data – we check it for you free.
Selection checklist
Six facts we need to size your ID fan
Do not worry if you do not have all of them. Send what you know – our engineers will help with the rest.
1Gas flow
Actual volume in m³/hr at the operating temperature – not normal m³/hr.
2Pressure
Total static pressure the fan must overcome, in mmWC or Pa, from ducts, filter and stack.
3Temperature
Normal and peak gas temperature. It sets density, materials and bearing cooling.
4Gas & dust
Moisture, corrosive gases, dust load and particle type – they decide blade shape and lining.
5Drive
Direct, belt or coupling drive, speed control (VFD) and motor rating.
6Site
Altitude, ambient temperature, hours of running and available space.
Watch it work
Inside an ID fan, part by part
Our exploded-view video shows every part of the fan and blower – from the motor to the blades.
Want a fan like this for your plant?
Send your duty details – we recommend a fan and quote it.
More videos on our YouTube channel
FAQ
Technical questions
How do I size an ID fan?
You need the gas flow (actual m³/hr at operating temperature), the total static pressure the fan must overcome, the gas temperature, the dust and moisture content, the altitude and the drive arrangement. Send what you know and our engineers will work out the rest.
Why does gas temperature matter?
Hot gas is lighter. The same fan produces less pressure on hot gas than on cold air, and the shaft, bearings and impeller materials must handle the temperature. We size the fan on your real operating conditions.
Which blade type is right for dusty gas?
Radial or paddle blades are self-cleaning and wear-resistant, so they suit heavy dust, sticky or abrasive gas. Backward-curved blades are more efficient but better on clean to lightly dusty gas, usually after a dust collector.
How can I reduce the power my ID fan uses?
Check that the fan is not over-sized, fit a variable frequency drive instead of damper control, keep the impeller clean and balanced, and reduce pressure loss in the ducts and filters. Our duty-point calculator shows what the power costs.
What causes ID fan vibration?
Most often dust build-up or wear on the impeller (imbalance), loose foundation bolts, worn bearings, misalignment or operating far from the design point. Balancing the impeller on site and checking the bearings usually solves it.
How often should an ID fan be inspected?
Check bearing temperature and vibration every shift, and inspect the impeller, liners and casing at each planned shutdown. Dusty duties need more frequent checks. Our maintenance guides have a practical checklist.
Tell us your duty. We will size your ID fan.
Share air flow, pressure, temperature and gas – get a fan recommendation and price within one working day.