Induced Draft (ID) Fan Manufacturer: Sizing for Hot Flue Gas, Draft and Dust
NextAir Systems, Faridabad, India. Last reviewed October 2026.
An induced draft fan sits at the end of a combustion or process line and pulls the gas through, holding the furnace or boiler slightly below atmospheric pressure so that the flame and the gas stay where they belong. It handles the hottest, dirtiest and most variable gas in the plant, and it runs for years without a break. The questions that decide its selection are about the gas, not the fan.
NextAir builds centrifugal blowers up to 800 HP and 500 mm WC, with high-temperature construction for furnaces, kilns and ovens, and we apply that range to induced-draft duties that fall inside it. This page works through how the gas condition sizes the fan, and states what we do not build.
At a glance
- At 250 °C, 100,000 normal m³/h of flue gas occupies about 53.2 m³/s, and at 2,500 Pa and 70% efficiency the fan needs about 190 kW. The same gas at 450 °C needs 263 kW.
- A 30 m stack with 250 °C gas supplies about 139 Pa of natural draft against the ambient air, which reduces what the fan must do.
- We build up to 800 HP and 500 mm WC; pressures above that, radial-blade material-handling duty and ATEX blowers are outside what we build.
- The dust in the gas decides the wheel type, and a heavy ash load may fall outside our range.
What an ID fan does
In a boiler or furnace the gas is pushed through by the combustion air fan, pulled by the induced draft fan, or both. An ID fan holds the combustion chamber at a slight negative pressure and moves the gas through the heat recovery equipment, any filters or scrubbers, and up the stack. What the fan sees is the pressure drop of everything between the furnace and the stack, less any natural draft from the stack itself, and the volume of gas at its inlet temperature.
Gas condition sets the volume
A process fixes the mass flow of flue gas, usually quoted in normal cubic metres per hour. Moving the actual volume at the inlet temperature, which is larger, is the fan’s job. Here we convert 100,000 normal m³/h to actual flow at four flue gas temperatures, and gives the shaft power at an assumed total system pressure of 2,500 Pa and 70% efficiency, using the density of dry air at the same temperature as an approximation for the gas.
| Flue gas temperature (°C) | Density (kg/m³, as air) | Actual flow (m³/s) | Shaft power at 2,500 Pa and 70% (kW) |
|---|---|---|---|
| 150 | 0.83 | 43.0 | 154 |
| 250 | 0.67 | 53.2 | 190 |
| 350 | 0.56 | 63.4 | 226 |
| 450 | 0.49 | 73.5 | 263 |
The real gas is not air: composition shifts the density, as in gas composition and moisture, and the figures use air for illustration. The pattern is clear. Doubling the temperature raises the actual volume by about 60%, and the power with it, since the pressure the system needs does not change with temperature. The conversion between normal and actual flow is explained in reading a blower datasheet.
Draft from the stack
A stack is a natural draft device. The pressure difference it produces is its height times gravity times the density difference between the ambient air and the hot gas inside. For a 30 m stack with 250 °C gas and 35 °C ambient air (1.145 kg/m³) that is about 139 Pa. In a hot climate the ambient air is less dense, so the stack draft falls, and on a hot day the fan has more to do than on a cool one. The system pressure the fan must supply is the total drop minus this draft.
Dust, ash and wear
Flue gas from solid fuels carries ash, and the ash wears the wheel. That is the factor that most changes the fan: wheel type, material, thickness, wear liners and the speed all depend on the dust loading. We build backward-curved and forward-curved wheels and do not build radial-blade material-handling duty, so a fan for a heavy ash load may fall outside our range. Ask a supplier for the maximum dust loading the proposed wheel is designed for, and how the wheel is protected. For clean gas, such as from gas or light-oil firing, the duty is straightforward.
Think about shutdown as well as running. When the combustion stops, the gas path cools and the fan should keep running through a cool-down, or the hot shaft and wheel can distort in still hot gas. A trip in the middle of operation can leave the furnace under positive pressure, so interlocks between the fans, the burners and the dampers are part of the design and belong to the plant’s safety philosophy. Ask the supplier how the fan behaves on a power failure, and what it assumes of the interlock scheme.
Control, start-up and the motor
- Control is by an inlet damper or a variable frequency drive, and a drive saves energy at part load, as the true cost of a blower shows for large fans.
- Cold gas is denser, so power is highest at start-up, and the motor and starting method are planned for it, as in motors, drives and starting.
- For hot gas, temperature means the materials, bearing cooling and speed limit are engineered for the duty; see high-temperature blowers.
- On safety, ATEX and spark-resistant blowers are outside our range, so say if the gas can be flammable.
For the supplier checklist see how to choose a centrifugal blower manufacturer, and for the combustion side combustion air blower sizing.
What to send us for a quotation
- Flue gas flow in normal m³/h and the temperature at the fan inlet
- Gas composition, moisture and dust or ash loading
- Total system pressure, and the stack height and temperature if natural draft helps
- Fuel and the combustion arrangement
- Control method: damper or variable frequency drive
- Voltage, frequency and the drive arrangement
Send these to us through our contact page and we will tell you plainly whether the duty is inside our range. See the centrifugal blower range, our guide to centrifugal blowers and the power generation and cement plants pages. Our notes for India cover local practice, and the exhaust may need an oxidizer.
Types and variants we build
- High-Pressure Centrifugal Blower Supplier: Up to 500 mm WC, and What That Takes at the Wheel
High-pressure centrifugal blowers up to 500 mm WC (about 4.9 kPa): the tip speed that pressure needs, a calculated table, and where our range ends. - Induced Draft (ID) Fan Manufacturer: Sizing for Hot Flue Gas, Draft and Dust
Induced draft fans pull hot flue gas through a boiler or process: how gas temperature sets volume and power, what a stack adds, and what ID duty asks. - Stainless Steel Centrifugal Blower Manufacturer: When Stainless Pays and When It Does Not
Stainless steel centrifugal blowers for corrosive, humid, coastal and washdown duty: a cost-per-year comparison with mild steel and what else to specify.
Frequently Asked Questions
Do you make induced draft fans?
We build centrifugal blowers up to 800 HP and 500 mm WC, with high-temperature construction for furnaces, kilns and ovens, and apply that range to induced-draft duties that fall inside it.
How do I size an ID fan?
Convert the normal flow to actual flow at the inlet temperature, add up the system pressure drop less any stack draft, and apply the efficiency to get shaft power.
What gas dust loading can you handle?
Dust loading decides the wheel type and wear protection. We do not build radial-blade material-handling duty, so a heavy ash load may be outside our range. Tell us the loading.
Why does a stack help the fan?
A hot stack produces natural draft from the density difference between the ambient air and the hot gas, reducing the pressure the fan must supply.
Do you build ATEX ID fans?
No. ATEX and spark-resistant blowers are outside what we build.
Talk to Our Engineering Team
Call +91-9311805618 or use our contact form for a facility-specific recommendation.
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