ID Fans and FD Fans: Selection Guide for Plant Engineers and Procurement Teams
Most plants that come to us for ID or FD fan replacements have the same complaint: the fan was undersized for the actual flue gas temperature, or the material grade wasn’t matched to the dust loading. The result is premature bearing failure, impeller erosion, or vibration that shuts the line down.
This guide covers how induced draft fans and forced draft fans work, where each belongs, how to select the right configuration for your process, and what separates a fan that runs for 10 years from one that needs attention in 18 months.
AS Engineers has manufactured centrifugal blowers and industrial fans from Ahmedabad since 1997. We test every fan to IS 4894 and balance impellers to G6.3 or G2.5 depending on operating speed. For deeper technical reference, visit our ID fan knowledge hub at idfan.in.
What Is an Induced Draft (ID) Fan?
An ID fan sits at the outlet side of a combustion or process system, typically between the dust collection equipment and the chimney stack. It pulls flue gas or process exhaust through the system by creating negative pressure (suction) upstream.
Because the ID fan handles gas that has already passed through the combustion chamber, it operates under these conditions:
- Gas temperature ranging from 150°C to 450°C depending on the process and upstream cooling
- Dust and particulate loading that varies with the upstream collector efficiency
- Negative pressure at the fan inlet — which affects shaft seal design and bearing arrangement
- Corrosive or abrasive constituents in cement, chemical, and power plant applications
The fan must handle these conditions continuously, at varying load points, without structural fatigue or bearing overheating.
What Is a Forced Draft (FD) Fan?
An FD fan operates at the inlet side — it pushes ambient air into the combustion chamber or process zone under positive pressure. Since it handles clean, ambient air, FD fans operate under fundamentally different conditions:
- Inlet gas is ambient temperature (30°C to 50°C in Indian conditions)
- Clean air with minimal dust loading
- Positive pressure at the outlet — requires robust casing and inlet box design to prevent leakage
- Higher volumetric efficiency required since combustion stoichiometry depends on accurate air delivery
FD fans are sized based on the combustion air requirement at the rated fuel input, with a design margin for variation in fuel quality and ambient conditions.
ID Fan vs. FD Fan: Key Differences
| Parameter | ID Fan | FD Fan |
|---|---|---|
| Location in system | Outlet — between collector and chimney | Inlet — before combustion chamber |
| Operating pressure | Negative (suction) | Positive (pressure) |
| Gas handled | Hot, dust-laden flue gas | Ambient, clean air |
| Operating temperature | 150°C to 450°C (process dependent) | Ambient (30°C to 50°C) |
| MOC priority | Temperature + corrosion resistance | Structural rigidity, efficiency |
| Impeller wear concern | High — abrasion from particulate | Low — clean air application |
| Shaft seal design | Critical — negative pressure requires robust sealing | Standard positive pressure sealing |
Application-to-Configuration Matrix: Which Fan for Which Process
This is where generic guides fall short. The right fan configuration depends on your industry and what the fan is handling — not just whether it’s on the inlet or outlet.
| Industry | Application | Recommended Type | MOC Guidance |
|---|---|---|---|
| Thermal power | Boiler ID fan (flue gas) | Radial or backward inclined | SA516 Gr.70 or alloy for high temp |
| Cement | Kiln ID fan (hot, abrasive gas) | Radial blade — wear-resistant | Hard-faced impeller, MS casing |
| Cement | Raw mill FD fan (combustion air) | Backward curved | MS standard |
| Chemical | Reactor exhaust fan | Backward inclined or radial | SS 304 or SS 316 for corrosive media |
| Fertilizer | Prilling tower exhaust | Radial | MS with corrosion-resistant lining |
| Steel | Blast furnace ID fan | Radial — heavy duty | Alloy steel, high-temperature bearing |
| Food / Pharma | Process exhaust | Backward curved | SS 316L, GMP-compliant finish |
The cement kiln ID fan is among the most demanding applications. It handles gas at 250°C to 350°C with significant lime dust after the cyclone separator. A standard MS impeller without hard facing will erode within 12 to 18 months in this duty. Specifying the correct impeller geometry and surface protection is not optional — it is what determines service life.
Why Axial Fans Are Not the Right Choice for Draft Duty
Plant engineers sometimes consider axial fans for induced or forced draft applications because they move large air volumes at low power. For draft applications in power, cement, or chemical plants, axial fans have specific limitations:
- Axial fans generate lower static pressure — typically below 150 mmWC. Most boiler and kiln ID applications require 200 to 600 mmWC.
- They are more sensitive to system resistance changes. Draft systems see resistance variation with load — centrifugal fan performance curves handle this better.
- Axial fans are less tolerant of dust-laden gas without specialized blade protection.
For axial fan applications involving cooling towers, forced ventilation, or low-resistance exhaust ducts, axial fans remain the right choice. For thermal draft and process exhaust with any meaningful static pressure requirement, centrifugal fan geometry — specifically radial or backward inclined — is the correct selection.
Material Selection for ID Fans: What the Specification Sheet Doesn’t Tell You
MOC selection for ID fans is where many procurement decisions go wrong. The specification might state “MS construction” without accounting for the actual gas composition or temperature cycling.
- Mild steel (MS/IS 2062): Suitable for temperatures below 200°C with low dust loading and neutral pH flue gas. Not suitable for cement kiln or sulfur-containing flue gas.
- Carbon steel with SA516 Gr.70: Suitable for pressure vessel-grade construction in high-temperature power plant duty.
- SS 304 / SS 316: Required for corrosive gas streams — fertilizer plants, chemical reactors, sulfur-rich combustion products.
- Hard facing: Applied to impeller blades in high-abrasion cement, coal, and mineral processing applications. Extends blade life by 2 to 3 times compared to plain MS.
- Alloy steel (EN-8, EN-24): Used for shafts in high-temperature, high-load applications.
Our high temperature plug blower range is specifically designed for operating temperatures up to 650°C, with appropriate shaft and bearing isolation from the hot gas stream. For heavy-duty radial configurations handling dust-laden exhaust, our impellers are dynamically balanced to G6.3 minimum and G2.5 for high-speed applications.
Testing and Standards: What to Demand from Any ID/FD Fan Supplier
In India, the applicable standard for centrifugal fan performance testing is IS 4894 (Centrifugal Fans — Methods of Performance Testing). When evaluating any ID or FD fan supplier, the test certificate should confirm:
- Actual airflow (m³/hr) measured at test conditions
- Static pressure (mmWC or Pa) achieved at the rated duty point
- Shaft power (kW) consumed at the test point
- Speed (RPM) at which the test was conducted
- Impeller dynamic balance report — G2.5 or G6.3 per IS 13280
The Bureau of Energy Efficiency (BEE) under MoP has also been progressively bringing large industrial fans under energy efficiency norms. For motors driving ID/FD fans above 15 kW, IE3 efficiency class motors are now mandatory under BEE Star Labelling Programme for agricultural pumps and motors. Confirm your drive motor specification with your procurement team.
For facilities covered under CPCB stack emission norms, the ID fan capacity directly determines whether flue gas exits the stack at required velocity and temperature for dispersion compliance. Undersizing the ID fan to save cost creates a compliance risk, not just a performance risk.
What to Specify When Requesting a Quotation
A complete RFQ for an ID or FD fan should include:
- Actual gas volume at inlet conditions (m³/hr at actual temperature, not at NTP)
- Gas temperature at fan inlet (minimum, normal, maximum)
- Static pressure required (mmWC)
- Gas composition — dust loading (g/m³), moisture content (%), any corrosive components
- Installation arrangement — horizontal shaft, drive arrangement (belt/direct), space constraints
- MOC preference or acceptance of MOC recommendation based on duty
- Relevant site conditions — altitude above MSL, ambient temperature range
Without this data, any quotation is a guess. We review the process conditions before sizing — which is why our fans are designed to the actual duty point, not selected from a standard catalog range.
AS Engineers: What We Manufacture
AS Engineers is an ISO 9001:2015 certified manufacturer of centrifugal fans and blowers operating from GIDC Vatva, Ahmedabad. Our ID and FD fan range covers:
- Airflow: 500 m³/hr to 2,50,000 m³/hr
- Static pressure: 25 mmWC to 2,100 mmWC
- Operating temperature: up to 650°C with appropriate configuration
- MOC: MS, SS 304, SS 316, SS 316L, SA516 Gr.70, alloy steels, hard-faced impellers
- Drive: belt-driven or direct-coupled
- Testing: IS 4894, dynamic balancing to IS 13280 (G2.5/G6.3)
We supply ID and FD fans to power plants, cement plants, steel mills, chemical processors, fertilizer manufacturers, and ETP operators. Every fan is manufactured to the specific process conditions provided — not selected from a catalog.
Blower and fan service after supply includes performance analysis, impeller replacement, shaft alignment, and on-site dynamic balancing.
Discuss Your ID Fan or FD Fan Requirement
If you have a replacement fan requirement or are specifying for a new project, share your process conditions with our engineering team. We review gas temperature, dust loading, required pressure, and MOC, then confirm the right configuration before quoting.
Send us your process data and request a quotation
Phone: +91 99090 33851 | +91 82386 77554 Email: info@theasengineers.com
Frequently Asked Questions
What is the difference between an ID fan and an FD fan?
An ID fan (induced draft fan) pulls flue gas out of the system by creating suction at the outlet — typically between the dust collector and the chimney. An FD fan (forced draft fan) pushes clean combustion air into the system under positive pressure at the inlet. They handle different gases at different temperatures, so MOC and impeller design requirements differ significantly.
What MOC should be specified for a cement kiln ID fan?
Cement kiln ID fans handle hot, abrasive gas carrying lime dust after the cyclone pre-heater. Standard MS is inadequate beyond 12 to 18 months in this duty. The correct specification is a hard-faced impeller (Stellite or equivalent wear coating on leading edges) with an MS or alloy steel casing rated for the operating temperature, typically 250°C to 350°C.
Which standard governs ID fan performance testing in India?
IS 4894 governs centrifugal fan performance testing. The test certificate should confirm actual airflow, static pressure, shaft power, and speed at the rated duty point. Dynamic balance certification should reference IS 13280 (G2.5 for high-speed; G6.3 for standard).
Can an axial fan replace a centrifugal fan in an ID fan application?
Generally no. Axial fans are limited to approximately 150 mmWC static pressure. Most industrial ID fan applications — boilers, kilns, chemical reactors — require 200 mmWC to 600 mmWC. Centrifugal radial or backward inclined impeller designs are the correct geometry for these duties.
What information is needed to size an ID or FD fan correctly?
Actual gas volume at inlet temperature, gas temperature (min/normal/max), static pressure required, gas composition including dust loading and corrosive components, and installation arrangement. Sizing from NTP volumes without adjusting for actual gas density at operating temperature is a common error that results in undersized fans.
