Boiler Fan and ID Fan: Types, Selection Criteria, and What to Specify

A boiler system has three distinct fan duties – induced draft, forced draft, and primary air. Each operates under different temperature, pressure, and air composition conditions. Selecting the wrong fan type, or under-specifying the duty conditions to a manufacturer, creates performance problems that are expensive to fix once the unit is installed and commissioned.

This guide covers how each fan type works, what the specification parameters are, and what material and construction choices matter for long-term reliability in boiler service.

The Three Fan Duties in a Boiler System

Most plant engineers and purchase managers searching for a “boiler ID fan” are actually dealing with one of three distinct requirements. Understanding which one applies determines everything about the fan design.

Induced Draft (ID) Fan

The ID fan sits on the outlet side of the boiler, downstream of the heat exchangers and economizer. It draws flue gas from the furnace and exhausts it through the stack. Because it handles combustion exhaust, the ID fan operates in the most demanding conditions of the three fan duties:

• Gas temperature: typically 150°C to 350°C depending on boiler design and whether an air pre-heater is installed
• Gas composition: combustion products including CO₂, water vapour, SO₂ (in coal or oil-fired boilers), and fly ash particulates
• Pressure duty: negative pressure (suction) on the furnace side, discharge against stack resistance

The fly ash content in the gas stream for coal-fired boilers causes progressive blade erosion. This is the most common cause of ID fan performance degradation and unplanned shutdowns in power and process plants.

Forced Draft (FD) Fan

The FD fan supplies combustion air to the boiler burner from the ambient side. Unlike the ID fan, it handles clean ambient air at near-ambient temperature. Operating conditions are far less severe: air is clean, cool, and non-corrosive. FD fans run at higher speed and generate higher static pressure because they push air into a pressurized furnace plenum.

Primary Air (PA) Fan

In pulverized coal-fired boilers, a PA fan handles primary air that transports pulverized coal from the mill to the burners. PA fans operate at very high static pressure — often 700 to 1,200 mmWC or higher — because they must overcome the resistance of the coal pipe runs and burner nozzles. They handle warm air (after the air pre-heater) and must deliver consistent flow regardless of mill load changes.

Fan Type Comparison: Boiler Air System

Why ID Fan Selection Is More Complex Than It Appears

Most plant engineers focus on the airflow and pressure numbers. These are necessary but not sufficient. For an ID fan on a coal-fired boiler, the fly ash concentration in the flue gas is the parameter that determines impeller life and maintenance frequency.

Coal fly ash is abrasive. Impeller blades on a radial or backward inclined design experience continuous particle impact, primarily on the leading edge and pressure face of the blade. Without the right material or surface treatment, a new ID fan impeller can show measurable erosion within 12 to 18 months of operation on a high-ash coal boiler.

The corrective approaches are:

• Hard-facing weld overlay on blade leading edges and pressure faces — extends blade life significantly in moderate ash loading
• Ceramic tile lining on the blade surface — higher initial cost, longer life in severe erosion duty
• Radial blade (paddle blade) impeller design — sacrifices some efficiency but distributes erosion more evenly than backward curved
• Wear-resistant MOC: SA516 Gr.70 pressure vessel steel for casings, abrasion-resistant steel (AR400 or equivalent) for blade wear zones

AS Engineers manufactures high-temperature centrifugal blowers and ID fans capable of handling gas temperatures up to 650°C, with static pressure up to 2,100 mmWC and airflow up to 2,50,000 m³/hr. Impeller diameter range: 250 to 2,200 mm.

Material of Construction: Matching MOC to Duty Conditions

Getting the MOC wrong is the second most common source of premature ID fan failure after erosion. The selection criteria are straightforward when the duty conditions are clearly defined.

For clean or mildly dusty flue gas below 250°C, MS fabricated impellers with a protective coating are standard. For temperatures between 250°C and 400°C on coal-fired duty, CS (carbon steel) with hard-facing on blade edges is the baseline. For temperatures above 400°C or for sulphur-bearing fuels (high-sulphur coal, fuel oil, process gas), SS 304 or SS 316 construction prevents acid dew-point corrosion on cooler sections of the casing.

Available MOC options from AS Engineers:

• Mild Steel (MS) — general duty, clean gas, ambient to 250°C
• Carbon Steel (CS) — moderate temperature, general coal duty
• SA516 Gr.70 — pressure-rated casing material for high-temperature, pressurized applications
• SS 304 / 316 / 321 — corrosive gas streams, sulphur-bearing fuels, acid dew-point risk
• Hard Facing (weld overlay) — blade leading edges for abrasive fly ash duty
• Duplex SS 2205 — aggressive corrosion environments in chemical and petrochemical boiler service

Performance Testing and Quality Standards

AS Engineers manufactures fans to IS 4894 (centrifugal fans — methods for performance testing). Dynamic balancing is carried out to G6.3 or G2.5 grade depending on operating speed, per ISO 1940-1. For boiler ID fans, dynamic balancing is particularly important because unbalance forces at operating temperature differ from room-temperature assembly conditions. Thermal growth of the shaft and impeller must be accounted for in the balancing specification.

Material test certificates (MTC) and performance test certificates are supplied with each fan as standard. For power sector and critical process applications, third-party inspection and witness testing can be arranged.

Industries and Applications

Industrial Exhauster Radial Blowers and High Temperature Plug Blowers from AS Engineers are configured for the following boiler and combustion air system duties:

• Power plants and captive power units — ID and FD fans for coal, biomass, and gas-fired boilers; PA fans for pulverized coal burner systems
• Cement plants — kiln ID fans handling high-temperature, high-dust gas from the clinker cooler and kiln exit; coal mill PA fans
• Fertilizer and chemical plants — process heater ID fans handling sulphur-bearing combustion products; FD fans for steam generation boilers
• Steel and metals — reheating furnace exhaust fans, sinter plant windbox fans, blast furnace stove exhaust
• Paper and pulp — recovery boiler fans handling smelt-bearing flue gas; lime kiln ID fans
• Refineries and petrochemicals — fired heater FD/ID fans; flare gas blowers; process furnace combustion air supply

For more detailed technical content on ID fan selection and boiler air system design, the idfan.in knowledge hub covers impeller selection, performance curves, and boiler application specifics.

After-Sales Support for Boiler Fans

Boiler ID fans are run-or-fail equipment in most plants. Unplanned downtime is not an option. AS Engineers’ centrifugal blower service team provides on-site dynamic balancing, impeller inspection and hard-facing renewal, bearing replacement, and performance testing for fans already in service — including fans originally supplied by other manufacturers. Spare impellers, shafts, and bearing housings are available for standard range fans.

Annual Maintenance Contracts (AMC) covering periodic inspection, balancing, and critical spare pre-positioning are available for plants that require guaranteed response time.

What to Submit for an Accurate ID Fan Quotation

Incomplete specifications are the single biggest cause of delays in boiler fan procurement. Submitting the following data enables AS Engineers to return a technically specific offer — not a range price with caveats.

Required inputs for ID fan sizing:

• Volumetric flow at fan inlet conditions: m³/hr (specify if at actual or standard conditions)
• Static pressure required: mmWC (at fan total or fan static, specify which)
• Gas temperature at fan inlet: °C
• Gas composition: flue gas from coal/oil/gas/biomass, approximate fly ash loading (g/Nm³ if known)
• Gas density or molecular weight if available
• Available motor power, voltage, and frequency
• Drive arrangement preference: direct or belt-driven; if direct, coupling type
• Site altitude above sea level: m (affects gas density)
• Available floor space or installation constraints

Submit your boiler fan specification to AS Engineers for a sizing and technical offer. Phone: +91 99090 33851 / +91 82386 77554.

Frequently Asked Questions

What is the difference between a boiler ID fan and an FD fan?

An ID fan (Induced Draft) draws flue gas from the boiler furnace and exhausts it through the chimney. An FD fan (Forced Draft) pushes fresh combustion air into the furnace from the ambient side. The ID fan handles hot, potentially dusty and corrosive flue gas; the FD fan handles clean ambient air. They operate at different temperatures, gas compositions, and pressure duties, so they require different impeller designs and material specifications.

What causes ID fan impeller erosion and how is it prevented?

Fly ash in the flue gas stream causes erosion, primarily on blade leading edges and pressure faces. The erosion rate depends on ash loading (g/Nm³), particle hardness, and gas velocity through the impeller. Prevention measures include hard-facing weld overlay on blade edges, radial (paddle) blade impeller design that distributes erosion more evenly, and abrasion-resistant blade materials (AR400 or equivalent). For severe duty, ceramic tile lining on blade surfaces is used.

What material is used for boiler ID fans on high-sulphur coal?

High-sulphur coal produces SO₂ in the flue gas. When flue gas temperature drops below the acid dew point (typically 120 to 150°C depending on sulphur content and moisture), sulphuric acid condenses on fan surfaces and causes rapid corrosion. For these applications, SS 304, SS 316, or SS 321 construction for the impeller and casing is specified. Casing drain points are also added to remove condensate.

What IS standard applies to boiler fans?

IS 4894 covers the performance testing of centrifugal fans including boiler ID and FD fans. Dynamic balancing is performed per ISO 1940-1 to G6.3 or G2.5 grade. For high-temperature applications, the fan design also references IS 2825 or ASME Section VIII Div.1 for pressure-rated casing components where applicable.

Can AS Engineers retrofit or replace ID fans originally supplied by another manufacturer?

Yes. With the original performance data (flow, pressure, speed) and the available motor details, AS Engineers can design a replacement impeller or complete fan assembly for retrofit into an existing installation. On-site dynamic balancing after installation is included in the commissioning scope.