Fans for Power Plants: Selection Guide for ID, FD, PA, SA, and Auxiliary Fans

In a coal-fired thermal power plant, fans account for roughly 60% to 70% of total auxiliary power consumption. The ID fan alone — managing flue gas from the boiler to the stack — can consume 5 MW or more in a 210 MW plant. When a plant’s heat rate target is under pressure, or when CPCB’s revised emission norms for thermal power plants are pushing tighter stack compliance, the fan system is not a peripheral concern. It sits at the centre of both efficiency and regulatory performance.

This guide covers the six principal fan types used in thermal power plants, their operating conditions, material selection logic, sizing parameters, and what engineers and procurement teams should verify when specifying or replacing them.

AS Engineers manufactures centrifugal blowers and industrial fans for power plants from Ahmedabad, ISO 9001:2015 certified, testing to IS 4894. For ID fan technical reference, see also idfan.in.

The Regulatory Context Driving Fan Upgrades in Indian Power Plants

Two regulatory developments are directly influencing fan specifications in Indian thermal power plants right now.

First, the Ministry of Environment, Forest and Climate Change notified revised emission standards for coal-based thermal power plants in 2015, with compliance timelines that have been extended but not withdrawn. Stricter particulate matter limits (30 mg/Nm³ for newer units) require higher-efficiency ESP performance, which in turn requires the ID fan to maintain consistent negative pressure across wider load ranges without efficiency loss.

Second, the Bureau of Energy Efficiency’s Perform Achieve and Trade (PAT) scheme targets specific energy consumption in thermal power stations. Auxiliary power consumption — which fans dominate — is a direct variable in the SEC target. Plants replacing legacy fans with correctly sized, IS 4894-tested replacements routinely reduce auxiliary power consumption in the fan circuit by 8% to 15% through better duty-point matching.

The CEA’s Technical Standards for Construction of Electrical Plants and Electric Lines prescribe requirements for auxiliary equipment including fans. Any new installation or major retrofit in a CEA-regulated plant must meet these standards in addition to manufacturer test certifications.

Power Plant Fan Types: Function, Operating Conditions, and MOC

Induced Draft Fan (ID Fan / ESP Fan)

The ID fan sits between the electrostatic precipitator (ESP) and the chimney stack. It draws flue gas through the entire boiler gas path — economiser, air pre-heater, ESP — by maintaining suction (negative pressure) at the boiler outlet. In a balanced draft boiler, the ID fan works in tandem with the FD fan to hold furnace pressure at a slightly negative value, typically between -2 mmWC and -5 mmWC at the furnace top.

Operating conditions that determine ID fan design:

• Gas temperature at fan inlet: 120°C to 180°C under normal operation; up to 250°C during tripping transients or APH bypass
• Dust loading at fan inlet: 50 to 200 mg/Nm³ after ESP (but can spike significantly during ESP trips)
• Static pressure requirement: 200 to 600 mmWC depending on plant size and gas path resistance
• Gas volume: proportional to plant MW rating and excess air level

MOC selection logic: The standard for utility-scale ID fans is carbon steel casing with SA516 Gr.70 for pressure parts, combined with wear-resistant impeller construction. Where fly ash abrasion is high (high-ash Indian coal typically runs 30% to 45% ash content), impeller blades are hard-faced on the leading edges — Stellite or equivalent — to extend service life. Radial or backward-inclined impeller geometry is standard; forward-curved impellers are not used in ID applications due to their non-stall-proof curve shape.

For the broader ID fan selection framework, see our ID and FD fan guide.

Forced Draft Fan (FD Fan)

The FD fan pushes ambient air into the air pre-heater and from there into the furnace as primary and secondary combustion air. It operates under positive pressure, handling clean ambient air — the mechanically simplest duty among the major power plant fans.

Operating conditions:

• Inlet gas: ambient air at 30°C to 50°C (Indian site conditions)
• Static pressure: 300 to 700 mmWC, depending on air pre-heater resistance and furnace pressure
• Duty: continuous at rated capacity; damper-controlled at part load in older plants, variable frequency drive (VFD) controlled in modern units

MOC selection: Standard MS (IS 2062 Grade B) casing and impeller is adequate for FD duty. The design priority for FD fans is aerodynamic efficiency and curve stability across the operating range, since the FD fan directly influences the air-to-fuel ratio and combustion efficiency. A poorly selected FD fan operating away from its best efficiency point (BEP) increases auxiliary power consumption and can contribute to flame instability at low load.

Backward-curved impeller geometry is the standard selection for FD fans where efficiency is the primary criterion.

Primary Air Fan (PA Fan)

The PA fan is the highest-pressure fan in the coal-fired boiler circuit. Its job is to draw hot air from the air pre-heater and push it through the coal pulverizer (bowl mill or ball mill) at sufficient velocity to dry the pulverized coal and carry it in suspension to the furnace burners.

Operating conditions:

• Air temperature at inlet: 250°C to 350°C (hot primary air from APH)
• Static pressure: 800 to 1,500 mmWC — significantly higher than FD fan pressure
• Dust loading at outlet: heavy (coal particles in the conveying stream)

MOC selection: The PA fan handles hot air at elevated temperature, which eliminates standard MS as the casing material for most applications. Casing in SA516 Gr.70 or equivalent pressure vessel grade carbon steel is standard. Shaft material moves to alloy steel (EN-8 or EN-24) to handle the combination of temperature and mechanical load. Impeller geometry is typically radial blade — sacrificing efficiency for erosion resistance, since the coal-laden stream at the outlet causes progressive blade wear. Hard facing on the radial blades is specified where coal hardness (HGI) is low.

The PA fan is frequently the first fan to require impeller replacement in a coal plant, due to the erosive duty. Stocking a spare impeller and maintaining shaft alignment records is standard practice for plants running on high-ash, low-HGI Indian coal.

Secondary Air Fan (SA Fan)

The SA fan — sometimes called the overfire air fan — supplies additional combustion air above the primary combustion zone to complete burnout of unburned carbon and control NOx formation through staged combustion. In older plants, secondary air is supplied directly from the FD fan through a separate dampered duct. In newer plants with low-NOx burner systems, a dedicated SA fan at higher pressure is used to control the secondary air registers independently.

Operating conditions:

• Air temperature: ambient to 80°C (unheated secondary air in many configurations)
• Static pressure: 400 to 800 mmWC
• Duty: subject to frequent load changes as operators adjust combustion staging

MOC selection: Standard MS construction for ambient-temperature SA fans. Where SA fans handle pre-heated air, the same temperature-appropriate material selection as the PA fan applies.

Scanner Cooling Fan

Flame scanners monitor the burner flame in the furnace. They operate in a radiant heat environment and require a continuous supply of cooling air to prevent scanner lens contamination and body overheating. The scanner cooling fan supplies this air — a relatively small-volume, low-pressure duty but one where reliability is non-negotiable, since a failed scanner cooling fan triggers a unit trip on flame failure protection.

Operating conditions:

• Duty: continuous; flow typically 1,000 to 5,000 m³/hr per burner header
• Pressure: 300 to 600 mmWC
• Gas: clean ambient air

MOC selection: Standard MS. The critical parameter is reliability and clean air delivery — not pressure or temperature capability. These fans are specified with N+1 redundancy in most plant designs.

Seal Air Fan

While not always listed alongside ID/FD/PA fans, the seal air fan is a practical necessity in any plant with bowl mills or tube mills. It supplies pressurised clean air to the mill classifier body and bearing seals, preventing coal dust and pulverizer pressure from escaping into the motor and bearing housings.

Loss of seal air pressure is a common cause of premature mill bearing failure and is a recurring maintenance issue in plants without proper seal air fan monitoring. Seal air fans are typically small — 2,000 to 8,000 m³/hr at 500 to 800 mmWC — but their impact on mill availability is disproportionate to their size.

Power Plant Fan Summary: Selection Reference

What to Verify in a Power Plant Fan Specification

The following parameters must be confirmed before any fan can be correctly sized and offered. Vendors who quote without this data are working from assumptions.

• Gas volume at actual inlet conditions (m³/hr at actual temperature, density, and altitude — not at NTP)
• Static pressure required — including system curve at rated load and at minimum stable load
• Gas temperature — minimum, normal operating, and maximum transient (e.g., during APH bypass or boiler trip)
• Dust loading and particle characteristics — for ID and PA fans especially: particle size distribution, ash hardness (in coal applications)
• Site elevation above MSL — affects air density and therefore fan sizing
• Drive arrangement — direct-coupled or belt-driven; motor frame and coupling details for retrofit
• Existing casing dimensions and bolt circle — for like-for-like replacement
• IS 4894 test certificate confirming performance at the rated duty point
• Dynamic balance report to IS 13280 (G2.5 or G6.3 as applicable)

AS Engineers: Power Plant Fan Manufacturing

AS Engineers manufactures centrifugal fans for power plant duty from GIDC Vatva, Ahmedabad, ISO 9001:2015 certified. Our power plant fan range covers:

• Airflow: up to 2,50,000 m³/hr
• Static pressure: 25 to 2,100 mmWC
• Operating temperature: up to 650°C with appropriate configuration
• MOC: MS, SA516 Gr.70, SS 304/316, alloy steels (EN-8/EN-24), hard-faced impellers
• Testing: IS 4894 performance test; dynamic balancing to IS 13280 (G2.5/G6.3)
• Drive: belt-driven or direct-coupled

We supply fans for both new project installations and replacement/retrofit duty in operating plants. For replacement projects, we review the existing installation drawings and operating data before confirming the replacement specification — reducing the risk of a second undersized or mismatched fan.

After-supply support includes performance analysis, impeller replacement, on-site dynamic balancing, and shaft alignment.

For high-temperature applications including PA fans and high-temperature ID fan service, see our high temperature plug blower configurations. For heavy-duty radial configurations for ID and exhaust duty, see our radial blade industrial exhausters.

Discuss Your Power Plant Fan Requirement

If you are specifying fans for a new project or sourcing a replacement for an operating unit, share your process data with our engineering team. We review gas conditions, system pressure drop, temperature transients, and MOC requirements before sizing — not after.

Send your specification and request a quotation

Phone: +91 99090 33851 | +91 82386 77554 Email: info@theasengineers.com

Frequently Asked Questions

What is the difference between a PA fan and an FD fan in a power plant?

Both supply air to the boiler, but their duties are distinct. The FD fan pushes ambient air through the air pre-heater and into the furnace as combustion air, operating at 300 to 700 mmWC. The PA fan handles hot air from the APH outlet and pushes it through the pulverizer at much higher pressure — typically 800 to 1,500 mmWC — to dry and transport pulverised coal to the burners. PA fans operate at higher temperature and face coal dust erosion at the impeller; FD fans handle clean air and are designed for efficiency rather than erosion resistance.

Why does the ID fan consume more power than the FD fan in the same boiler?

The ID fan handles a larger gas volume than the FD fan. Flue gas volume is greater than the combustion air volume entering the furnace because it includes the products of combustion — CO₂, water vapour, and excess nitrogen — in addition to unburned gases. Combined with higher gas density at elevated temperature, the ID fan moves more mass per unit time, requiring more shaft power. In plants running on high-excess-air conditions, the gap between ID and FD power consumption widens further.

What MOC is correct for an ID fan in a high-ash Indian coal plant?

Casing: SA516 Gr.70 or equivalent carbon steel for the pressure-retaining parts. Impeller: radial or backward-inclined blade geometry in carbon steel, with hard-facing (Stellite or equivalent chromium carbide overlay) on the leading blade edges and inlet shroud where ash abrasion is highest. Standard MS impellers without hard-facing require replacement every 12 to 24 months in high-ash duty. Hard-faced impellers typically extend this to 36 to 48 months, depending on dust loading and particle hardness.

What Indian standard governs centrifugal fan testing for power plant supply?

IS 4894 (Centrifugal Fans — Methods of Performance Testing) is the applicable standard. The test certificate should confirm airflow, static pressure, shaft power, and speed at the rated duty point. Dynamic balance certification references IS 13280 — G2.5 for high-speed applications or G6.3 for standard speed. These documents are required for CEA-regulated installations and should be requested at the time of RFQ, not after order placement.

Can AS Engineers supply replacement fans for existing power plant installations?

Yes. For replacement duty, we require the existing fan drawing (or dimensional survey), the original duty specification, and ideally the performance test record from the installed fan. Where performance records are unavailable, we review the current operating parameters against the boiler design data. Replacement fans are designed to match the existing installation’s bolt circle, shaft centre height, and casing dimensions while correcting any duty mismatch from the original specification.