How supplementary fired hRSGs work
A supplementary fired HRSG adds a duct burner between the gas turbine exhaust and the heat transfer surfaces. Turbine exhaust still holds 13 to 15% oxygen, so the burner fires natural gas or liquid fuel into it without extra air, raising the gas to about 700 to 900°C and the steam output well above that of an unfired unit. Many process plants also need steam when the turbine is down. For that, a fresh air fan supplies ambient air to the burner and the HRSG runs as a conventional fired boiler, known as fresh air firing. In normal operation the turbine pushes the gas through the HRSG within its back pressure limit; during fresh air firing the fresh air fan does that job and the boiler is pressurised.
Where the ID fan sits
- Gas turbine or fresh air fan
- Duct burner
- Superheater and evaporator
- Economiser
- SCR (if fitted)
- ID fan
- Stack
Where fitted, the fan sits after the economiser, handling clean gas at about 120 to 180°C. In turbine mode it holds the turbine exhaust pressure within the allowance; in fresh air firing it holds the furnace, the duct burner section, at a slight suction or near atmospheric. The two modes give very different gas volumes and pressures, and the fan must cover both, with the control switching its reference from turbine exhaust pressure to furnace pressure.
Gas at the ID fan
| Gas temperature, normal | 120 to 180°C after the economiser |
| Maximum or upset | Higher at peak supplementary firing with fouled surfaces, typically up to 220°C |
| Dust at the fan | Negligible on gas; light soot when the burner fires liquid fuel |
| Moisture and dew point | Water vapour rises with supplementary firing, often 10 to 14%; acid dew point only with sulphur-bearing liquid fuel |
| Operating modes | Unfired, supplementary fired and fresh air fired, each with its own volume and pressure |
| Density at the design temperature | About 0.82 kg/m³ at 160°C, 68% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
ID Fan selection for supplementary fired hRSGs
| Blade form | Backward curved aerofoil, selected so both operating modes fall in a stable region |
| Material of construction | Mild steel; corrosion allowance where liquid fuel with sulphur is fired |
| Drive and arrangement | Direct coupled, arrangement 8, double inlet on large units |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive, with control reference switched between turbine exhaust pressure and furnace pressure by mode |
| Also specified | Interlocks with the diverter damper, fresh air fan and burner management system |
What this ID fan has to cope with
Two operating modes, one fan
In turbine mode the gas volume is large and the burner adds little; in fresh air firing the volume is set by the fresh air fan and the firing rate. A fan selected only for one mode may surge or run inefficiently in the other. Plotting both duty points on the curve and using speed control keeps the fan stable in each.
Transfer between modes
When the turbine trips, the plant must switch to fresh air firing without losing the flame or over-pressurising the boiler. The ID fan, fresh air fan and dampers move together under the burner management system. The fan's speed range and response time are part of that sequence and are checked during commissioning.
Back pressure at peak firing
Supplementary firing raises gas volume through the HRSG and therefore its pressure drop. At peak firing the turbine back pressure can exceed its allowance. A fan sized for that peak, rather than for unfired operation, prevents the turbine losing output when steam demand is highest.
Energy and control
In a process cogeneration plant, supplementary firing follows the steam demand while the turbine follows the power demand, so the fan duty changes with both. Speed control from the appropriate pressure reference in each mode keeps fan power to what each mode requires. Where the fan is only needed at peak firing, it can idle at low speed for much of the year.
ID fan and FD fan in supplementary fired hRSGs
| ID fan | FD fan | |
|---|---|---|
| Position | After the economiser, before the stack, only where needed | Beside the turbine transition duct, ahead of the duct burner |
| What it handles | Clean exhaust and combustion gas | Filtered ambient air |
| Temperature at the fan | 120 to 180°C | Ambient |
| Main design concern | Two operating modes and turbine back pressure | Fast takeover and hot gas backflow |
| Control | Turbine exhaust or furnace pressure by mode | Burner firing rate in fresh air mode |
See also the Forced Draft (FD) Fan for supplementary fired hRSGs.
Questions buyers ask
Does a supplementary fired HRSG need an ID fan?
Often not. The gas turbine pushes the exhaust through in normal operation, and the fresh air fan does so during fresh air firing. An ID fan is added where peak firing, extra surfaces or SCR push the resistance beyond the turbine's back pressure allowance, or where balanced draft is preferred in fresh air mode.
How does the ID fan work during fresh air firing?
When the turbine is down, the fresh air fan supplies combustion air to the duct burner. If an ID fan is fitted, it switches to controlling furnace pressure, just as on a conventional balanced draft boiler. The burner management system coordinates both fans and the dampers during the changeover.
Why does HRSG back pressure rise with supplementary firing?
The burner raises gas temperature and adds combustion products, so the actual gas volume through the superheater and evaporator increases and so does the pressure drop. The turbine sees this as higher back pressure, which must stay within its allowance at maximum firing. Checking it at peak firing, not only unfired, is essential.
Information needed to quote an ID fan for supplementary fired hRSGs
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- Operating modes. Gas flow, temperature and required pressure in unfired, supplementary fired and fresh air fired modes.
- Burner and turbine data. Duct burner rating and fuels, and the turbine back pressure allowance.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.