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 FD fan sits
- Inlet filter and silencer
- FD (fresh air) fan
- Fresh air damper
- Turbine exhaust duct
- Duct burner
- HRSG
The fresh air fan sits beside the transition duct between the turbine and the HRSG, discharging through an isolating damper into the duct upstream of the duct burner. In turbine mode the damper is shut and the fan is either stopped, or running at low speed to supply burner cooling air. When the turbine trips, the damper opens and the fan provides the full combustion air for the burner, against the resistance of the burner grid and the whole HRSG.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Pressure at the fan outlet | Burner grid plus full HRSG gas-side resistance in fresh air mode |
| Dust | Filtered air |
| Changeover time | Must reach duty quickly after a turbine trip so the burner can stay lit or relight |
| Isolation | Damper must seal against hot turbine exhaust in normal operation |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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. |
FD Fan selection for supplementary fired hRSGs
| Blade form | Backward curved, for stable flow across the burner turndown |
| Material of construction | Mild steel; heat shield on the discharge side near the hot duct |
| Drive and arrangement | Direct coupled, with a motor and starter able to accelerate fast to duty |
| Bearings and cooling | Standard pedestal bearings; no cooling needed for the fan, but the discharge damper must be tight against hot backflow |
| Flow control | Variable speed drive or inlet vanes, linked to burner firing rate in fresh air mode |
| Also specified | Inlet filter and silencer, tight isolation damper, and burner cooling air connection |
What this FD fan has to cope with
Hot gas backflow into the fan
In turbine mode the fan discharge sees turbine exhaust at 450 to 600°C behind the isolating damper. If the damper leaks and the fan is stopped, hot gas flows back into the fan and can distort the impeller. A tight damper, a seal air or cooling air bleed, and running the fan at low speed prevent it.
Fast takeover on a turbine trip
Steam users in refineries and chemical plants cannot wait for a slow restart. The fan, damper and burner must reach fresh air firing in a short, defined sequence. The fan motor, starting method and acceleration time are selected so that the air arrives within the time the burner management system allows.
Wide range of air demand
In fresh air mode the burner may fire from low to full rate, and the air must follow. A fan selected for full fresh air firing operates far down its curve at low fire. Speed control, or inlet vanes, keeps the fan stable and the air-fuel ratio correct across the range.
Energy and control
The fresh air fan may run for only a few hundred hours a year, during turbine outages and trips, but at that time it carries the whole steam supply. Its efficiency matters less than its readiness. Where it runs continuously at low speed to provide burner cooling air and keep the damper cool, a variable speed drive keeps that standby power small.
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 Induced Draft (ID) Fan for supplementary fired hRSGs.
Questions buyers ask
What is fresh air firing on an HRSG?
Fresh air firing lets an HRSG with a duct burner raise steam when the gas turbine is not running. A fresh air fan supplies ambient air to the burner in place of the turbine exhaust, and the HRSG works as a conventional fired boiler. It keeps process steam available during turbine outages.
Does a supplementary fired HRSG always need a fresh air fan?
No. If the plant can do without HRSG steam when the turbine is down, or has auxiliary boilers, the duct burner simply uses turbine exhaust oxygen and no fresh air fan is fitted. The fan is added where steam must continue through turbine trips and outages.
How fast must the fresh air fan start after a turbine trip?
Fast enough for the burner management system to keep or re-establish the flame within its allowed sequence, often within tens of seconds. The fan, motor, starter and damper are selected and tested together to meet the time set by the HRSG and burner supplier. The changeover is proven at commissioning.
Information needed to quote an FD fan for supplementary fired hRSGs
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Fresh air mode. Maximum firing rate in fresh air mode and the air flow and pressure the burner needs.
- Changeover. Required time from turbine trip to full fresh air flow, and whether the fan runs on standby.
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.