How gas turbine heat recovery steam generators work
A gas turbine heat recovery steam generator takes the turbine exhaust, typically 450 to 600°C with 13 to 15% oxygen, and passes it through superheater, evaporator and economiser sections, often with a condensate preheater, before the stack at about 90 to 150°C. The gas turbine discharges at a small positive pressure, and the HRSG, ducts and stack must stay within the turbine's back pressure allowance, typically 250 to 400 mmWC, beyond which turbine output falls. A diverter damper and bypass stack let the turbine run in simple cycle. An unfired HRSG has no combustion of its own, so it has no FD fan in the usual sense; the fans around it are seal air fans for the dampers, and ID or booster fans only in special cases.
Where the ID fan sits
- Gas turbine exhaust
- Diverter damper
- Superheater and evaporator
- Economiser
- Condensate preheater
- ID (booster) fan
- Stack
Where an ID fan is used, it sits at the cold end after the economiser or condensate preheater, handling clean exhaust at about 100 to 180°C. It takes away the resistance of added surfaces or SCR catalyst so the turbine exhaust flange stays within its back pressure limit. Control is from turbine exhaust pressure. Because the gas volume is very large and the pressure rise small, the fan is large in diameter and low in head.
Gas at the ID fan
| Gas temperature, normal | 100 to 180°C after the economiser |
| Maximum or upset | Up to turbine exhaust temperature, 450 to 600°C, if the HRSG is bypassed through the fan route |
| Dust at the fan | Negligible on natural gas; light soot on liquid fuel firing |
| Moisture and dew point | About 7 to 10% water vapour, water dew point near 45°C; acid dew point only on sulphur-bearing liquid fuel |
| Pressure rise | Small, typically a fraction of the turbine back pressure allowance |
| Density at the design temperature | About 0.84 kg/m³ at 150°C, 70% 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 gas turbine heat recovery steam generators
| Blade form | Backward curved aerofoil, large diameter and low head for efficiency on clean gas |
| Material of construction | Mild steel for normal duty; heat-resisting steel if the fan can see bypassed exhaust |
| Drive and arrangement | Direct coupled, double inlet where the volume is very large |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on turbine exhaust pressure |
| Also specified | Interlock with the diverter damper so a fan trip sends exhaust to the bypass stack immediately |
What this ID fan has to cope with
Every millimetre of back pressure costs power
Gas turbine output and efficiency fall as exhaust back pressure rises. The first design choice is an HRSG with low gas-side resistance. A booster fan is justified only where that is not enough, and the fan power then has to be weighed against the turbine output recovered.
Turbine trip and fast transients
On a turbine trip the exhaust flow disappears in seconds, and on a start it ramps quickly. A booster fan must follow these transients without pulling the turbine exhaust into suction. Speed control with a fast pressure loop and an interlocked diverter damper protect both machines.
Very large volume at low head
Turbine exhaust volumes are large for the power involved. The fan is large in diameter, which brings attention to rotor stiffness, balancing and the first critical speed. Balancing to ISO 21940-11 and a stiff pedestal design keep vibration low. Double inlet designs reduce the diameter for the same volume, at the cost of a more complex inlet box and ductwork.
Energy and control
Gas turbine plants typically run at base load for long periods with some part-load operation at night or in hot weather. Where a booster fan exists, speed control from exhaust pressure keeps its power to what is needed. Because the gas is clean and the duty steady, an aerofoil wheel selected near peak efficiency is worth the cost.
Questions buyers ask
Does a gas turbine HRSG need an ID fan?
Usually no. The gas turbine discharges its exhaust at enough pressure to push it through a well-designed HRSG and stack. An ID or booster fan is only used where the HRSG, catalyst and ducts together exceed the turbine's back pressure allowance, typically on retrofits. New HRSGs are designed to avoid the need.
What happens to gas turbine output if HRSG back pressure is too high?
Higher back pressure reduces the expansion through the turbine, so output and efficiency fall and exhaust temperature rises. The turbine maker gives the allowable value. The HRSG designer keeps within it; a booster fan is a last resort. Any added surface or catalyst should be checked against that limit before installation.
Where is the booster fan placed on an HRSG?
At the cold end, after the economiser or condensate preheater, where the gas is coolest and densest. This keeps the fan small for the duty and in mild steel construction. It also keeps the fan away from the hot inlet ducts and the diverter damper. Control is from the pressure at the turbine exhaust flange, so the fan offloads only the excess resistance.
Information needed to quote an ID fan for gas turbine heat recovery steam generators
- 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.
- Turbine data. Turbine model, exhaust flow and temperature across the load range, and the back pressure allowance.
- HRSG resistance. Gas-side pressure drop of the HRSG, SCR and stack, and the shortfall the fan must make up.
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.