How natural gas fired water tube boilers work
Natural gas fired water tube boilers, usually D-type or O-type package units or field-erected units of 30 to 200 TPH, supply high pressure steam in fertiliser plants, refineries and gas processing. Gas burns in a membrane-wall furnace, and the gas passes the superheater and convection bank, then an economiser and, on some units, an air heater or SCR. Clean gas allows closely spaced tubes and high efficiency. Many of these boilers fire under positive pressure with FD fans only. Balanced draft with an ID fan is used where the boiler was converted from oil or coal, where the casing is not fully gas-tight, or where heat recovery and NOx control add resistance after the boiler.
Where the FD fan sits
- Air intake and silencer
- FD fan
- Air heater, where fitted
- Windbox
- Low NOx burners
- Furnace
One or two FD fans stand at ground level and discharge into the windbox, directly or through an air heater. On pressurised firing the fan must overcome the windbox and burners plus the entire gas path, giving pressures of 400 to 700 mmWC on large units. On balanced draft units the ID fan takes the gas side and the FD fan pressure is much lower. Many plants run two FD fans at about 60% each so the boiler can continue at reduced load on one.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Pressure at the fan outlet | Typically 400 to 700 mmWC pressurised, 150 to 300 mmWC balanced draft |
| Air after the air heater | 200 to 300°C where fitted, downstream of the fan |
| Dust | Clean air; urea dust near prilling towers in fertiliser plants |
| Recirculated gas | Some low NOx designs mix flue gas into the FD fan inlet |
| 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 natural gas fired water tube boilers
| Blade form | Aerofoil backward curved, double inlet on large units |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled motor, medium voltage on large units |
| Bearings and cooling | Pedestal bearings, oil lubricated on large fans |
| Flow control | VFD or inlet guide vanes on air flow, cross-limited with fuel |
| Also specified | Inlet silencer and screen, fast outlet isolation damper, low air flow trip |
What this FD fan has to cope with
One fan tripping in two-fan operation
If one FD fan trips, the other must take over fast enough to avoid a fuel trip, and the tripped fan's outlet damper must close to stop air flowing backwards through it. Fan curves checked for single-fan operation, fast-closing dampers and runback logic in the control keep the boiler on line at reduced load.
Urea and process dust at the intake
In fertiliser plants urea dust and ammonia-laden air can reach the FD fan intake. Urea deposits on the blades absorb moisture, corrode the steel and unbalance the impeller. Siting the intake away from prilling towers and bagging plants, an inlet screen and periodic washing protect the fan.
Noise at high pressure
Pressurised FD fans running at 500 mmWC and above generate high noise at the intake and from the casing. Intake silencers sized for the blade-pass frequency, acoustic lagging on the casing and selections at lower tip speed bring noise down to plant limits without a large pressure penalty.
Energy and control
With continuous high-load running, FD fan efficiency matters as much as ID fan efficiency, and on pressurised units the FD fan is the larger consumer. An aerofoil fan selected for its best efficiency at normal load, rather than at the design margin point, plus VFD control for the remaining range, keeps power low. Air heater and windbox leaks are worth checking before any fan change.
ID fan and FD fan in natural gas fired water tube boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the economiser or air heater, before the stack | At the windbox, ahead of the burners |
| When used | Balanced draft units only | Every unit |
| What it handles | Clean, moist flue gas | Ambient air |
| Temperature at the fan | 120 to 150°C | Ambient, 20 to 45°C |
| Main design concern | Efficiency and condensation | Pressure and two-fan operation |
| Control | Furnace pressure | Air flow in ratio with fuel |
See also the Induced Draft (ID) Fan for natural gas fired water tube boilers.
Questions buyers ask
What pressure does an FD fan need on a pressurised gas fired water tube boiler?
It must cover the burners, windbox and the full gas path to the stack, typically 400 to 700 mmWC on large units. On a balanced draft boiler it covers only the air side, usually 150 to 300 mmWC. The burner and boiler resistance data set the exact figure, with a margin agreed with the plant.
Why do fertiliser plant boilers have two FD fans?
Two fans at about 60% each let the boiler keep running at reduced load if one trips, which protects steam supply to the ammonia and urea plants. The trade-off is the need for careful parallel operation, tight isolation dampers and runback logic in the combustion control.
Can an FD fan handle flue gas recirculation on a gas boiler?
Yes, many low NOx designs draw recirculated flue gas into the FD fan inlet. The fan then handles a warm, moist mixture and must be sized for the extra flow and lower density, with drains and corrosion protection at the inlet and a motor rated for cold air at start-up.
Information needed to quote an FD fan for natural gas fired water tube boilers
- 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.
- Firing mode. Pressurised or balanced draft, and the number of fans with their duty split.
- Intake location. Where the intake is and what process dust or vapour is near it.
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.