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 ID fan sits
- Furnace
- Superheater
- Convection bank
- Economiser
- Air heater or SCR
- ID fan
- Stack
The ID fan, one or two per boiler, sits at ground level after the economiser or air heater and discharges to the stack. There is no collector. Suction covers furnace draft, the superheater and convection bank, the economiser and any SCR or air heater, typically 150 to 300 mmWC on a large unit. Placing the fan after all heat recovery gives the smallest gas volume, but gas temperature can be as low as 110 to 130°C, which makes casing insulation and condensate drainage necessary.
Gas at the ID fan
| Gas temperature, normal | 120 to 150°C after the economiser |
| Maximum or upset | 200 to 250°C with the economiser bypassed |
| Dust at the fan | None; occasional rust scale from ducts |
| Moisture and dew point | About 17 to 19% water vapour; water dew point 55 to 60°C |
| Corrosive species | Negligible sulphur; small ammonia slip where SCR is fitted |
| Density at the design temperature | About 0.86 kg/m³ at 140°C, 72% 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 natural gas fired water tube boilers
| Blade form | Aerofoil backward curved, for peak efficiency on clean gas |
| Material of construction | Mild steel casing and impeller; stainless steel drains |
| Drive and arrangement | Direct coupled, arrangement 8, double inlet on large units |
| Bearings and cooling | Pedestal bearings with cooling disc, oil lubricated on large fans |
| Flow control | VFD on furnace pressure, coordinated with the FD fans |
| Also specified | Insulated casing, casing drains, outlet isolation damper for two-fan operation |
What this ID fan has to cope with
Condensation when banked or idle
Gas firing produces a lot of water vapour. When the boiler is banked or cold, moisture condenses in the casing and on the impeller, and rust forms on surfaces that would otherwise stay clean for years. Insulation, drains at the casing low point and a closed outlet damper during shutdowns prevent this slow corrosion.
Oversized fans after conversion
Many gas fired water tube boilers were designed for oil or coal. The original ID fan, sized for a larger gas volume and a dust collector, runs at low flow, sometimes near stall and with poor efficiency. A new aerofoil impeller in the existing casing, sized for the actual gas flow, is a common efficiency retrofit.
Draft swings on fast load changes
In fertiliser and refinery service, steam demand can change quickly, for example when a large compressor trips. The ID fan must follow furnace pressure without overshoot, or the furnace trips on high or low pressure. A VFD with feedforward from the FD fan demand responds faster and more smoothly than inlet vanes.
Energy and control
Gas fired water tube boilers often run continuously at 70 to 100% load for years between overhauls. Over these hours a few points of fan efficiency are worth a great deal: an aerofoil impeller can run several points above a plain backward curved wheel and well above a radial one. A VFD adds the saving at part load and during start-up, and the clean gas means the efficiency is kept over the life of the fan.
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 Forced Draft (FD) Fan for natural gas fired water tube boilers.
Questions buyers ask
Do natural gas fired water tube boilers need an ID fan?
Many do not; they fire under pressure with FD fans alone. An ID fan is used on balanced draft designs, on boilers converted from oil or coal, and where SCR, an air heater or condensing heat recovery adds significant resistance after the boiler that the FD fan should not carry.
What material is needed for a natural gas boiler ID fan?
Mild steel is usually enough because the gas carries no ash and almost no sulphur. The real risk is water condensation at low gas temperature or during shutdowns, so casing insulation, drains and dry lay-up matter more than the material. Stainless drains and outlet sections are a sensible addition.
How much energy can an aerofoil ID fan save on a gas fired boiler?
Compared with a radial or worn backward curved fan, an aerofoil impeller can raise fan efficiency by several points, and more if the old fan is oversized and throttled. On a boiler running most of the year the saving is usually large enough to justify a new impeller on the existing casing.
Information needed to quote an ID fan for natural gas fired water tube boilers
- 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.
- Heat recovery train. Economiser, air heater, SCR or condensing unit, with the gas-side resistance of each.
- Operating hours. Hours per year and the typical load range.
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.