How natural gas fired package boilers work
Natural gas package boilers, shell or compact water tube, of 1 to 30 TPH serve chemical, pharmaceutical and food plants wherever piped gas is available. Gas arrives from the city gas network, is reduced to burner pressure, and burns with about 9.5 to 10 m³ of air per m³ of gas before excess air. There is no ash and almost no sulphur, but the flue gas carries about 18% water vapour, with a water dew point near 55 to 60°C. The boiler fires under positive pressure from the burner fan, with an economiser and sometimes a condensing heat recovery unit before the stack. Modern burners use low NOx heads and modulate over 5:1 to 10:1.
Where the FD fan sits
- Boiler house air
- FD fan
- Burner head
- Furnace
- Boiler passes
- Economiser
- Stack
The fan is mounted on the burner on smaller units, or on a separate base with ducting to the burner on larger ones. It takes air from the boiler house, sometimes through a ducted intake from outside. Its pressure covers the burner head, which on low NOx burners can be a large part of the total, the furnace, the tube passes, the economiser and any condensing unit. On a 10 TPH boiler with economiser the total is typically 150 to 300 mmWC.
Air at the FD fan
| Air temperature at the fan | Boiler house air, 25 to 45°C |
| Pressure at the fan outlet | Typically 150 to 300 mmWC, higher with condensing heat recovery |
| Turndown | 5:1 to 10:1 on modulating gas burners |
| Dust | Clean air; process dust in food plants if the intake is not ducted |
| Excess air | About 2 to 3% O₂ with O₂ trim, 4 to 5% on fixed linkage |
| 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 package boilers
| Blade form | Backward curved or aerofoil, for efficiency and a stable curve to low flow |
| Material of construction | Mild steel or aluminium |
| Drive and arrangement | Direct mounted on an inverter-rated motor |
| Bearings and cooling | Motor bearings |
| Flow control | VFD with servo air damper under the burner management system and O₂ trim |
| Also specified | Inlet silencer, air pressure switch, ducted outside intake where boiler house air is hot or dusty |
What this FD fan has to cope with
Low NOx head pressure
Low NOx gas burners stage the gas and air or recirculate gas inside the head, and need higher air pressure at the head than older burners. When a boiler is upgraded to a low NOx burner, the existing fan often lacks pressure at full fire. The new fan is sized on the burner's pressure demand plus the boiler and economiser resistance.
Instability at low fire
At 10 to 20% load the fan runs far down its curve. A fan with a dip in its pressure curve can hunt, and the flame lifts or pulses. A backward curved fan with a smooth curve, sized for the real maximum rather than with a large margin, and VFD control instead of heavy damper throttling keep low fire steady.
Hot boiler house air
Boiler houses in summer can reach 45 to 50°C, making the air about 5 to 10% lighter than at a 25 to 30°C design. The fan delivers less mass flow, excess air falls at full fire and CO appears. Selecting the fan at the maximum intake temperature, or ducting cooler air from outside, avoids derating the boiler on hot days.
Energy and control
Gas package boilers in food and pharmaceutical plants follow process demand that can swing sharply between shifts and batches. A burner fan on VFD saves most in the long low-fire hours, where damper control wastes power. Fan energy is small next to fuel, so the larger gain is O₂ trim holding excess air near 2 to 3% O₂, and avoiding burner cycling, since every start needs a full-air purge.
Questions buyers ask
What size fan does a natural gas package boiler burner need?
A burner needs roughly 10 m³ of air per m³ of natural gas plus excess air, at a pressure covering the burner head, furnace, tube passes and economiser. For a 10 TPH boiler this is typically around 9,000 to 11,000 m³/h at 150 to 300 mmWC. We size from the burner and boiler data.
Is a VFD worth it on a gas boiler burner fan?
Usually yes for boilers that run long hours at part load. The VFD cuts fan power at low fire, lowers noise, and with O₂ trim gives tighter excess air control. On small boilers that run mostly at full fire the saving is small, and a good servo damper with O₂ trim may be enough.
Why does a natural gas package boiler need a burner fan if it has no ID fan?
The burner fan supplies all the combustion air and also pressurises the furnace, so the gas flows through the boiler and economiser and out of the stack. Package boilers are designed to run under positive pressure with sealed casings, so no ID fan is needed in normal practice.
Information needed to quote an FD fan for natural gas fired package 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.
- Burner type. Conventional or low NOx, with the head pressure at full fire.
- Heat recovery. Economiser or condensing unit after the boiler, and its gas-side resistance.
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.