How LPG fired boilers work
LPG fired boilers are small shell or coil-type package units, typically 0.5 to 5 TPH, used where piped natural gas is not available, mainly in food processing, bakeries, dairies and hotels. LPG, a mixture of propane and butane, is stored as liquid and vaporised before the burner, either naturally from cylinders or a bullet, or in a vaporiser for larger draws. It has a calorific value of about 11,000 to 11,800 kcal/kg and needs about 24 to 31 m³ of air per m³ of vapour, depending on the propane to butane ratio. Combustion is clean and practically sulphur-free. The burner fan pressurises the boiler, and there is no ID fan in normal practice.
Where the FD fan sits
- Raised air intake
- FD fan
- Burner head
- Furnace
- Boiler passes
- Chimney
The fan is usually part of a monoblock gas burner on the boiler front, drawing air from the boiler room. Because leaked LPG vapour sinks and spreads along the floor, the fan inlet should not draw from low points, pits or trenches. Fan pressure covers the burner head, the furnace and the tube passes, typically 50 to 150 mmWC on small boilers. Before each start the fan purges the furnace and passes with several volume changes of air to clear any vapour.
Air at the FD fan
| Air temperature at the fan | Boiler room air, 25 to 45°C |
| Pressure at the fan outlet | Typically 50 to 150 mmWC |
| Air requirement | About 24 to 31 m³ of air per m³ of LPG vapour, plus excess air |
| Dust | Clean; flour and food dust in bakeries |
| Turndown | About 3:1 to 6:1, limited by vaporiser and burner |
| 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 LPG fired boilers
| Blade form | Backward curved, or forward curved on small monoblock burners |
| Material of construction | Aluminium or mild steel |
| Drive and arrangement | Direct mounted on the burner motor |
| Bearings and cooling | Motor bearings |
| Flow control | Servo or two-stage air damper; VFD on modulating burners |
| Also specified | Air pressure switch, purge timer, inlet screen, intake raised above floor level and drains |
What this FD fan has to cope with
Purge adequacy
LPG that leaks into a cold furnace settles in the lowest pass and is hard to clear. The fan must deliver its rated purge flow with the dampers fully open for long enough to give the required number of volume changes. An air proving switch set against the actual purge pressure confirms this at every start rather than assuming it.
Changing propane and butane mix
Supplied LPG varies between propane-rich and butane-rich, and butane needs about 30% more air per cubic metre than propane. A fixed air setting then runs lean on one gas and rich on the other. O₂ trim, or setting excess air with margin for the richer gas, avoids CO and soot on the heating surfaces.
Flour and fine dust on the impeller
In bakeries and snack plants, flour dust settles on the impeller and air damper, cutting air flow and unbalancing the fan. A screened intake from a cleaner area and regular cleaning keep the air supply steady, and a clean damper keeps the air setting where it was left at commissioning.
Energy and control
LPG is an expensive fuel, so fan energy matters far less than excess air and cycling. Small LPG boilers with on-off or two-stage burners cycle often at low demand and lose heat in every purge. A modulating burner with a VFD fan and O₂ trim keeps the boiler firing steadily and holds excess air close to 2 to 3% O₂, cutting fuel use more than any change to the fan itself.
Questions buyers ask
Is the burner fan different on an LPG boiler than on a natural gas boiler?
The fan itself is similar, but LPG needs roughly two and a half to three times more air per cubic metre of gas than natural gas, and the purge and intake location need more care because LPG vapour is heavier than air. The burner head and gas train are specific to LPG.
Why does my LPG boiler burner trip on air pressure?
The air pressure switch trips if the fan cannot hold the set pressure, usually because of a blocked inlet or dirty impeller, a weak motor, or a switch set too close to the actual low-fire pressure. Cleaning the fan and checking the pressure at low fire against the switch setting normally solves it.
Does an LPG fired boiler need an ID fan?
No, not in normal practice. The burner fan pressurises the furnace and pushes the gas to the chimney. An ID fan is considered only if there is significant resistance after the boiler, such as heat recovery equipment, or the flue cannot discharge the gas on its own draft.
Information needed to quote an FD fan for LPG fired 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.
- LPG supply. Vaporiser or direct draw, and the propane to butane ratio if known.
- Boiler room layout. Where the fan can take its air and whether pits or trenches are near the burner.
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.