How oil fired hot water generators work
An oil fired hot water generator burns light diesel oil, or less often furnace oil, in a forced draft burner firing into a fire tube or coil type pressure vessel. It supplies hot water at 80 to 95°C, or pressurised hot water up to about 120 to 140°C, for hotels, hospitals, laundries, food plants and process heating. The furnace is pressurised: the burner fan pushes the flame gas through the passes and up the chimney, and there is no ID fan in normal practice. Gas leaves at about 180 to 230°C. Because return water can be cool, the fire side can run near the acid dew point on furnace oil, which is mainly a concern for the generator rather than the fan.
Where the FD fan sits
- Inlet silencer
- FD (combustion air) fan
- Air damper
- Burner head
- Pressurised furnace and passes
- Chimney
On a monoblock burner the fan is in the burner housing on the generator front; on a dual-block burner it stands beside the generator and is ducted to the wind box. Because the furnace is pressurised, the fan must overcome the burner head, the furnace and tube passes and the chimney draught loss, typically 100 to 250 mmWC. Installations in hotels and hospitals often sit in basements, where air supply and noise matter.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C; warmer in enclosed plant rooms |
| Pressure at the fan outlet | Typically 100 to 250 mmWC |
| Dust | Clean air; lint in laundries |
| Turndown | Two-stage or modulating burners, typically 2:1 to 4:1 |
| Duty | Pre-purge, then firing with the hot water demand |
| 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 oil fired hot water generators
| Blade form | Backward curved, for a stable curve and good efficiency |
| Material of construction | Mild steel or aluminium in packaged burners |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings; no cooling needed |
| Flow control | Air damper linked to the fuel, or variable speed drive with oxygen trim |
| Also specified | Inlet silencer, air pressure switch and fresh air supply to the plant room |
What this FD fan has to cope with
Noise in occupied buildings
Burner fans in hotels, hospitals and apartment blocks are close to guest rooms, wards and offices. High speed fans produce tonal noise that carries through the structure. An inlet silencer, a fan selected near its best efficiency point and anti-vibration mounts reduce the noise at source.
Short cycling and purge losses
Hot water demand is intermittent, and a burner that cycles on and off many times an hour spends a large share of its time in pre-purge, blowing cold air through the generator. A modulating burner with a speed controlled fan runs longer at low fire, reducing purge losses and soot from repeated starts.
Starved air in basement plant rooms
Generators in basements and enclosed rooms often lack enough fresh air. The fan then draws from a room under suction and delivers less air than it should, and the burner smokes. A ducted fresh air supply to the fan inlet or the plant room fixes this.
Energy and control
An oil fired hot water generator often runs at low average load with peaks in the morning and evening. A variable speed combustion air fan with electronic air-fuel ratio control and oxygen trim cuts fan power and keeps excess air low across these swings. Reducing on-off cycling cuts purge losses, and in many installations the fuel saved is worth more than the electricity.
Questions buyers ask
Does an oil fired hot water generator need an ID fan?
Normally no. The furnace is pressurised and the burner fan pushes the flame gas through the generator and up the chimney. An ID fan is only considered when a long or restricted flue, a heat recovery unit or a shared chimney adds resistance the burner fan cannot overcome.
Why is the burner on my hot water generator so noisy?
Burner fans run at high speed, and in basement plant rooms their noise carries through the building. An inlet silencer, operating the fan near its best efficiency point and isolating the generator from the structure reduce noise. A separate, slower dual-block fan is an option on larger units.
What does the combustion air fan on a hot water generator need to overcome?
The burner head pressure, the furnace and tube passes of the pressurised generator, and any flue and chimney losses, typically 100 to 250 mmWC in total. The burner maker gives the head pressure, and the generator maker gives the furnace back pressure. A long horizontal flue or a heat recovery unit adds to it and must be included.
Information needed to quote an FD fan for oil fired hot water generators
- 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. Monoblock or dual-block, firing range and furnace back pressure.
- Installation. Plant room location, fresh air supply and any noise limit at the nearest occupied area.
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.