How oil fired hot air generators work
An oil fired hot air generator burns light diesel oil or furnace oil in a pressure jet or rotary cup burner inside a refractory or water-jacketed combustion chamber. On direct fired units the flame gas is mixed with dilution air to the dryer inlet temperature, typically 150 to 450°C, and passes into the dryer for chemicals, minerals, detergent, fertiliser or foundry sand. On indirect units, used for pharmaceutical, food and heat-sensitive products, the gas heats clean air through a stainless or mild steel tube exchanger and leaves at about 200 to 300°C. Furnace oil carries 2 to 4% sulphur, so on direct units the dryer exhaust contains SO₂ and SO₃ as well as product dust and moisture.
Where the FD fan sits
- Inlet silencer
- FD (combustion air) fan
- Burner
- Combustion chamber
- Mixing chamber with dilution air
- Dryer
The combustion air fan feeds the burner wind box, from a monoblock burner or as a separate fan, at 150 to 300 mmWC. Where dilution air is blown rather than drawn, a second fan supplies ambient air to an annular jacket around the combustion chamber, cooling the shell before mixing with the flame gas. The dilution fan moves several times the combustion air volume at lower pressure.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Pressure at the fan outlet | 150 to 300 mmWC combustion air; 50 to 150 mmWC dilution air |
| Dust | Clean air; filter where the dryer handles sensitive products |
| Air ratio | Dilution air often three to ten times combustion air |
| Duty | Pre-purge, then modulating with dryer inlet temperature |
| 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 air generators
| Blade form | Backward curved for both combustion and dilution air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings; no cooling needed |
| Flow control | Combustion air linked to the burner; dilution air speed controlled on dryer inlet temperature |
| Also specified | Inlet silencer, air pressure switch and purge interlock |
What this FD fan has to cope with
Burner and dilution fan fighting
If combustion air and dilution air are both controlled on the dryer inlet temperature, the two loops interact and the temperature hunts up and down. A better scheme lets the burner modulate on heat demand while the dilution fan holds a fixed ratio or mixing chamber temperature, which gives a stable inlet temperature and a steady product moisture.
Chamber shell overheating
Where dilution air also cools the combustion chamber shell through an annular jacket, cutting it back at low load lets the shell overheat and the refractory or steel shell crack. The dilution fan is given a minimum speed to protect the chamber, and the burner turndown is set so that this minimum air does not overcool the dryer.
Soot from poor air control
An oil burner on worn linkage or with a dirty inlet runs short of air at some loads, and soot goes straight into the product on a direct fired dryer, discolouring it. Electronic air-fuel ratio control, an oxygen check after the mixing chamber and a clean inlet filter keep combustion complete across the firing range.
Energy and control
The combustion air fan is small; the dilution air fan can be several times larger. Excess dilution air means heating more air than the dryer needs, so speed control on the dilution fan saves both fuel and power. Reducing the dryer inlet temperature only where the product allows, and recirculating exhaust air where it is clean enough, cut the total air the fans must move.
ID fan and FD fan in oil fired hot air generators
| ID fan | FD fan | |
|---|---|---|
| Position | After the dryer bag filter | Ahead of the burner and mixing chamber |
| What it handles | Humid exhaust with SO₃ and product fines | Clean ambient air |
| Temperature at the fan | 80 to 130°C | Ambient |
| Main design concern | Acid condensation | Stable dryer inlet temperature |
| Control | Dryer pressure | Burner firing and dilution ratio |
See also the Induced Draft (ID) Fan for oil fired hot air generators.
Questions buyers ask
What fans does an oil fired hot air generator need?
A combustion air fan for the burner, usually part of the burner or a separate FD fan, and on many units a dilution air fan that blows ambient air into the mixing chamber. The dryer exhaust fan downstream pulls the whole system under suction. Indirect units also need a process air fan on the clean side of the exchanger.
Why use a separate dilution air fan on an oil fired hot air generator?
It gives positive control of the air mixed with the flame gas, holds a steady dryer inlet temperature and can cool the combustion chamber shell. Drawing dilution air through a damper by the exhaust fan is cheaper but less precise, and the mixing chamber pressure then changes with every change at the exhaust fan.
What pressure does the combustion air fan on an oil fired hot air generator need?
Typically 150 to 300 mmWC, depending on the burner head pressure and the combustion chamber pressure. The burner maker's data gives the required wind box pressure at full fire; the duct, any air preheater and the chamber back pressure are added. Rotary cup burners usually need less than pressure jet burners with high turbulence heads.
Information needed to quote an FD fan for oil fired hot air 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. Burner type, firing range and required wind box pressure.
- Dilution. Dilution air volume and whether it is blown by a fan or drawn in by the exhaust fan.
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.