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 ID fan sits
- Oil burner and chamber
- Mixing chamber
- Dryer
- Cyclone
- Bag filter
- ID (exhaust) fan
- Stack
The fan sits after the bag filter, which is the usual product collector on chemical and mineral dryers, and pulls the whole system under suction. It handles exhaust at 80 to 130°C, carrying the evaporated water and the combustion products of the oil. Suction includes the dryer, cyclone, bag filter and ducts, typically 150 to 300 mmWC. On indirect units a separate small flue gas fan, where fitted, handles the burner side.
Gas at the ID fan
| Gas temperature, normal | 80 to 130°C after the bag filter |
| Maximum or upset | Up to 250°C if dryer feed stops |
| Dust at the fan | Product fines after the bag filter, usually low |
| Moisture and dew point | Water dew point 55 to 70°C; acid dew point with furnace oil can be above 120°C |
| Corrosive species | SO₃ and sulphuric acid on furnace oil; chlorides or acids from some chemical products |
| Density at the design temperature | About 0.93 kg/m³ at 110°C, 77% 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 oil fired hot air generators
| Blade form | Backward curved, on filtered exhaust |
| Material of construction | SS 304 or SS 316 for furnace oil firing or corrosive products; mild steel with coating on LDO and benign products |
| Drive and arrangement | Direct coupled or belt driven |
| Bearings and cooling | Pedestal bearings outside the gas stream with a shaft seal |
| Flow control | Variable speed drive on dryer pressure |
| Also specified | Insulated casing, casing drain and inspection door |
What this ID fan has to cope with
Acid condensation at the fan
On furnace oil the exhaust may already be below its acid dew point at normal operation, and certainly at start-up and shutdown. Sulphuric acid forms on the casing, impeller and bag filter. Stainless construction, insulation and running the burner on LDO during start-up and shutdown reduce the attack.
Product dust and acid together
Chemical and fertiliser dusts that pass the bag filter can be hygroscopic. Mixed with acid condensate they form a corrosive paste that sticks to the blades and grows unevenly. A well-maintained filter, a stainless wheel with smooth surfaces and washing at each stop prevent hard deposits, corrosion under the deposit and unbalance.
Hot exhaust on feed loss
If the dryer feed stops while the burner fires, the exhaust temperature rises quickly towards the dryer inlet temperature. Bag filter media and fan bearings are at risk. A high temperature trip that cuts the burner to low fire or opens a cold air bleed protects both.
Energy and control
Oil fired dryers usually run continuously through a production campaign, at a load set by product throughput. A variable speed exhaust fan holding dryer pressure saves energy whenever throughput is below design. Lowering the exhaust temperature saves oil, but on furnace oil it cannot go below the acid dew point safely; switching to a low-sulphur fuel removes this limit and lets the dryer, and the fan, run cooler.
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 Forced Draft (FD) Fan for oil fired hot air generators.
Questions buyers ask
Why is the exhaust fan on my furnace oil fired dryer corroding?
Furnace oil produces SO₃, which condenses as sulphuric acid wherever the humid exhaust falls below its acid dew point, often above 120°C. Dryer exhausts usually run below that, so the casing and impeller see acid every day, and more at start-up. Stainless construction, casing insulation and a drain are the normal response.
Is a direct oil fired hot air generator suitable for food or pharmaceutical drying?
Usually not with furnace oil. Combustion products, soot and sulphur reach the product directly. Food and pharmaceutical dryers use indirect generators, where the oil flame heats clean air through a heat exchanger, or direct gas firing. The fan arrangement differs accordingly, with a separate flue gas fan on indirect units.
What material should the ID fan be on an LDO fired hot air generator?
LDO is low in sulphur, so acid condensation is far less severe than with furnace oil. Mild steel with a suitable coating is often adequate if the product is not corrosive. Where the product is acidic or hygroscopic, stainless steel is still the safer choice.
Information needed to quote an ID fan for oil fired hot air generators
- 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.
- Fuel oil. LDO, furnace oil or LSHS, and the sulphur content.
- Product. Product being dried and whether it is corrosive, hygroscopic or food grade.
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.