How dehydrated onion and garlic dryers work
Onion and garlic are peeled, sliced and spread on multi-stage conveyor belt dryers, where hot air passes up or down through the bed as the product moves from stage to stage. Fresh onion is about 85 to 88% water and is dried to 4 to 6%. Air temperature starts at about 75 to 90°C in the first stage and is reduced to about 55 to 60°C in the final stages, because onion browns and loses pungency if heated too far at low moisture. Air is heated by steam or thermic fluid coils, or by an indirect fired air heater, so no combustion gas touches the product. Each stage has supply and exhaust fans, and a typical dryer has many of them.
Where the ID fan sits
- Drying stages
- Exhaust hood
- Exhaust duct
- Exhaust (ID) fan
- Stack
Exhaust fans sit on the roof or the side of each dryer stage, drawing from the space above or below the belt. In the wet first stage most of the air is exhausted; in later stages more of it is recirculated and only a small part is exhausted. Pressure is low, typically 30 to 100 mmWC, as the fan overcomes only the hood, the duct and the stack, but the flow is large on the wet end.
Gas at the ID fan
| Air temperature, normal | 45 to 70°C, highest at the dry end |
| Maximum or upset | About 90°C if the belt stops with the heaters on |
| Dust at the fan | Papery onion skins and dry fines, mainly from the later stages |
| Moisture and dew point | Very humid at the wet end, with condensation in cold ducts |
| Corrosive species | Organic sulphur compounds from onion and garlic; mildly corrosive condensate |
| Density at the design temperature | About 1.06 kg/m³ at 60°C, 88% 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 dehydrated onion and garlic dryers
| Blade form | Backward inclined or axial, sized for large flow at low pressure |
| Material of construction | SS 304 or galvanised steel; SS 304 at the wet end |
| Drive and arrangement | Direct coupled or belt driven, with a variable speed drive |
| Bearings and cooling | Bearings outside the air stream; no cooling needed |
| Flow control | Variable speed drive on stage humidity or exhaust temperature |
| Also specified | Inlet screen for skins, casing drain and cleaning access |
What this ID fan has to cope with
Skins and fines on the impeller
Light onion skins lifted from the belt collect on the inlet and the blades and reduce flow. In the dry stages fine onion powder, which contains sugars, absorbs moisture and sticks. An inlet screen, a plain blade and cleaning at each product changeover keep the fans clean.
Over-exhausting the dry end
Exhausting too much air from the final stages wastes heat, while exhausting too little lets humidity rise so the product will not reach its final moisture. Speed control per stage, on humidity or temperature, gives the balance.
Odour and corrosion
Onion and garlic release sulphur compounds that give a pungent odour and a mildly corrosive condensate. Stainless fans and ducts at the wet end, with drains, last much longer than painted mild steel.
Energy and control
Onion dehydration is seasonal and runs hard for months. Exhaust air carries most of the heat lost from the dryer, so exhausting only what each stage needs, with variable speed control on humidity, saves steam or fuel as well as fan power. Recirculating more air in the later stages, where the air leaves only partly loaded with moisture, cuts heat use further.
ID fan and FD fan in dehydrated onion and garlic dryers
| ID fan | FD fan | |
|---|---|---|
| Position | On the stage exhaust hood | On the stage side, before the heater coils |
| What it handles | Humid air with skins and sulphur odour | Fresh and recirculated air |
| Temperature at the fan | 45 to 70°C | Ambient to 60°C |
| Main design concern | Deposits, corrosion and humidity control | Gentle, even flow through the bed |
| Control | Stage humidity | Stage air flow |
See also the Forced Draft (FD) Fan for dehydrated onion and garlic dryers.
Questions buyers ask
How many exhaust fans does an onion dehydration belt dryer have?
Usually one exhaust fan per stage or per group of zones. A large multi-stage onion dryer can have six to twelve exhaust fans, with the largest at the wet end where most moisture is removed.
Why does the onion dryer exhaust fan need stainless steel?
The wet end exhaust is humid and carries sulphur compounds from the onion and garlic, which condense on cold surfaces. Stainless steel resists this condensate and is easy to clean, which matters in a food plant.
Can the exhaust fan speed affect onion colour?
Yes. The exhaust rate controls stage humidity and so the effective drying temperature at the product. Too little exhaust in the final stages keeps the product warm and moist for longer, which can lead to browning.
Information needed to quote an ID fan for dehydrated onion and garlic dryers
- 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.
- Dryer layout. Number of stages, belt width and the evaporation in each stage.
- Product. Onion, garlic or both, and the product forms such as flakes, kibbled or powder.
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.