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 FD fan sits
- Fresh air intake
- Recirculation damper
- FD (supply) fan
- Steam or thermic fluid coils
- Plenum
- Product bed on belt
Each stage has one or more fans mounted on the side of the dryer, drawing fresh air and recirculated air and pushing it through the heater coils into a plenum under or above the belt. The pressure covers the coils, the plenum, the belt and the bed, typically 50 to 150 mmWC. Air velocity through the bed is limited so that light slices are not blown about.
Air at the FD fan
| Air temperature at the fan | Ambient to 60°C, depending on how much air is recirculated |
| Air after the coils | 55 to 90°C, highest at the wet end |
| Dust | Fresh air with some skins and fines in the recirculated air |
| Humidity | Seasonal ambient humidity changes drying rate |
| Pressure at the fan outlet | Typically 50 to 150 mmWC |
| Density at the design temperature | About 1.11 kg/m³ at 45°C, 93% 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 dehydrated onion and garlic dryers
| Blade form | Backward curved or backward inclined, for steady flow |
| Material of construction | Galvanised or SS 304, to suit food plant hygiene |
| Drive and arrangement | Direct coupled or belt driven, with a variable speed drive |
| Bearings and cooling | Standard bearings outside the air stream |
| Flow control | Variable speed drive per stage |
| Also specified | Inlet filter on fresh air and access to the coil face |
What this FD fan has to cope with
Skins blinding the coils
Onion skins and fines in the recirculated air collect on the face of the heater coils, raising resistance and cutting heat transfer. They can also char on hot coils. Filters on the recirculation path and regular coil cleaning protect the heaters and keep the fan on its duty.
Lifting the product
Too high a velocity through the bed lifts light onion slices and blows them about, giving uneven drying. The supply fan is sized so the bed velocity stays within the product's limit, with speed control to set each stage.
Uneven air across the belt
A poorly designed plenum or a fan throttled far from its best point gives uneven flow across the belt width, with wet edges and over-dried centres. Correct fan selection and plenum baffles fix this.
Energy and control
Supply fans run for the whole season on a large dryer, so efficiency matters. Variable speed drives on each stage let operators match air flow to the product moisture at each point, and more recirculation in later stages saves heat. High-efficiency wheels and correctly sized motors give the lowest running cost.
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 Induced Draft (ID) Fan for dehydrated onion and garlic dryers.
Questions buyers ask
Is the supply fan in an onion dryer an FD fan?
It does the job of a forced draft fan for each stage, pushing air through the heaters and the product. It usually handles a mix of fresh and recirculated air, so its inlet can be warmer than ambient.
How is the air heated in an onion dehydration plant?
Most plants use steam or thermic fluid coils, or an indirect fired air heater. Direct firing is avoided because combustion products would contaminate the onion and garlic.
Why is air speed limited in an onion belt dryer?
Onion slices and flakes are light, and too high an air velocity lifts them off the belt and causes uneven drying. The supply fan is selected so that the bed velocity suits the product.
Information needed to quote an FD fan for dehydrated onion and garlic dryers
- 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.
- Stage data. Air flow and temperature for each stage, and the share of recirculated air.
- Heater. Coil type and air side pressure drop, clean and fouled.
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.