How fruit and tomato powder spray dryers work
Fruit, vegetable and tomato powders are dried from juice concentrates or pulps rich in sugars and organic acids. Sugars such as fructose have a low glass transition temperature, so the powder turns sticky in warm, moist air. Most plants add a carrier such as maltodextrin and dry with dehumidified air at moderate inlet temperatures of about 140 to 180°C and outlets of 60 to 80°C. Chambers often have cooled walls and cone, a cool, dry air sweep and dehumidified conveying air. Tomato paste is diluted from about 28 to 30 brix before drying. Exhaust air passes cyclones and a bag filter. Because the powder is acidic and hygroscopic, the exhaust side is hygienic stainless steel and kept dry.
Where the FD fan sits
- Air intake
- Filters
- Cooling coil or desiccant dehumidifier
- FD (supply) fan
- Air heaters
- Air disperser
- Drying chamber
The supply fan follows the filters and dehumidifier and feeds the heaters, so it handles cool, dry air. Dehumidification adds a chilled-water coil, a desiccant wheel or both, each with its own pressure drop, and the desiccant wheel needs a separate regeneration air fan. Separate fans supply cone cooling and conveying air, usually drawing from the same dehumidified air. Its pressure therefore runs higher than on a plain filter-and-heater supply.
Air at the FD fan
| Air temperature at the fan | 15 to 40°C after dehumidification |
| Air after the heaters | 140 to 180°C, downstream of the fan |
| Humidity | Dehumidified to a low dew point to limit powder stickiness |
| Dust | Filtered air |
| Pressure | Filters, dehumidifier, heaters and disperser |
| Density at the design temperature | About 1.15 kg/m³ at 35°C, 96% 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 fruit and tomato powder spray dryers
| Blade form | Backward curved or aerofoil |
| Material of construction | Stainless steel or food-grade coated mild steel |
| Drive and arrangement | Direct driven with variable speed drive |
| Bearings and cooling | Standard bearings outside the air stream |
| Flow control | Variable speed drive on measured flow |
| Also specified | Separate cone cooling and conveying air fans |
What this FD fan has to cope with
Dehumidifier pressure drop
A chilled coil and desiccant wheel can double the supply side pressure compared to a plain filter and heater. The fan duty includes the full train at its dirty condition, or the fan runs short of flow once the filters and coil foul.
Sticky powder when air drifts
Small rises in inlet air moisture or a fall in flow push the powder past its sticky point within minutes. The supply fan holds the flow constant, and the dehumidifier holds the dew point, so the outlet conditions stay inside the window the product allows.
Cooling air fan failure
If the cone cooling or conveying air fan stops, powder sticks to the cone and caramelises within minutes, and the dryer must be stopped and cleaned. Those fans are interlocked with the feed pump, and their flow is monitored.
Energy and control
Fruit powder dryers spend energy on dehumidifying and cooling air as well as heating it, and the chiller and desiccant regeneration can rival the heater. A variable speed drive on the supply fan and heat recovery from the exhaust reduce both. Dehumidifying only as far as the product needs saves chiller and regeneration energy.
ID fan and FD fan in fruit and tomato powder spray dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | After the dehumidifier, before the heaters |
| What it handles | Exhaust with sticky acidic fines | Dehumidified filtered air |
| Temperature at the fan | 60 to 80°C | 15 to 40°C |
| Main design concern | Syrup film and acid corrosion | Dry air and steady flow |
| Control | Chamber pressure | Drying air flow |
See also the Induced Draft (ID) Fan for fruit and tomato powder spray dryers.
Questions buyers ask
Why dehumidify the inlet air for fruit powder?
Dry inlet air lets the dryer reach a low powder moisture at a lower outlet temperature, which keeps the sugar-rich powder below its sticky point. It also keeps output steady in humid weather.
Is the supply fan before or after the dehumidifier?
Usually after the dehumidifier and before the heaters, so it handles cool, dry air at high density. The fan pressure must cover the filters and the dehumidifier coil or wheel on its suction side, as well as the heaters and disperser on its discharge.
How many fans does a fruit powder spray dryer have?
At least a supply fan and an exhaust fan, plus cone cooling, wall sweep and conveying fans, and a regeneration fan where a desiccant wheel is used. The supply and exhaust fans carry most of the power, but the smaller fans are just as important to keeping the powder from sticking.
Information needed to quote an FD fan for fruit and tomato powder spray 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.
- Dehumidification. Chiller coil, desiccant wheel or both, with pressure drops and target dew point.
- Auxiliary air. Cone cooling and conveying air flows required.
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.