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 ID fan sits
- Drying chamber with cooled cone
- Cyclones
- Bag filter
- Exhaust (ID) fan
- Stack
The fan follows the bag filter. Besides the drying air, it removes the cone cooling, wall sweep and conveying air, which together can add a significant share to the total volume. The suction covers the chamber outlet, cyclones and filter, typically 250 to 400 mmWC. The chamber is held at a slight suction to contain the powder, and the fan casing is insulated so the exhaust does not cool on its way through.
Gas at the ID fan
| Air temperature, normal | 60 to 80°C at the fan inlet |
| Maximum or upset | Up to about 120°C if feed stops |
| Dust at the fan | Sticky, hygroscopic sugar and acid fines; low after the bag filter |
| Moisture and dew point | Moderate, as inlet air is dehumidified; condensate is acidic |
| Corrosive species | Citric, malic and other fruit acids; tomato condensate at pH about 4 |
| Density at the design temperature | About 1.01 kg/m³ at 75°C, 85% 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 fruit and tomato powder spray dryers
| Blade form | Backward curved, smooth, steep blade angle that sheds deposits |
| Material of construction | SS 316 for acidic fruit and tomato fines |
| Drive and arrangement | Direct or belt driven, variable speed |
| Bearings and cooling | Bearings outside the air stream |
| Flow control | Variable speed drive on chamber pressure |
| Also specified | Hot water wash nozzles, drain, insulated casing and inspection door |
What this ID fan has to cope with
Syrup film on blades
Fruit fines that meet warm, moist surfaces pass their sticky point and melt into a syrup film that traps more powder. Keeping the fan casing insulated and warm, the exhaust dry, and washing with hot water at each stop prevents build-up.
Acid corrosion
Fruit acids such as citric and malic acid dissolve in the condensate that forms at shutdown, and attack mild steel and, over time, SS 304 welds. SS 316 wetted parts, a drain at the lowest point and a rinse after each campaign avoid pitting.
Cooling air volume underestimated
Cone cooling, wall sweep and conveying air all reach the exhaust fan. If they are left out of the duty, the fan cannot hold chamber suction and powder leaks. The exhaust duty is built up from every air stream.
Energy and control
Fruit dryers spend energy on dehumidifying and cooling air as well as heating it. The exhaust fan on a variable speed drive follows the total air. Raising carrier ratio or feed solids reduces stickiness and air volume, but changes the product, so the balance is set by the product specification.
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 Forced Draft (FD) Fan for fruit and tomato powder spray dryers.
Questions buyers ask
Why is fruit powder hard to spray dry?
Fruit sugars have a low glass transition temperature, so the powder becomes sticky in warm, humid air. Plants use carriers such as maltodextrin, dehumidified inlet air, cooled walls and low outlet temperatures to keep the powder below its sticky point.
What material should a tomato powder dryer exhaust fan be?
SS 316 for wetted parts. Tomato and fruit fines are acidic, and the condensate that forms at shutdown attacks mild steel and can pit SS 304 welds. A drain at the lowest point and insulation of the casing reduce the condensate further.
Does the exhaust fan handle the cone cooling air too?
Yes. All air entering the chamber, including cone cooling, wall sweep and conveying air, leaves through the exhaust fan, so its duty is the sum of every stream plus the evaporated water. Leaving any stream out leaves the fan unable to hold chamber suction.
Information needed to quote an ID fan for fruit and tomato powder spray 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.
- Air streams. Flows of drying, cone cooling, wall sweep and conveying air entering the chamber.
- Product. Fruit or vegetable, carrier ratio and the acidity of the feed.
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.