How guar gum flash dryers work
Guar splits, the endosperm separated from the husk and germ of the guar seed, are soaked to about 50 to 60% moisture, flaked, and then ground and dried at the same time in a mill-dryer. Hot air at typically 150 to 250°C, usually from a thermic fluid or indirect fired air heater, sweeps through the mill and lifts the ground powder up a flash duct, drying it to about 10 to 12% moisture. Cyclones and a bag filter collect the powder, which is then sieved and blended to the required mesh and viscosity grade. An exhaust fan after the bag filter pulls the air through the mill and collectors, and on larger lines a supply fan pushes air through the heater.
Where the ID fan sits
- Air heater
- Flaker and mill-dryer
- Flash duct
- Cyclones
- Bag filter
- Exhaust (ID) fan
- Stack
The fan sits after the bag filter, so it handles warm humid air with only the finest powder that passes the bags. It pulls air through the heater, the mill and the collectors, and because the mill itself has significant air resistance, the total suction is typically 450 to 700 mmWC. The suction also prevents powder escaping at the mill seals.
Gas at the ID fan
| Air temperature, normal | 60 to 85°C at the fan inlet |
| Maximum or upset | Above 130°C if the split feed stops |
| Dust at the fan | Very fine guar powder, low after a bag filter |
| Moisture and dew point | Humid air with a dew point about 45 to 55°C |
| Fire and explosion | Guar powder is a combustible organic dust |
| 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 guar gum flash dryers
| Blade form | Backward curved with smooth blades for high pressure and efficiency |
| Material of construction | SS 304 for food and pharma grades, mild steel with food-grade coating for industrial grades |
| Drive and arrangement | Direct coupled with variable speed drive |
| Bearings and cooling | Bearings outside the air stream with a shaft seal |
| Flow control | Variable speed drive on mill suction or outlet temperature |
| Also specified | Earthing, casing drain, inspection door and inclusion in the explosion protection design |
What this ID fan has to cope with
Gel formation
Guar powder absorbs water rapidly and becomes a sticky gel. Fines in the casing that meet condensate form a rubbery layer that is hard to remove and unbalances the rotor. Holding the outlet temperature above dew point, insulation and dry purging at shutdown prevent it.
High system pressure
The mill and flash duct give much higher resistance than a simple flash dryer. A fan selected without the mill loss struggles to lift the powder and drying suffers. Measured site suction or mill supplier data is used for the duty. A fan with a steep pressure curve holds flow as the bag filter loads.
Combustible dust
Fine guar is explosible. The fan is earthed, built to avoid rubbing, and included in the plant's venting or isolation arrangement with the bag filter. Housekeeping around the fan matters too, since settled guar dust on ducts and casings can feed a secondary explosion. The fan inlet and outlet flanges are bonded across flexible connections.
Energy and control
Guar plants run long campaigns on varying grades, where mesh size and viscosity targets change the mill speed and air flow. A variable speed drive on the exhaust fan matches each grade and saves power compared with a damper. Better soaking control, which keeps split moisture consistent, lets the dryer run with less air.
ID fan and FD fan in guar gum flash dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | Before the air heater |
| What it handles | Humid air with fine guar powder | Filtered ambient air |
| Temperature at the fan | 60 to 85°C | Ambient |
| Main design concern | Gel deposits and high mill suction | Filter loading and pressure balance |
| Control | Mill suction | Air flow |
See also the Forced Draft (FD) Fan for guar gum flash dryers.
Questions buyers ask
Why does the guar gum dryer fan get a rubbery coating?
Guar powder forms a gel with water. If the fan casing cools near the dew point, fines absorb condensate and stick to the blades. Insulation, keeping the outlet temperature up and a dry purge at shutdown prevent it. A wash with warm water after a dry scrape removes stubborn layers.
How much suction does a guar gum mill-dryer need?
Typically 450 to 700 mmWC including the mill, flash duct, cyclones and bag filter. The mill resistance varies by design, so it is checked with the mill supplier or by site measurement. A fan without enough margin on suction will also struggle as the bag filter loads, so the selection includes the filter at its dirty condition.
Does a guar gum flash dryer fan need to be stainless steel?
For food and pharmaceutical grades, stainless steel is usual. For industrial grades used in oil drilling, mild steel with a food-grade coating is often acceptable. Mild steel fans need good coating and dry shutdowns, since rust flakes in the casing contaminate fines that may be returned to the product.
Information needed to quote an ID fan for guar gum flash 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.
- Mill-dryer. Mill make or type and the air flow and suction at the mill.
- Grades. Grades produced, mesh and viscosity targets, and food or industrial use.
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.