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 FD fan sits
- Air filter
- FD (supply air) fan
- Thermic fluid air heater
- Hot air duct
- Mill-dryer inlet
The supply fan sits before the air heater, so it handles cool air and pushes it through the heater and into the mill. On smaller lines without a supply fan, the exhaust fan pulls air through the heater. A supply fan is added when the total system loss is too high for one fan or when the mill inlet must be held close to atmospheric pressure.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the heater | 150 to 250°C, downstream of the fan |
| Dust | Filtered air; desert dust in summer loads filters quickly |
| Humidity | Low humidity in dry seasons helps drying |
| Pressure at the fan outlet | Set by the heater and mill inlet, typically 150 to 300 mmWC |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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 guar gum flash dryers
| Blade form | Backward curved |
| Material of construction | Mild steel with food-grade coating, or stainless steel where specified |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on air flow |
| Also specified | Filter differential pressure gauge and inspection door |
What this FD fan has to cope with
Dust-laden intake air
In dry, dusty regions the air filters load fast, and the supply fan loses flow. Speed control on measured air flow and regular filter changes keep the drying stable. An intake placed away from roads and yards also helps. A filter differential pressure gauge tells the operator when to change them.
Balancing with the exhaust fan
If the supply fan pushes harder than the exhaust fan pulls, powder escapes at the mill seals. The exhaust fan controls mill suction and the supply fan controls air flow. An interlock stops the supply fan if the exhaust fan trips. Both fans on speed control make this simple.
Heater fouling
Dust passing poor filters fouls the heater coils, raising resistance and lowering heat transfer. Good filtration protects both heater and fan duty. A thermic fluid coil that fouls also loses heat output, which forces the heater to run hotter and stresses the fluid. Pressure drop measurement across the coil shows when it needs cleaning.
Energy and control
The supply fan handles all the drying air across long campaigns. An efficient backward curved wheel with speed control is the standard choice. Keeping the filters and heater clean reduces the pressure and power needed. Heat recovery from the exhaust adds a coil pressure drop to the supply fan duty but reduces thermic fluid load across the campaign.
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 Induced Draft (ID) Fan for guar gum flash dryers.
Questions buyers ask
Does a guar gum dryer always have an FD fan?
No. Many lines use only an exhaust fan, pulling air through the heater and mill. A supply fan is added on larger lines or where the mill needs near-atmospheric inlet pressure. The supply fan is sized with the exhaust fan, so that each covers its share of the system resistance and the mill inlet stays close to atmospheric.
Should the supply fan be before or after the thermic fluid heater?
Before, so it handles cool, dense air and needs no high-temperature construction. The heater is then on the pressure side, and the fan bearings and casing stay at ambient temperature. A filter before the fan keeps desert dust off both the fan and the heater coil.
How is the supply fan controlled?
On drying air flow, with the exhaust fan controlling suction at the mill, both through variable speed drives. The two drives are interlocked so the supply fan cannot run alone, and each grade has its own air flow setting to suit mesh size and viscosity targets.
Information needed to quote an FD fan for guar gum flash 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.
- Heater. Thermic fluid or indirect heater type and its air-side pressure drop.
- Site conditions. Ambient dust and temperature range at the intake.
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.