How fluid bed dryers and coolers work
In a fluid bed dryer, heated air is blown up through a perforated or slotted distributor plate and lifts a bed of granules, crystals or powder so that it behaves like a boiling liquid. Heat and mass transfer are fast and even. Continuous plug-flow units dry salt, sugar, fertilizer, minerals and chemicals, often with a cooling section using ambient air at the discharge end; batch units in pharmaceutical plants dry granules from wet granulation. Inlet air is typically 80 to 200°C, heated by steam coils, thermic fluid, electric heaters or an indirect or direct-fired air heater. The exhaust passes a cyclone and bag filter, or a bag filter built into the dryer top, and an exhaust fan draws it out while a supply fan pushes the air through the plate.
Where the FD fan sits
- Air filter
- FD (fluidising air) fan
- Air heater
- Plenum
- Distributor plate
- Product bed
The supply fan sits after the air filter and before the heater, so it handles ambient air and pushes it through the heater into the plenum below the plate. The plate pressure drop, the bed weight per unit area and the heater together make up the duty, typically 300 to 700 mmWC. Cooler sections have their own fan, or a branch of the main fan, with unheated air.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Air after the heater | 80 to 200°C, downstream of the fan |
| Dust | Filtered air; pharma plants use fine and HEPA filtration |
| Humidity | Some pharma and food plants dehumidify the supply air |
| Pressure at the fan outlet | Typically 300 to 700 mmWC through heater, plate and bed |
| 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 fluid bed dryers and coolers
| Blade form | Backward curved, high-pressure design with a steep curve |
| Material of construction | Mild steel, or stainless steel for pharma and food |
| Drive and arrangement | Direct coupled with variable speed drive |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on air flow or bed pressure drop |
| Also specified | Inlet filter, silencer for high-pressure fans |
What this FD fan has to cope with
Defluidisation
If the fan cannot hold flow as the bed depth or wet product load rises, parts of the bed stop fluidising and lumps form. A fan with a steep curve and margin on pressure holds the flow. The fan is selected near the peak of its efficiency at normal load, with pressure margin above it, rather than at the edge of its curve.
Noise from high-pressure fans
High-pressure supply fans are noisy, especially in enclosed pharma suites. Inlet silencers and acoustic lagging keep noise within limits. They also produce a low-frequency tone that travels along the ducts into the dryer. Inlet silencers and acoustic lagging keep noise within limits, and a flexible connection at the outlet stops vibration reaching the plenum and plate.
Plate blinding
Fines falling through or sticking to the plate raise its resistance over time. The fan selection allows for a fouled plate, and plate cleaning is scheduled. A differential pressure reading across the plate and bed, trended over weeks, shows when cleaning is due before fluidisation is affected.
Energy and control
The fluidising fan works at high pressure and runs whenever the dryer runs, so it is often the largest fan in the plant. Speed control matches air to the bed, especially in batch dryers where less air is needed as drying proceeds. Using the minimum fluidising velocity with an adequate margin saves both fan power and heater energy.
ID fan and FD fan in fluid bed dryers and coolers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone or bag filter | Before the heater and plate |
| What it handles | Moist air with product fines | Filtered ambient air |
| Temperature at the fan | 50 to 90°C | Ambient |
| Main design concern | Carry-over, abrasion or explosion risk | High pressure and stable fluidisation |
| Control | Hood pressure | Air flow or bed pressure |
See also the Induced Draft (ID) Fan for fluid bed dryers and coolers.
Questions buyers ask
How much pressure does a fluid bed dryer fan need?
Typically 300 to 700 mmWC, made up of the heater, the distributor plate and the bed weight. Deep beds and fine plates need the higher end. Pharma batch units with fine filters on the inlet need more. A fan selected without the bed weight cannot fluidise a fully loaded batch, so bed area and depth are part of every enquiry.
Why does my fluid bed dryer form lumps?
Often because the supply fan does not give enough air to keep the bed fluidised, especially with wet feed. Checking the fan flow and the plate condition is the first step. Wet feed that is too sticky also forms lumps, so feed moisture and back-mixing with dry product are checked at the same time.
Is the supply fan before or after the heater in a fluid bed dryer?
Usually before, so it handles cool air and is smaller. The heater is on the pressure side. Placing the fan before the heater also keeps its bearings and motor at ambient temperature. Where the plant recirculates exhaust air, the fan may handle warm air and is then selected at that temperature.
Information needed to quote an FD fan for fluid bed dryers and coolers
- 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.
- Bed and plate. Bed area, bed depth and plate type.
- Air treatment. Filtration and any dehumidification 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.