How salt drying and refining plants work
Salt refineries wash solar salt or crystallise vacuum salt from brine, centrifuge it to about 2 to 5% moisture, then dry it in a fluidised bed dryer or a rotary dryer before iodisation, addition of a free-flowing agent and packing. Fluid bed dryers are the most common: hot air from a steam, thermic fluid or hot air generator heater passes through a perforated distributor plate and fluidises the salt, and a cooling section at the end cools the product with ambient air. Inlet air is about 150 to 250°C and exhaust about 60 to 90°C. The exhaust carries fine salt dust and is cleaned in cyclones and a wet scrubber, with the brine returned to the process. Chlorides, often in a coastal atmosphere, make corrosion the dominant concern.
Where the FD fan sits
- Air intake
- Filter
- FD (supply) fan
- Air heater
- Fluid bed plenum
- Distributor plate
- Salt bed
The supply fan takes ambient air, often from a salty coastal atmosphere, through a filter and pushes it through a steam, thermic fluid or hot air generator heater into the fluid bed plenum. Its pressure covers the filter, heater, distributor plate and bed, typically 400 to 700 mmWC, which is higher than most dryer supply fans. The cooling section has its own fan blowing ambient air under the cooling end of the bed.
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 | Ambient air with salt aerosol in coastal plants |
| Pressure | High for fluid beds, covering distributor plate and bed |
| Heat source | Steam, thermic fluid or hot air generator |
| 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 salt drying and refining plants
| Blade form | Backward curved, high-pressure |
| Material of construction | Coated mild steel or stainless steel, for a salty atmosphere |
| Drive and arrangement | Direct driven, variable speed |
| Bearings and cooling | Standard bearings with good sealing |
| Flow control | Variable speed drive on air flow |
| Also specified | Intake filter and corrosion-protected exterior |
What this FD fan has to cope with
Fluidisation stability
Too little air and the bed defluidises, salt lumps form and drying stops; too much and fines blow over to the cyclones. The supply fan must hold flow steady as the bed load changes, which calls for a steep fan curve and flow-controlled speed.
Salty intake air
Coastal air carries salt aerosol, and the plant itself adds salt dust. Drawn into the fan, it corrodes the wheel and casing and deposits on the heater. The intake is filtered and placed away from packing, and the fan is coated or stainless.
Heater coil fouling and corrosion
Salt that passes the intake filter collects on the fins of steam or thermic fluid coils, corroding them and raising the air side pressure drop. The fan duty includes a fouled coil, and filtration ahead of the fan protects the heater.
Energy and control
Fluid bed supply fans work at high pressure for the whole operating time, so their power is significant. An efficient high-pressure wheel and a variable speed drive holding the fluidising air flow save power as bed load changes. Using warm air from the cooling section as part of the drying air recovers some of the heat removed from the salt.
ID fan and FD fan in salt drying and refining plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the wet scrubber | Before the heater and fluid bed |
| What it handles | Saturated air with brine droplets | Ambient air |
| Temperature at the fan | 40 to 55°C | Ambient |
| Main design concern | Chloride corrosion and salt crust | Fluidisation pressure |
| Control | Dryer suction | Air flow |
See also the Induced Draft (ID) Fan for salt drying and refining plants.
Questions buyers ask
What pressure does a salt fluid bed dryer fan need?
Typically 400 to 700 mmWC, covering the intake filter, heater, distributor plate and the bed itself. The exact figure depends on bed depth, salt particle size and plate design, so it is taken from the dryer supplier's air balance rather than from a rule of thumb.
Does a salt dryer FD fan need to be stainless steel?
It handles ambient air, but in coastal salt works that air is salty and the plant adds salt dust. Coated mild steel with a marine-grade paint system or stainless steel is used for durability, with stainless fasteners.
Does the salt dryer cooling section need its own fan?
Usually yes. A separate fan supplies ambient air to the cooling end of the fluid bed, sized to bring the salt close to ambient before iodisation and packing. It runs at a similar pressure to the supply fan because it also passes the plate and bed.
Information needed to quote an FD fan for salt drying and refining plants
- 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.
- Fluid bed. Bed area, depth and distributor plate pressure drop.
- Heater. Heater type and pressure drop.
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.