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 ID fan sits
- Fluid bed or rotary dryer
- Cyclones
- Wet scrubber
- Mist eliminator
- Exhaust (ID) fan
- Stack
The fan usually follows the wet scrubber, so it handles saturated air at 40 to 55°C carrying brine droplets that pass the mist eliminator. In some plants the fan sits between the cyclones and the scrubber and handles hot, dry air with fine salt dust. The suction covers the fluid bed or drum, the cyclones, scrubber and mist eliminator, typically 250 to 450 mmWC, and the dryer hood is held just below atmospheric.
Gas at the ID fan
| Air temperature, normal | 40 to 55°C after the wet scrubber; 60 to 90°C after cyclones |
| Maximum or upset | Up to about 120°C if scrubber water fails |
| Dust at the fan | Fine salt dust or brine mist; salt crusts form where droplets dry |
| Moisture and dew point | Saturated after the scrubber |
| Corrosive species | Sodium chloride, with magnesium and calcium chlorides in less pure salt |
| Density at the design temperature | About 1.09 kg/m³ at 50°C, 91% 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 salt drying and refining plants
| Blade form | Backward curved, smooth, with an open shape that sheds crust |
| Material of construction | FRP, SS 316L or duplex stainless steel, or rubber-lined mild steel |
| Drive and arrangement | Belt or direct driven, variable speed |
| Bearings and cooling | Bearings outside the air stream with a tight shaft seal |
| Flow control | Variable speed drive on dryer suction |
| Also specified | Wash nozzles, drain, sealed casing, and corrosion-resistant fasteners and base frame |
What this ID fan has to cope with
Chloride pitting and crevice corrosion
Salt deposits and brine attack mild steel within months and pit SS 304, especially under deposits and at welds. FRP, SS 316L, duplex stainless or rubber lining are used for wetted parts, chosen for the gas temperature and the magnesium and calcium chloride content of the salt.
Salt crust on blades
Brine droplets that pass the mist eliminator dry on the warm blades as hard crusts that grow unevenly and unbalance the rotor. A well-maintained mist eliminator, water wash nozzles in the fan inlet and a short wash at each stop dissolve the crusts before they grow.
Corrosion of the outside of the fan
Coastal salt works and refineries corrode the fan exterior, base frame, fasteners and motor as fast as the wetted parts. Coatings suited to marine exposure, stainless fasteners, a sealed motor and a base frame that does not trap brine are part of the specification.
Energy and control
Salt dryers run continuously, with throughput set by the centrifuges and the packing line. A variable speed drive on the ID fan follows the feed rate and keeps the bed suction steady. Lower cake moisture from well-maintained centrifuges reduces the air the dryer needs. Heat recovery from the exhaust is limited by its low temperature and salt content.
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 Forced Draft (FD) Fan for salt drying and refining plants.
Questions buyers ask
What material should a salt dryer exhaust fan be made of?
For wet scrubber service, FRP, SS 316L, duplex stainless steel or rubber-lined mild steel. SS 304 pits quickly in chloride service and mild steel fails within months. For dry service after cyclones, SS 316L is common. The choice depends on gas temperature and salt purity.
Why do salt dryer exhaust fans go out of balance?
Brine droplets carried past the scrubber dry on the blades into hard salt crusts that build up and break off unevenly. A good mist eliminator, water wash nozzles and washing at each stop dissolve the crusts before they cause vibration.
Is the ID fan before or after the scrubber in a salt plant?
Usually after, so it handles cooled, saturated air, which suits FRP or rubber-lined construction. Placed before the scrubber, the fan handles hot, dry salt dust and must be stainless, with wear and deposits from the dust to manage.
Information needed to quote an ID fan for salt drying and refining plants
- 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.
- Dryer type. Fluid bed or rotary dryer and the scrubber arrangement.
- Salt quality. Purity and impurities such as magnesium chloride.
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.