How ammonium sulphate dryers work
Ammonium sulphate is crystallised from ammonia and sulphuric acid, either in a saturator on coke oven gas or in an evaporative crystalliser on caprolactam by-product liquor. The slurry is centrifuged to about 1 to 3% moisture and dried in a fluid bed or rotary dryer. Drying air is heated to typically 120 to 180°C in a steam coil or an indirect oil or gas fired air heater and leaves at 70 to 90°C, carrying fine crystals. Product is cooled to below about 50°C in a fluid bed cooler to prevent caking in storage. Exhaust air passes cyclones and a wet scrubber, which return dissolved salt to the crystalliser, before the exhaust fan discharges to atmosphere.
Where the FD fan sits
- Inlet filter
- FD fan
- Steam or indirect air heater
- Plenum
- Distributor plate
- Fluid bed
The fan sits before the air heater so it handles cool, dense ambient air and needs no high temperature construction. Downstream it must overcome the heater, plenum, distributor plate and the bed itself. A fluid bed of ammonium sulphate crystals typically needs 300 to 600 mmWC at the fan outlet, more than a rotary dryer. A separate fan, usually smaller, supplies ambient air to the fluid bed cooler section.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the heater | 120 to 180°C, downstream of the fan |
| Dust | Ambient air in a fertiliser plant can carry salt dust; inlet filtered |
| Pressure at the fan outlet | Typically 300 to 600 mmWC for a fluid bed |
| Humidity | Monsoon humidity raises the drying load but not the fan duty |
| 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 ammonium sulphate dryers
| Blade form | Backward curved, chosen for a steeply rising pressure curve |
| Material of construction | Mild steel with a corrosion-resistant coating; stainless steel inlet parts where salt dust is heavy |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal or motor bearings |
| Flow control | Variable speed drive on air flow, with an inlet damper for start-up |
| Also specified | Inlet filter and silencer, and air flow measurement for the dryer control |
What this FD fan has to cope with
Loss of fluidisation
If air flow falls as the distributor plate blinds with salt, the bed slumps, wet crystals form lumps and the dryer must be stopped and cleaned. A fan with a steep curve and flow control holds air flow as plate resistance rises. Trending plate pressure drop shows when cleaning is due.
Salt dust in the intake
Fertiliser plant air carries fine ammonium sulphate, which deposits in the fan, coats the steam coil fins and corrodes uncoated steel. An inlet filter drawing air from outside the bagging and handling area protects both fan and heater. The filter pressure drop is included in the fan duty, with margin for a loaded filter between changes.
Start-up with a heavy bed
A settled bed of damp crystals needs more pressure to break into fluidisation than a running bed. The fan is selected with pressure margin for start-up, and the drive ramps flow up gradually so the bed lifts evenly. The cooler fan needs the same check, since cooled product can settle hard if the line stops with a full bed.
Energy and control
Fluid bed dryers need a minimum air velocity to stay fluidised, so the FD fan cannot simply follow production down to low rates. The saving comes from running at the true fluidising flow rather than at the design flow with a damper partly closed. A variable speed drive set on measured air flow, with the setpoint tied to bed pressure drop, typically cuts energy noticeably on plants that run below nameplate capacity.
ID fan and FD fan in ammonium sulphate dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclones and wet scrubber | Before the air heater, feeding the bed plenum |
| What it handles | Humid air with salt droplets | Clean ambient air |
| Temperature at the fan | 40 to 60°C, near saturation | Ambient |
| Main design concern | Corrosion and salt build-up | Steady fluidising pressure |
| Control | Dryer suction | Air flow and bed pressure drop |
See also the Induced Draft (ID) Fan for ammonium sulphate dryers.
Questions buyers ask
How much pressure does a fluid bed dryer supply fan need?
For an ammonium sulphate fluid bed dryer, the supply or FD fan typically needs 300 to 600 mmWC to cover the air heater, plenum, distributor plate and bed. The actual figure depends on bed depth, crystal size and the distributor design. Measure bed and plate pressure drop on the running dryer.
Should the drying air fan be before or after the steam heater?
Before. The forced draft fan then handles cool, dense ambient air, so it is smaller and needs no high temperature materials or bearing cooling. The heater sits on the pressure side of the fan. A fan placed after the heater would need heat-resisting construction, cooled bearings and a larger size for the same mass flow.
Why does my fluid bed dryer lose fluidisation over time?
Usually because the distributor plate holes are blinding with salt, so resistance rises and air flow falls. A fan controlled on air flow compensates for a while, but the plate still needs regular cleaning. Clean the plate at each scheduled stop, and check the inlet filter and steam coil fins, which also add resistance as they load with salt dust.
Information needed to quote an FD fan for ammonium sulphate 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.
- Bed data. Bed area, depth and the measured pressure drop of bed and distributor plate.
- Heater. Type of air heater and its air side 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.