How pharmaceutical spray dryers work
Pharmaceutical spray dryers produce APIs, amorphous solid dispersions, excipients and inhalable particles, usually in small to medium chambers with two-fluid or pressure nozzles. Aqueous feeds dry in open-cycle units: HEPA-filtered air is heated by steam or electric heaters to about 120 to 200°C and leaves at 50 to 90°C through a cyclone and a cartridge or bag filter with a HEPA police filter. Feeds in organic solvents such as acetone, methanol or dichloromethane dry in closed-cycle units with nitrogen: the gas leaves the chamber, passes the cyclone and filter, is cooled in a condenser that recovers the solvent, and is reheated and recirculated by a single blower. Containment of potent compounds, product purity and, for solvents, oxygen levels and explosion risk set the design.
Where the FD fan sits
- Air intake
- Pre-filter and fine filter
- FD (supply) fan
- Heater
- HEPA filter
- Air disperser
- Drying chamber
The supply fan follows the pre-filters and sits before the heater and terminal HEPA filter, so it handles cool, filtered air. Because the HEPA filter is downstream, anything from the fan itself is caught before the chamber. The fan pressure covers the filters at their dirty pressure drop, the heater, the HEPA filter and the disperser. In closed cycle, this role is taken by the recirculation blower.
Air at the FD fan
| Air temperature at the fan | Ambient or conditioned room air, 15 to 35°C |
| Air after the heater | 120 to 200°C, downstream of the fan |
| Dust | Pre-filtered air; terminal HEPA filtration after the heater |
| Humidity | Some plants dehumidify inlet air for consistent product moisture |
| Pressure | Filters, heater, HEPA filter and disperser |
| Density at the design temperature | About 1.15 kg/m³ at 35°C, 96% 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 pharmaceutical spray dryers
| Blade form | Backward curved for clean air |
| Material of construction | SS 304 or SS 316L |
| Drive and arrangement | Direct driven with a variable speed drive |
| Bearings and cooling | Sealed bearings outside the air path |
| Flow control | Variable speed drive on measured air flow |
| Also specified | Cleanable casing, documented materials, low-shedding gaskets |
What this FD fan has to cope with
Batch-to-batch repeatability
Particle size and moisture depend on the gas flow, so the supply fan must deliver the same flow every batch as filters load. Flow measurement and speed control hold it, and the setpoint is part of the batch record.
Wide turndown
One dryer may run small development batches and full production batches. The fan is selected to stay stable across the full flow range, not just at maximum, so low flows do not push it into unstable operation.
Closed-cycle confusion
Specifying a supply fan for a closed nitrogen cycle duplicates the recirculation blower and adds a leak path. The cycle type is fixed first, and the fans are then chosen to match it.
Energy and control
Fan energy is small next to heating, dehumidification and, in closed cycle, solvent recovery. A variable speed drive suits the batch pattern and the cleaning cycle and lets the fan run slowly during warm-up. Dehumidifying the inlet air raises energy use but improves batch consistency, and is often required by the process.
ID fan and FD fan in pharmaceutical spray dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the product and HEPA police filters | After pre-filters, before heater and HEPA |
| What it handles | Moist air or solvent-laden nitrogen | Clean filtered air (open cycle only) |
| Temperature at the fan | 50 to 90°C open cycle; cool in closed cycle | Room temperature |
| Main design concern | Gas-tightness and containment | Repeatable flow |
| Control | Chamber pressure or loop flow | Gas flow |
See also the Induced Draft (ID) Fan for pharmaceutical spray dryers.
Questions buyers ask
Does a closed-loop spray dryer have an FD fan?
Usually not a separate one. One recirculation blower drives the nitrogen round the loop through the chamber, filter, condenser and heater. Some large units use two blowers, but there is no ambient air supply.
Why is HEPA filtration after the heater?
So the air entering the chamber is free of particles from the duct, heater and fan. The supply fan therefore handles pre-filtered air, and the terminal HEPA filter protects the product.
How is gas flow controlled on a pharma spray dryer?
By a flow measurement and a variable speed drive on the supply or recirculation fan, with the setpoint fixed in the batch recipe so it is the same every batch.
Information needed to quote an FD fan for pharmaceutical spray 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.
- Filtration and heating. Filter grades including HEPA, and heater type and pressure drop.
- Turndown. Lowest and highest gas flows the dryer will run.
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.