Self-powered, so the layout stays changeable
Once a ducted supply system is built, the supply points are fixed. Semiconductor and display fabs reconfigure lines every two or three years, and every reconfiguration means touching the ductwork — significant cost and downtime.
An FFU puts the fan inside the module, making each unit an independent supply point. Air is delivered wherever a module sits, and capacity is set by how many run. When the line changes, modules move; the ceiling grid does not.
Units taken out of service can be sealed and stored for the next project rather than written off.
Why the plenum runs at negative pressure
An FFU generates its own static pressure, holding the plenum above it below the pressure of the clean zone. Any gap therefore leaks from the cleanroom into the plenum, not the other way round — accumulated dust in the plenum is never driven into the clean space.
A positive-pressure ducted system behaves in exactly the opposite way: any imperfect joint pushes accumulated particles out into the room. This is the structural advantage of FFUs at high cleanliness classes, and the reason sealing work can be simplified.
Three numbers that drive selection
- **Face velocity**: vertical unidirectional-flow cleanrooms are typically set between 0.35 and 0.55 m/s. Velocity determines whether flow stays unidirectional and carries particles away, and therefore how many units are required.
- **Noise**: an FFU runs continuously, two or three metres from operators. There is roughly 12 dB between the high and low settings, so medium is usually the working set point, with high reserved for start-up and recovery.
- **Filter grade**: HEPA and ULPA differ in pressure drop. The fan must overcome the filter resistance plus the resistance of the whole recirculation path; the wrong grade shows up immediately as insufficient airflow.
Dry-seal versus liquid-seal
Dry-seal FFUs use gasket sealing. They install quickly and cost less, and are the standard choice for most electronics facilities.
Liquid-seal units run a channel of sealing gel around the grid; the filter frame sits into that channel, forming a continuous liquid seal. This gives higher sealing reliability for pharmaceutical and high-grade clean zones with strict leakage limits, at the price of a more complex grid system and attention to the gel level during maintenance.
