Fan filter units vs ducted HEPA systems

Every cleanroom has to move a large volume of air through HEPA or ULPA filters into the room. There are two main ways to do it. In a ducted system, central air handlers push air through ductwork to passive terminal filter housings in the ceiling. In a fan filter unit (FFU) system, each ceiling module contains its own small fan and filter, drawing air from a ceiling plenum and pushing it through the filter into the room.

Both approaches can achieve any cleanliness class. The right choice depends on room size, class, ceiling space, noise, heat, controls and how the owner will operate and maintain the facility. Many facilities combine them.

How each system works

Ducted terminal HEPA

A recirculating air handling unit, or a combination of makeup air and recirculation units, provides all the fan power. Supply ducts run to each ceiling filter housing, and return air comes back through low wall returns and return ducts or chases. Airflow to each filter is set by balancing dampers. The central fans must overcome duct losses plus the pressure drop of the filters.

Fan filter units

Each FFU is a self-contained module, commonly sized to fit a standard ceiling grid opening such as 600 x 1200 mm (2 x 4 ft) or 1200 x 1200 mm (4 x 4 ft). A small fan, usually with an electronically commutated (EC) motor, draws air from a pressurized or neutral plenum above the ceiling and pushes it through the filter. Return air reaches the plenum through wall chases or return grilles. A separate makeup air unit supplies conditioned outdoor air to the plenum, and sensible cooling coils may be located in the return path or in dry cooling coils within the plenum.

Side-by-side comparison

Energy

Energy performance is the most debated point. Central air handlers with large, efficient fans or fan arrays can achieve high efficiency, but ductwork, dampers and fittings add pressure drop that the fans must overcome. FFUs avoid supply ductwork entirely, but each small fan is less efficient than a large one.

In practice, results depend heavily on the specific design:

• Modern EC-motor FFUs with low-pressure-drop filters can compete well with ducted systems, especially in small and medium rooms.
• Well-designed ducted systems with low duct velocities, short runs and high-efficiency fans can outperform FFUs in large facilities.
• Older FFUs with AC motors and poor fan selection can use significantly more energy than either.

Compare options using fan power per unit airflow, often expressed in watts per L/s or W per cfm, for the actual system, including filter pressure drop at mid-life. ISO 14644-16 recommends this kind of analysis. See cleanroom energy efficiency for wider strategies.

Fan heat

All fan energy ends up as heat. In a ducted system, part of the fan heat is added at the air handler, upstream of cooling coils. In an FFU system, fan heat is released in the ceiling plenum and must be removed by the cleanroom's cooling coils. This should be included in the cooling load and affects coil sizing.

Noise

A single FFU is quiet, but a ceiling full of them can produce noticeable sound levels, especially at high speeds. Manufacturers publish sound data at various airflows; check the combined effect for the number of units in the room and the room's acoustic properties. Ducted systems can place fans far from the room and use attenuators, which makes low noise targets easier to meet.

Controls and monitoring

Networked FFU control systems allow each unit's speed to be set and monitored. Advantages include:

• Fine adjustment of airflow across the ceiling for uniform velocity.
• Setback to lower speeds during unoccupied periods.
• Alarms when a unit fails or slows.
• Data on running hours and power for maintenance planning.

Ducted systems typically control airflow centrally with variable speed drives and airflow measurement, with filter airflows set by balancing dampers. Changes in one branch can affect others, so rebalancing may be needed after modifications.

When each is a good fit

FFUs are often a good fit for:

• Small to medium cleanrooms where ductwork would be costly or difficult.
• Retrofits into existing buildings with limited ceiling space for ducts.
• Unidirectional ISO 5 zones within a larger room.
• Modular cleanrooms and facilities expected to change layout.
• Semiconductor and electronics ballrooms with very large filter ceilings.

Ducted systems are often a good fit for:

• Pharmaceutical facilities where all maintenance should occur outside the classified area.
• Rooms with strict noise limits.
• Facilities where centralized maintenance and fewer moving parts are preferred.
• Large, stable facilities where efficient central fans and well-designed ductwork pay off.

Hybrids

Many facilities use ducted HEPA filters for background rooms and FFUs for local unidirectional zones, or FFUs supplied by a central makeup air unit with dry cooling coils in the return plenum. Combining approaches lets each area use the most suitable system.

Design and installation points

1. Size the plenum so that pressure is uniform across all FFUs, avoiding starved units far from the makeup air entry.
2. Select filters with low pressure drop; FFU fans have limited static pressure capability.
3. Make the ceiling grid and seals airtight, since leaks bypass filters.
4. Plan access for FFU replacement, ideally room-side.
5. Include fan heat in the cooling load.
6. For ducted systems, keep duct velocities low and balance carefully, then verify each filter's airflow during certification. See HEPA and ULPA filter systems for leak testing.

Key takeaways

• Both FFUs and ducted HEPA systems can achieve any cleanliness class; the choice depends on size, noise, heat, controls and maintenance.
• FFUs avoid supply ducts and allow flexible layouts, but add fan heat and noise inside the room envelope.
• Ducted systems keep fans and maintenance outside the room and suit strict noise or pharmaceutical maintenance requirements.
• Compare energy using system fan power per unit airflow, including filter pressure drop at mid-life.
• Hybrid designs often combine the strengths of both.