The purpose of FFUs

Filter fan units (FFUs) purify recirculating air. Their high efficiency at removing particulate contaminants and their flexible installation and operation options have made them increasingly popular. They filter airborne particles by using HEPA or ULPA filters, a fan with a control unit and often a pre-filter. A control and monitoring system is used to adjust the motor speed of individual units and check their status.

FFUs are usually found in the floor or ceiling grid of the room, and hundreds or even thousands may be installed in large spaces. However, they are usually more cost effective where relatively few units are needed because noise, automation costs and capital costs increase for every unit. Both standard units and customised designs tailored to attain specific airflows, velocities and noise restrictions are available on the market.

FFU applications

FFUs are used in clean rooms and microenvironments of different sizes and configurations and with many applications. They can be especially useful where overhead clearance is limited, depending on the space necessary for HVAC components, the required FFU placement density, the space available and roof and floor support. Since sufficient ducting to provide the large airflows required by cleanrooms is a major limiting factor in usable room size, FFUs can be an invaluable element of cleanroom design. An air handling unit (AHU) can provide conditioned air to a plenum above the ceiling, where FFUs deliver it to the cleanroom after filtering it through HEPA filters, together with some recirculation air. Upgrading the cleanroom to a higher cleanliness specification can often be done simply by installing more FFUs.

FFU advantages

A fan-powered FFU solution can have the following benefits:

• Having many redundant individual FFUs prevents problems when one or several units do not function.
• It can result in lower costs than the more usual ducted supply system if fewer than twenty filters are to be used.
• FFUs are ideal when space restrictions (especially height restrictions) limit the area available.
• Energy efficiency may be improved because every FFU can be separately controlled.
• Being stand-alone units, they are completely salvageable.
• FFUs make for excellent room design flexibility: more units can be added to upgrade the facility or exchanged for panels or lights if the need arises. A gel-track ceiling is not necessary as it would be for other solutions.
• Cleanroom ventilation design is relatively simple.
• The individual units have a low noise level.

HVAC systems in cleanrooms take up a significant proportion of total energy use. The precise energy use of the system is governed by many factors, including the contamination limits of the cleanroom, HVAC layout and air component resistance. FFUs that are used in cleanrooms likewise vary widely in energy efficiency. At full fan speeds, larger units are often more energy efficient than smaller ones. Energy efficiency may also change significantly for a specific unit, depending on airflow velocity and pressure rises in the unit given specific operating conditions. It is therefore important to consider the particular conditions in a cleanroom before selecting the appropriate FFU.

FFU construction and options

FFUs may be constructed of aluminium or stainless steel and may be powder coated. Accessories include optional pre-filters and heat exchangers. Newer units usually have DC motors, which are more efficient but carry higher capital costs.

These may be electronically commutated motors (EC motor or ECM) that can completely control and monitor the unit. Such motors are highly efficient, produce relatively little waste heat, and also maintain their efficiency at part speed. They are quiet and have a wider operating range than traditional motors, with a life that equals or exceeds that of AC motors. Because they are a form of DC drive, their speed is infinitely variable. The motors are thus very cost effective to operate and can deliver considerable energy savings. However, they have a higher capital cost.