Components

Fan filter unit

A cleanroom ceiling module that combines a HEPA or ULPA filter with its own small fan and motor, drawing air from the plenum above the ceiling and pushing it down through the filter into the room. A ceiling of fan filter units replaces supply ductwork to individual filters.

Also called FFU, fan-powered HEPA module

Direct answer

What is a fan filter unit?

A fan filter unit, or FFU, is a cleanroom ceiling module with its own fan and HEPA filter that recirculates room air from the ceiling plenum. All of a fan filter unit's electrical input becomes heat in the cleanroom, so a unit drawing 250 watts adds about 850 BTU per hour.

Fan filter unit: how it works

Room air returns through low-wall chases into the plenum above the ceiling. Each fan filter unit pulls from that plenum and pushes air down through its filter, so the plenum sits slightly below room pressure and any ceiling leak flows out of the room. The units only recirculate and filter; cooling, dehumidification, and room pressurization come from a separate coil and a makeup air unit.

Where you meet the term

  • In a cleanroom ceiling grid, as a filter module with a small fan housing above it and power or control wiring.
  • In a cleanroom heat load calculation, where each unit's input watts are entered as room sensible heat.
  • In a controls schedule, where networked units report speed, status, and faults individually.

How to check it

  1. Check the unit status on the controller. Networked units show running status and faults on a central controller, where a stopped unit appears before anyone notices it in the room. Owner check
  2. Measure airflow below each unit. A certifier measures velocity or airflow under each filter and adjusts unit speed to bring it within the acceptance band. Technician
Read this twice

Fan filter unit: what it is not

  • A fan filter unit does not cool the room. It adds heat, and the cooling has to come from a separate coil in the recirculation path.
  • A fan filter unit does not pressurize the room. It recirculates room air, so room pressure is set by makeup air and exhaust.

Fan filter unit questions

How much airflow does a fan filter unit move?

Airflow depends on the unit size, its fan, its speed setting, and how loaded its filter is, so it comes from the manufacturer's data at the chosen operating point. The HVAC Bench cleanroom heat load example uses units at 625 cfm, and the unit count follows from design airflow divided by that figure.

Can fan filter units run at reduced speed at night?

They can where the room's requirements allow lower airflow when unoccupied, which cuts fan energy and heat. The room has to return to its classified condition before work resumes, and a regulated room cannot drop below its required air changes.

What happens when one fan filter unit fails?

The room loses that unit's share of airflow over one area while the rest of the ceiling keeps running. If the failed unit sits over critical work, that work stops until the unit is replaced or the area is shown to meet its class.

Where this definition comes from

Written from standards body publication, government guidance material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.

  • ISO 14644-4:2022, Cleanrooms and associated controlled environments, Part 4: Design, construction and start-upInternational Organization for Standardization · standards body publicationSecond edition, replacing the 2001 edition, giving design, construction, and start-up guidance for new and modified cleanrooms, with checklists of requirements and performance parameters to be agreed between customer and supplier, energy management design approaches, and guidance that pressure differences between adjacent cleanrooms of different cleanliness lie in a range of about 5 to 20 pascals so doors can be opened without unintended cross-flow.
  • Airflow Design for Cleanrooms and its Economic Implications (LBNL-51549)Lawrence Berkeley National Laboratory · government guidanceMeasured performance of HVAC systems in seven ISO 5 cleanrooms, reporting that IEST-RP-CC012.1 recommended 240 to 480 air changes per hour for ISO 5, equal to 40 to 80 feet per minute with full ceiling coverage, that measured rooms ran at roughly 100 to 480 air changes per hour and 20 to 80 feet per minute, that many fell below the recommended lower limits, and that a cleanliness class cannot be tied simply to one air velocity or air change rate.
  • ASHRAE Handbook, FundamentalsASHRAE · standards body publicationASHRAE reference volume carrying the psychrometric relationships, the climatic design condition tables and their annual percentile basis, duct and pipe fitting loss coefficients, the bin and degree-day methods used for seasonal energy estimation, the table of heat gain from occupants by activity including light bench work at 275 BTU per hour sensible and 475 latent, the standard-air sensible and latent heat factors, and the orifice flow relationship used to estimate airflow through openings and gaps from pressure difference.