Most cleanrooms today are built with HEPA fan filter units in ceiling to provide air filtration for cleanroom class. The advantages of the negative pressure plenum design modular cleanrooms over the old Air Handling units ducted cleanroom design include lower cost, easier install, more robust, and easier to upgrade.

Advantages of using HEPA fan filter units in a negative pressure plenum:

• Lower cost. Elimination of long lead time expensive custom air handlers. The HEPA fan filter unit provides the filtered air movement so lower cost short lead time commercial HVAC DX systems can be used to cool the cleanroom.
• Easier installation and less maintenance: Each HEPA fan filter unit has its own fan pushing ~ 600 CFM of HEPA filtered air locally, eliminating the problem of balancing HEPA air flow for long cleanroom duct runs.
• More robust: The cleanroom is not dependent on single piece of custom equipment (air handler) for filtration air flow. If a HEPA fan filter unit fails the other HEPA fan filter units in the cleanroom will continue to provide HEPA filtered air. The failed HEPA fan filter unit is easily replaced.
• Lower cost: There is no need for ducting to each HEPA fan filter unit. The air easily flows thru the cleanroom plenum to each unit.
• Easy to Upgrade: The cleanroom can be easily upgraded to higher cleanroom classification by adding more HEPA fan filter units into plenum and more return air walls
• Options: special variants of HEPA fan filter units for cleanrooms include
  • ULPA filters
  • Room side replaceable filters
  • 2’x2’, 2’x4’ and 4’x4’ size
  • Energy saving DC motors
  • Test ports
  • Indictor lights

Disadvantages of air handler based ducted cleanrooms:

• Long lead times and high cost for custom cleanroom air handler units to drive high air volume thru HEPA filters. Standard commercial air handlers will not work.
• Hard to get air to flow evenly to HEPA filters long distance from air handler
• Cleanroom Air handler is single point of failure
• Cleanroom requires frequent balancing
• Cost of ducting to each HEPA filter
• Very expensive and difficult to upgrade cleanroom class

Situations where air handler based ducted HEPA filter cleanrooms still used:

• Hazard chemical cleanrooms where explosion risk dictates that electric motors be located away from air stream
• Tight temperature tolerance cleanrooms (+1 F) where the temperature of the air can be affected by presence of electric motors in HEPA fan filter units
• Non critical ISO-8 cleanrooms where single HEPA filter can be placed in Air Handler. The Air Handler must be custom designed for correct CFM and static pressure drop

Summary

Most cleanrooms today are built with HEPA fan filter units in ceiling to provide air filtration for cleanroom class. The advantages of the negative pressure plenum design modular cleanrooms over the old Air Handling units ducted cleanroom design include lower cost, easier install, more robust, and easier to upgrade. There are still some circumstances where air handler unit centric ducted HEPA filtration is used.