Cleanroom HEPA Filter Overview

Cleanroom HEPA Filter Overview

Cleanrooms vary in just how clean they need to be, and what is meant by clean.  Typically Cleanrooms in the United States are designed by using US FED STD 209E (which has long been discontinued but still referenced) and ISO 14644-1.  There is no set cleanroom HEPA filter based on the classification, in fact the same filter can be used across different classifications.  The ISO standard is a bit more rigorous and the lower classes in ISO 14644-1 also allow for cleaner spaces than what can be achieved following the 209E standard.

 US FED STD 209E has particle count thresholds based on Cubic Feet

Class0.1µ Threshold0.2µ Threshold0.3µ Threshold0.5µ Threshold5.0µ Threshold

ISO 14644-1 has particle count thresholds based on Cubic Meters

Class0.1µ Threshold0.2µ Threshold0.3µ Threshold0.5µ Threshold1.0µ Threshold5.0µ Threshold
ISO 110n/an/an/an/an/a
ISO 21002410n/an/an/a
ISO 31,00023710235n/an/a
ISO 410,0002,3701,02035283n/a
ISO 5100,00023,70010,2003,520832n/a
ISO 61,000,000237,000102,00035,2008,320293
ISO 7n/an/an/a352,00083,2002,930
ISO 8n/an/an/a3,520,000832,00029,300
ISO 9n/an/an/a35,200,0008,320,000293,000
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 Notice how all of these standards are only measuring particles, and not addressing molecular pollutants.  Depending on what the owner is doing inside the room there could be a compelling reason to address molecular pollutants.  For example, if there is risk of damage from oxidizing agents like Ozone or Nitrogen Oxides (both are fairly common in atmospheric air) then having molecular filters that go above and beyond what is described in ISO 14644-1 is advisable.   Control of molecular pollutants is particularly of interest in Microelectronics manufacturing facilities where things like superconductors, flat panel displays, hard drives, and solar panels are made.

 Energy Efficiency?  With how much expensive cleanrooms are to build and how much energy they consume it is often difficult for owners to spend extra money when building the facility, especially when it comes to filters which are not typically seen as a potential energy saving opportunity.  The truth is you can have a large impact on energy consumption with the selection of a HEPA filters or ULPA filters by paying attention to two main areas: Pleat Spacing and Pack Depth.  An easy way for a company to “win” a bid is to put a smaller media pack in a bigger filter depth or space pleats farther apart in order to lower costs and offer a lower price.  Though this is clearly unethical most people do not pay much attention to the filters as there are a lot of other moving parts within these projects.

 Pack depth usually is between 20mm and 100mm with 41mm and 53mm depths being the most common.  When optimizing for energy efficiency and filter life the 100mm packs always offer the premium solution, but due to higher costs even sustainability minded designers settle for a 70mm pack depth.

Static Pressure Comparison in inches w.c.
Pack Depth53mm70mm100mm
99.99% @ 0.3µ0.490.380.29
99.9995% @ MPPS0.680.480.36
Improvement Over 53mm Depth
Pack Depth70mm100mm
99.99% @ 0.3µ22%41%
99.9995% @ MPPS29%47%

Clearly the 100mm pack depth should be used more often during the design phase, as they offer 41% to 47% lower resistance to airflow than the more common 53mm packs. 

 Fan Powered Vs Ducted:  Depending on the size of the space the cleanroom filters could be either ducted and get their air from a larger HVAC or Make-up air system or from drawing from an interstitial space with a fan powered unit.  Either way the use of a ceiling mounted terminal diffuser is advised as opposed to traditional 12” deep filters that would be installed in the ductwork of the supply air.  Ceiling HEPA diffusers offer easier testing/certification process and a higher assurance that the air filling the room will be treated by the filter.  Because of the possibility of infiltration that ductwork has (even if it’s sealed or positive pressure) or can develop over time can be avoided by simply using a terminal unit in the ceiling that incorporates both a diffuser for laminar flow and an ULPA or HEPA filter to combine two needed items into one.

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