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What Air Filter(s) Do I Need?

May 31, 2013

What Air Filter(s) Do I Need?

How to choose the correct air filters for CRAC units within a Data Center

As previously discussed, air filters used in data center CRAC units are specialized for particle removal efficiency and resistance to air flow (pressure drop).  Similar to our comments about the filters in home heating and air conditioning systems, proper maintenance is required for problem free operation in both cases. But when it’s time to change the filter, how do you know which to select from the myriad of choices?

To reiterate, removing particles is important to insure the heat exchanger is keep clean and working at optimal efficiency.  It's therefore important to choose filters that meet the manufacturer’s particle removal specifications. These specifications are often in terms that are not commonly used or understood, so what do they mean in simple terms? The specifications are typically describing the industry standard for testing of the "particle removal efficiency" of air filters.These are geographically based suggestions, and may vary based on location. The terms that are relevant for most applications are the following:

  • MERV: Minimum Efficiency Reporting Value from ASHRAE Std. 52.2 - 2007 Testing Method
    • Filters rated from MERV 1 to MERV 16, commonly used in USA
  • CEN: European Committee for Standardization from EN779:2012 Testing Method
    • Filters are rated G, M, F, and U (clean room applications), commonly used in EU
  • ISO: International Organization for Standardization from ISO 14644-1
    • Filters are rated ISO 1 to ISO 9, International Standard often used in clean rooms, frequently referenced by electronic equipment manufacturers

When selecting an air filter to replace the existing CRAC unit filters, use of the OEM filter will insure that the filter meets both particle removal and pressure drop specifications as required. However, because the particle types and concentration in the air can vary from location to location, the life of the filter will need to be determined by monitoring the pressure instrumentation of the CRAC unit. For example, arid climates may have a greater loading of sandy dust, whereas wetter environments may find many pollen and plant fibers.  The number of individuals that enter or leave the IT space, the leak integrity of the room, and the particle removal effectiveness of the makeup air handler can all play an important role in the actual life of the filters. CRAC OEMs can provide guidance about when to change filters and options for extended filter life (if needed).  Third party suppliers can also offer a wealth of information. One can select the optimum filter for any particular location by using comparative filter testing data and gathering information on vendors and available filter types.

If an alternate filter is selected, it's important to monitor the operating parameters of the CRAC unit.  Filters with greater pressure drop will require more fan capacity (energy). CRAC units with VFD fans will automatically accommodate the change within the control range. On the opposite end, Non-VFD units may require manual adjustment.  The ultimate goal is to ensure the fan is not operating at 100% capacity with a new filter, since there will be no additional capacity to overcome the increase in pressure drop as the filter accumulates dust. 

The following images are not necessarily from CRAC filter suppliers. They are intended to assist the reader in discussions about replacement air filters.


Different types of air filters that may be employed. 

Source: Link to Image 1

 Various particle capture mechanisms that may be employed in air filters. Generally smaller particles are removed more efficiently by mechanisms at the bottom of the chart.

Source: Link to Image 2

Graphical visualization of the types of particles found in households compared to various filter types.  CRAC air filters are generally in the Standard Filter category.  The source of this image markets Electrostatic Air Purifiers to control allergens. 

Source: Link to Image 3

References: Generally from air filter, equipment or cleaning services suppliers (Standards can be ordered from the respective organizations.)




  1. Flanders Filters: ASHRAE Std. 52.1 Comparison to Std. 52.2
  2. Camfil Farr Technical Services Bulletin ASHRAE Testing for HVAC Air Filtration A Review of Standards 52.1-1992 & 52.2-1999
  3. Camfil Farr presentation describing ASHRAE 52.2 with good technical  background information on particle sizes and particle removal mechanisms



  1. AAF International: EN779:2012 New European Standard for General Ventilation Filters
  2. Filtrair B.V The Netherlands presentation European Air Filter Test Standard EN779:2012 describing the background for filter testing and rating with a good technical description of test methods for both CEN and ASHRAE tests.
  3. CEN and ASHRAE  cross reference charts from:
    1. Camfil Farr
    2. Flanders Filters



  1. Fujitsu FTS Specification “Gaseous and Particulate Contamination Guidelines for Data Centers”
  2. IBM Systems Hardware information, Environmental design criteria
  3. Camfil Farr Clean Room Design Standards and Energy Optimization, describes ISO, ASHRAE and CEN particle specifications for pharmaceutical manufacturers, good technical particle source and control discussions for clean room environments with lessons for data center operators


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