HEPA filters are used in cleanrooms in many differentindustries, including semiconductor, pharmaceutical medicaldevices, nuclear, and biotechnology. The main function of a HEPAfilter is to provide clean air to the cleanroom. The HEPA filter isconstructed with many pleated layers of filter media paper; thisdesign prevents particles from freely passing through the filter asthey become trapped and stick onto the filter fibers (Figure 1).There are four mechanisms at work: capture by straining, impaction,interception, and diffusion (Figure 2). Straining/sieving isdefined as when a particle is too large and becomes trapped betweentwo filter fibers. Impaction is when a particle of relativelygreater mass is unable to follow the curved streamline around thefiber and, as a result of momentum, travels in a straight line intothe filter fiber and sticks. Interception occurs when a section ofa particle “runs into” a filter fiber. Diffusion capture occurswhen particles leave the streamline due to random collisions withthe surrounding fluid molecules and strike the fibers, where theyagain stick.
LEAK TESTING
ISO 14644-2 outlines the frequency of cleanroom validationaccording to cleanroom classification. Part of this validationincludes leak testing of the HEPAfilter. ISO 14644-3 outlines thetesting procedure to follow.
EQUIPMENT REQUIRED
Aerosol Generator: used to produce an aerosol upstream of the HEPAfilter.A stable test aerosol has particles that have the followingdistribution:
- More than 20% by mass of particles less than 0.5 µm
- More than 50% by mass of particles less than 0.7 µm
- More than 75% by mass of particles less than 1.0 µm
An annual output test should be carried out to verify that theaerosol meets this distribution.
Photometer: used to measure the upstream aerosolconcentration and downstream penetration of the HEPA filter by theaerosol; it should be calibrated at least annually.
Particle Counter: used to measure the concentrations ofparticles of different sizes downstream of the HEPA filter; itshould be calibrated at least annually.
Figure 1: Filter media magnification x500
Figure 2: Filter mechanisms at work
AEROSOL GENERATOR OUTPUT TEST
The aerosol generator output test, carried out annually by thesupplier, verifies that the aerosol generator is capable ofproducing a stable distribution.The photometer verifies that thesensor/optics and flow rates are within tolerances. Some newdigital photometers now on the market have the ability to verify on“power up” the condition of the sensor/optics and flow rates aspart of a self-diagnostics program and, therefore, improves thereliability of the unit continually instead of waiting annually forthe calibration and hoping it passes. Photometer operators can besatisfied that the photometeris free from contamination around thesensor/optics, the flow path is unobstructed,the unit is performingcorrectly, and that the results are accurate.
TEST METHOD
A linear photometer may be used to measure upstream concentrationsprovidedit is calibrated.
This upstream concentration may be used as the 100% reference,enabling percentage penetration downstream to be measured directly.The linear photometer may be used to test filters to 0.01%.
TEST REQUIREMENTS
Each filter requires an upstream challenge of 20–100 µg/L for bestresults. One must find the area of the filter and the airflowthrough the filter before calculating the throughput of the filter.The product of throughput and concentration are required to givethe aerosol output required. Alternatively,the aerosol can beadjusted to reference the photometer at 100%.
TEST PROCEDURE
Disperse the test aerosol upstream of the HEPA filter to produce auniform challenge concentration in the region of 20–100 µg/L. Usingthephotometer, measure the upstream challenge concentration.
Maintain this concentration throughout the test. Adjust theaerosol generator such that the challenge concentration at theupstream filter face is at a level such that the photometer can beset and maintained at a stable reading of 100%. The photometer isthen set at 100%.
Using the same photometer, scan the entirety of the downstreamface and perimeter inclusive of the sealing device with thesampling probe. Hold the probe approx 25 mm away from the areatested and pass over the entire area in slightly overlappingstrokes, at a rate of 5 cm/sec. Record the location of any steady,repeatable reading of the photometer that exceeds 0.01% for gradesA through D in the relevant class of environmental cleanliness.Refer to ISO 14644-3 section B.6.2.5 for a more detailed procedure.
GUIDELINES
Care should be taken when generating the upstream aerosol as toomuch aerosolwill over-concentrate the filter and filter replacementwill be necessary; use too little aerosol and there may beinsufficient aerosol to effectively scan the filter sufficiently.Discuss these scenarios with to reach an agreementregarding theupstream concentration.
Once a leak has been detected, repair of the filter will benecessary, following the manufacturer’s procedure regarding thetype of repair media and guidelines on resealing the filter andperforming tests after the repair. After successful testing, aparticle counter positioned under the filter can evaluate thecondition of the filterand can be used as a back up to thephotometer.
References
- ISO 14644-3
- ISO 14644-2
- IEST-RP-CC034
- IEST-RP-CC007
Jason Kelly is the Director of OptiCal Sciences,IrelandLtd.and has been working in the semiconductor andpharmaceutical/medical device cleanroom industry for the last tenyears.He can be reached at www.optical-sciences.ie.