HEPA filters are used in cleanrooms in many different industries, including semiconductor, pharmaceutical medical devices, nuclear, and biotechnology. The main function of a HEPA filter is to provide clean air to the cleanroom. The HEPA filter is constructed with many pleated layers of filter media paper; this design prevents particles from freely passing through the filter as they 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 is defined as when a particle is too large and becomes trapped between two filter fibers. Impaction is when a particle of relatively greater mass is unable to follow the curved streamline around the fiber and, as a result of momentum, travels in a straight line into the filter fiber and sticks. Interception occurs when a section of a particle “runs into” a filter fiber. Diffusion capture occurs when particles leave the streamline due to random collisions with the surrounding fluid molecules and strike the fibers, where they again stick.

LEAK TESTING
ISO 14644-2 outlines the frequency of cleanroom validation according to cleanroom classification. Part of this validation includes leak testing of the HEPAfilter. ISO 14644-3 outlines the testing procedure to follow.

EQUIPMENT REQUIRED
Aerosol Generator: used to produce an aerosol upstream of the HEPA filter.A stable test aerosol has particles that have the following distribution:

• 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 the aerosol meets this distribution.

Photometer: used to measure the upstream aerosol concentration and downstream penetration of the HEPA filter by the aerosol; it should be calibrated at least annually.

Particle Counter: used to measure the concentrations of particles of different sizes downstream of the HEPA filter; it should 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 the supplier, verifies that the aerosol generator is capable of producing a stable distribution.The photometer verifies that the sensor/optics and flow rates are within tolerances. Some new digital photometers now on the market have the ability to verify on “power up” the condition of the sensor/optics and flow rates as part of a self-diagnostics program and, therefore, improves the reliability of the unit continually instead of waiting annually for the calibration and hoping it passes. Photometer operators can be satisfied that the photometeris free from contamination around the sensor/optics, the flow path is unobstructed,the unit is performing correctly, and that the results are accurate.