Filter performance

The NAFA Guide to Air Filtration (1993) describes a method for evaluating HEPA filter performance.

The DOP [dioctyl phthalate] test does not measure efficiency, rather it measures penetration, i.e., the fraction of DOP which passes through the filter. Efficiency is determined by subtracting the penetration (percent) from 100%. The original maximum [HEPA] penetration was 0.5% so that the efficiency was 99.95% (100-0.05). Later improvements in media manufacture and materials allowed efficiency to be extended to 99.97% without any increase in pressure drop.

For every filter operating at a specific cfm rating, there is a most-penetrating particle size (mpps). This is the size particle on which the filter efficiency is the lowest. Efficiency will be higher on particles both larger and smaller than this mpps. This is not a significant concern for typical ventilation filters, but is important wherever high collection efficiency on small particles is required, such as in the case of HEPA filters. Particles 0.3 micron in size were originally chosen as the test challenge because it had been calculated that this would be the most-penetrating particle size for HEPA filters. Filter media used in HEPA filters actually have a higher efficiency (lower penetration) than that guaranteed for the filter. This difference allows for the pinhole leaks which can occur in the media or the leaks where the pack is sealed to the filter frame. As a result, the guaranteed efficiency makes no claim as to where the penetration is coming from.

When laminar flow equipment was introduced for cleanrooms, the penetration location in HEPA filters became a matter of concern. Wherever there was leakage, a stream of unfiltered air could pass through these leakage openings. Because the flow was "laminar" (non-turbulent) the stream of dirty air remained intact, potentially contaminating a product in the "clean" air stream. This resulted in the requirement that a HEPA filter for use in a laminar flow application had to be leak-free. When leak-free filters were made, their minimum efficiency was 99.99% Leakage came either from "pinhole" leaks too small to detect or from the limited capability of the filter media.

From: A Design Guide for Energy-Efficient Research Laboratories

Cleanroom Related Links   A-Team | EETD | LBNL | Search the EETD Server