Hepa and Ulpa Filters
HEPA and ULPA Filters are two components of our NBC filtration system. Many air filtration systems use HEPA and ULPA Filters as a stand alone filtration system. Rising S Bunkers uses HEPA & ULPA Filters along with TEDA carbon to create the best solution for your NBC filtration needs.
HEPA stands for High Efficiency Particulate Air filter. A HEPA filter is a type of air filter meeting certain standards of efficiency such as those set by the United States Department of Energy. To qualify as HEPA by US government standards, an air filter must remove (from the air that passes through) 99.97% of particles that have a size of 0.3 micrometres. (American Society of Mechanical Engineers, ASME AG-1a–2004, “Addenda to ASME AG-1–2003 Code on Nuclear Air and Gas Treatment”, 2004)
HEPA filters are composed of a mat of randomly arranged fibers. The fibers are typically composed of fiberglass and possess diameters between 0.5 and 2.0 micrometers. Key factors affecting function are fiber diameter, filter thickness, and face velocity. The air space between HEPA filter fibers is much greater than 0.3 μm. The common assumption that a HEPA filter acts like a sieve where particles smaller than the largest opening can pass through is incorrect. Unlike membrane filters at this pore size, where particles as wide as the largest opening or distance between fibers cannot pass in between them at all, HEPA filters are designed to target much smaller pollutants and particles. These particles are trapped (they stick to a fiber) through a combination of the following three mechanisms:
1. Interception, where particles following a line of flow in the air stream come within one radius of a fiber and adhere to it.
2. Impaction, where larger particles are unable to avoid fibers by following the curving contours of the air stream and are forced to embed in one of them directly; this effect increases with diminishing fiber separation and higher air flow velocity.
3. Diffusion, an enhancing mechanism that is a result of the collision with gas molecules by the smallest particles, especially those below 0.1 µm in diameter, which are thereby impeded and delayed in their path through the filter; this behavior is similar to Brownian motion and raises the probability that a particle will be stopped by either of the two mechanisms above; it becomes dominant at lower air flow velocities. Diffusion predominates below the 0.1 μm diameter particle size. Impaction and interception predominate above 0.4 μm. In between, near the most penetrating particle size (MPPS) 0.3 μm, both diffusion and interception are comparatively inefficient. Because this is the weakest point in the filter’s performance, the HEPA specifications use the retention of these particles to classify the filter.
Lastly, it is important to note that HEPA filters are designed to arrest very fine particles effectively, but they do not filter out gasses and odor molecules. Circumstances requiring filtration of volatile organic compounds, chemical vapors, cigarette, pet, and/or flatulence odors call for the use of an activated carbon (charcoal) filter instead of or in addition to a HEPA filter.
ULPA is an acronym for “ultra-low penetration air” Or “ultra-low particulate air”. A ULPA filter can remove from the air at least 99.999% of dust, pollen, mold, bacteria and any airborne particles with a size of 120 nanometres (0.12 micron) or larger. Even anthrax (which has one of the smallest particulates among the toxins) can’t pass through a ULPA filter.
WHY DOES IT MATTER IF ANTHRAX CAN BE FILTERED BY YOUR AIR FILTRATION SYSTEM?