A high efficiency particulate air or HEPA (pronounced /ˈhɛpə/) filter is a type of high-efficiency air filter. HEPA filters, by definition, remove at least 99.97% of airborne particles 0.3 micrometers (µm) in diameter.
HEPA filters are composed of a mat of randomly arranged fibers. The fibers are typical composed of fiberglass and possess diameters between 0.5 and 2.0 micron. Key metrics 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, where particles as wide as the largest opening or distance between fibres 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 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 0.3 μm MPPS, diffusion and interception predominate. The filter resistance to flow is known as its pressure drop.
HEPA filters are critical in the prevention of the spread of airborne bacterial and viral organisms and, therefore, infection. Typically, medical-use HEPA filtration systems also incorporate high-energy ultra-violet light units to kill off the live bacteria and viruses trapped by the filter media. Some of the best-rated HEPA units have an efficiency rating of 99.995%, which assures a very high level of protection against airborne disease transmission.
PM400 - MiracleAir
CM12 - MiracleAir
Second Source Parts
Second Source Service