HEPA : High Efficiency Particulate Air Filters. ULPA: Ultra Low Penetration Air.
HEPA filters have been recognized by the Center for Disease Control (CDC) to play an important role in the containment of airborne infectious pathogens and harmful airborne particles.
Mikropor HEPA filters are designed for use in hospitals, medical facilities, laboratories, and commercial areas for highly efficient particulate and pathogen removal.
HEPA filters can remove at least 99.97% of airborne particles 0.3 micrometers (μm) in diameter. Particles of this size are the most difficult to filter and are thus considered the most penetrating particle size (MPPS). Particles that are larger or smaller are filtered with even higher efficiency.
HEPA filters are composed of a mat of randomly arranged fibres. Key metrics affecting function are fibre density and diameter, and filter thickness. The air space between HEPA filter fibres 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. Just as for membrane filters, particles so large that they are as wide as the largest opening or distance between fibres cannot pass in between them at all. But HEPA filters are designed to target much smaller pollutants and particles are mainly trapped (they stick to a fibre) by one of the following three mechanisms:
- Interception, where particles following a line of flow in the air stream come within one radius of a fibre and adhere to it.
- Impaction, where larger particles are unable to avoid fibres by following the curving contours of the air stream and are forced to embed in one of them directly; this effect increases with diminishing fibre separation and higher air flow velocity.
- 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 behaviour 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 initial filter air flow resistance and final filter air flow resistance are typically measured as pressure drop across the filters.
An ULPA filter (theoretically) can remove from the air at least 99.999% of dust, pollen, mold, bacteria and any airborne particles with a size of 120 nanometres or larger.
High‐efficiency particulate air (HEPA) filters and ultra‐low particulate air (ULPA) filters are designed to screen particles larger than 0.3 μm. HEPA filters are 99.99% efficient in removing particles 0.3 μm and larger. ULPA filters are 99.9995% efficient in removing particles 0.12 μm and larger. When used with a prefilter, high‐efficiency particulate air (HEPA) filters and ultra‐low particulate air (ULPA) filters can last for several years in a cleanroom environment. Moreover, because airflow capacity decreases and static pressure increases over time, HEPA filters and ULPA filters can become more efficient as the filter loads.
High‐efficiency particulate air (HEPA) filters and ultra‐low particulate air (ULPA) filters are made of a spun‐hooked glass fiber that is rolled into a paper‐like material which is then pleated to increase its surface area. Both HEPA filters and ULPA filters are available from a variety of suppliers, many of whom comply with standards from organizations such as the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE); the American Institute of Architects; (AIA) and the Joint Commission for the Accreditation of Healthcare Organizations (JCAHO).
High‐efficiency particulate air (HEPA) filters and ultra‐low particulate air (ULPA) filters are used in air filtration and purification systems to control airborne particulate levels and to stop the spread of toxic agents and infectious diseases. HEPA filters are often used in HVAC systems, biological safety cabinets, laminar flow hoods, and healthcare facilities. Biological safety cabinets use HEPA filtration to exhaust contaminated air and prevent air from a surrounding area from contacting a work surface.
High‐efficiency particulate air (HEPA) filters and ultra‐low particulate air (ULPA) filters that are used with laminar flow hoods perform a similar function, but are not designed to control infectious diseases or toxins. Medical facilities such as isolation rooms, intensive care units (ICU), and operating rooms also use HEPA filters and ULPA filters to remove infection and contamination by airborne microorganisms such as molds and spores.
There are several important specifications to consider when looking for high‐efficiency particulate air (HEPA) filters and ultra‐low particulate air (ULPA) filters. These include porosity, efficiency, filter length, height and width or ring size, and ply. Filter porosity is measured in average number of pores‐ per‐linear‐inch (ppi). The range of ppi provides an open cell with little air restriction to fine‐celled foam for maximum particulate filtration. Efficiency is the ability of HEPA filters and ULPA filters to remove a certain amount of particles of a certain size at certain airflow. Ply refers to the layers of media contained in the filter. Ply can be anywhere from one up to five and sometimes greater than five.
- Microelectronics (eg. semiconductor cleanrooms)
- Bio and gene technology
- Chemical industry
- Nuclear air ventilation
- Waste incinerators
- Hospital operating rooms
- Emergency burn centers
- Medical industry
- Food industry
- Optical industry