O ar de sangria é filtrado usando filtros de ar. Várias empresas oferecem muitos filtros de ar, mas o HEPA (High-efficiency particulate arrestance) é um filtro bastante comum usado para filtrar o ar sangrado.
Seu trabalho é como abaixo:
HEPA filters are composed of a mat of randomly arranged fibres. The
fibres are typically composed of fiberglass and possess diameters
between 0.5 and 2.0 micrometers. Key factors affecting function are
fibre diameter, filter thickness, and face velocity. 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. Unlike
membrane filters at this pore size, 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 fibre)
through a combination 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 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 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 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. (ZAND)
O HEPA realmente se parece com isso:
As especificações da HEPA são as seguintes:
HEPA filters, as defined by the DOE standard adopted by most American
industries, remove at least 99.97% of airborne particles 0.3
micrometers (µm) in diameter. The filter's minimal resistance to
airflow, or pressure drop, is usually specified around 300 Pa at its
nominal flow rate.
The specification usually used in the European Union is the European
Norm EN 1822:2009. It defines several classes of HEPA filters by their
retention at the given most penetrating particle size (MPPS):
particle size (MPPS):
HEPA class retention (total) retention (local)
E10 >85% ---
E11 >95% ---
E12 >99.5% ---
H13 >99.95% >99.75%
H14 >99.995% >99.975%
U15 >99.9995% >99.9975%
U16 >99.99995% >99.99975%
U17 >99.999995% >99.9999%
The original HEPA filter was designed in the 1940s and was used in the
Manhattan Project to prevent the spread of airborne radioactive
contaminants. It was commercialized in the 1950s, and the original
term became a registered trademark and a generic term for highly
efficient filters. Over the decades filters have evolved to satisfy
the higher and higher demands for air quality in various high
technology industries, such as aerospace, pharmaceutical processing,
hospitals, health care, nuclear fuels, nuclear power, and electronic
microcircuitry (computer chips).
Today, a HEPA filter rating is applicable to any highly efficient air
filter that can attain the same filter efficiency performance
standards as a minimum and is equivalent to the more recent NIOSH N100
rating for respirator filters. The United States Department of Energy
(DOE) has specific requirements for HEPA filters in DOE regulated
applications. In addition, companies have begun using a marketing term
known as "True HEPA" to give consumers assurance that their air
filters are indeed certified to meet the HEPA standard.[4]
Products that claim to be "HEPA-type", "HEPA-like", "HEPA-style" or
"99% HEPA" do not satisfy these requirements and may not have been
tested in independent laboratories. Some of these sub-par quality
filters may come reasonably close to HEPA filtration, while others
will fall significantly short, making them truly inferior.[5]
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