High efficiency particulate air (HEPA) filters and ultra-low particulate air (ULPA) filters are air filters designed to trap a vast majority of very small particulate contaminants from an air stream.
Definition and Efficiency:- Air filters must satisfy certain standards of efficiency most commonly those developed by the US Department of Energy (DOE) in order to qualify as a HEPA filter. The US standard (DOE-STD-3020-2005) requires that a HEPA filter be capable of removing 99.97% of contaminant particles 0.3 μm in diameter. Most standards also specify that HEPA filters must feature minimal pressure drop and maximum airflow when in operation.
A filter’s percent efficiency can be calculated using the simple equation below.:-
E = percent efficiency
D = downstream concentration (of contaminants)
U = upstream concentration (of contaminants)
Particle Size and Filtration Method:- While the US HEPA standard usage of 0.3 micrometer particles to describe efficiency may seem arbitrary, particles of this size are actually the most difficult to filter, rendering them a kind of “worst-case scenario” reference particle. The reasons for this difficulty in filtration are described below.
HEPA filter media is made up of countless randomly-arranged fibers which together form a dense mat; when air flows through the filter, the media captures and contains contaminant particles throughout its depth.
Filter fibers trap contaminants using three primary methods:
- Interception takes place when a contaminant particle passes within the distance equal to one particle’s radius of a filter fiber, resulting in it touching the fiber and being removed from the airflow. Particles further than one particle radius from a fiber will not be trapped.
- Inertial impaction occurs when a large particle, unable to adjust to the change in air direction near a filter fiber, becomes trapped on the fiber. The particle’s inertia ensures that it continues along its original path instead of circumventing the fiber, resulting in its capture.
- Diffusion relies on the Brownian motion of gas particles. Small particles (typically 0.1 μm or less) tend to travel on a streamline in an erratic fashion, making random motions as they interact with gas molecules. This erratic motion causes the contaminant particles to become stuck to filter fibers.Understanding these three methods makes it clear why particles around 0.3 micrometers are most difficult to filter. Particles less than 0.1 micrometers are easily trapped due to diffusion while particles larger than 0.4 micrometers are trapped by inertial impaction. Particles between 0.1 and 0.4 μm are therefore too large for effective diffusion and too small for inertial impaction and efficient interception, so that the filter’s efficiency drops within this range. By specifying a HEPA filter’s efficiency at 0.3 μm, standards bodies are really describing a variant of the filter’s minimum efficiency.
Authenticity:- While HEPA products designed for industrial, military, and government applications are explicitly certified, some cheaper consumer air purifiers and filters are marketed as “HEPA-type” and are often touted as capable of “removing 99.97% of dust and allergens” without specifying particle size. The growth of these marketing devices has led some manufacturers to use the term “True HEPA” to describe filters and purifiers manufactured and tested to DOE or EN standards.
ULPA Basics:- Ultra-low particulate (or sometimes “penetration”) air (ULPA) filters are closely related to HEPA filters but are even more efficient. ULPA filters are specified to remove 99.999% of contaminants 0.12 μm or larger in diameter. The chart below shows the overlap in the capabilities of ULPA and HEPA filters.
European Standards for HEPA and ULPA: – The European Union standard for both HEPA and ULPA filters EN 1822 classifies filters into different classes depending on their efficiency. All EN 1822 specifications are based on a filter’s ability to trap and contain the most penetrating particle size (MPPS) particular to the filter. The MPPS is typically determined by a laser spectrometer or electrostatic classifier. European filter classes and relevant specs are listed in the table below. Note the radical difference between the European and American definition of HEPA efficiency, particularly that the EU standard permits HEPA filters with efficiencies as low as 85%.
Classification Filter type Percentage efficiency at MPPS E10 HEPA ≥ 85 E11 HEPA ≥ 95 E12 HEPA ≥ 99.5 H13 HEPA ≥ 99.95 H14 HEPA ≥ 99.995 U15 ULPA ≥ 99.9995 U16 ULPA ≥ 99.99995 U17 ULPA ≥ 99.99999
MPPS:- It stands for most penetrating particle size. This refers to the dimensions of those particles that are the most difficult to trap. It generally lies in the region of 0.1 to 0.2 microns (µm). The MPPS must be established before subjecting a filter to tests.
Applications:- HEPA and ULPA filters are used in applications requiring very efficient filtering of airborne pathogens which can cause aggravate asthma and cause allergies or disease. These filters are also useful in manufacturing environments which require very clean air. Some applications include:
- Airline cabin air purifiers
- Biomedical air filtration
- Electronics manufacturing
- Pharmaceutical manufacturing
- Vacuum cleaner filters
Standards:- Standards are particularly important to the manufacturing, use, and testing of HEPA and ULPA filters. The standards listed below are commonly referenced.
- IEST RP-CC001:- HEPA and ULPA filters
- IEST-RP-CC007:- Testing ULPA filters
- ASTM F1471:- Standard Test Method for Air Cleaning Performance of a High-Efficiency Particulate Air Filter System
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