hepa filters and ulpa filters selection guide     hepa filters and ulpa filters selection guide

Image credit: N.R. Murphy Ltd. | ProVent

 

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.

 

HEPA Basics

 

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.

 

 

where:

 

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.

 

hepa and ulpa filters selection guide

A fibrous filter's media as seen through an electron microscope.

Image credit: ECAT

 

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.

 

hepa and ulpa filters selection guide

The three filtration methods described above (plus an electrostatic method).

Image credit: Totobobo

 

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.

 

hepa and ulpa filters selection guide

A HEPA performance graph, showing the steep drop in efficiency around 0.1 μm.

Image credit: Berriman

 

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.

 

hepa and ulpa filters selection guide

Image credit: Sentry Air Systems

 

 

Specifications

 

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

≥ 99.999995

 

Table source: AAF International  

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.

 

References

 

Camfil Farr - A science-based approach to selecting air filters (pdf)

 

John Larzelere - New and novel technologies in particulate filtration (pdf)