What is a HEPA Filter?

WHAT IS A HEPA FILTER?

HEPA is the acronym for “High Efficiency Particulate Air” or “High Efficiency Particulate Arrestance”. The HEPA filter is a type of mechanical filter. It works by providing a barrier in the form of a very dense filter media which is made up of very fine fibers that trap practically all particles.

HEPA technology helps to protect advanced and sensitive manufacturing processes and protect people from microbiological contamination in research laboratories. HEPA filters are also used to eliminate infectious pollutants from the air in the health sector where the risk of infection is high such as operating theatres. They also protect the environment by eliminating polluting particles from industrial extraction systems.

HOW DOES A HEPA FILTER WORK?

Different types of filters use different mechanisms to capture particles. There are many theoretical and experimental studies on air filtration using fibrous media. A filter made up of fibres uses various mechanisms to trap particles, which are described below:

1. Straining: The straining mechanism is the effect that intuitively comes to our minds when we talk about an air filter. When a particle is larger than the distance between the fibres, this particle cannot pass through the gap and it gets captured by the filter. The straining mechanism is effective on particles greater than 5μ and typically only captures 1% of the particles in the airflow.

2. Inertia: The second filtration mechanism is known as inertia. Here, the momentum generated by the airflow causes the particle to hit the front part of the filter fibres. Large and high-density particles tend to be trapped by inertia. When the airflow passes through the filter media, it passes around the fibres. The rapid change of direction of the airflow and the principle of inertia make the particle separate from the air stream and hit the fibre. This principle occurs when there is a large concentration of coarse particles. The inertial mechanism is effective on particles greater than 1μm and captures 1% of the particles in the airflow.

3. Interception: The third filtration mechanism is called Interception. To understand this concept, one must take into account how medium and small particles interact with the fibres. All particles and fibres have a small “positive or negative charge” and therefore have an inherent attraction for each other. This principle is known as “van der Waal’s law”. This mechanism occurs to a greater extent in synthetic materials. The particle follows the direction of the air flow. The particle follows the direction of the air flow. When the particle approaches the fibre at a distance smaller than the radius of the particle, it rubs against the filter material and gets retained by it. The particles trapped by this method adhere to all parts of the fibres: the front, back and sides. The interception mechanism is effective on 0.2 to 3 μm particles and captures 30-40% of the particles in the airflow.

4. Diffusion: The smallest particles in the airflow are trapped by the diffusion effect. These tiny particles travel in irregular paths due to the impacts that happen between them and with other molecules, in a similar way to gases. This movement is known as Brownian motion. Brownian motion is a mathematical model used to describe how particles collide with each other when moving at different speeds and in different, random directions. These irregular movements increase the chances of the particle coming into contact with the fibres and becoming trapped. The diffusion mechanism is effective on submicron particles between 0.001 and 0.2 μm and captures 60-70% of the particles in the airflow.

5. Electrostatic effect (it does not have an effect on HEPA filters) The fifth mechanism is called “electrostatic charge or effect”. The filter fibres are charged in a way that attracts the particles. This effect has a higher efficiency on smaller particles and a lower filter resistance. HEPA filters are rarely designed and manufactured to take advantage of this mechanism. Therefore, this mechanism falls outside the scope of this document.

HOW HEPA FILTERS ARE CLASSIFIED AND REGULATED

We know that the most important feature of an air filter is its efficiency, which measures its ability to remove airborne pollutants, such as dust, particles and gases. Other essential properties of a filter include pressure drop and dust holding capacity.

ALSO READ: Popular Questions and Answers for HVAC

To measure these properties, the air filtration industry needs regulations to ensure that filter testing is performed consistently and reliably. Standards based on documented knowledge allows users to classify and compare filters from different manufacturers.

These regulations help us to classify HEPA filters in different classes according to the Regulations of reference: 

1. EN1822 (Europe)
2. IEST-PR-CC001 (USA)
3. ISO 29463 (INTERNATIONAL)

In Europe, the EN 1822 standard is used. This standard divides the EPA, HEPA and ULPA high efficiency air filter tests into five parts. High-efficiency air filters are classified based on the MPPS (Most Penetrating Particle Size). Mandatory efficiency tests (leak) are required for class H13 filters and higher.

In the past it was believed that the most difficult particle size to filter was 0.3μm. The appearance of electronic microscopes and optical particle counters determined that the MPPS was between 0.12 and 0.25μm.

“ISO 29463 - High efficiency filters and filters to remove particles in the air” - is an ISO standard based on EN 1822. It is divided into five parts which are named in the same way as in EN 1822, although ISO 29463 includes additional classifications, i.e. 99.90%, which is ISO 30E.

IEST, an international engineering society based in the United States, has established various test methods. IEST-RP-CC001, 007, 021 and 034 refer to high efficiency air filters. This standard covers different areas, such as filter media performance, classification, design requirements, and filter media testing requirements.

ALSO READ: SOP for Cleaning and Operation of Air Handling Unit Pre-Filters

COMPONENTS OF A HEPA FILTER

Filter frame – The frames of HEPA filters can be made using a range of different material including aluminium, galvanised steel, plastic, stainless steel and wood. The frame construction can be determined by the application requirements.

Filter gasket – The filter gasket helps to seal your HEPA filter and to reduce and eliminate bypass by creating an air tight connection. Gaskets widely used are solid gaskets like PU, neoprene and silicone or gel type gasket.

Media separators – The media separators are used to open up the pleats to allow for higher dust holding capacity and less filter resistance. Aluminium, hot melt and glass fibre string are common media separators.

Filter sealant – The filter sealant is used to bind the filter media to the frame and seal up any bypasses. Polyurethane, silicone and ceramic sealants are all commonly used for HEPA filters.

FILTER MEDIA

The HEPA filter media is the fundamental part of the final HEPA filter and its here where the filtration properties are most important. For HEPA filters there are three different materials used that can be summed up below:

Glass fibre is the traditional HEPA media that has been used since the 1950s. This media has been the preferred media as it maintains its filtration efficiency throughout its lifetime and has a high dust holding capacity to ensure an optimal performance and increased product lifetime. Fragility of the media requires very careful and experienced handling as a heavy touch can damage the media and allow for filter leakages.

Membrane media was developed in the late 1990s. The premises of the introduction of this media was to bring a lower initial pressure drop for low energy consumption. Efficiency stability and lifetime often come as concerns with this media. Due to its inconsistent oil and particulate loading capabilities the efficiency and pressure drop ratings can be compromised during routine on site testing.

Multi-fibre polymeric media or Energuard media is the most recently developed media on the market. The goal of this media is to combine the benefits of glass fibre and membrane media and have none of their faults. Multi fibre polymeric media is typified by its low energy consumption, long filter lifetime and durability.

PROPERTIES OF A HEPA FILTER

The primary function of a HEPA filter is to capture the particulates that are in the air stream and ensure that the air provided within the environment is optimal. Not all HEPA filters are created equal and for this reason, HEPA filters should be evaluated on a number of key properties.

ALSO READ: Airflow Balance in HVAC