How does a HEPA filter work?

A HEPA filter is constructed such that air has to go through a thick fabric of fine fibers to enter the filter. Airborne particles are filtered by a variety of means like interception, inertial impact, and diffusion. H13 filters have 99.97% efficiency when filtering air with particles of 0.3 μm size which is a size of the maximum difficulty for the filter. Particles which are either greater or smaller have a lower difficulty rate because the processes of diffusion and impact become more successful.

What is "HEPA"?

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But what exactly is a HEPA filter? The description you see in the box around the paragraph is the short answer as to how a HEPA filter operates. What lies behind it surprises almost everyone the first time they stumble upon this. Most people think of it as some form of sieve — holes smaller than the particles we want to stop, but that's not exactly how it works.

Thus, the knowledge of how HEPA filter works leads to an understanding of a number of surprising things, such as why it works for particles smaller than 0.3 μm, why it "fails" at 0.3 μm, and why frame, sealant, and airflow is as important as the paper used inside. That’s why before making comparisons between products and prices, it pays to know how the HEPA filtration works.

The 99.97% Rule and Its Origin

The 99.97% @ 0.3 μm threshold can be traced back to the specification of the U.S. Department of Energy (DOE), which serves as the American benchmark for HEPA classifications. The equivalent definition of performance in Europe appears in EN 1822 [1] and on an international scale in ISO 29463 [2]. Among other things, the United States has IEST-RP-CC001 which serves as the standard for HEPA and ULPA filter performance. However, it should be noted that ASHRAE 52.2 [4] is a performance standard (MERV 1-16) for general ventilation filters, failing to provide a definition for HEPA classes thus preventing it from being used as a HEPA qualification standard.

According to the DOE standard, a HEPA filter must be able to capture 99.97% of airborne particles of 0.3 micrometer (μm) size. In accordance with EN 1822/ISO 29463, the efficiency is determined at the Most Penetrating Particle Size (MPPS) which falls in the range of 0.15 to 0.25 μm making it slightly lower than the classical 0.3 μm figure. The two approACHes fully coincide as their meanings coincide, with the only difference being the 0.3 μm parameter being usually preferred in the United States and MPPS approach being the official European and international method.

Why is this important? Because it is at the point of 0.3 μm (or at MPPS point) where a HEPA filter shows the lowest performance. If the filter works at the worst potential point, then everything else should go even better. The criteria used has been adopted to show the worst case of HEPA filters operation making it possible to answer the question “how do HEPA filters work in the worst conditions?”

Classes H10–U17 of HEPA

EN 1822 [1] classifies high-efficiency filters into groups, and each group is characterized by efficiency at MPPS:

One aspect to mention is the fact that efficiency is measured at the MPPS (typically 0.15-0.25 µm) and not at the steady rate of 0.3 µm in accordance with EN 1822/ISO 29463. 0.3 µm is used throughout this article because it is a considered to be a traditional DOE. The two methods show the same weakness in terms of performance, and the HEPA technology is the same in practice. H13 has been widely used in cleanrooms and also in HVAC field. Deiiang has developed HEPA membranes using H13 media with efficiency that ranges from 99.97% to 99.99% at 0.3 micron, which is a technology belonging to category H13 but at some instances it might be referred to H14 technology as well. Further studies on HEPA filters fall into ULPA category, since they work with particles of 0.12 microns and smaller which we will touch upon a bit later.

THE INDUSTRY TRAP "HEPA-type", "HEPA-like", and "HEPA at 99%" are the slogans that have no regulations. Normally this means availability of E10-E12 media with no possible confirmation of their properties. If the data sheet does not provide EN 1822 or ISO 29463 certificate confirming MPPS efficiency and the number, we can regard the HEPA box as a trademark.

HEPA Filter's Operational Principle: Mechanisms of Capture

Airlock Door. A High-Efficiency Particulate Air (HEPA) filter employs three ways of removing particles from the air at various sizes. The HEPA media includes a random mat of fibers that have a diameter that runs from 0.5 to 2 micrometers. The chaos present allows the air to pass through the filter fibers, without really passing through a single fiber – but instead passing through numerous fibers. 

What contributes to the efficiency of the HEPA filter is the contribution of all three mechanisms which make it possible for HEPA filters to work regardless of the diameter of the particles. 

Interception 

The mechanism of interception occurs because the particles can be moving in the air and may contact the fiber, thus becoming ‘stuck’ to it. The particles that undergo interception will have the diameter that ranges from 0.1 to 0.4 micrometers. These particles are not overly large, thus being able to make contact with fibers, but still are not overly small following the air. 

Inertial impaction 

When talking about the inertial impaction mechanism, one can say that the particles that tend to be bigger (with the diameter greater than 1 micrometer) have greater inertia. When the air is moving near the fiber, the particles continue moving straight ahead and hit the fiber. 

Diffusion 

The smallest particles (those with the diameter up to 0.1 micrometers) are randomly moving through Brownian motion that allows for their chances of hitting a fiber being much higher when compared to larger particles. 

What Makes 0.3 μm the Most Penetrating Particle Size (MPPS)

When particles reach 0.3 μm they switch from being able to diffuse well and having good size for inertial impacts as a well as engaging on the fibers through interception. Therefore, this is why the particles here have the biggest efficiency in penetration.

If someone wants to know how the HEPA filter works, it is better to show this curve first to make them understand the fact.

Inside a HEPA Filter: What Is There

When it comes to frame construction, the principle of physics is unaffected; What matters most here is the manufacturing quality which leads to the efficiency of the filter.

Media: Ultra-Fine Glass Fiber Paper

Such smooth transitions help explain the high efficiency of HEPA filters available to filter all kinds of particles.

Deiiang has introduced ultra-thin glass fiber papers for the HEPA and ULPA filters; in the case of ULPA filters U15-U17, American HV fiber sheets are employed featuring the smallest pores.

Pleats, separators, and geometry

Using flat media would result in having to ｒｅｐｌａｃｅ the filter very often, owing to the fact the mediumpaperhas been properly designed to utilize pleated structure allowing HEPA’s contact area with air to be multiplied up to 20-30 times.

Deep pleated HEPA filter cartridge, side view highlighting the pleats — this geometry of pleats makes it possible to have the surface area multiplied.

Hot-melt EVA ribbon separators provide free airflow, ensuring that air is directed to every tiny piece of tape, and its thickness defines the trapping capacity and the pressure drop.

Good pleat geometry is one of the reasons a HEPA filter can serve its purpose for years instead of weeks in a properly organized system.

Frames, Sealants, and Gaskets

The media is placed into a rigid frame and sealed completely — if there is even a minute bypass path, then the filter will not get its 99.97% rating. Deiiang's DOP integrated HEPA filter is a prime example of typical construction:

Moreover, the design includes an integral DOP test port for in-situ leak testing. Even a small detail that saves tremendous effort during cleanroom commissioning.

Frame and sealant types do not change the way that the HEPA filter operates mechanically. They help decide if rated efficiency is preserved during transport, installation, and years of temperature cycling.

DOP integrated HEPA air supply unit — metallic frame, continuous seal and integral testing port which makes in-situ DOP scanning possible.

What HEPA Filter Can – and Cannot Capture

Particle Types Captured

As catching is of purely mechanical type, because of HEPA filter there is practically no particle type in gas stream that cannot be captured. We've gathered this list based on what facility managers would like to have addressed at audits:

• Dust and coarse particulate matter (PM10 and PM2.5)
• Pollen (10 to 100 μm) and mold spores (2 to 10 μm)
• Bacteria (0.3 to 10 μm) usually carried by bigger droplets
• Virus-bearing aerosol droplets typically above 0.3 μm
• Combustion particles, toner and textile fibers
• Metal dusts in typical industry

Gases and VOCs - commonly held misconception

To eliminate odors, gases and VOCs activated and carbon filtration and chemisorption are used usually in combination with HEPA filters. If your supplier claims that HEPA filter can deal with scents on its own, keep it in mind as a warning. There is nothing soft about the way HEPA works — it works by mechanical capture only.

Sizes and How Much Air it Can Move

Below are some common configurations of Deiiang DOP HEPA:

Note how deeper pleats of 150 mm can accommodate air flow rates that are 1.5 – 2.5 times that of pleat sizes of 120mm needed for the same design. When a project requires more air in a lower ceiling space, it is more economical to go with deep pleats than using more housings.

Pressure Drop and Energy Cost

An H13 filtering system can typically start at about 200 – 250Pa of pressure drop at figure before rising toward a change-out figure of about 2 or 3 times the initial specific resistance of around 450 to 600Pa. Since the level of fan power is dependent on pressure, and many fans are installed deep into their charging curve, cost accounting is quite a saver.

The equation looks like this: increase in fan power = airflow in m³/s times the increase of retained pressure in Pa divided with fan efficiency.

• For example: 10,000 m³/h = 2.78 m³/s; and assuming an added 300 Pa ÷ 0.6 of the fan efficiency, the power will equal 1.4 kW
• Operating at 1.4 kW for 24 hours can result in annual energy consumption of about 12,200 kWh, which happens to cost over a set of new filters

Pre-Filters: How the HEPA Filter lasts longer

The cheapest way to ensure safety of the most costly filter in the system is to install lower-cost pre-filters in front of it. Coarse dust and fine dust constitute more than 95% of the total load of particles in typical recirculated air and each gram of it that settles on the HEPA filter media shortens the life of the filter. The principle of operation remains unchanged when a pre-filter is used — the HEPA filter works in the same way, but receives much less dirt.

The traditional approach is to use EN 779 bag filter stage F5–F9. Deiiang's medium effect bag filters are produced from synthetic or fiberglass filtering material, the largest size being rated 592×592×381 at 2,050 m³/h rate.

In this case, a lighter dust load is transferred to the HEPA filtering stage that only handles the particles that managed to go through it and mostly sub-micron particles. Guidelines from the industry:

• The F7 pre-filter can prolong the life of HEPA filter for two up to three times if there are a lot of dust particles present
• The process of replacement of pre-filter will cost just a small part of the price of HEPA filter replacement
• The omission of pre-filter replacement is among the most frequent design errors in pharma and electronics sector, and being one of the costliest
• F5-F6 is matched with all general ventilation systems, F7-F8 with all clean rooms, F9 with heavy loads from production plants

Comparison of ULPA and HEPA filters: is better always better?

ULPA filters (U15-U17, according to EN 1822 [1]) have a remarkable efficiency of more than 99.9995% when it comes to filtering of 0.12 micron particles. The working principle is the same as that of the HEPA filter, which ensures that both filter systems consist of fine fibers.

Here is the trade off:

Comparing HEPA filters. Two HEPA filter cartridges side by side. Even two HEPA filter cartridges with the same class ranking may have different pleat depth and filtration efficiency, thus having very different filtering capacity.

According to the cleanliness criteria applicable to iso 6-8 cleanrooms (iso 14644-1 [6]), HEPA H13 filter is adequate and economical. As far as U15 is concerned, it is used in semiconductor lithography, hard disk production and research where particles smaller than 0.1 micron can create difficulties.

If you want to know which of the two types of filters is the best, there simply is no better filter than the lower class filter that meets cleanliness requirements and is installed properly. A properly installed and tested H13 filter will work better than a badly installed U15 filter.

To obtain the proof: DOP testing

The Scan Test and the DOP Test Port

The efficiency standards are meaningless without verification on the spot. The standard approach is the DOP (Di-octyl phthalate) testing method. A polydisperse aerosol of DOP/PAO droplets in the size of 0.3 microns is directed to the air flow above the filter, and then the photometer is used to scan the filter face downstream.

By using the DOP/PAO scan test method, it is possible to find out if the filter is really a 99.97% one, and to learn how it functions with all the seals and gaskets in place. If the key to how HEPA filter works lies in DOP testing, the same logic was applied by Deiiang while implementing the test port in the filter housing. A HEPA filter may seem to work correctly right after it leaves the factory, but the only effective way of determining whether it actually operates well once installed is to carry out an in-situ test.

What Should Information on a Real Certificate Include

• Test method used (EN 1822-1 [1] / ISO 29463 [2]) and MPPS determining method
• Effective test efficiency of the filter with the reported efficiency like H13 ≥ 99.95% being mentioned
• Amount of airflow and initial pressure drop values
• Filtration media and frame material used
• Certificate serial number is easily tracked back to the HEPA filter installed

Real-Life Cases and Experiences of Deiiang Company

Where HEPA Filters Work

• Cleanrooms of iso classes 5 to 8 used in the electronics and pharmaceutical sectors — areas in which HEPA filters are at the heart of the system
• Isolation wards, operating rooms, and biosafety cabinets
• FFUs in modular cleanroom ceilings
• Final filtration in food and beverage filling
• Laboratory animal facilities and R&D laboratories

Experience of Retrofitting 300 m² Electronics Assembly Room

Common FFU HEPA module used in cleanroom ceiling. FFU/HEPA module in cleanroom ceiling — the same terminal-filtration approach used in the location retrofitting.

A medium-sized electronic assembly company contacted Deiiang™ after not passing particle count audit ISO 14644-1 [6] in their 300 m² assembly room. The facility had some basic panel filters with no effective pre-filtering. The HEPA filters were loaded within a few months, the pressure went above 600 Pa, and the particle count was out of spec.

Three factors caused complications with the project: lack of ceiling height for an additional filter bank, an immovable fan setup, and a completion schedule with only a 72-hour break.

Deiiang's head product designer Jason.peng created a refurbishment which avoided all three shortcomings. First, the old terminal filters were changed for 48 DOP integrated H13 filters (99.97% for 0.3 μm, 610×610×150 mm with 2500 m³/h each) capable of bringing the system back to the project design value of 25 air changes per hour. Second, there was included a pre-filtering F7 stage to all the circulation loops. Finally, differential pressure was tracked at both stages of filtration with alarming limits.

Maintenance and Lifespan

Assuming that the HEPA filter is operational, it does not deteriorate in terms of materials, but only collects dirt on its surface. The service life varies significantly:

• HEPA filters for cleanrooms (iso 5-8; with proper pre-filtration): service life of 3 to 5 years in normal conditions, or much longer if pre-filtration is kept in good maintenance and differential pressure does not exceed design parameters.

• HEPA cartridges for commercial or portable air purifiers: life expectancy of 6-12 months, as there are no proper pre-filtration and much higher dust load per square foot supported by the air filtering element.

• Heavy industrial environments without pre-filtration: as short as 6 months, driven almost entirely by dust loading rather than media degradation.

In all cases, differential pressure is the reliable indicator — not the calendar. Here's a procedure to follow:

Always remember to check delta pressure of each filter stage, not just HEPA filter

Change your HEPA filter once the pressure differential equals approximately 2–3 times the pressure differential in clean state

Never wash HEPA filter as this could tear fibers in the filter surface and tear the seal between medium and frame

Regularly check the condition and ｒｅｐｌａｃｅ pre-filters even if upstream filter surface seems clean

Do not forget to keep records of certifications

The molar mass, vibration, and density are the parameters that we examine most often regarding HEPA filtration performance.

Frequently Asked Questions

How does HEPA work?

Air is blown through a thick layer of randomly arranged glass fibers. It stops large dust particles, moderately-sized ones cause catching, and small ones only move randomly until they reach fibers. All three methods provide at least 99.97% efficiency at 0.3-micron particles level.

Do HEPA filters actually stop particles smaller than 0.3 microns?

It is certainly possible. At around 0.1 microns small dust particles are moved randomly thanks to Brownian movement, which helps them touch fibers. The 0.3-micron level is used only because at this point all three mechanisms are at the weakest.

Why is 0.3 microns considered a standard test size?

That is because it is the MPP — most penetrating particle size. This means that testing at this point guarantees HEPA operation in all disparate fractions. Note that under EN 1822 / ISO 29463 the MPPS is typically 0.15–0.25 μm, while 0.3 μm is the traditional DOE convention.

Can HEPA filter be washed and reused after being cleaned?

Unfortunately no. Washing destroys fibers of the filter and breaks the media-to-frame seal that cannot be repaired. You should consider getting a new filter.

Does HEPA filter work for gases and odors?

It is absolute mechanical filtration, using the principle of trapping particles without touching gases, so the answer is no.

What is the lifespan for a typical HEPA filter?

In cleanroom terminal business (ISO 5–8, with appropriate pre-filtration), as a rule, HEPA filters last for 3-5 years. Commercial air-purifying cartridges last anywhere from 6 months to a year. Continuous heavy industry operations without pre-filters can force a filter to work for only 6 months or so. Always check the situation with differential pressure and not a date on the calendar.

What distinguishes HEPA from ULPA classification?

Both kinds of filter are glass fiber filters, but HEPA (H13 to H14) is rated 0.3 microns with 99.95% efficiency or more, while ULPA (U15 to U17) covers even 0.12 microns, working with efficiency of 99.9995% or more but with higher operating costs.

References

1. EN 1822-1: High efficiency air filters (EPA, HEPA, ULPA) — Classification, performance testing, marking
2. ISO 29463: High-efficiency filters and filter media for removing particles in air
3. EN 779: Particulate air filters for general ventilation — Determination of the filtration performance
4. U.S. Department of Energy (DOE) HEPA specification — 99.97% @ 0.3 μm
5. iso 14644-1/-2: Cleanrooms and associated controlled environments — Classification and test methods
6. IEST-RP-CC001: HEPA and ULPA Filters — Recommended practice for performance specification and testing