How does a medium filter work?

The question of how does a medium filter work comes down to a simple fact: air passes through a porous filtering material, where particles are trapped via interception, impaction, and diffusion. The filtering material isn't a sieve, since mesh openings are much larger than the particles being trapped.

While that statement indicates that air filtration takes place, in practice, it is important to understand the particulate size, the diameter of the fibers, the structure of the filter, and the face velocity because every filter utilizes three principles to function at the same time.

This article explains how does a medium filter work by describing eACH of the principles, the definitions of medium air filters, their mechanisms, the description of the construction of the filters, the standards of the tests, as well as the pressure drop in the selection, installation, and review process. Each paragraph indicates the importance of understanding a concept for a user to specify or maintain a medium air filter.

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What Is a Medium Air Filter?

The medium air filter is a type of general ventilation-type filter that is located between the coarse pre-filter and the high-efficiency filter. The word medium relates to a market and stage of application; it does not refer to a class of filters that have the same efficiency range [2][3].

This does indicate that the location of the filter and what type of function it will have. However, based on that information, the user can obtain the efficiency rating of a filter since this value will depend on the testing methodology and the size of particles [1][3]. Choosing a medium air filter therefore means fixing a position in the filtration train and a grade, not picking one universal class.

Table 1 indicates appropriate standards for different filters, however, it does not imply that the aforementioned ratings are convertible amongst filter types.

Table 1: HVAC filtration stages in an air-handling system and criteria for assessment of the different processes

How “Medium” Describes a Filter

Medium is a relative term. A medium air filter in one setting may be classified as fine in another based on upstream and downstream filters.

The legacy standard EN 779 classified air filters for general ventilation into the G1-G4 categories and F5-F9 and reported the average efficiency or average arrestance of filters averaged over a loading sequence [2]. This legacy standard has been withdrawn and should only be referred to in a historical context.

The new standard ISO 16890 indicates the amount of test aerosol mass size removed by different filters in the filter's ePM1, ePM2.5, and ePM10 categories [3]. Both standards are not directly interchangeable.

Based on the Deiiang categorized data, the product line of medium-efficiency air filters is listed under the legacy standard grade F5-F9 with the equivalent range for pleated media MERV 8-13 [1][2]. Readers should verify this information model-wise since this information is taken from catalogs and thus not independently validated.

Where Medium Filters Fit in HVAC and Cleanroom Systems

In HVAC filtration stages the medium filter always comes after the coarse prefilter and before the final filter, thus serving the same function as a cleanroom prefilter in cleanroom operations. That, in one line, is how does a medium filter work inside a real system: it lowers the load the final filter has to carry.

The intended outcome of well-chosen HVAC filtration stages is to reduce the speed of the loading rate of the final filter allowing the more costly HEPA or ULPA filter to reach its replacement point later. The magnitude of this benefit is based on particle loads to the filter chosen for the application.

The position and function of an HVAC medium filter dictates how users specify them, followed with the grade. Catalog information supplied by Deiiang includes bag, W-bank, V-bank carbon, and pleated panel style filters.

See Figure 1 for a representation of where these HVAC filtration stages sit in relation to each other, and where a cleanroom prefilter fits into the same train.

How Does a Medium Filter Work?

In the medium filter air enters the filter face and is forced through the pleat openings into the air filter media. Inside the media the air is separated into a large number of different small streams as it moves around the fibers.

Particles that are small enough to follow the air flow still leave the flow. How does a medium filter work at this level? Three mechanisms operate on the particles simultaneously and produce a composite result of filter particle capture throughout the depth of the media.

Each mechanism works on a particle size, which is the reason why efficiency is usually specified as a curve rather than just one number.

Interception, Impaction, and Diffusion

Interception is a process where a particle in its streamline travels within one particle radius of a fiber and sticks to it. Interception is most significant for particles that have a size of a few tenths of a micrometre.

Impaction occurs in a situation in which the inertia of a particle does not allow it to turn with the airflow and causes it to leave its streamline and hit the fibre. The impact becomes more significant as the size or dimensions of the particles starts to increase above one micrometre.

Diffusion is Brownian movement. Very small particles will travel randomly and will reach the fibres regardless of the fact that they would be travelling past them under normal airflow movement. The impact appears to be the greatest for particles that have a size of less than 0.1 μm.

However, the dimensions listed above should be treated as two illustrative examples but not fixed parameters. Fibre diameter, packing density, pleating geometry, and face velocity change them in every specific filter design, which means that it is impossible to create one single diagram for all filter models.

At the same time, all three processes occur simultaneously, so filter particle capture depends on the particle size as much as on the design of the media. The extent of the impact of each process is determined by the particle size, fibre diameter, type of material used to manufacture the media, and flow velocity.

Most Penetrating Particle Size (M.P.P.S.) is not a constant indicator as well. It is determined by the manufacturing method and working conditions, especially in case of EN 1822 M.P.P.S. classification system.

How Media Structure and Pleats Affect Capture

More pleats and smaller spacing result in an increase in area, which normally translates to a reduction in both velocity and resistance. In actuality, the reduction in velocity and resistance can only occur if all of the following variables are also in place: airflow, pleat geometry, media permeability, and loading.

In the case of an overly dense pleat configuration, airflow cannot properly distribute, and as such, area becomes ineffective. In addition, if media permeability is low, then an increase in area alone will not reduce pressure drop.

The correlation between media area and medium filter pressure drop is not straightforward, as media area is only one determining factor. By holding pleat spacing constant, spacers ensure that resistance remains fairly evenly distributed along the filter face.

For comparison, the high-efficiency Deiiang H14 combined filter has an area of 20.04 m² and is rated for 2500 m³/h flow rate at a pressure drop of 592×592×292 mm. It is included for comparison purposes only, and is not reflective of how medium filters will perform.

Filter Media and Construction

Medium-efficiency air filter media typically consist of glass fibre or synthetic meltblown fibre depending on temperature and humidity limitations as well as costs and material availability.

The following information is derived from the Deiiang catalogue and applies specifically to the glass-fiber bag filter line. Please verify with the manufacturer's data sheet before applying filter specifications.

According to the Deiiang catalogue, glass-fiber bag filters are made with media supplied by the JM Company of America and mounted on aluminum (or stainless steel) frames with PU (polyurethane) two-component AB adhesive sealants and board dividers of 120 g/m².

For the glass-fiber versions of a medium-efficiency air filter, the rated maximum temperature of operations is 150 °C and the maximum permitted humidity is 100%. Aluminium frame version has a catalogue maximum temperature of 80 °C.

Media Types, Support Layers, and Filter Frames

The selection of air filter media is based on the duty instead of general preferences. For example, glass fiber is used for high-temperature duty whereas synthetic meltblown media meets standard HVAC specifications at lower prices.

A medium-efficiency air filter includes upstream protective mesh and filter frame with pleat geometry. If they are absent, medium may deform or migrate under high face velocity.

• Glass fiber bag has a catalogue maximum temperature of 150 °C and 100 percent relative humidity (Deiiang range)
• Synthetic bag has a catalogue maximum temperature of 80 °C (Deiiang range)
• Pleated panel catalogue refers MERV Code of 8-13 for regular purposes in HVAC system
• V bank activated charcoal media has a capacity of 3700 m³/h for gas and odour removal purposes

Seals, Bypass Leakage, and Installation Fit

The efficiency of an HVAC medium filter assumes that all air is going through the medium. Gap between housing and frame allows unfiltered air to go around the medium.

Bypass depends on the size of the gap between two parts, pressure equalization across the filter, and flow conditions. This value is variable and increases with pressure losses through the filtering medium during operating time.

Inspect the entire periphery of the filter as not only visible corners, because a cleanroom prefilter that bypasses at one corner loses its rated performance. Gasket made of EVA or neoprene should provide even compression against mounting flange.

Efficiency, Particle Size, and Test Standards

Efficiency is a term that holds no meaning unless the particle size included and the test method used is identified. One filter may be rated at 85% efficiency when tested at 0.4 μm under one methodology but may not have that same rating when tested at 0.3 μm under a different methodology [1][2][3].

High-efficiency classifications and methods of general ventilation are two separate categories that yield different results. The error of treating their results as one continuous scale is a mistake commonly made when comparing filters which results in an incorrect selection of filter.

In Table 2 below, the scope of each method is indicated and the limitations on how the results can be utilized.

Table 2: Scope and efficiency ratings of the main filter testing methods

Understanding MERV, EN 779, and Filter Efficiency

The ASHRAE 52.2 assigns MERV levels based on the documented efficiency of removal for each of the three defined particle size bands with the lowest band being 0.3 and 1.0 μm [1]. For the MERV 13 classification, a specific minimum composite efficiency must be achieved in the lowest band.

Since the definitions of bands and their reported values are revised in different editions of the standards, verify the ASHRAE 52.2 edition referenced in the project specifications before using a threshold figure.

EN 779 has been withdrawn. Class F values from the older reference suggest the average arrestance or average efficiency value developed over a loading test under that standard, but does not apply to a classification that exists today [2].

ISO 16890 provides the current general ventilation standard but does not create a direct correlation between classes in EN 779 [3]. Deiiang's F5-F9 and MERV 8-13 designations apply to some of the products they offer, but should not be universally used in making a comparison.

What EN 1822 and ISO 29463 Do—and Do Not—Rate

EN 1822 categorizes the EPA, HEPA, and ULPA filters using a particle count at or near the most penetrating particle size developed for that filter [4]. The MPPS is not a fixed number, since it varies according to the design and test conditions.

ISO 29463 introduces parallel terminology for classifying high-efficiency filters and media [5]. Although the two standards share similarities in intent, the actual class names should not be used interchangeably without checking division information first.

Deiiang lists H13 as having 99.97% - 99.99% and H14 with rates of 99.995% - 99.999% efficiency at 0.3 μm for their high-efficiency filters. These are not integral or local efficiency level designations but are ranges that need verification with the current edition of the standard.

Neither EN 1822 nor ISO 29463 provides useful information on medium-efficiency air filters. A medium filter's grade is derived from the ASHRAE 52.2, ISO 16890, or older EN 779 testing standards [1][2][3].

Pressure Drop, Airflow, and System Performance

The medium filter pressure drop is the observed resistance across the filter under some given airflow condition, and for an HVAC medium filter it is the number that decides fan duty. It will naturally increase as it fills with dust, and can also increase again if the airflow condition exceeds the specifications.

The pressure drop curve is not linear in practical situation because the permeability of the media, the geometry of the pleats and the pattern of loading behave differently as the filter becomes loaded.

For an HVAC medium filter, energy impact is additive to fan power consumption. A simple illustration of the relationship can be expressed by the relationship: fan power ≈ airflow × pressure drop ÷ fan efficiency, where units must be of the same type. For example, with 1 m³/s at 200 Pa and 0.6 fan efficiency, the power that the fan should provide is approximately 333 W.

In practice, the actual consumption is variable because it is also a function of the airflow, the fan efficiency, controls, and running time of the system. Therefore, the value calculated should be seen as a rough estimate rather than an accurate prediction. For actual savings to have value, a proper method of measuring them must be determined.

Clean and Final Pressure Drop

The initial pressure drop reading takes place with a clean filter operating at rated airflow conditions; this is the baseline for every later medium filter pressure drop reading. The final pressure drop reading will be at a factory specified replacement point for a given filter model.

The data from the Deiiang catalogue for the 592 x 592 x 381 mm six-bag medium efficiency filter shows a maximum air flow capacity of 2050 m³/h along with a total media area of 3.18 m². When at F5, the level of initial resistance has to be less than 50 Pa while at F8 it must be less than 100 Pa. The applicable end-of-life resistance for the filter is between 250 and 400 Pa.

For comparison, the H14 high efficiency filter made by Deiiang, rated at 592 x 592 x 292 mm, has a maximum air flow value of 2500 m³/h and a total media area of 20.04 m². The initial resistance level for this filter is ≤ 220 Pa with an end-of-life resistance between 400-600 Pa. All of these values are related to high efficiency filters.

Selecting Airflow Without Compromising System Performance

For a medium air filter, the rated airflow is the pressure drop that has been measured according to the catalogue and should serve as a reference, rather than aimed to be surpassed.

When operating above the rated flow, the face velocity and pressure drop will increase and the filter media will sustain the risk of damage.

When a cleanroom prefilter is involved, using a larger or deeper pleat pack will lead to a reduction in face velocity while retaining the efficiency class of the filter. It is also necessary to measure and note the pressure drop at the commissioning to relate later measurements to this point.

Selecting a Medium Filter and Reviewing Deiiang Project Data

The first step in selecting a filter involves matching the filter specification to the application, then verifying airflow and pressure drop together as a pair. Table 3 provides an overview of the input selections and a specific illustration of the Deiiang catalogue, permitting direct side-by-side comparison. Prior to specification, ensure both are examined against the model-specific data sheet.

Table 3: Medium-filter input selections and catalogue example from Deiiang

Source: Deiiang catalogue data, medium-efficiency bag filter range, current catalogue revision. Confirm against the model datasheet before specification.

Match Filter Specifications to the Application

The principles of conditional guidance given as a principle as opposed to a rule which states that commercial ventilation normally employs a mixture of medium grades in the beginning of the range but cleanrooms equipped with HEPA filters are known to apply medium grades of a higher caliber depending on the jurisdiction and design basis.

Details from the Deiiang catalogue concerning the other medium-format items: the glass-fiber bag filter with a capacity of 1700 m³/h (F5 and F8) and 2050 m³/h (F6), and the synthetic bag filter with a capacity of 1000 m³/h (F6, F7, F9) and 1700 m³/h (F5 and F9).

The range also includes the W-bank combination filters in sizes of 23×23×11.5 in (F7) and 11.3×11.3×11.5 in (F8), plus the combination activated carbon filter rated at 3700 m³/h.

The activated carbon combination filter can provide adsorption of such gases as well as odors; however, it cannot serve as a substitute for the use of filters for particulate matter but can be used in conjunction with them.

Real-Project Challenge, Deiiang Solution, and Verification

The following is taken from the Deiiang project historical record. Note that this should not be construed as being validated external reference material, as the information is presented in the form of fundamentals for anonymity, and any information regarding these figures presented externally is subject to verification with Deiiang records.

The reported issue: a cleanroom that packages pharmaceuticals where the requirement for a class 1000 / iso 6 cleanroom can be met using a system where the ambient air load exceeds maximum allowable limits without creating excessive pressure drop.

Reported solution: Deiiang supplied the low-profile H14 combined filters with HEPA efficiency rating in the capacity of 592×592×292 mm at the air volume of 2500 m³/h with the initial resistance of ≤220 Pa at 20.04 m² surface area for the media and the F7 pre-filters with silicone free sealing material.

It was confirmed that DOP scan testing had been carried out before shipment and that particulate counts had changed from the baseline of iso 7 to iso 6 in project-reported results.

In addition, Deiiang has also stated that the fan power has been less with the lower starting pressure drop and stated that the actual amounts depend upon the fan efficiencies, airflow through the fan, controls, and hours of operation, which is not quantified in this report.

Installation, Maintenance, and Replacement

Proper installation ensures rated performance while incorrect seating voids it. Maintenance serves to provide monitoring of the pressure drop across the medium filter in comparison to the acceptable pressures for the model to which they both belong.

Installation Checks That Prevent Bypass

Properly seat filter ensuring that gasket compresses uniformly against housing flange or else user can have a situation with a loose corner gasket or a damaged gasket leading to availability of a path too excessive to where the rating of the media is valid.

Confirm that there has not been overtightening of retaining clips and that points of distortion along the frame may lead to openings formed along the seal even when filter appears to be seated as it should.

Whenever there is a HEPA or ULPA stage then it is necessary to confirm whether or not the medium filter has properly seated prior to conducting the integrity test on the output filter [4][8].

Maintenance Triggers and Safe Replacement Practices

ｒｅｐｌａｃｅ when the pressure drop meets specific standards and not merely on schedule. A lightly loaded filter operating in a clean environment can exceed a nominal interval by a very large amount.

Follow the product documentation for cleaning because only filters specifically certified for reuse can be cleaned according to the manufacturer's instructions.

Cleaning could affect the efficiencies, pressure drops and structural integrity of the filter regardless of whether the media is functional after the process.

Please do not attempt this cleaning process without approval from the manufacturer.

Before servicing the air-handling unit, isolate the unit in accordance with site practices, handle the filter according to their handling practices, and dispose of expired filters in accordance with local regulations.

Cleanroom Performance and Verification

The medium filter is just one part of a total HVAC and cleanroom design that considers air changes, pressurisation and the terminal filter.

The medium filter cannot be responsible for maintaining room cleanliness alone.

Testing verifies that the system that is installed functioned as intended by the cleanroom plan, not just based on the labels of the filters.

The Medium Filter’s Role in a Filtration Train

The standard design of a cleanroom train is a first deposit with a coarse prefilter and a second deposit with either a HEPA or ULPA terminal filter. The entire design aims at reducing the total load which is present in each element before passing into the next unit.

The rationale is to ensure that there is a delay in the time that is taken to ｒｅｐｌａｃｅ the expensive HEPA filters, depending on the level of loading in the filters.

Deiiang's U15–U17 ULPA filters are high-efficiency devices that fall beyond the ambit of medium-filter ratings.

Relating Filter Performance to iso 14644-1/-3

ISO 14644-1 categorizes cleanrooms into classes based on the level of particles in the air [6]. The standard sets out the classification goal but does not provide any assessment of individual filter performance.

iso 14644-3 describes protocols for evaluating cleanroom systems. This includes air flow inside the clean room, testing for leaks, and clean up [7]. It lays out protocol for testing some cleanroom systems, but it does not specify protocols for each cleanroom based on its class.

Frequently Asked Questions

How does a medium filter work?

The air goes through porous filter media and parts of air pollutants are trapped by the process of interception, impaction and diffusion all together. The relative contributions of each of these processes varies as a function of the size of particle, diameter of the air filter media and the airflow velocity.

What is a medium air filter used for?

Medium filters serve as second stage media in HVAC systems and as an intermediate filter upstream of HEPA/ULPA filters in cleanrooms. Medium filters are primarily designed to capture fine dust and prevent overloading the HEPA filter.

How is a medium filter different from a HEPA filter?

Medium efficiency filters are rated using ASHRAE 52.2, ISO 16890, or old EN 779 standards; HEPA filters are rated by using either EN 1822 or ISO 29463 using MPPS particle counting. These classes cannot be converted.

Does a medium filter restrict airflow?

All filters create resistance, but typically medium efficiency filters create about tens to low hundreds of pascals at its rated flow. Increasing the surface area or choosing a deeper pleat pack will help lower the face velocities and reduce the pressure drop of the medium efficiency filter within the fan's available pressure drop budget.

How often should a medium filter be replaced?

ｒｅｐｌａｃｅ the filter when the pressure drop reaches the manufacturer maximum for that model, which is stated to be 250-400 pascals for the Deiiang bag filters. The calendar time is a fallback position; the actual pressure drop is the better choice for knowing when to change the filters.

Can a medium filter be washed and reused?

Only if the medium efficiency filter is specifically designed and approved for reuse and the manufacturer's cleaning instructions are followed. Washing could alter the medium efficiency filter efficiency, pressure drop, or structural integrity, therefore washing outside of specifications could result in a loss of the rated performance parameters.

References

• [1] ANSI/ASHRAE Standard 52.2, Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size. Confirm current edition before referring to MERV rating.
• [2] EN 779:2012, Particulate air filters for general ventilation — Determination of the filtration performance. Withdrawn; cited for legacy purposes only.
• [3] ISO 16890-1, Air filters for general ventilation — Part 1: Technical specifications, requirements, and classification system based on particulate matter efficiency (ePM). The current general ventilation standard.
• [4] EN 1822-1, High efficiency air filters (EPA, HEPA, ULPA) — Part 1: Classification, performance testing, marking. Confirm current part and edition.
• [5] ISO 29463-1, High-efficiency filters and filter media for removing particles in air — Part 1: Classification. Confirm current part and edition.
• [6] ISO 14644-1, Cleanrooms and associated controlled environments — Part 1: Classification of air cleanliness by particle concentration. Confirm current edition for size and limits.
• [7] ISO 14644-3, Cleanrooms and associated controlled environments — Part 3: Test methods. Confirm current edition.
• [8] IEST-RP-CC001, HEPA and ULPA Filters. Cited for high-efficiency filter acceptance practices; confirm scope and edition.
• [9] MIL-STD-282, Filter Units, Protective Clothing, Gas-Mask Components, and Related Products: Performance-Test Methods. Cited for the applicability of the DOP test method; confirm.
• [10] Deiiang product catalogue, medium efficiency bag filter range and combined high-efficiency filter range, current revision.