What types of contaminants can a medium filter capture?

The medium filter has the capability of capturing the various suspended particulate matter such as dust, lint, pollen, large number of mold spores, textile fibers, and some biological material associated with suspended particles, and these are the medium filter contaminants that matter most.

The percentage of medium filter contaminants captured will depend upon the size of the suspended particles, filter's efficiency, filter test range, and the conditions under which it has been installed.

There is no specific lower range below which a medium filter cannot capture contaminants, so what can a medium filter capture is really a question of distribution rather than a hard cutoff. The contaminants present in medium filter can be said to have a distribution of sorts: there will be higher percentage of larger particles caught as compared to that of smaller particles.

O2Wittr7Kng

What a Medium Filter Captures

A medium filter works through the process of capturing suspended particles through straining, interception, inertial impaction, and diffusion processes. The efficiency of eACH mechanism used will vary according to the particle size.

The reason for the different performance of two filters with the same nominal rating lies in how the medium filter contaminants are distributed across particle sizes.

In typical HVAC operation, a medium efficiency filter functions as a dust and pollen filter for the airstream and also supports mold spore filtration, since many spores are in the size range that is effectively captured by a medium filter.

The figure gives the approximate size ranges for the various types of particles filtered by medium efficiency filters ranging from the size of lint of over 10 microns to the size of fine dust close to 0.3 microns.

Common airborne particles retained

Laboratory removal efficiency for medium filter contaminants is based on the rating and size of the airborne particle. It should be noted that MERV and EN 779 F-class ratings use different techniques and therefore should not be compared directly.

Table 1: Laboratory removal efficiency based on rating and size range. Reports are based on laboratory data and may not be compared directly to one another.

Presently ISO 16890 [3] is the overarching general-ventilation standard and is reported in terms of ePM1, ePM2.5 and ePM10. The data from the EN 779 [2] should only be used as historical reference.

Fine particles and biological contaminants

The lower the size of particles, the greater the dependence on the media. With particles below one micron, interception and diffusion are the predominant mechanisms used for removal.

It is possible to observe considerable differences between a glass-fibre media and a synthetic media with nominally identical performance characteristics.

In airborne particle filtration the removal of biological material mostly happens because bacteria and viruses are carried inside droplets or droplet nuclei. Usually a one-micron bacterium carried in a droplet which is five-microns will be treated as belonging to five micron particle group.

Thus, the size of an organism is not accurate when predicting removal efficiencies and the size of the media particle is much better determinant of medium filter efficiency for design purposes.

The Deiiang catalogue states that the efficiency grades of the fabric bag filters are F5 being 45 per cent, F6 being 65 per cent, F7 being 85 per cent, F8 being 90 per cent, and F9 being 95 per cent for particle size of 0.5 micron. Note that these are nominal catalogue values and must be confirmed prior to using as reference.

Class labels of EN 779 do not substantiate the stated value since purchase justification cannot be made based only on these class labels. One must request the specifications for the specific model before relying on these values.

It is important to note that the fabrics do not sterilise the air and that organisms that are captured will remain in the air. Thus correct procedure must be adhered to when replacing the media.

How Filter Efficiency Relates to Particle Size

The medium filter efficiency is the function of particle size, face velocity, loading, and test aerosol. For fibrous media efficiency will often decline to a point where neither interference nor impaction works well.

Medium filter efficiency varies between different media types, fiber diameters, packing densities, and airflows. Other than that, the value of 0.3 µm is not always applicable to all kinds of filters.

MPPS is applied as a recognized term in EN 1822 for the classification of high-efficiency filters. However, ISO 16890 presents particle size efficiencies in the use of general ventilation filters.

The figure is conceptual. It shows efficiency generally changing across particle sizes, with a mid-range dip drawn illustratively. It does not assign a fixed minimum to any specific product.

Particle size, efficiency, and test conditions

According to ASHRAE 52.2, a medium-efficiency air filter is tested with the aid of potassium chloride aerosol going through twelve different sizes from 0.3 µm to 10 µm. EN 779 uses different types of a test aerosol, resulting in average efficiency.

The classification systems are not related to one another, thus converting a filter from one system to another through a simple formula is impossible.

Face velocity affects medium filter efficiency and pressure drop in a media and design specific manner. Higher velocity does not mean higher capture because it can lead to greater inertial impaction and shorter diffusion time.

Proper measurements must be taken in order to assess the performance of every specific filter in particular housing. The Deiiang catalog values indicate that the resistance rate is less than 50 Pa for F5 glass fiber bag filters and less than 100 Pa for F8.

However, initial resistance alone does not allow one to predict medium filter efficiency over the life of the filter. The effective service life depends on the pollutant type, dust holding capacity, and air velocity.

What efficiency ratings do—and do not—tell buyers

MERV and F ratings refer to performance of the filter in a laboratory setting, under a specific performance test. They do not tell buyers how the filter has performed in practice once installed.

For example, bypass leakage can cause a dramatic reduction in system performance. A filter rated highly on MERV or F, when placed in a poorly fitted frame, will not perform anywhere near that rating in actual use.

A buyer must check the tightness of the gaskets, the frame fit, and the installation with the design specifications. Sealing is a property of the system, not the filter.

Also, efficiency ratings do not tell a buyer anything about gas-phase removal. Odour or VOC control requires an adsorbent stage of filtration that has its own performance ratings.

Deiiang suggests that in airborne particle filtration, when a buyer looks at performance scores, the buyer ask which standard defined that performance number. For general ventilation, ask for ISO 16890 results and treat EN 779 data as obsolete.

Medium Filters Versus Other Filter Stages

An HVAC medium filter is not typically the only filter used in a filtration situation; it sits between coarse prefilters and highly efficient final filters. Each filter provides a unique type of filtration.

The filter staging process is a practical basis in airborne particle filtration design, and it is also where the choice of an HVAC medium filter is usually settled.

Table 2: Filtration stages, roles, target contaminants, and rating systems. Rating systems are not interchangeable between rows.

Figure 3 illustrates how air traverses every stage. Particle concentration decreases with each stage, while total pressure loss at the filter bank accumulates.

Prefilters, medium filters, and HEPA filters

A prefilter is a coarser filter (normally MERV 6 – 8 or G4). It filters out lint and large dust and, in doing so, guards the medium filter stage from rapid loading.

A medium-efficiency air filter can catch most particles from 0.3 to 10 μm depending on the classification, and mold spore filtration is one of its practical strengths. This filter does not ｒｅｐｌａｃｅ the ultimate high efficiency filter – it extends its life span.

High-rate filters are classified according to their performance at the MPPS, per the EN 1822 [4]. H13 and H14 classifications are based on performance at the MPPS, not on overall 0.3 µm effectiveness.

A common HEPA filter reference figure is 99.97% at 0.3 µm. This figure derives from a different test basis which is historically associated with IEST-RP-CC001 [8] and MIL-STD-282 [9]. Always make sure to reference that basis.

According to Deiiang, catalogue information gives H13 as 99.97 to 99.99% and H14 as 99.995 to 99.999% at 0.3 µm for their combined efficiencies. These should be verified with the datasheet for the model and the relevant EN 1822 [4] or ISO 29463 [7] report.

Choosing a stage for the system

There will be differences in how selections are made, but the order in which these considerations take place remains fundamentally similar across each kind of system.

• Commercial and retail businesses: MERV 8 to MERV 11.
• Medical, pharmaceutical and electronics industries using high-efficiency filters as their upper limit: MERV 11 to MERV 13.
• Cleanroom last stages: based on materials and process risk.

In cleanroom air filtration the class by itself does not specify a prefilter grade; the choice must include process risk, flow rate, terminal filter specification, pressure budget and facility requirements.

The item selection criteria prescribed by Deiiang utilize this model. According to Jason.Peng, designer for Deiiang, the prefilter selection should occur at the same time as the final filter should occur instead of in a sequence.

Contaminants Medium Filters Do Not Reliably Remove

The standard filter medium is not designed to mitigate gaseous pollutants. Odours, VOCs, ozone and gaseous chemicals are unaffected by the mechanical sludge because there is little ability for them to be removed.

It is not merely a matter of size. Gaseous molecules are not "smaller particles" that can be screened by a finer filter.

In order to remove gaseous pollutants, either adsorption or chemisorption is needed on the proper substrate with sufficient time to remedy the pollutant of concern.

In Figure 4, particles are located on one side of the particulate filter while gases are located on the other side of the filter. The filtration system will include a separate gas-phase filtration section for odour and vapours control as well.

Table 3: Characteristics of the two methods of filtration—particle filtration vs gas-phase filtration. There are differences in how each works and what each requires.

Odors, gases, and chemical vapors

A standard dust and pollen filter will not work for cooking odours, solvents, or vehicle exhausts. For this situation, one would need to use activated carbon or any other chemisorbent.

Deiiang catalogue indicates a combination of activated carbon medium filter in V-bank configuration with capacity 3700 m³/h for gas and odour applications. Buyers must confirm the exact part number, type of carbon, bed depth and the rated capacity in the current datasheet.

Adsorption can be influenced by the kind of contaminant, concentration, time of contact and capacity. Saturated carbon will stop working regardless of the normal-looking performance parameters hence carbon must be replaced based on monitoring rather than pressure drop.

Viruses, bacteria, and claims about removal

Bacteria-bearing particles are subject to removal in the case of standard medium (MERV 13), so what can a medium filter capture includes many biological aerosols. Viruses are typically carried in droplets, so removal depends on droplet size rather than organism size.

A particle filter does not have the ability to identify, inactivate or kill pathogens, as the claims for removability and sterilisation are based on different evidence.

Where pathogen control is vital, high efficiency filtration and effective mold spore filtration can enhance the removal of virus laden aerosols if properly chosen, sealed, and maintained. However, it will neither prevent infections by itself nor ｒｅｐｌａｃｅ other means of controlling the situation.

Cleanroom and HVAC Applications

An HVAC medium filter covers two major applications: general usage in commercial buildings and pre-filtration for cleanroom air filtration systems. The criteria for selection in both cases differ.

In HVAC, the medium filter used as an HVAC medium filter protects the duct from dust accumulation on the coils and heaters. Clean coils will ensure stable airflow and less cleaning frequency.

In cleanroom air filtration, medium filters protect the final HEPA filter from fast overload. A well-matched pre-filter can prolong the life-span of the HEPA filter and reduce the total filtration costs.

The figure shows the application chart for filter stages according to the zone of the facility, starting from ordinary HVAC to cleanroom zones.

HVAC systems and cleanroom prefiltration

Deiiang provides the medium-efficiency air filter range as glass-fibre bag filters from F5 to F9 and synthetic-fibre bag filters over the same span. The air flow for catalogue products is 1700 m³/h and 2050 m³/h for the glass filter and 1000 m³/h and 1700 m³/h for synthetic ones.

The six-bag box with the size of 592×592×381 mm has an area of the media of 3.18 m² and final pressure drop of 250-400 Pa. In the catalogue data offered by Deiiang, one can choose the use of glass-fibre up to a temperature of 150 °C and synthetic up to a temperature of a maximum of 80 °C. The temperature is, however, a data catalogued for each machine.

Matching filtration to room requirements

According to the iso 14644-1 [5], cleanrooms are classified in terms of the maximum concentration of particle allowable. The 2015 edition of the standard permits a maximum of 1,000,000 particles within a cubic meter for iso 6 when measured at a ≤0.1 µm level and a maximum of 35,200 particle concentration for particles measured at ≥0.5 µm.

However, for cleanroom air filtration iso 14644-3 [6] provides the test procedure used for proving the correctness of the expected performance through particle counting and airflow visualization. ISO 14644-1 provides the classification while ISO 14644-3 provides the verification procedure.

Downstream of the medium stage, Deiiang catalogue data lists U15–U17 baffle-free filters with efficiency values quoted at 0.12 µm. Those values require a model-specific report and an EN 1822 [4] or ISO 29463 [7] basis before use in a specification.

For medium-filter selection, the practical point is simpler: the final stage defines the required prefilter performance, not the other way round.

Selecting a Medium Filter for a Project

The selection process for the medium filter begins with a survey of the medium filter contaminants to remove and the efficiency required of the filter for the specified particle size.

Before deciding on an HVAC medium filter model, the following should be confirmed by buyers: particle size distribution, target removal percentage, pressure drop, as well as installation dimensions.

In order to make the right choice, it is important to confirm medium filter efficiency from the recorded resistance and airflow rate under the use condition.

The decision sequence from the contaminant to the validated grade broken down by verification stages is depicted in Figure 6.

Key specification and compatibility checks

Deiiang provides a specification checklist that buyers can complete and submit along with their enquiry. A briefer checklist is given below.

The frame and media specifications, which together set medium filter efficiency, are based on configurations. Deiiang glass-fibre bag filters are available in aluminium frames or in certain cases stainless steel frames where the said media is procured from a known supplier.

The buyers need to check for these specifications with up-to-date manuals provided by Deiiang rather than referring to specifications for other products.

Make the differentiation between the catalogue specifications for an HVAC medium filter and those applicable to a specific project. The media area is fixed at 3.18 m² according to the catalogue, whereas the silicone-free gasket is project dependent and should be explicitly specified on the order.

Deiiang catalogue data to verify before purchase

Table 4: Deiiang catalogue medium-filter data to be checked. Each value needs to be verified against the current model-specific datasheet.

The 1700 m³/h and 2050 m³/h ratings are for different grades and bag configurations within one same frame family; they are not two different numbers for one model. Please verify the model number, bag count, and grade prior to making quote comparisons.

Deiiang suggests acquiring ISO 16890 ePM data for general ventilation and considering F-classes as obsolete data.

Testing, Standards, and Performance Verification

The testing of filter products and proving if a system complies with standards address different things. The factory test is the report on the filter in controlled conditions. However, the system validation is about the filter in application.

Problems arise because of combining both concepts into one. A filter can pass the factory test, yet still fail to work in situ if the sealing around the filter frame has some leakage or the gasket is compressed improperly or the airflow rate in operation is above the rated flow.

The figure shows the sequence of verification steps, from media and filter testing through installation checks, airflow balancing, and cleanroom particle counting.

Filter ratings and applicable test standards

Different classes of filters are subject to various standards. The wrong standard used leads to invalid comparisons. Citing a particular standard cannot ensure the validity of a product claim.

• General ventilation: ASHRAE 52.2 for MERV and ISO 16890 for ePM data.
• Historical European general ventilation data: EN 779, withdrawn, shows only the historical context.
• HEPA and ULPA filters: use EN 1822 and ISO 29463.
• Classifications of cleanrooms: iso 14644-1.
• Testing methods in cleanrooms: ISO 14644-3.
• Testing of HEPA and ULPA filters: use IEST-RP-CC001 (confirm current revision) and MIL-STD-282.

The product has to be tested to the mentioned standard; the report has to be traceable to the model and size that the delivery was made.

Installation checks and facility-level verification

In airborne particle filtration there are installation features that belong to Deiiang products, project specifications, or commissioning practice that must not be referred to universally.

• Deiiang product feature: a DOP test port that has been placed in the air outlet side of the DOP integrated filter family, plugged off with a rubber cap in this range of models.
• Deiiang product practice: individual scan testing of the high-efficiency filters that has to be carried out before their delivery according to the testing procedure.
• Project specification: silicone-free sealing gaskets in cases where there is a limit to silicone contamination by the process.
• Recommended commissioning practice: frame seal and inspection of the airflow measurement as well as counting of particles in the facility.

Furthermore, there should be necessary checks for gasket compression and frame sealing along with filter seating as per the design of the equipment. Facility level checking should follow the practices mentioned in ISO 14644-3 [6] in case the classification within the cleanroom is declared.

Maintenance, Pressure Drop, and Real-Project Evidence

It is essential to conduct HVAC medium filter maintenance based on readings rather than on a fixed interval. Pressure drop and value of airflow will tell how the filter has been loaded and if there has been any change prior to the current operation.

A graphic showing a pressure drop reading can be quite helpful in case it contains the straight (normal) slope of loading, the change point of this slope as well as the value when terminal pressure drop limit is reached.

Figure 8 shows a representative change-out of the kind described in the scenario below; it is an illustrative image and not a measurement record.

Maintenance practices that preserve capture performance

• Monitor pressure drop according to a schedule established at commissioning based on the trends established through operation.
• Ensure airflow stays within the design range after each filter change.
• Check gaskets, frames, and filter seating for leakage at every service visit.
• Substitute damaged filters right away, as a ripped media filter will allow unfiltered air downstream.
• Document the filter's model, type, installation date, and the pressure drop.

The appropriate resistance for some Deiiang medium bag filters is 250–400 Pa in the catalogue, but this is not a blanket replacement guideline. Please use the exact specification of the manufacturer, considering airflow and maintenance limits of the factory.

Deiiang project case study and documented results

This airborne particle filtration example is non-identified given the nature of the project. It was used as a situation so that documented measurement records cannot be disclosed.

The facility sought ISO 6 (class 1000) with a high ambient particulate load, low plenum height, and a strict pressure drop limit for laminar flow. Deiiang supplied low-profile H14 combined high-efficiency filters, with dimensions of 592 mm x 592 mm x 292 mm.

These filters have specifications of 2500 m³/h airflow, a media area of 20.04 m², and catalogue initial resistance of ≤220 Pa, and the filters come with F7 pre-filters. In the project, there was an indication of reduction in the iso 7 particle count to an ISO 6 particle count with energy benefits due to the low initial resistance.

In order for this evidence to be accepted, the following information must be provided: initial and final particle counts including sampling methods and locations, measured airflow, and pressure drop at commissioning and during operation; and filter make and model.

Frequently Asked Questions

What contaminants can a medium filter capture?

The medium filter captures airborne contaminants like dust, lint, pollen, mold spores, textile fibers, and biological materials. The percentage of medium filter contaminants captured is based on particle size and the classification of the filters.

Can a medium filter capture bacteria and viruses?

Medium filters will remove some bacteria-laden particles; this is especially true when the filter classification is high like MERV 13. In general, viruses are carried by droplets and, therefore, their removal will be dependent on the size of the droplet.

Does a medium filter remove odors or gases?

No. Conventional particle filters, however good their mold spore filtration, do not offer protection from gases, odors, volatile organic compounds (VOCs), or chemical vapors. For these various threats, filters with activated carbon or some chemical sorbent, depending on the type of gas, should be utilized.

How does a medium filter compare with a HEPA filter?

A medium-efficiency air filter acts on particles sized approximately 0.3 - 10 microns, but the degree of medium filter efficiency depends on the specific type of medium-efficiency air filter used and is tested according to specific test conditions.

The HEPA filter has a specific performance standard that identifies both the most penetrating particle size and the fraction of that size that can be removed. Usually, they are executed in the system in different stages.

Can a medium filter be used in a cleanroom?

A medium filter may be used for prefiltration ahead of the high-efficiency filter in a cleanroom air filtration train, removing particulates downstream and prolonging the life of the final filter.

How often should a medium filter be replaced?

A dust and pollen filter is typically changed according to the pressure drop value recommended by the filter manufacturer, air flow, and local operations for the given air filter. A pressure drop of 250-400 Pa limit is given in catalog data, although this is not universal.

The replacement set point should be established based on the performance data recorded during the time of commissioning rather than based on a fixed schedule.

References

• [1] ANSI/ASHRAE Standard 52.2-2017, Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size. ASHRAE.
• [2] EN 779:2012, Particulate Air Filters for General Ventilation — Determination of the Filtration Performance. Withdrawn and superseded by the ISO 16890 series; cited as legacy context only.
• [3] ISO 16890-1:2016, Air Filters for General Ventilation — Part 1: Technical Specifications, Requirements and Classification System Based upon Particulate Matter Efficiency (ePM). ISO.
• [4] EN 1822-1:2019, High Efficiency Air Filters (EPA, HEPA and ULPA) — Part 1: Classification, Performance Testing, Marking. CEN.
• [5] iso 14644-1:2015, Cleanrooms and Associated Controlled Environments — Part 1: Classification of Air Cleanliness by Particle Concentration. ISO.
• [6] iso 14644-3:2019, Cleanrooms and Associated Controlled Environments — Part 3: Test Methods. ISO.
• [7] ISO 29463-1:2017, High-Efficiency Filters and Filter Media for Removing Particles from Air — Part 1: Classification, Performance Testing and Marking. ISO.
• [8] IEST-RP-CC001, HEPA and ULPA Filters. Institute of Environmental Sciences and Technology; confirm the current revision in force.
• [9] MIL-STD-282, Filter Units, Protective Clothing, Gas-Mask Components and Related Products: Performance Test Methods. US Department of Defense, 1956, with later amendments.