How do I check if my medium filter needs replacing?

This article presents a method to follow in order to make an informed decision regarding medium filter replacement in cleanroom and commercial HVAC systems. It explains how to check a medium filter by using pressure measurements, visual inspections and checking airflow.

Deiiang catalogue data and a reported case history demonstrate the process, with Jason.peng assisting in developing the verification procedure. Each section leads to a documented verification and medium filter replacement record, preserved for the protection of both the filter and downstream equipment.

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What a medium filter does in a cleanroom HVAC system

Medium efficiency filtration refers to air filtration of 1-5 µm size particles sandwiched between a coarse prefilter and the final HEPA or ULPA stage in the filtration process. Their role is to protect the high efficiency filter from loading prematurely and, therefore, prolong its service life and reduce how often it is replaced.

The medium stage of filtration absorbs much of the airborne particulates prior to final filtration in cleanroom HVAC systems.

This figure shows a clear path of airflow through the prefilter, medium-efficiency air filter and HEPA stage, which is the sequence HVAC medium filter maintenance has to protect, identifying points where HVAC medium filter maintenance is critical and where pressure taps need to be used.

As per Deiiang catalogue data, medium-efficiency bag filters, W-bank combined filters and pleated panel filters are rated from F5 to F9 according to the EN 779 legacy scale, which roughly converts to MERV levels of 8 to 13 as per ASHRAE standard 52.2.

The 592 mm x 592 mm x 381 mm six-bag model is rated for 2050 m3/h and has a media area of 3.18 m2. The initial resistance stays below 50 Pa for F5 and is less than 100 Pa for F8, while the final resistance rating is in the range of 250 to 400 Pa.

Where medium-efficiency filters are installed

The medium efficiency air filters have to be installed in air handling units, usually downstream from the prefilter and upstream from the high-efficiency stage, because HVAC medium filter maintenance depends on that position. In cleanroom applications, they can exist within a separate bank of filters or in combination with other filter units.

Deiiang medium filters W-bank with plastic frame can easily fit 23 x 23 x 11.5 inch slots and the 11.3 x 11.3 x 11.5 inch slots for the grades of F7 and F8 respectively. The V-bank combined activated carbon medium filter can easily manage 3700 m3/h gas and odour requirements.

How filter efficiency relates to cleanroom performance

Filter efficiency impacts how much particulate mass will get through to other stages. An F7 filter that has 85 percent arrestance at 0.5 µm will catch 850 particles out of 1000 from that particle size range. An F9 filter that has 95 percent arrestance at the same particle size will trap 950 particles out of 1000.

Because of the efficiencies presented by medium filters, HEPA loading is reduced along with less energy spent overcoming downstream resistance to ensure cleaner air. Deiiang has selected guidance stating that commercial facilities should choose MERV 8-11, or F5-F7, while medical, pharmaceutical and electronics facilities should operate on MERV 11-13, or F7-F9.

Establish a baseline before inspecting the filter

Without a baseline for evaluating your filter pressure drop reading on a filter, you will have difficulty interpreting the results, so the first answer to how to check a medium filter is to record that baseline. The baseline must be the clean filter pressure drop measurement at commissioning with a fixed airflow rate. Document this number on the filter label and in your maintenance records.

Initial resistance measurements are good data to consult, but you should expect some variation between the two baselines of 10 to 20 percent due to duct layout and fan curve. Always take a measurement; don't just assume.

The table sets out the baseline and current readings that must be compared, so that the same parameters are recorded on every visit and the trend stays valid.

Table 1: Measurement point, baseline reading, current reading and interpretation for medium filter assessment.

Record pressure drop and airflow at commissioning or replacement

During commissioning, record three values: clean filter pressure drop, airflow and temperature. Deiiang 2500 m3/h H14 combined HEPA filters record an initial resistance of equal to or less than 220 Pa and a final resistance of 400-600 Pa.

The same process for the medium filter upstream must be followed. Write the installation date and opening pressure drop on the filter frame in permanent marker.

Confirm the filter model and manufacturer's limits

Confirm the filter ID with the equipment documentation before interpreting the reading. A Deiiang glass fibre bag filter at 1700 m3/h rated F5 may have a different final resistance than a Deiiang glass fibre bag filter rated F6 at 2050 m3/h.

The recorded final resistance for a six-bag family of filters can vary from 250-400 Pa. If the ID is unreadable, obtain the filter size and compare it to the current catalogue.

Check pressure drop and airflow

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The filter pressure drop across the filter refers to the resistance to airflow, measured as the difference in static pressure between the upstream (before the filter) and downstream (after the filter) sides of the filter. A clean medium filter may register between 50 to 100 Pa, while a dirty filter can go up to 250 to 400 Pa depending on the type and grade of the filter medium used.

This relationship is non-linear, which is why how to check a medium filter cannot rely on a single reading: the filter pressure drop increases very slowly at the beginning, and then increases rapidly as dust builds up.

This chart indicates the filter pressure drop over time for a medium efficiency filter. The graph marks the stable monitoring band and the final resistance limit at which replacement is due. A filter with a pressure drop of 85 Pa that went up to 200 Pa in a three month period is loading up much faster than one that took eight months to get to the same reading.

Measure pressure drop across the filter correctly

Pressure drops should always be measured under the same conditions of fan operation. If the fan speed changed between readings then that reading cannot be compared. The initial resistance for the Deiiang U15 baffle-free filter is at or below 150 to 170 Pa at 450 m3/h for the 610 x 610 x 50 mm size.

Whenever the use of taps occurs upstream and downstream of duct, it is important to place taps at a distance of 1 duct diameter upstream and 2 duct diameters downstream of the filter. In general cases of bag filters, taps are normally positioned on or near the filter frame.

Identify airflow changes that warrant investigation

Any deviation of airflow by more than 10% from commissioning level is going to be a source of concern and should thus be investigated. HVAC airflow problems with respect to airflow may be attributable to filter loading, fan failure, damper or duct leakage.

Before assuming HVAC airflow problems come from the filter, check fan amperage and damper position to determine the nature of the actual situation. If airflow drops below 1800 m3/h when the filter pressure drop reaches 300 Pa, filter blockage is indicated.

Inspect the filter and its installation

Visual assessment should reveal deficiencies not visible through pressure measurements, and it is a required part of cleanroom air filter inspection. Many filters having low pressure drop can exhibit damage through tearing of filter media allowing raw air to bypass the filtered medium layer.

Cleanroom air filter inspection requires assessment of filter media, filter frame and gasket in conjunction with cleanliness of the cleanroom. Remember never to evaluate the condition of a filter by visual inspection only.

This figure describes the areas requiring inspection on medium filters while providing details related to defects that require monitoring during the normal routine assessment period.

Recognize visible damage and abnormal loading

Tears larger than 2 cm in filter media allow unfiltered air to flow through the tear, which is one of the signs an air filter needs replacing. A tear of 1 percent of the face area of the filter can pass 20.5 m3/h of unfiltered air with a total airflow of 2050 m3/h.

This amount is enough to cause downstream particle counts to rise and neutralize the medium stage. Abnormal loading will present itself as a dark line along the area of uneven air entering the filter, and it is one of the clearest signs an air filter needs replacing.

Check seals, frames and filter-housing fit

Gasket leaks and housing bypass are both frequent causes of apparent filter underperformance. Deiiang sells PU two-component polyurethane sealant and EVA or neoprene gaskets for its medium and high-efficiency filters.

Press lightly on the gasket during your inspection; it should feel flexible, not brittle or compressed, because a hardened gasket is one of the signs an air filter needs replacing. Look for any corrosion or debris within the housing that may inhibit the gasket from seating properly. If there is a 3 mm space along a 600 mm edge of the filter, it corresponds to a bypass area of 1800 mm2.

Interpret warning signs and troubleshoot system effects

Filter pressure drop and airflow readings must be interpreted together and not separately, because HVAC airflow problems and pressure changes share the same causes. If the pressure drop is rising while airflow remains constant, this indicates that the fan is compensating and drawing more power.

If the pressure drop increases and airflow decreases, it means that the fan has hit its limits and the system is essentially starving. One variation allows an extended period before medium filter replacement is necessary, while the other requires immediate intervention.

Table 2: Observed symptom, possible cause, verification action and response for medium filter troubleshooting.

This figure illustrates the process from observed symptom to verification procedure leading to course of action. It also differentiates between problems associated with filter restriction and fan, damper and duct problems, so one does not mistakenly pick filter replacement when it is not necessary.

Separate filter restriction from other HVAC problems

The medium filter functions as one of the several parts of the whole, which includes the preliminary filters, coils, fans, dampers and ductwork. Prior to replacing the medium filter due to a high pressure drop, one should first ensure that the pressure drop was measured across the filter and not overall, in the air handling unit.

A dirty cooling coil or a blocked prefilter can produce similar symptoms, so the reading must be taken at the filter taps before blaming the medium stage.

Assess cleanroom impact and escalation criteria

The functioning of a cleanroom can always be compromised if the pressure drop across the medium filter continues to increase, which is why cleanroom air filter inspection tracks the trend and not a single reading.

Specifically, if the pressure drop exceeds that defined for the final filter stage by more than 20 percent (e.g. a pressure drop of 480 Pa across the medium filter and a maximum defined pressure drop of 400 Pa), then the HEPA filter may experience an increase in particle loading of between 30 and 50 percent.

Routine replacement should be carried out whenever the particle counts at workstations exceed iso 14644-1 class limits [1].

Set a replacement decision and maintenance schedule

The replacement decision should be based on the simple premise of medium filter replacement once the filter has reached its final resistance, if damage is present, or when airflow is below system specifications. Never delay replacement of a damaged filter because failure of filter integrity will immediately jeopardize cleanroom classification.

An air filter replacement schedule should be developed based on operating conditions and not predetermined calendar dates.

The comparison guide above lists the conditions that justify continued monitoring and those that require replacement, starting from the baseline measurement and the stated final resistance.

In Deiiang's glass-fibre medium filter series, the final resistance is designated as being between 250 and 400 Pa. The synthetic variety reaches its final resistance at a lower dust holding capacity compared to the glass-fibre version.

Acceptance criteria may specify less than this in order to preserve fan energy. By changing the pressure set point from 400 Pa to 300 Pa, fan energy consumption can be reduced by approximately 8-12% throughout the life of the filter.

Apply manufacturer limits and site acceptance criteria

Manufacturer limits define the maximum pressure drop allowed before the filter media is damaged or the frame fails. However, the site acceptance criteria may have a lower pressure to ensure energy optimization or prevent loss of air.

Therefore, one must use and document both levels. If the manufacturer limit is 400 Pa but the site's limit is 300 Pa, the user should ｒｅｐｌａｃｅ the filter at 300 Pa. If a 2500 m3/h high-efficiency filter is rated between 400 to 600 Pa, it should be replaced at 450 Pa.

Build a repeatable inspection and replacement record

A record of how to check a medium filter should include the date of inspection, the model of the filter being inspected, the pressure drop, the airflow rate, who conducted the inspection, and what actions were recommended. The filter label and the pressure gauge should be photographed during the inspection.

This information can be used to determine whether pressure drops are linear or accelerating, which will help dictate the next air filter replacement schedule interval. This information was used in the Deiiang pharmaceutical case study, which incorporated DOP testing and commissioning pressure logs on a per unit basis.

Verify replacement filters and standards

Standards stipulate how filters should be tested and classified; however, they do not guarantee a filter will perform in an application. The standards regarding HEPA and ULPA classification are EN 1822 and ISO 29463 [2], cleanroom classification and testing standards are ISO 14644-1 and iso 14644-3 [1], legacy HEPA testing standards are IEST-RP-CC001 and MIL-STD-282 [3], and MERV ratings that govern general ventilation filters are defined by ASHRAE 52.2 [4].

This figure groups the standards by intention: general ventilation classifications, high-efficiency classifications, cleanroom classifications, and the component and application rules.

It must be noted that EN 779 has been removed and replaced by ISO 16890 for general ventilation purposes [5]. The reference for EN 779 should only be mentioned as legacy commentary to show that ISO 16890 with its categories of ePM1, ePM2.5 and ePM10 forms the current standard for general ventilation and not EN 779. EN 1822 and ISO 29463 are still the key documents for HEPA and ULPA classification.

Table 3: Standard, scope and relevance to medium-filter selection or verification.

Match filter performance requirements to the application

Usually, the MERV ratings for medium filters vary from 8 to 13, whereas HEPA applications are not required. The product range of Deiiang's medium filters consists of glass-fibre bag filters ranging from 1700 m3/h for F5 and F8 filters to 2050 m3/h for F6 filters.

In addition, synthetic bag filters offer 1000 m3/h for F6, F7 and F9 filters, and 1700 m3/h for F5 and F9 filters, while V-bank activated carbon filters can achieve airflow rates of 3700 m3/h. Ensure that the rated airflow performance does not exceed the design rated airflow by more than 10%.

Understand what standards do and do not demonstrate

A product rated at F7 according to EN 779 only proves the filter's collection ability of 85% for particles of 0.5 µm while under laboratory conditions. However, it will not assure any type of performance in an actual project based on your configuration, the temperature in which it is working and in terms of the types of dust and how much of it will accumulate over time.

The use of standards will allow for comparison only; it is not a guarantee. If the product is claimed to meet a particular standard, verify product specifications and ask for documented and current test results.

Apply the method to Deiiang products and a real project

The methodology outlined above can be applied to the catalogues of Deiiang's medium filter range. The catalogue allows for baseline information and actual site measurements provide the most current information, allowing for the calculation of the comparison, hence providing the information for the decision-making process.

An example of this methodology in practice is demonstrated below in a cleanroom at a pharmaceutical packaging site that wanted to meet an iso 6 class. The site had high ambient particle concentration, short plenum heights and an absolute limitation for pressure drops in order to maintain laminar flow. The baseline particle count at the facility was determined to be an iso 7 class.

The image is a general illustration of filter instrumentation and not a photograph of the project described below. It is included to show the kind of differential-pressure readout that the monitoring method depends on.

Deiiang had supplied low-profile high-efficiency H14 combined filters having dimensions of 592 mm x 592 mm x 292 mm and an airflow rating of 2500 m3/h. The initial resistance should not be higher than 220 Pa and the media area should be 20.04 m2.

The supply package was inclusive of silicone-free sealing gaskets and F7 pre-filters, which underwent a DOP scan to check for any defects in the product. As confirmed by the project report, particle counts had fallen from the iso 7 baseline to stable ISO 6 levels, and there are energy savings associated with the low initial pressure drop.

Select a Deiiang medium filter using verified specifications

In the particular case of pharmaceuticals, the F7 pre-filter safeguarded the H14 final stage filter and ensured that the pressure drop is within the limits of height of the plenum. The catalogue F7 glass fibre bag filter has an airflow rating of 2050 m3/h and possesses 3.18 m2 of filter media and initial resistance of less than 100 Pa.

The 592 mm x 592 mm x 381 mm configuration is compatible with standard air handling tracks. It is advisable to use the catalogue airflow rating as a point of reference, followed by verification concerning system design airflow.

Show a documented project inspection and replacement decision

The project inspection record details the initial pressure drop at installation, monthly measurements, and replacement procedures when it reached a pressure drop of 350 Pa. This figure is less than the 400 Pa final resistance but more than the 300 Pa at which energy optimisation was achieved.

The decision was based on three key factors: pressure drop trend, airflow stability and visual inspection showing even dust loading on the filter without damage to the media. The data from this record is a useful reference for the application of similar filters in other installations.

Frequently Asked Questions

How can I tell if my medium filter needs replacing?

To determine whether your filter is due for medium filter replacement, compare the current pressure drop to the baseline clean-filter and the final resistance value from the filter manufacturer. Once pressure drop has approached the final resistance or airflow has been reduced by 10% or more of the designed rate, the filter must be changed. Visibly damaged filters, including tears, gaps between gaskets or frame cracks, should also be replaced.

How often should a medium-efficiency filter be replaced?

The air filter replacement schedule depends on operating conditions and cannot be predicted strictly according to a calendar. In high-load cleanrooms, the filter may need to be changed every three to four months, while in low-load commercial buildings the medium-efficiency air filter can last for up to a year before needing to be replaced. The pressure drop should be monitored monthly and replaced when it gets to the predetermined acceptance threshold.

Can I tell a medium filter is clogged by looking at it?

You cannot tell if the filter is clogged by visual means alone, so treat visible dirt as one of the signs an air filter needs replacing rather than as proof. The filter may not look very dirty but could be at an acceptable pressure drop or vice-versa. Use airflow measurement and filter pressure drop as primary signs, and visual assessment as a secondary sign for damage or bypass.

What pressure drop means a medium filter needs replacing?

The replacement pressure drop is the one indicated as maximum resistance by the manufacturer, which can range from 250 to 400 Pa for medium filters by Deiiang. Some facilities have a lesser amount, such as 300 Pa in lieu of 400 Pa, to cut down fan energy consumption. One should always take care not to exceed the maximum resistance stipulated, as media damage and frame rupture are likely to occur.

What happens if I keep using a dirty medium filter?

The pressurized medium filter causes the rise of filter pressure drop, reduces airflow and makes the fan consume increased amounts of energy. It also accelerates the loading of downstream HEPA filters, leading to a shortened lifespan of those filters. In cleanroom facilities, the rise of particle count can go beyond the limits specified under the ISO 14644-1 standard.

Can a medium filter be cleaned and reused?

Medium-efficiency filters are designed to be discarded once they reach the prescribed limit, thus cleaning them will not be advisable as it can ruin the fibers, make tears or disrupt pleat spacing. The filter should be replaced once it has reached the specified resistance or ended up damaged. Deiiang media filtration products feature PU sealants and EVA hot melt spacers that cannot be cleaned more than once.

References

• [1] iso 14644-1:2015, Cleanrooms and associated controlled environments, Part 1: Classification of air cleanliness by means of concentration of airborne particles. iso 14644-3:2019, Methods of test.
• [2] EN 1822-1:2019, High-efficiency air filters (EPA, HEPA and ULPA), Part 1: Classification, performance testing and marking. ISO 29463-1, High-efficiency filters and filter media for removing particles in air, Part 1: Classification, performance, testing and marking.
• [3] IEST-RP-CC001, HEPA and ULPA Filters. MIL-STD-282, Method 102.9.1, DOP smoke penetration test.
• [4] ASHRAE Standard 52.2, Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size.
• [5] ISO 16890-1:2016, Air filters for general ventilation, Part 1: Technical specifications, requirements and classification system based upon particulate matter efficiency (ePM). Supersedes EN 779:2012.