What size medium filter do I need?

Do not ever order using nominal description solely as filters that are nominally considered 24 x 24 x 2 in would be closer to 23.5 x 23.5 x 1.75 in once measured.

A good example of this is if a 592 x 592 x 381 mm six bag F7 filter is required to handle an airflow of 2,050 m³/h at a resistance of less than 100 Pa, you will order using this number assuming your filter slot is 590 x 590 x 380 mm or will need custom air filter dimensions if the filter slot you have measures 570 x 570 x 370 mm.

medium filter size combines three independent variables. First is physical fit and secondly airflow capacity which must be combined with filtration efficiency. If your filter fits but restricts airflow, it will ultimately choke your system. Ordering a filter rated at 3,400 m³/h but inserted in a duct rated at 1,700 m³/h creates an inefficient system and ultimately wastes money.

The following procedure answers the question of what size medium filter do I need through the steps of measurement, compatibility testing, ordering, and validation. EACH section builds on one another, with tables and figures for flexible reference. It was created by Deiiang™ product designer Jason.peng based on the specific medium-efficiency air filter catalogue available through Deiiang, specifically for F5–F9 grades.

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How to determine the correct medium filter size

Determining the correct medium filter size starts with three physical measurements and one system check. The three measurements are frame width, frame height, and frame depth. The system inspection process is about airflow capacity against your fan curve. If you get any of these wrong the filter will seat incorrectly, bypass air, or create a much greater pressure drop than the fan can compensate.

Figure 1 indicates the measurement convention; width runs left to right across the face, the height runs from top to bottom, and the depth runs from front to back along the airflow direction. Inside the housing are points indicating where to make measurements.

For instance, the medium air filter dimensions will follow the W×H×D format in catalogues. When an Deiiang glass-fibre bag filter is specified as 592×592×381 mm, the filter is 592 mm wide, 592 mm high, and 381 mm deep from the frame face to the tips of the bags.

Note that measurements are to be made into the filter housing as oppose to measuring the old filter which would give deferent dimensions. As a rule, Use a steel tape and measure into the housing and record whichever point along each dimension is the smallest. That reading is your true HVAC filter replacement size.

The housing may be deformed over years of service; gaskets are compressed, and an old filter has probably been forced into an undersized space. In this case you are merely measuring a previous persons mistake instead of correcting it.

Which filter dimensions should I measure?

You should measure the filter housing opening from inner edge to inner edge, because medium air filter dimensions are always taken at the tightest point of each axis. Front access and loading track systems you measure track width, track height, and the distance from front retaining lip to rear stop. In side access air filters, you measure the additional slot depth from the gasket seat to the back wall of the housing.

Measure four parameters: the true width, true height, the actual depth of the board frame and the two diagonal checks, because together they define your HVAC filter replacement size. A diagonal check can test rack distortion in your frame. If the two diagonals are more than three mm apart from each other, then the frame is distorted, and therefore the rectangular filter won't close properly.

How do nominal and actual dimensions differ?

Nominal size refers to the rounded figure which is quoted. Actual size measures the absolute figure in nature. A filter which is nominally 24 inches by 24 inches by 2 inches will normally measure about 23.5 inches wide by 23.5 inches tall by 1.75 inches deep – which is about half an inch smaller than the nominal size in terms of depth and width and about one quarter of an inch smaller in depth, respectively [1].

The difference arises between the nominal dimension and the actual one solely to allow the filter to slide freely into its respective track without its getting stuck. When you buy filters according to HVAC medium filter sizes, always verify whether the dimensions are quoted in nominal or actuals. Thus, Deiiang's catalog uses the actual measurements in millimeter for every liter.

Getting medium air filter dimensions right here avoids ordering a medium filter size that will not seat. Many HVAC filter replacement size errors trace back to confusing nominal and actual figures, and this is where most contractors lose time and money.

Identify the filter type and housing

Correct cleanroom air filter sizing depends on recognizing the type of filter beforehand, since the filter type determines how the depth will be measured, how the frame seals will be performed and the air in the filter must pass through the housing.

Figure 2 shows the basic geometric structures of the panel air filter and air filter for pocketing while it also illustrates the various shapes of different types of air filters. Also, the figure presents a clearer view of the entire array of air filter profiles and the airflow as shown in the respective figure. Bag filters consist of a header frame and long bags of fabric usually 300–600 mm in depth.

Is the filter a panel, pocket, or bag type?

Panel filters can be installed for limited depth of less than 100 mm, so cleanroom air filter sizing for panel units rarely goes beyond 96 mm. The W-bank of Deiiang is a medium filter made of plastic frame that fits into this category with nominal catalogue sizes of 23×23×11.5 inches and 11.3×11.3×11.5 inches.

Pocket and bag filters will trade depth for media area, so pocket filter size and bag filter dimensions both grow once you add pockets. The six-bag Deiiang F7 filter having dimensions of 592×592×381 mm can deliver up to 3.18 m² of media area with an airflow of 2,050 m³/h, which is approximately three times more than what panel type filter having the same face area could deliver at the same pressure drop.

The pocket number and bag length determine the pocket filter size and bag filter dimensions. More pockets would result in more area of media for a face area, which will provide more media area while decreases the initial pressure drop and increases depth. Getting pocket filter size wrong is a common cause of bypass, and bag filter dimensions must always be measured with the bags hanging free so the HVAC medium filter sizes you order actually match the housing.

Which housing and frame details affect fit?

The design of the frame should also correspond to the type of housing track. The different styles of frames are box frame, single-header frame, and double-header frame. The type of seal is also important, and thus the EVA gaskets, neoprene gaskets, and PU sealant beads will all have different requirements for compression gaps.

Before making your order, you need to check three different aspects of the housing:

For both pocket and bag filters, orientation is important. The airflow through the open pocket needs to enter through the end of the pocket and exit from the fabric side. If you reverse the air filter, it will cause the pockets to collapse against the frame.

Table 1: Filter type, frame dimensions, depth and key compatibility checks. The dimensions mentioned in the catalog are specific to Deiiang products.

Match size to airflow and pressure drop

In a medium-efficiency air filter the relationship between airflow capacity and pressure drop is determined by the media area and the resistance curve of the filter. A bigger filter has more media area and can accommodate a larger flow rate at a lower pressure drop. Conversely, a smaller filter has a more rapid face velocity, which translates into more pressure drop and quicker loading.

Figure 3 illustrates the relationship between air flow and pressure drop. At 2,050 m³/h, the six bag F7 filter from Deiiang has an initial resistance of less than 100 Pa. At 1,500 m³/h, the same filter reaches around 55–65 Pa. At 2,500 m³/h, the pressure increases to above 130 Pa.

As illustrated in the pharmaceutical case study later in the article, the choice of low pressure drop filters is essential for maintaining the necessary laminar flow for iso 6 compliance.

How do I check airflow capacity?

Use the fan curve at the operating point of the filter. If the rated airflow of the filter exceeds the maximum airflow capacity of the fan at a constant static pressure, the fan will then operate at the extreme ends of its curve consuming a lot of energy and not achieving much air movement than what is intended.

There is an easy way to perform the determination, which is by dividing the airflow into the area of the filter face. For instance, the filter, which measures 592 mm x 592 mm has an area of 0.35 m². Using air flow rate, say 2,050 m³/h, the face velocity becomes 2,050 ÷ (0.35 × 3,600) = 1.63 m/s. Average filters run at face velocities of about 1.5 m/s to 2.5 m/s.

How can pressure drop affect filter selection?

The drop in pressure increases the workload on the fan, which may mean that a filter that puts on an extra pressure of 50 pascals (Pa) would force the fan to operate outside of its design point and reduce the total airflow by as high as 20% in most situations.

For instance, the F5 bag filter from Deiiang operating below 50 Pa and the F8 model which shares the same dimension of 592 mm x 592 mm x 381 mm for each of its six bags starts with a pressure drop of above 100 Pa. Therefore it is crucial to ensure that the fan can operate using the appropriate maximum pressure points.

Table 2: Filter dimensions, air flow and initial pressure drop of glass fiber and synthetic bag filters made of Deiiang.

Select filtration efficiency for the application

The efficiency class is the classification of air filter goods since this rating of filtration will define the particle capture efficiency and downstream HEPA stage diameter. The efficiency class of F7 filters will remove 85% of 0.5 µm particles. The efficiency class of F9 filters will remove over 95% of 0.5 µm particles.

The numbers reported in the above section were generated according to the legacy EN 779 testing, but ISO 16890 has since superseded this process for all ventilating use of filters [2].

The methodology demonstrated visually in Figure 4 does not claim equivalence between the various rating systems. Therefore, there is a legacy classification for EN 779 from F5 to F9. ISO 16890 has established a different classification system using ePM1, ePM2.5, and ePM10. ASHRAE 52.2 employs ratings of MERV 8 through MERV 13 illustrating the same area of responsibility. Finally, the HEPA and ULPA classes can be rated above MERV 16 by utilizing EN 1822 or ISO 29463.

Which efficiency rating does my system require?

Deiiang establishes selection guidelines to match the application with the rating level. General commercial building classification is equivalent to MERV 8 through MERV 11 rating or F5 through F7. Likewise, for a medical-pharmaceutical-electronic facility ahead of HEPA, the rating will be MERV 11 through MERV 13 or F7 through F9.

Hence, in cases where there is a pre-filter rating of MERV 8 leading to HEPA filtration, the medium class will be rated at either MERV 13 or F8 in order to maximize the life of HEPA through the reduction of the amount of particles reaching the HEPA filter. An improvement of 10% in medium-stage arrestance can lead to an extension of HEPA replacement intervals by 15% to 25%.

Do cleanroom requirements change the filter choice?

Medium stage sizes of cleanroom air filter depend on the selected ISO. iso 6 cleanroom (class 1000) shows 35,200 particles/m³ at ≥0.5 µm [3]. The medium filter should lower the ambient load sufficiently for the HEPA stage to be effective within that standard and not overloaded.

EN 1822 maintains HEPA filter classifications as per their MPPS efficiencies. H13 efficiency is ≥99.95%, and H14 efficiency is ≥99.995% at MPPS [4]; while ISO 29463 uses the same classifications from ISO 35 H through iso 75 U.

Table 3: Classification systems, efficiencies, standards, and applications.

Confirm cleanroom and HVAC compatibility

Compatibility means filter fits housing, fan can overcome pressure drop, seal matches frame, and efficiency meets required cleanliness standards. Filter resizing for the cleanroom adds one more step: your medium stage must not shed fibers and/or particles that cannot be captured by the HEPA stage. Correct cleanroom air filter sizing therefore also protects the final HEPA stage.

Figure 5 outlines the decision process: what ISO you need to start with, what HEPA filtration efficiency is needed, and how to size the pre-filter so as to minimize loading. iso 14644-1 and iso 14644-3 provide the standard and test method [3].

What cleanroom requirements should I check?

The following are the cleaning room standards you should check before selecting a medium filter.

How do I verify the filter will fit the existing system?

You need to verify the fit by following these 3 steps. First, you need to verify the filter dimension against the opening minus any gasket pressure. Second, check the frame design against the parameters of the track. Finally, you need to check clearance for installation: a bag filter that is 600 mm long requires 650 mm of clearance at the front.

When you change filters, do not fall into the convenience of reasoning that two filters with exactly the same face dimensions can be used interchangeably since they may not have the same depth. A filter with measurements of 592 x 592 mm with a depth of 96 mm cannot take the place of filter 592 x 592 mm with a depth of 381 mm in a track for a bag filter only.

Measure, specify, and order the replacement

Choosing the right medium filter size can be explained through a series of 6 steps in a sequential manner. If you skip any of the steps, you run the risk of simply ordering a filter that will not fit or perform. Before placing an order with the supplier, follow the six steps outlined below in figure 6.

The first step is to measure the inside dimensions of the housing at three different points. The minimum measurement should be recorded. The second step is to take a picture of the existing filter and its nameplate, including its model number, if visible. The third step is to compare the filter type with the housing type.

The fourth step is to check the fan curve against the air filter specification to ascertain the required static pressure. The fifth step is to select the desired efficiency class. The sixth step is to provide information regarding dimensions, frame type, seal type, and orientation.

What information should I provide when ordering?

You need to send seven pieces of information to the supplier:

• Actual width, height, and depth in mm.
• The nominal dimensions, if known.
• The type of filter (panel, pocket, bag, V-bank filter).
• Efficiency rating of the filter (F7, MERV 13, ePM1 50%, etc.)
• Required air flow in m³/h.
• Type of frame and seal.
• Mention operating temperature and humidity limits.

In the case of ordering Deiiang products, mention the catalogue number, if available. The six-bag F7 filter with dimensions of 592×592×381 mm is commonly available. However, if the required depth is between 381 mm and 600 mm, it may not be available.

What should I do if the size is nonstandard?

Non-standard sizes are commonplace in retrofit projects, which is exactly where custom air filter dimensions earn their place. Deiiang customized air filters to meet customer specifications. Provide the size of the housing, filter type, and efficiency class. Lead time for customization is 2–4 weeks, and this is how custom air filter dimensions become a practical option rather than a last resort.

Housing which is critically warped or otherwise damaged is not a suitable environment for forcing custom air filter dimensions into an inappropriate track; it may be more advantageous to ｒｅｐｌａｃｅ the housing instead. Typically, a 5 mm warping of the housing will lead to a 30–50% reduction in seal efficiency and lead to unfiltered air entering the system as bypass air.

ｒｅｐｌａｃｅ and validate the installed filter

Verifying the replacement medium filter size is a controlled process rather than a quick substitution. It is time-consuming to ensure proper seating, sealing and orientation. After installation, confirmation should be sought using both a pressure drop reading and a visual inspection.

Figure 7 outlines the procedure to be followed which consists of five steps: isolation of system, removal of old filter, inspection of housing, reinstallation of new filter and recording of initial pressure drop reading.

Standard safety procedures should be observed at all times including de-energizing the fan and locking out the electrical supply and allowing the system to depressurize.

After the installation of the filter check proper seating by running your hand along the gasket. Any gap larger than 1 mm will allow the unfiltered air to enter by bypassing the filter.

How can I confirm the replacement is installed correctly?

There are four ways to check for proper installation:

The reading on a calibrated manometer for a Deiiang F7 six bag filter at 2,050 m³/h should be less than 100 Pa.

When should I recheck airflow and pressure drop?

Re-evaluate the pressure drop after 24 hours of operation, and then again monthly. If the pressure drop reaches its final resistance of 250–400 Pa for medium bag filters then the filter needs to be changed. For instance, in the case of a Deiiang F5 Model bag filtering system it means the pressure should rise from lower than 50 Pa to 250 Pa (an increase of 5 times).

Document every reading with date, airflow, and pressure drop. The trend line predicts remaining service life. A filter rising 20 Pa per month reaches 250 Pa in 10 months from a 50 Pa start.

Deiiang project case study and product data

A cleanroom for pharmaceutical packaging with class 1000 / ISO 6 specifications required a solution in the medium stage of filtration. The existing baseline was ISO 7 which was high with ambient particle concentration and limited space in the plenum.

Documented evidence of the installation showing the low-profile H14 filter array and F7 pre-filter bank can be viewed in Figure 8.

What sizing challenge did the project team face?

The limitation of plenum design made it impossible to use filters longer than 292 mm, and this forced cleanroom air filter sizing toward a low-profile arrangement. Standard 381 mm bag filters could not be used. The allowable static pressure budget was only 220 Pa at the medium and final filter stages.

A 592 mm x 592 mm x 381 mm bag filter was too deep; at the same time, a standard 292 mm HEPA filter did not have the required dust holding capacity to handle the amount of particles in the air.

How did the selected filter perform in the application?

Deiiang delivered low-profile H14 combination high-efficiency filters with the dimension of 592 mm x 592 mm x 292 mm. Each filter unit operated at 2,500 m³/h at an initial pressure drop of ≤220 Pa, with a media area of 20.04 m². The project also utilized silicone-free sealing gaskets and F7 pre-filters.

The 2,500 m³/h filter can hold about 1,200 grams of dust, so the replacement of filters can be delayed compared to the equivalent filter size of lesser thickness.

Frequently Asked Questions

How do I find the size of my medium filter?

One should take out the existing filter, check if there is a label on the frame. If it is not visible, the individual should measure the housing openings. The width, length, and depth should be noted down in millimeters so that you can match them against the HVAC medium filter sizes in the supplier's catalogue and confirm your medium filter size.

Should I use nominal or actual filter dimensions when ordering?

One should use actual size. Nominal sizes are labels that are not always the same as the actual size that has a variance of more than 1/2 inch. If you only receive quotation of nominal sizes from your supplier, please confirm with them to make sure that you will get the actual size and the correct HVAC medium filter sizes.

Can I ｒｅｐｌａｃｅ a medium filter with one of the same size but a different type?

Yes. If your frame style setup, seals, depth, and airflow ratings are the same, then you may ｒｅｐｌａｃｅ a bag filter with a panel filter of the same face size. The only condition for this is that the housing track can accept the narrow frame and the airflow for each unit of face need to remain within the fan specifications.

How do I measure a pocket or bag filter?

To measure a bag filter, you need to take the width of the filter frame in the same way as for panel filters. As for measuring the depth, you need to measure the bag from the frame to the tip of the bag while allowing it to hang.

Never take measurements with a filter that is in place as airflow would alter the effective measurement by inflating the bag, and this keeps both your pocket filter size and bag filter dimensions reliable.

Does a larger medium filter improve system performance?

A larger medium-efficiency air filter will have more media area, therefore, the velocity on the face of the medium filter will decrease. Besides, it will also have less initial pressure and longer life. The doubling of the media area results in a 30–50% decrease in the initial pressure for the same airflow level, which in turn reduces the energy for the fan accordingly.

What if my required medium filter size is not standard?

In the event that you have a required medium filter size that is not common, you can get it custom-made. Just provide accurate details of your housing, type of filter, efficiency rating, and airflow, and the Deiiang will supply you a custom-built filter in a 2–4 week period.

References

• [1] American Standard Air. Explanation of Furnace filter sizes. Quantification of nominal and actual dimensions.
• [2] ISO 16890. Classification of air filters based on ePM1, ePM2.5, and ePM10.
• [3] iso 14644-1:2015 & ISO 14644-3. Cleanrooms/corresponding controlled environments classification on air cleanliness based on percentage of particle concentration; test procedure.
• [4] EN 1822-1:2019. High Efficiency Air filters (EPA, HEPA, ULPA) — classification/testing.
• [5] ISO 29463. High Efficiency filters and media made of filters for particle removal from the air – classification/testing.
• [6] ASHRAE 52.2. General ventilation air cleaning devices performance testing for particle removal efficiency.
• [7] EN 779:2012 (Withdrawn). Particulate Air filters operation performance. Please refer to ISO 16890.
• [8] IEST-RP-CC001. HEPA and ULPA filters, recommended practices for classification/testing.
• [9] MIL-STD-282. Tests performance testing methods of filter units, protective garments, gas masks.
• [10] Deiiang Clean Room and Air filter Manufacturer product catalogue. Medium Efficiency Filters (F5–F9); combined high efficiency filters H13 and H14; DOP integrated filter; super high efficiency baffle free filter U15 and U17.