What are the benefits of using a medium filter?

On the other hand, medium efficiency air filters don't have one performance number for user rating. Particle capture will vary based on the filter's nominal efficiency rating, type of filter media, face velocity, and other factors.

For a medium efficiency air filter, eACH efficiency rating and pressure drop is unique to the equipment and should be evaluated based on the testing method used.

In this article we will explain the medium air filter benefits, compare efficiency ratings, and explain how to choose a medium air filter for HVAC, cleanroom, and industrial applications. The information on the Deiiang catalogue may be referenced as such in this document.

How medium filters Improve Air Quality

A medium efficiency air filter improves facility air quality by specifically addressing the particle sizes that are relevant to comfort levels as well as protecting processes. Those are the two headline medium air filter benefits, and both follow from the same capture mechanism. The filter helps eliminate most of the coarse and fine dust particles before those particles reach the final filtration step.

This figure is conceptual, not a measured Deiiang test result. It illustrates how particles are progressively intercepted by fibrous media, with impaction and interception dominating for larger particles and diffusion for the smallest.

What Particles Can a Medium Filter Capture?

The medium filter may capture pollen, mold spores, dust from concrete, textile threads, and some part of fine dust based on the effectiveness of the filter and working conditions. These airborne pollutants are illustrative examples, and their capture will differ based on working conditions.

A medium efficiency air filter cannot alone guarantee the removal of pathogens, disinfection, or classification as a clean room. These tasks require separate design, testing, and validation under the respective standards.

How Do Efficiency Ratings Describe Filter Performance?

When we have a rating for air filtration efficiency, it converts the numbers that we obtain in the performance of filters into a more understandable number, but the rating systems are not interchangeable. To illustrate this point, EN 779 F grades worked on average efficiency for fine filters at 0.4 µm.

For example, F7 sits at a minimum efficiency of 35% and an average efficiency of 80–90%.

As of now, EN 779 has been removed and replaced with ISO 16890 for ventilation filters. ISO 16890 gives different methodologies for categorizing filters as ePM1, ePM2.5, and ePM10.

Furthermore, there are ASHRAE 52.2 MERV ratings that work with particle sizes based on three different efficiency bands. MERV and EN 779 F grades are not fixed conversions, and no universal equivalence should be assumed.

Table 1: Standards and classification bases for air filters.

Protecting HVAC Equipment and Downstream Filters

Out of many benefits, HVAC equipment protection is one of the main advantages of medium filters as a whole. An HVAC medium filter located upstream of both the cooling coil and fan motor as well as heat exchangers ensures that those components remain cleaner for a longer duration.

This keeps heat exchange efficiency at an optimum level while decreasing the need for cleaning over time, which is where the value of an HVAC medium filter shows up first.

The figure shows a three-stage arrangement: a coarse prefilter, a medium efficiency air filter, and a final HEPA stage. Each stage removes a progressively finer particle fraction, so downstream filters see a lower dust load per unit of airflow.

How Does a Medium Filter Reduce Dust Loading on Other Filters?

The accumulative effect of dust loading is a significant factor with respect to both downstream element longevity and HVAC equipment protection, as it will depend on the type of dust present, concentration of dust, volumetric flow rates, filter surface area, and the pressure drop criterion for filter replacement.

If a medium filter removes a particular size and volume of dust from the airstream, it allows less mass of dust to pass through to the HEPA stage. That lowers the rate of pressure increase for that HEPA filter.

Lower rates of dust build-up on the final filter are the user's practical benefit, not a fixed service-life multiplier, and they are the clearest measurable form of HVAC equipment protection.

Which HVAC Components Can Benefit from Improved Upstream Filtration?

The most benefit goes to systems with a high cost of replacement or loss of operating efficiency due to dust build-up. Examples are:

• Cooling coils, which are fouled with dust and have a negative effect on heat transfer.
• Fan impellers and motors, which lose performance and efficiency due to dust build-up.
• Heat recovery wheels, which are negatively impacted by particulate blockage.
• HEPA or ULPA filters, which have a shortened service life due to dust build-up.

According to ASHRAE guidance, the use of MERV 8 filtration will protect heating and cooling coils in mechanical equipment rooms and similar areas [7]. For energy savings to exist, measured system data is needed.

Maintenance benefits should never be confused with energy benefits in the absence of site-specific data, even though both count as medium air filter benefits on paper.

Balancing Filtration, Airflow, and Energy Use

Every filter has a pressure drop, and a pressure drop, no matter how small, requires fan energy. The engineering challenge is to find the lowest level of filtration suitable for meeting a facility air quality goal, then ensure the pressure drop for the HVAC medium filter fits into the static pressure budget for the fan.

For a given facility air quality target, the trade-off can be assessed when limited parameters are met. For a constant fan speed and constant impeller diameter, the actual power required to drive the fan shaft is proportional to the cube of the fan rotational speed according to affinity laws [4].

However, in practice, this rule of thumb is not applicable for all fan types, control schemes, and operational requirements.

How Does Pressure Drop Affect System Performance?

The pressure drop plays a significant part in airflow and energy consumption. As the filter loads, the resistance of the filter increases, and therefore the fan will be able to generate less airflow if there is no capacity reserve.

In a constant-volume system, the fan will need to exert considerably more effort to maintain the airflow rate, and the HVAC medium filter is the component whose loading drives that increase.

When a system is constructed using a medium filter with a low initial resistance rating and a HEPA stage with a certain initial resistance rating, the whole system works at a joint initial pressure ratio. As both filters load, the total working pressure of the fan will define how much airflow is lost.

How Can Engineers Balance Filtration Needs and Energy Consumption?

The filtration energy consumption is regulated by engineers by establishing three parameters prior to the selection of the necessary elements, starting from the facility air quality target the system has to hold.

In general practice engineers opt for the medium filter type with a minimal effectiveness level so that the selected target can be met, and reconfirm the initial pressure level against the fan pressure chart.

For instance, Deiiang's F7 W-bank filter is rated at 85% efficiency at 0.5 µm with an initial resistance below 100 Pa.

Table 2: Comparison of medium filter products of Deiiang (catalogue data).

Applications in Cleanrooms and Other Facilities

Clean rooms typically utilize the medium filter for cleanrooms as the first stage of filtration to ensure that the incoming count of particulate matter is decreased before entering the HEPA or ULPA stages.

This is a main contributor to the classification of clean rooms, which is determined by measuring airborne particle concentration under iso 14644-1 [1].

The above shows different placements of the same family of medium filters as per the requirements of cleanroom facilities, including different methodologies for performance ratings of other filters, including gas-phase filter systems.

Where Are Medium Filters Used in Cleanroom Systems?

The medium filters are utilized in clean room systems in three different positions. The very first use is in the fresh air intake AHU, where the medium stage protects the HEPA filters from dust coming in from the surroundings.

The second placement of a medium filter for cleanrooms is in the return air path, where this filter stops generated contaminants from entering the air conditioning system again before recirculation.

A medium filter for cleanrooms in recirculation AHUs is the third situation, and it is an application where a medium filter increases the time interval for changing the HEPA filter.

Because ISO 14644-1 defines classifications according to the level of particles in the air, the factors that influence the filtration series include the cleanliness goal, process outputs, outdoor air quantity, air circulation pattern, types of final filters used, and supervision measures [1].

How Should Application Requirements Guide Filter Selection?

The choice of filter must be made according to the degree of risk associated with the application rather than habit. A good starting point, as outlined in Deiiang's suggestions, is MERV 8–11 or F5–F7 for general commercial establishments.

Medical, pharmaceutical, and electronics industries use MERV 11–13 or F7–F9.

These parameters are recommendations and not hard and fast rules, but they should be based on the applicable requirements or confirmed by Deiiang's verification of the selection process.

Filters made of activated carbon are designed to remove gases and odors, which is a different type of contaminant from particle-type filters.

Choosing the Right Medium Air Filter

In order to choose a medium air filter, one must take four factors into consideration: application type, required efficiency, the airflow involved, and the measurements. The first step is to find out the application type since it determines the lowest efficiency level selected. Then, one must check the airflow amount as well as available side space.

Table 3: Selection checklist for medium air filters.

Which Filter Specifications Should Buyers Compare?

When buyers ask how to choose a medium air filter, five specifications should be compared directly, including the air filtration efficiency classification and testing basis, rated airflow, initial pressure drop at airflow, final pressure drop, and media area.

Media area is important, but other factors matter too, including media structure, dust-holding capacity, airflow, aerosol type, and final pressure drop criteria. The performance of a given media cannot simply be concluded from the media area alone; it requires comparable test data from Deiiang that includes the test procedure.

What Information Should Buyers Confirm with a Manufacturer?

Four items should be confirmed by the buyer prior to purchasing. The first is the actual initial pressure drop at the specified airflow instead of a nominal figure. The second is the final pressure drop when replacement will be required.

The third is the maximum operating temperature and humidity of the complete assembled filter. The fourth is whether the frame and gasket are compliant with the AHU.

According to catalogue information, Deiiang glass fibre bag filters are rated at a maximum operating temperature of 150 °C, while synthetic filters are rated for 80 °C. An aluminum folding frame, glass-fiber filter media, and two-component polyurethane sealant, along with 120 g/m² adhesive board dividers, are used in the glass-fiber construction, per catalogue specifications.

Installation, Monitoring, and Replacement

Medium filter maintenance is mainly based on the reduction in pressure. Once the filter has been installed and sealed, pressure drop monitoring is performed until it reaches the predetermined level indicated by the manufacturer.

The diagram above indicates the four inspection points used to check the efficient operation of medium filters. Bypass of the filter media is evidence of a misconfigured installation.

How Should Medium Filters Be Installed and Inspected?

Installation requires confirming three things: how well the filter fits in the frame, whether the seal is intact, and in which direction the airflow arrow will point. All gaps should be eliminated to ensure a tight fit.

Once the filter is installed, an initial pressure drop must be noted, along with the location of the measurements. This establishes the baseline pressure drop for the unit.

If the reading exceeds the maximum levels according to catalog specifications, either sealing was not done correctly or a problem is noted with the blower motor.

When Should a Medium Filter Be Replaced?

Replacement dates vary depending on the facility's medium filter maintenance procedures and the specifications established by the company for that individual medium filter, as there is no standard pressure drop threshold that applies to all filters.

Deiiang recommends using pressures within the range of 250 to 400 Pa, with actual numbers depending on the model type.

The replacement checklist should contain relevant information on differential pressure at baseline, location of measurement, airflow status, as well as initial checkpoint data. Calendar-life estimates are used as a general planning tool and not to determine when an HVAC product should be replaced.

Standards, Testing, and Product Verification

Standards determine what classification scheme is used and how a product is validated. For medium filters, ISO 16890 for ePM classifications, ASHRAE 52.2 for MERV, and the now-obsolete EN 779 apply. The EN 1822 and ISO 29463 standards apply to HEPA and ULPA filters.

Table 4: Standards relevant to filter classification and verification.

Which Standards Apply to Medium Filter Classification and Testing?

Medium filters for general ventilation are assigned to ISO 16890 since 2018, when it replaced EN 779. ASHRAE 52.2 MERV ratings are also used extensively. Both measure air filtration efficiency through specific ranges.

HEPA and ULPA filters are categorized under EN 1822 or ISO 29463. These standards categorize filters not by a universal 0.3 µm test point but through MPPS-based testing [10][11].

How Should Test Data Be Interpreted for Cleanroom Use?

Test data should be interpreted with respect to the target iso class of the cleanroom. iso 14644-1 defines classification of airborne particles; however, it does not define a particular filter arrangement [1].

A HEPA or ULPA filter stage can be used to achieve high cleanliness levels but must be tested against the process and particle count criteria. The medium filter's contribution is defined through decreased dust loading and a stable pressure drop across the HEPA or ULPA filter stage.

Deiiang Product Data and Project Experience

Deiiang is a manufacturer of clean rooms and air filters. Jason Peng is the product designer. The Deiiang range of medium filters includes glass fiber bag filters, synthetic bag filters, W-bank combination filters, activated carbon V-bank filters, and pleated panel filters.

The figure illustrates the installed filter configuration and the DOP scan of the filtration installation from a pharmaceutical packaging cleanroom project.

What Should a Medium Filter Project Case Study Demonstrate?

A case study should discuss the challenge, the filtration train used, and the outcome with conditions. The cleanroom project for pharmaceutical packaging is constrained by three main factors: a very high amount of particulate contamination in the air, a limited height in the plenum, and the requirement of low pressure drops to ensure laminar airflow.

The design required that the initial pressure drop of the system be low enough for laminar airflow to occur. Deiiang provided low-profile H14 combined high-efficiency filters (592 × 592 × 292 mm, rated at 2500 m³/h, ≤ 220 Pa initial resistance, and 20.04 m² media area) as well as silicone-free sealing gaskets and F7 pre-filters.

Each one of the H14 units was tested with a DOP scan test on a per-unit basis before it was shipped.

The project-reported outcome is a transition from the baseline particle counts to stable target counts, plus energy savings due to the low initial pressure drop achieved with the H14 stage. Full test records, measurement criteria, and baseline data ought to be requested from Deiiang for confirmation.

How Can Buyers Evaluate Deiiang Product Data for Their Facility?

Customers need to match the catalogue airflow to the design airflow and compare initial and final pressure drops against the fan curve. The Deiiang catalogue includes model dimensions, efficiencies, airflow rates, and pressure drops for all products by family.

For the medium range, the 592 × 592 × 381 mm six-bag model provides 2050 m³/h with 3.18 m² media area and an initial resistance of less than 50 Pa (F5) to less than 100 Pa (F8), while final resistance is 250–400 Pa.

Frequently Asked Questions

What Is a Medium Air Filter?

A medium air filter can be seen as an intermediate ventilation filter that lies somewhere between a coarse filter and a HEPA filter, and is referred to as either an ISO 16890 filter or an ASHRAE 52.2 filter. They come in several types, including bag filters, V-bank filters, and pleated panel filters, and are characterized by particular properties of the media as well as the operating conditions under which they work.

What Are the Main Benefits of Using a Medium Filter?

Some of the medium air filter benefits include better indoor air quality for your facility, protection of HVAC equipment, and increased endurance of filters located behind the medium air filter. More so, medium air filters help reduce the rate of increase in pressure drop at the last filter stage, thereby maintaining stable airflow and energy usage.

Where Are Medium Air Filters Commonly Used?

Some of the areas where medium air filters are applied include commercial HVAC systems, hospital AHUs, laboratory AHUs, Pharmaceutical cleanroom pre-filtration, and industrial dust removal systems. They can also be used for particle and gas removal ahead of activated carbon filters.

Can a Medium Filter Help Extend the Life of a HEPA Filter?

When a medium filter is located upstream from a HEPA filter, it removes a certain percentage of particles that would otherwise cause loading on the HEPA medium and impact its life expectancy. The precise extension depends on the kind of particle involved, its concentration level, the flow of air, the capacity of the filter being used, and the criteria set for changing the media.

How Often Should a Medium Air Filter Be Replaced?

The replacement of a medium filter is determined by the prescribed manufacturer limit and the medium filter maintenance procedures of the specific establishment. For example, for Deiiang's medium filtration systems, replacement is due when the pressure drop reaches its final value of between 250 and 400 Pa, depending on the design of the filter.

How Do I Choose the Right Medium Air Filter?

The process of selecting an appropriate medium filtration element consists of four steps: determine the intended application and the required efficiency rating, confirm the airflow at the design stage, ensure that all dimensions fit the air handling unit, and verify the initial pressure loss. You should also use ISO 16890 ePM ratings for current projects.

References

• [1] iso 14644-1:2015 — Cleanrooms and associated controlled environments — Classification of air cleanliness by particle concentration.
• [2] ISO 16890-1:2016 — Air filters for general ventilation — Technical specifications, requirements and classification system based upon particulate matter (PM).
• [3] EN 779:2012 — Particulate air filters for general ventilation — Determination of the filtration performance (withdrawn; superseded by ISO 16890).
• [4] ASHRAE Standard 52.2-2017 — Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size.
• [5] EN 1822-1:2019 — High efficiency air filters (EPA, HEPA and ULPA) — Classification, performance testing, marking.
• [6] ISO 29463-1:2017 — High-efficiency filters and filter media for removing particles in air — Classification, performance testing, marking.
• [7] ASHRAE Handbook — HVAC Systems and Equipment (current edition) — Air-cleaning devices and coil protection guidance.
• [8] Fan Affinity Laws — Engineering reference for fan power and pressure relationships (see standard fan engineering texts).
• [9] ISO 16890-2:2016 — Air filters for general ventilation — Measurement of fractional efficiency and air flow resistance.
• [10] IEST-RP-CC001 — HEPA and ULPA Filters (test method reference for high-efficiency filters).
• [11] MIL-STD-282 — Filter Units, Protective Clothing, Gas-Mask Components and Related Products: Performance-Test Methods (DOP test method).