HEPA filters outperform standard filters

Air filtration improves indoor air quality by targeting two main types of pollutants:
particulate matter and gases. Parth Harshad Gandhi, Airside Application Expert
at Trane Technologies India, says that technologies, including HEPA filters and
innovative filtration mechanisms, capture contaminants, improving healthier
environments and indoor air quality, however, ISO 16890 and ASHRAE 62.2 are
valuable resources for understanding filter selection and system design.

How do the types of air filters used in air conditioning systems differ in application?

To understand the types of contaminants and the filtration mechanisms, it is recognised that air contains two main types of pollutants: particulate matter and gases. Particulate matter includes dust particles, pollen, pathogens, and other solid particles. The gaseous pollutants mainly consist of nitrogen oxides (NOx), sulfur oxides (SOx), carbon monoxide (CO), ozone (O3), ammonia (NH3), hydrogen sulfide (H2S), and volatile organic compounds (VOCs).

The five primary mechanisms by which particulate matter of different sizes is captured in air filters include: straining, diffusion, interception, inertial impaction, and electrostatic attraction. These mechanisms play a role in filtering and removing particles from the air.

Air filters are designed based on five fundamental principles. They are categorised by their ability to capture particles of varying sizes. Guidelines such as ISO 16890 provide standards for classifying filters according to their applications. Filters are categorised into four primary types. First, pre-filters capture larger particles, typically around 10 microns. Second, medium fine filters trap smaller particles, ranging from 10 to 3 microns. Third, fine filters capture particles in the 0.3 to 1 micron range.

Lastly, EPA, HEPA, and ULPA filters are used in sensitive environments, like cleanrooms used in pharmaceuticals, semiconductors, healthcare, nuclear power, research, precision manufacturing, and space exploration.

Pre-filters, medium fine filters, and fine filters are commonly used in air conditioning and general ventilation systems, including room air conditioners, centralized air conditioning, and air handling units. These systems serve a variety of indoor spaces, from homes and malls to airports, offices, schools, theatres, and even vehicles such as car cabins, aircraft, and trains.

Adsorption, a process involving chemisorption and physisorption, removes gaseous contaminants that cause toxicity, unpleasant odours, corrosiveness, or health hazards. Charcoal is a common, simple adsorbent material. Options like Activated Carbon and Alumina media (impregnated with chemicals like KMnO4, MgO, Sodium Thiosulfate, or Potassium Hydroxide) are available in various forms (pellets, balls, or gel). These chemical media are housed in different containers, including cassettes, cartridges, grid blocks, and disposable AMC filters, which can be installed in air handling units.

In specific applications, loose media is filled into a separate section of the Air Handling Unit (Deep Bed Scrubber), and this section functions as a chemical filtration element. These systems are used in a variety of environments. It helps remove oxidants, acids, bases, VOCs, formaldehyde, and ozone in IVF labs, commercial buildings, and airports. They eliminate acids, bases, sulfur compounds, amines, ozone, oxidants, and dopants in sewer treatment plants, data centres, paper and pulp manufacturing, and semiconductor cell or chip manufacturing.

They are essential for removing radioactive compounds like iodine, F-18, and Ga-68 in nuclear facilities, such as hospitals and power plants. These systems help filter phosphorus compounds, VOCs, bio-contaminants, and oxidants for warfare shelter ventilation. Other notable applications include fish processing units, emergency ventilation, cannabis processing, MRI labs, and any process that generates harmful gases. ISO 16890 and ASHRAE 62.2 are valuable resources for understanding filter selection and system design.

It is important to note that the efficiency of mechanical filters, including pre-filters and fine filters, is rated according to ISO 16890. The EPA, HEPA, and ULPA filters are classified based on EN 1822. Chemical media do not have efficiency ratings like mechanical filters. Therefore, when selecting equipment for gaseous contaminant removal, it is essential to consult with chemical media manufacturers to ensure optimal efficiency and performance.

How do HEPA filters compare to standard filters in terms of improving indoor air quality?

HEPA filters, capable of capturing particles as small as 0.1-0.3 microns with up to 99.999% efficiency, significantly outperform standard filters in improving indoor air quality. Standard filters cannot remove airborne pathogens, allergens, and other microscopic particles found in indoor environments.

HEPA filters are beneficial in applications such as healthcare facilities, where they help reduce the spread of airborne pathogens, and in research laboratories and connected offices, where they prevent contamination of sensitive experiments. They are also valuable in homes with occupants sensitive to airborne particles, such as those with allergies or respiratory conditions. While HEPA filters are more expensive than standard filters, their superior performance in critical applications justifies the investment. Improving indoor air quality (IAQ) should never be considered a cost. When the application demands a HEPA filter, it should be essential. However, it is important to note that the effectiveness of HEPA air cleaners can be compromised when windows are open for ventilation.

What factors mandate selecting an air filter to ensure optimal performance and energy efficiency?

Several factors affect filter performance and energy costs. First, the starting pressure drop across filters is crucial; a lower initial pressure drop is always preferable, so, it is essential to select filter media and size accordingly. The particle loading pattern in the filter media, especially for pre-fine filters, also plays a role. Depth-loading media, typically layered fibrous material, captures larger particles in the outermost layer and finer particles in the inner layers. This design allows for a higher level of dust holding with a lower operating pressure drop, resulting in a longer filter replacement cycle.

Additionally, a larger media area in a filter leads to a lower pressure drop. Contrary to common misconceptions, filters with excessive pleats may not always perform better. An overly dense pleat arrangement in HEPA filters can narrow airflow paths, increasing the pressure drop despite a larger media area. Using a simple formula, one can calculate or estimate the annual energy cost in INR for various filters, allowing the user to select the best-fitted filter for their system.

Filter cleaning or replacement cannot be determined only by a fixed schedule. Disposable filters maintain optimal air quality. Filters should replaced when they reach their final pressure drop or according to the specific design of the HVAC system.

What advancements in air filter technology are addressing air quality concerns?

Advancements in air filter technology are evolving to address air quality concerns. One of the developments is using Metal-Organic Frameworks (MOFs), which have 1,500 to 7,000 times more surface area than traditional chemical adsorbent materials. Additionally, aerosol-compatible membrane media is utilised in pharmaceutical and healthcare applications, offering numerous immediate and long-term benefits over conventional glass fibre technology.

Biofiltration is another innovative approach, employing living organisms like bacteria or plants to remove pollutants from the air to eliminate VOCs and odours. Filters incorporating nanoparticles, such as silver or titanium dioxide, can capture and neutralise harmful pollutants, including bacteria, viruses, and volatile organic compounds (VOCs). This makes them suitable for room air conditioners and standalone air purifiers.

Nanofiber filters, made from ultra-fine fibres, provide superior filtration efficiency and can capture smaller particles than traditional filters. Furthermore, smart air purifiers equipped with air quality monitoring technology, with sensors and multistage filtration options are improving the ability to maintain and improve indoor air quality.