Knowledge about Industrial Air Filters - Hencolin International |Industrial Air Filter Manufacturer

Definition of Air Filters

An air filter is a device used to remove suspended particles in the air, such as dust, pollen, bacteria, and viruses. Typically composed of a mesh-like material, it captures these particles to prevent them from entering indoor air, thereby improving indoor air quality. Air filters are widely used in residential, commercial, and industrial environments to protect people from pollution and harmful substances while reducing pollutants in the indoor air. Air filters are also used in air conditioning, heating systems, and automobiles to enhance indoor air quality and the health of occupants.

To determine the appropriate usage cycle, it is essential to understand the changes in resistance. First, the following definitions must be understood:

Rated Initial Resistance: The initial resistance provided by the filter sample, filter characteristic curve, or filter test report at the rated airflow.
Design Initial Resistance: The filter resistance at the system design airflow (should be provided by the HVAC system designer).
Operating Initial Resistance: The filter resistance at the beginning of system operation. If there are no pressure measuring instruments, the resistance at the design airflow is used as the operating initial resistance (the actual operating airflow cannot be exactly equal to the design airflow).

Classification of Filter Products

Primary and Medium Efficiency Filters

[American Efficiency Standards] ASHRAE 52.2 controls the test particle size within a specific range and divides it into three intervals: E1 (0.3-1.0 μm), E2 (1.0-3.0 μm), and E3 (3.0-10.0 μm).

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) issued the ASHRAE 52 standard, which specifies the testing methods for primary and medium efficiency air filters. Filters are classified into 16 levels based on their efficiency, also known as MERV (Minimum Efficiency Reporting Value) ratings.

ASHRAE has published version 52.2, which is currently the most commonly used filter classification standard.

In ASHRAE 52.2, two main methods are used:

Gravimetric Method (also known as AFI Method, proposed by the American Filter Institute)
Colorimetric/Dust Spot Method (also known as NBS Method, proposed by the National Bureau of Standards, now known as the National Institute of Standards and Technology, NIST)

Gravimetric Method (commonly used to test the efficiency of primary filters):

This method tests the efficiency of primary filters that mainly filter coarse dust particles (≥5 μm) using the AFI method (1960) and ASHRAE method (Standard 52.1—92). It is generally referred to as ASHRAE Arrestance or AFI, both representing the gravimetric method.

Wf: Amount of dust supplied upstream of the filter (g)
Wp: Weight of dust adhered to the filter (g)
Gravimetric Collection Efficiency (%) = (1 – Wp/Wf) × 100%

Colorimetric Method (commonly used to indicate the efficiency of medium and bag filters):

This method tests the efficiency of medium filters that mainly filter particles ≥1 μm. Identical filter papers are placed upstream and downstream of the filter. After sampling, the efficiency is calculated based on the change in transparency of the filter papers. Commonly used methods are the NBS method and ASHRAE method (Standard 52.1—92). After revisions and integration in 1968, it is generally referred to as ASHRAE Efficiency or NBS, both representing the colorimetric method.

Q1: Airflow passing through the upstream filter paper
Q2: Airflow passing through the downstream filter paper
O1: Opacity of the dust-coated upstream filter paper
O2: Opacity of the dust-coated downstream filter paper

Colorimetric Method Efficiency (%) = (1 – (Q1/Q2 × Q2/Q1)) × 100%

Note: There appears to be an error in the formula as written. Typically, colorimetric efficiency calculations involve opacity changes rather than airflow ratios. Please verify the correct formula.

[European Efficiency Standards] EN 779 & EN 1822

In the European Union, filter classification is primarily based on the EN 779 standard. This standard categorizes filter target substances into three main categories: “coarse,” “medium,” and “fine,” further divided into nine grades. For filters targeting larger particles, synthetic fibers are used for testing. Filters targeting medium and fine dust particles use 0.4 μm particles during testing.

Note: The earlier EU classification has been replaced and is no longer described.

[ISO 16890 Standard] (Replaces EN 779 in 2018): Filtration Efficiency Determined by Particle Classification

ISO 16890, introduced by the International Organization for Standardization, serves as a new filtration standard for testing and classifying general ventilation filters. The standard number is ISO 16890. It was unanimously adopted by all countries worldwide. The EN 779 standard for evaluating air filters will soon become obsolete. ISO 16890 began an 18-month transition period at the end of 2016 and replaced the old standard in 2018.

In the United States, ASHRAE 52.2 is the dominant standard. Europe implements EN 779, while Asia and the Middle East use both standards. China also has its own national standards. For many years, the World Health Organization and environmental authorities have used particle size to assess air quality, and the industry has followed this approach. With the introduction of the new ISO 16890 test standard, filter efficiency will be determined based on classifications such as PM1, PM2.5, and PM10. This ensures that filters are evaluated more specifically based on their actual performance, helping users select appropriate filters more effectively compared to the previous two major standards. ISO 16890 incorporates the advantages and eliminates the drawbacks of earlier standards, proposing a highly scientific testing standard and classification system. The most significant advantage is that the filter efficiency test must categorize particles, with efficiency determined by particle classifications PM1, PM2.5, and PM10.

In the future, this will ensure that filters are evaluated more based on their actual operational performance. Compared to the past, it helps users select suitable filters more targetedly. PM1 targets particles smaller than 1 μm, requiring a new filtration efficiency and post-electrostatic efficiency with a minimum limit of over 50% to be classified into the ePM1 efficiency group. This is crucial as increasing scientific reports indicate that PM1 particles are extremely harmful to human health.

High-Efficiency Filters

Regarding high-efficiency filters, the United States follows the IEST (Institute of Environmental Science and Technology) standard IEST RP-CC001.

This standard tests high-efficiency particulate air (HEPA) filters using 0.3 μm particles.

PAO Method (Used to Test HEPA Filters):

This method involves releasing PAO mineral oil upstream of the HEPA filter. A photometer detects the concentration, and a particle counter measures the PAO concentration downstream of the HEPA filter. The test instruments are placed about 25 mm from the HEPA discharge surface, and measurements are taken at a scanning speed of 50 mm/sec. The measurement range includes the HEPA filter material, the joint between the filter material and the frame, and the joint between the HEPA filter and the indoor environment. The measurement method itself is not specifically regulated; it only requires measuring the entire HEPA filter range. If particles exceeding 0.03 are detected, the test instrument must remain at that location for 10 seconds before moving back 100 mm to continue continuous measurement. If no particles above 0.03 are detected, the measurement can proceed.

PAO Efficiency (%) = (1 – (Particle Concentration Downstream of HEPA / Particle Concentration Upstream of HEPA)) × 100%

Note: Using higher-grade filters is not always better, as higher-grade filters incur greater costs (unnecessary energy consumption, waste generation, noise, etc.) and may produce high-quality products that harm the environment. Therefore, selecting appropriate filters based on actual needs is the correct and sustainable practice.

AHU Central Air Conditioning Systems vs. MAU Purification Air Conditioning Systems

AHU/Central Air Conditioning System: Primarily consists of a chiller, cooling water circulation system, refrigerant water circulation system, fan coil unit system, and cooling tower.

MAU/Purification Air Conditioning System: Essentially composed of three main parts: air handling and air transport equipment, and air distribution devices. Additionally, it includes refrigeration systems, heating systems, and automatic control systems.

Air Filtration Results

Central Air Conditioning:
- High indoor dust concentration
- Uses primary/medium efficiency filters
- No high-efficiency (above sub-high) air filters at the end of the air outlet
Purification Air Conditioning:
- Low indoor dust concentration
- Uses primary/medium/high-efficiency filters
- Must install sub-high or high-efficiency air filters at the end of the air outlet

Airflow Organization

Central Air Conditioning: Features turbulent airflow to achieve uniform indoor temperature and humidity fields through adequate ventilation.
Purification Air Conditioning: Utilizes vertical or horizontal laminar flow to reduce dust particle diffusion and minimize secondary airflow and vortices.

Indoor Pressure

Central Air Conditioning: No specific requirements
Purification Air Conditioning: Maintains a minimum pressure difference of 5–10 Pa or higher

Airflow Energy Consumption

Central Air Conditioning: Lower energy consumption with ventilation rates below 10 air changes per hour
Purification Air Conditioning: Higher energy consumption with ventilation rates above 12 air changes per hour