FAQ CATEGORY: Filtration Basics

Filtration Basics

What Is the MERV Rating System for Air Filters?

The MERV rating, which stands for Minimum Efficiency Reporting Value, is a standardized scale used to measure the effectiveness of air filters in capturing particles of specific sizes. The MERV rating system was developed by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE). ASHRAE has developed filter efficiency and testing standards that are used for all types of air filters, including HVAC filters and filters used in industrial air filtration and ventilation. For cartridge filters in dust collectors, the MERV rating indicates how efficiently the filter captures airborne particles. MERV ratings range from 1 (least efficient) to 20 (most efficient). (Note: MERV ratings above 16 are equivalent to HEPA filtration; the highest rating typically used for industrial air filtration is MERV 16.) The higher the MERV rating, the finer the particles the filter can capture. Manufacturers must have their cartridge filters tested by a third-party laboratory using ASHRAE-approved testing methods to use the MERV rating system.

MERV chart

What Is ASHRAE 52.2?

ASHRAE Standard 52.2, officially titled “Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size,” is a widely recognized standard developed by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE). It provides a standardized methodology for evaluating and determining the efficiency of air filters in removing particulate matter of specific sizes in general ventilation settings. ASHRAE 52.2 is critical in helping manufacturers, HVAC professionals, and end-users compare and select appropriate filters, ensuring that they perform adequately in mitigating particular airborne contaminants, thereby safeguarding indoor air quality (IAQ). For industrial dust collector filters, including baghouse and cartridge filters, ASHRAE 52.2 has been supplemented and updated by ASHRAE 199, which is specific to pulse-cleaned dust collection systems. Key aspects of ASHRAE 52.2 include:

  • MERV Ratings: The standard introduces the Minimum Efficiency Reporting Value (MERV) rating system, assigning filters a rating from 1 to 20 based on their capability to remove airborne particles in specific size ranges. (Note: 17 and above are equivalent to HEPA and rarely used.) A higher MERV rating signifies better filtration performance. 
  • Particle Size Ranges: It evaluates filter efficiency across multiple particle size ranges, specifically focusing on particles between 0.3 and 10 microns, to determine how well a filter can remove different types of particulates.
  • Testing Methodology: The standard outlines a specific procedure for testing filters, a means of measuring particle concentration, and a system for analyzing the data to calculate the filter’s performance.

What is ASHRAE 199?

ANSI/ASHRAE Standard 199 ("Method of Testing the Performance of Industrial Pulse Cleaned Dust Collectors") is a testing standard developed by ASHRAE specifically for evaluating the performance of industrial dust collection and fume extraction equipment. It provides a standardized and consistent method for measuring the performance of cartridge, baghouse or envelope-style industrial dust collectors, supplementing the general testing and rating requirements provided in ASHRAE 52.2. It applies specifically to dust collectors that use a filter pulsing mechanism to discharge dust from the filter surface while the collector remains online. This standard includes: 

  • Emissions Measurement: Determines the number of particulates (dust) escaping the system.
  • Pressure Drop Measurement: Evaluates the resistance to airflow within the dust collector.
  • Compressed Air Usage: Assesses how different systems utilize compressed air for pulse-cleaning of the filters.

What Is ASHRAE Standard 62.1?

ASHRAE Standard 62.1, titled “Ventilation for Acceptable Indoor Air Quality,” addresses criteria for achieving and maintaining acceptable indoor air quality (IAQ) in commercial or institutional buildings, including industrial environments. The standard provides guidelines regarding air cleaning and filtration to mitigate particulate and gaseous contaminants. While ASHRAE 62.1 is primarily concerned with HVAC and ventilation systems rather than industrial dust collection, there are principles that can apply more broadly to industrial air filtration. This includes the specification of minimum efficiencies for air filters (using MERV ratings), which can guide the selection of cartridge filters in dust collection systems within industrial settings. ASHRAE standards should be considered along with other requirements, such as OSHA Permissible Exposure Limits (PELs) for industrial air pollutants.

What Is ASHRAE 129?

ASHRAE 129-1997 (RA 2002), titled “Measuring Air Change Rates in Buildings,” provides methods for measuring air change rates in buildings and building spaces, which are essential for understanding the ventilation effectiveness of these environments. ASHRAE 129 focuses on defining repeatable methods of measuring air change rates, applicable in various settings, such as commercial, residential, and industrial buildings. The methods encompassed in the standard include tracer gas dilution methods, continuous-injection tracer gas method, occupied building or occupied space method, concentration decay method, and constant concentration method. While this standard does not directly reference cartridge air filters, understanding the required air change rate to meet air quality targets impacts dust collection and air filtration system design, including filter selection and dust collector sizing.

What Is a HEPA After-Filter, and When Do I Need One?

A HEPA after-filter is a high-efficiency filtration unit designed to capture extremely fine particles that might not be adequately trapped by standard cartridge air filters in a dust collection system. HEPA stands for High-Efficiency Particulate Air, and a filter must capture 99.97% of particles 0.3 microns in size to qualify as HEPA. In a dust collection system, an after-filter is typically positioned after the primary cartridge filter, serving as a secondary filtration stage. If you’re dealing with particularly fine or harmful dust or if you have stringent air quality requirements, a HEPA after-filter can be a valuable addition to your dust collection system. You might consider using a HEPA after-filter in situations where:

  • The process generates very fine particles that pose a health risk or can contaminate products.
  • Regulatory or industry standards require extremely high air quality.
  • The collected dust contains hazardous materials, allergens, or carcinogens.
  • You aim to recirculate filtered air back into the facility, necessitating cleaner air to improve indoor air quality.

What Is an Activated Carbon After-Filter, and When Do I Need One?

An activated carbon after-filter in a dust collection system is utilized primarily for odor control and removal of gaseous pollutants from the air. Activated carbon has a highly porous surface and excels at adsorbing a wide variety of odorous substances and volatile organic compounds (VOCs). After the dust collector’s primary filtration unit (e.g., the cartridge filter) captures particulate matter, the activated carbon after-filter engages to trap and remove unwanted odors and gases, ensuring that the air emitted from the system is not only free of particles but also smells clean and is void of gaseous contaminants. This additional step is particularly valuable in industries such as recycling or chemical manufacturing where odorous compounds or VOCs are prevalent, ensuring improved indoor air quality (IAQ) and compliance with air quality standards and regulations.

What Is Molecular Air Filtration?

Molecular air filtration refers to the process of removing gaseous, odorous, or otherwise molecular contaminants from the air using media that adsorb, absorb, or react/bond with the molecules. Rather than capturing particles, like typical air filters, molecular filtration targets airborne molecules through chemical or physical bonding (e.g., adsorption or absorption), often utilizing materials like activated carbon or specialized chemical media. This type of filtration is particularly useful in environments where clean, odor-free air is paramount or where harmful gaseous contaminants need to be controlled, such as laboratories or industrial settings, ensuring a safe and compliant atmosphere by addressing both particulate and molecular pollutants.