The removal of air contaminants, for example airborne particles, microorganisms and gaseous contaminants, plays a crucial role in achieving the desired indoor air quality level. The most common type of contaminant is airborne particle which, in most cases, can be effectively eliminated with the use of filters.

There are various filter types available in the market today, each suitable for different applications. There are many factors to consider when choosing the appropriate type of filter to install:

• The desired indoor air quality level, based on relevant standards, depending on the use of the conditioned space 
• The particle size of the contaminants
•  The particle concentration of the contaminants
•  The pressure drop that the filter will cause and the additional energy consumption that this will cause
•  The dimensions of the filter
•  The maintenance requirements of a specific filter type

The aim of this article is to present the main types of filters, which are most commonly used in HVAC systems, and their main features and applications.

Filter classes

Over the last decade, the class of a filter was usually defined according to standard EN 779 which set the criteria for the filter classes G, M and F, as follows:

A new standard, ISO 16890, is gradually replacing the old classification methods, as it introduces a new approach for classifying filter classes. This new approach examines the efficiency of a filter for the wider spectrum of diameters rather than just focusing on particles with a 0.4 µm diameter. ISO 16890 classifies the filters into 3 categories corresponding to 3 different particle size fractions:

• PM1: 0.3-1.0 µm
• PM2.5: 0.3-2.5 µm
• PM10: 0.3-10 µm

In order for a filter to be classified into one of the above categories, it must achieve an efficiency of at least 50 % in one of the particle size groups. If this is not achieved in any of the particle size groups, then the filter is classified as a Coarse filter.

HEPA Filters

HEPA (High Efficiency Particulate Air) filters are often found in HVAC systems serving spaces with very high indoor air quality requirements. It’s a type of filter made of interlaced glass fibers with very high filtering efficiency that is capable of removing up to 99.97% of solid particles with a size of at least 0.3 microns. The various classifications for HEPA filters are defined as follows:

• HEPA – E10: Efficiency at MPPS ≥ 85%
• HEPA – E11: Efficiency at MPPS ≥ 95%
• HEPA – E12: Efficiency at MPPS ≥ 99.5%
• HEPA – H13: Efficiency at MPPS ≥ 99.95%
• HEPA – H14: Efficiency at MPPS ≥ 99.995%

Besides their numerus applications in the medical industry, HEPA filters can also be found in household appliances, like vacuum cleaners and air purifiers, even in cars.

Carbon Filters

The primary use of carbon filters is the removal of gaseous contaminants from an airstream. This is achieved through the use of an activated carbon bed which eliminates volatile organic compounds and air odors. Carbon filters do not have the ability to eliminate fine solid particle contaminants, like mold or dust particles.

Electrostatic Filters

Electrostatic filters, as their name suggests, attract suspended solid particles from an airstream via electrostatic forces. In most cases they are made of two electrically charged plates, one positively and one negatively charged. The positively charged plate attracts the negatively charged solid particles whereas the negatively charged plate attracts the positively charged particles. These particles will remain attached to the two plates until these are washed.

AHU Filters

In most cases, an Air Handling Unit contains 2 filtering sections, often termed as primary and secondary section, which are positioned before the heating and cooling coils.

For the primary filtering section, synthetic panel filter are most commonly used which are very effective in capturing dust or other solid particles with a diameter of about 5 microns. It is very important to remove these relatively large diameter solid contaminants from the intake airstream in order to protect the ducting network and all electromechanical equipment inside them, for example fans, sensors and actuators. In addition, one of the most important task of the primary filter is to protect the often more expensive and more efficient secondary filter. The secondary filtering section in an AHU usually consists of synthetic bag filters and serve the task of capturing pollen, bacteria and other smaller solid contaminants.

Filter maintenance and replacement

The condition of an air filter is often determined through the use of pressure sensors. These sensors can be either digital, connected to a building management system, or a typical manometer and should indicate the pressure drop across a filter. The pressure drop across a filter increases as more solid contaminants get trapped in the filter thus increasing the resistance of airflow. Each filter manufacturer normally provides the pressure drop value that indicates the need for a filter replacement or maintenance.