When it comes to high-efficiency air filters, anything MERV-11 or higher, there are three prevailing medias (the part that catches particles) available. They are: Coarse Fiber/Synthetic, Airlaid Fine-Fiber, and Wetlaid Fine-Fiber. Coarse fiber/Synthetic air filter media is becoming more popular because it has low material and production costs; however ASHRAE, Eurovent, and most credible sources admit that it loses the ability to capture particles the longer it is in-service. ASHRAE has a disclaimer on page 3 of their 52.2–2012 air filter testing standard and Eurovent has a discharging step in its standard before particle removal efficiency testing is complete. ISO16890, which is the upcoming replacement for 52.2 and EN779, also includes a discharging step in their efficiency testing. There have been large gains in the discipline of nano-fibers and synthetic media may be able to overcome this efficiency loss problem in the near future, but because the purpose of an air filter is to capture particles it will not be discussed further.
This leaves two options for air filter medias that will maintain their efficiency throughout the filter’s life - Airlaid and Wetlaid fine-fibers. Each has their set of benefits and weaknesses so determining which is better for your facility will come down to the environmental conditions, desired performance properties, and to some extent personal preference.
Wetlaid air filter media is made similarly to the process to make paper using glass fibers instead of cellulose fibers and is typically used in compact 4” and 6” deep mini-pleat, V-Bed, Aluminum Separator, and HEPA/ULPA filters. Being able to handle very high temperatures and having particulate removal efficiencies of up to 99.99995% @ Most Penetrating Particle Size (MPPS usually 0.112μ) are the two most significant benefits of wetlaid air filter media. Also more media per square foot can be installed into a filter frame of the same size compared to airlaid media, but this is not just possible but needed because wetlaid media comes with a higher static pressure/resistance to airflow and lower dust holding capacity per square foot compared to airlaid media. A MERV-14 wetlaid air filter media will typically hold between 1.4 and 1.75 grams of ASHRAE dust per square foot of media. This relatively low dust holding capacity is due to the face loading properties of the media. Face loading is when debris is collected on the face of the media and is not able to penetrate into the media before being captured, which is called depth loading.
Close-up of Air-laid (left) and Wet-laid (right) Air Filter media:
Airlaid air filter media is made by melting glass pellets and drawing the liquefied glass through a strainer before it is collected at a uniform depth on a belt and dried. This media is only used in deep-pleated and bag/pocket style air filters. Having a lower static pressure/resistance to airflow and higher dust holding capacity per square foot of media are the main advantages of airlaid or “lofted” media. The lofted properties of this media allow depth loading of debris which is what allow the low static pressure and high dust holding properties. Depending on filter style and manufacturer the ASHRAE dust holding capacity of MERV-14 airlaid media ranges between 2.5 and 4.2 grams per square foot - roughly two to two and a half times higher than wetlaid media. Large particles are also handled better by airlaid media, so if your application has a high volume of large debris the lofted media will offer more filter longevity.
However, as shown by the limited filter styles airlaid media is available in does have some significant drawbacks; such as having a maximum removal efficiency of 85% on ASHRAE’s E-1 particle size range and the depth loading media needs to be held in place. Bag style filters are notorious for “shedding” debris because the pockets shake and the particles that wedged their way into the media are shaken through. Airlaid media is also subject to collapsing when it gets fully saturated with water, when this happens the benefit of having the lofted media goes away and the static pressure raises significantly.
In most HVAC applications either airlaid or wetlaid media will do a sufficient job with airlaid products usually offering a better price-to-life solution. Wetlaid products will give higher filtration efficiencies and with some configurations having 4–5 times more media than airlaid products the filter life and average static pressure will be much lower. With the large amount of options for high efficiency filters comparing and estimating life of different styles across different manufacturers can be nearly impossible, but understanding how the filter media is made and the benefits and limitations will help with evaluation.