The RxAir blog provides insight into factors that surround the industry of air purification.

Tuesday, May 18, 2010

Lower Pressure Drop HEPA Filtration?

An article in a hospital journal, suggesting that synthetic melt blown fibre filter material could be used in healthcare facilities to replace HEPA filtration, made me to do a little digging.

Every customer wants more for their money.
Every vendor wants to offer their customers more for their money.

Some companies offer what is known in the industry as synthetic HEPA filters. These filters meet the requirements of HEPA filtration (have an efficiency rating of 99.97% for removal of partials having a size of 0.3 microns) and have a lower pressure drop than true (mechanical) HEPA media. This improved performance is achieved using media that is more porous than true HEPA, with HEPA performance being achieved by applying an electrostatic charge to the media. The industry is aware that HEPA performance is achieved when the synthetic media filter is newly manufactured and also that this performance degrades as the electrostatic charge decays. Some published testing indicates that efficiency of meltblown synthetics can be reduced by as much as 30% when they are discharged and are no longer effective in the 0.3 to 1 micron range. We have carried out our own in house tests to evaluate synthetic HEPA materials and found the material as supplied achieved HEPA standards at increased airflow but when tested after a six month usage the filter had a 67% efficiency for removal of partials greater than 0.3 microns.

In today’s extremely tight economy we all endeavor to deliver better product for less cost, however, when it comes to medical grade air purification for Airborne Infection Isolation let us be ready to ask some questions regarding air filtration. Remember the CDC states that HEPA filtration at 99.97% removal efficiency, not 99.95% or 95%, is acceptable for air purification in isolation rooms.

What should we ask HEPA filter suppliers?
1)What filter media is used in your filter manufacture, pure mechanical HEPA or electrostatically enhanced synthetic?
2)If the supplier uses electrostatically enhanced synthetic media, ask for a statement of efficiency throughout filter life.

It is understood that electrically charged filter media has its application but we must be sure to understand what we are being offered and the nature of our application.

Further reading:
1) Approaches for Exposure Assessment and Control for Airborne Infectious Agents
University of Minnesota School of Public Health - Peter C. Raynor, Ph.D.
2) ANSI/ASHRAE Standard 52.2, Method of Testing General Ventilation Air Cleaning Devices for Removal Efficiency by Particle Size (Considers the effect of electrostatic filter discharge)
3) The Long-Term Performance of Electrically Charged Filters in a Ventilation System
Peter C. Raynor Soo Jae Chae
4) Dust loading on electrostatically charged filters in a standard test and a real HVAC system
Peter C. Raynor Soo Jae Chae

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