Wednesday, December 1, 2010

Benefits of Nanofiber Particulate Air Filters

Judy Driggans
Jayesh Doshi
Nanotechnology is an emerging technology being investigated in many industries and Nanofibers are an important part.
As nanotechnology emerges into the industry, many future products are expected to improve performance as a result of new scientific developments. Nanofibers will play an important role in this new emerging technology.
Nanofibers are a thousand times smaller than human hair and are made from polymers in the form of spider web like structures, with a very large surface area and a large number of pores with very small pore size. The nanofiber, when combined with conventional filter media, creates a new class of media defined as “nanomedia.” Nonwoven nanomedia are revolutionizing the field of liquid and air filtration, and a number of companies are beginning to offer new solutions to customers using state-of-the-art nanotechnology.
In the case of HVAC air filtration, the fine mesh of nanomedia increases filtration performance significantly, thereby blocking smaller airborne particulates with marginal increase in resistance to air flow. The nanomedia are very fine and do not increase the weight or change the mechanical characteristics of the conventional media. Nonwoven nanofiber media are typically made using an electrospin process, whereby nanofibers are laminated to the microfiber media similar to filter media widely used in today’s particulate air filters. These nanomedia are being used in dust cartridges, bag filtration, vacuum bags, chemical-biological warfare filters, and air conditioning filters as well as many other filtration applications.
How do the nanofibers improve HVAC filtration?
The thin layer of nanofiber web forms a barrier for particles while allowing the air to “slip” through the large volume of small pores. The large surface area of the nanofibers traps smaller particles, while air slips through the particles and filter medium.
Since nanofiber traps more submicron particles, will they “load up” the filter quickly and need frequent replacement?
The first nanofiber filter may load up very differently from conventional filters, however it really depends on the particle size and count in the air. For example, when three stage Minimum Efficiency Reporting Value1 (MERV) 11, MERV 13, MERV 14 nanofiber-based air filters were tested in a welding institute, and compared with a conventional filter, the nanofiber filters were becoming caked as most particles collected were condensed metal fume particles. Particle measurements of the air prior to filter installation were in several millions (0.3-0.5 microns particles). Most particles were generated as a result of welding fumes and the grinding of metal parts. A significant decrease in particle count was measured while air quality was improved. It is also important to note that, because of a higher surface area in these filters, we were able to replace very large air filters with smaller ones—for example a 24x24x12 filter was replaced with a 24x24x4. Hence, as a result of nanofiber technology we were able to improve the air quality and reduce the size of the filter, thereby making it easier for customers to handle the filter during the installation and disposal process.
During tests in a large hair salon, for example, we found that filter changes were needed on a monthly basis since a large number of haircuts, color treatments, and perms are performed at that location. The nanofiber filters may be the least expensive air filter available to trap short pieces of human hair. The nanofiber filters have been the only filters that actually stop human hair from getting into their ductwork. The added benefit of reducing fumes from the color and perm mixtures is a plus. Those fumes are typically in the E2 range where MERV 13 filters are at least 90% efficient.
In our field test(s) ranging from hospitals, manufacturing environments, cleanrooms, to allergy applications, our findings show that where smaller particles need to removed from air, nanofiber-based air filters have been superior in cleaning air.
Image 1
Image 2

Will the nanofibers come off and circulate in the air without being seen?
Since nanofibers are very small, only a few milligrams of spaghetti form are added onto a micron-sized substrate (i.e., conventional filter media) to achieve the desired performance. These continuous nanofibers are glued to, or encapsulated in, the microfiber media. When the media is assembled to form filters, the nanofibers are either “upstream” or “downstream” of the air flow depending upon the applications calling for “depth” or “surface” filtration.
Extensive testing on a 24x24x2 air filter in a wind tunnel at 2000 cfm air flow at 492 feet per minute face velocity per ASHRAE 52.2 test standards shows that when nanofibers are encapsulated or glued and are “upstream” of air flow, there is no evidence of nanofiber delamination and/or becoming airborne.
Nanofiber air filters improve E1 (0.3 to 1.0 microns) efficiency
Filters that begin to block submicron-sized particles (0.3 to 1.0 microns) are usually rated by the ASHRAE 52.2 test standard as MERV 13 or higher. Typical particles of submicron or nano size are: radon progeny, coal flue gas, oil smoke, resin smoke, tobacco smoke, sea salt, metallurgical dusts and fumes, carbon dust from graphite, viruses, bacteriophages, and cornstarch.2 An ASHRAE 52.2 test standard report will give a filter efficiency in three particle sizes. These Particle Size Efficiencies (PSE) are called E1, E2, and E3. The sub - micron-sized particles are in E1. Research demonstrates that nanofiber-based air filters trap more fine particles (E1) compared with conventional air filters.
Nanofibers Breathe
The ASHRAE 52.2 test standard allowed final pressure drop is 350 Pa (1.4 inches water column) for air filters rated at MERV 13 to MERV 16.3 Initial new filter pressures are usually about half the final pressure. Nanofiber particulate air filters rated at MERV 13 have an initial pressure drop of 0.34" W.G. Nanofibers are being used in special fabrics because they breathe while protecting skin with a microscopic filtering mesh.
Nanofiber Filters May Save System Energy
The higher the pressure drop, the harder your system fan will have to work to deliver conditioned air. The harder the fan works, the more you pay for energy. A rule of thumb is a 1 Pascal increase in pressure has an annual cost of 1 Euro in additional energy.4 A filter with only 50 Pascals (0.2 inches of water) pressure drop could cost over $60 more per year in system energy use.
Developing Carbon Nanofibers
Experiments are underway to develop manufacturing scale processes to turn electro spun nanofibers into Carbon nanofiber (CnF) and Activated Carbon nanofiber (ACnF). Activated carbon nanofibers can be tailored to capture volatile organic compounds (VOCs) and Toxic Industrial Compounds (TICs). Custom filters in the near future may not only be a special size, but they may also be manufactured to attract specific compounds using these nanofibers integrated with nanoparticle based adsorbents. These latest experiments to develop carbon and activated carbon nanofiber commercial scale manufacturing processes is being funded in part by a 2009 Technology Innovation Program (TIP) award from the National Institute of Standards & Technology (NIST).
What value does nano truly add and what is the value proposition to HVAC technology?
If nanofiltration is important, ask your filter suppliers “What is your E1?” That is short filter lingo for what is the efficiency of your filter in the 0.3 to 1.0 micron particle size? MERV ratings are exactly what they stand for—the Minimum Efficiency Reporting Value. The next question to ask is “What is your pressure drop?” The higher the pressure drop, the harder your system fan will have to work to deliver conditioned air. The harder the fan works, the more you pay for electricity.
As more and more companies develop and use nanotechnology in filtration, we expect filters with nanofibers, nanoparticles, and other nanomaterial to penetrate the market. We expect new nanotechnology integrated products to improve its performance in terms of particulate, odor, and toxic chemical filtration. Additionally, products are expected to become smaller, thinner, lighter, and have superior life cycle costs.
Photographs provided by eSpin Technologies, Inc. ©
  1. ANSI/ASHRAE 52.2-1999 (1999) “Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size,” published by Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.
  2. NAFA Guide to Air Filtration, Fourth Edition, 2007
  3. Zhou, Bin and Shen, Jinming. “Comparison of General Ventilation Air Filter Test Standards between America and Europe.” International Network for Information on Ventilation and Energy Performance. Date accessed 08Sept2010.
  4. accessed on 08Sept2010.

No comments: