Charged-Liquid Air Filtration Challenges HEPA Efficiency

Procter & Gamble

Electrostatic air cleaning is not new: Air is drawn through a charging mechanism where dust, dirt, and debris take on a slight charge. The dirt is attracted to a collecting plate while the clean air goes out a vent. The problem is that as dirt collects, the efficiency of the air cleaner dramatically decreases -- the dirt effectively insulates the collector. As efficiency decreases, power use increases. Intervals between cleanings can be as little as a few hours in dusty environments.

HEPA (High Efficiency Particulate Arresting) filters are known for their ability to filter 99% of particles smaller than a micron -- typically bacteria, smoke, pet dander, and household dust. HEPA filters have drawbacks, too, a primary one being expense. Home HVAC (heating, ventilating, air conditioning) systems are not powerful enough to draw air through a HEPA filter, thus preventing homes from enjoying that level of filtration. Nor are the human lungs powerful enough to draw air through a HEPA filter. Systems that can draw through a HEPA filter are often noisy -- 60-70dBa. Because air is compressible, it builds up on one side of the filter, requiring high power to overcome the backpressure.

A square meter of filter in 1ml of liquid

Scientists at Procter & Gamble have developed a system that uses a charged spray of non-aqueous, non-evaporative fluid to filter sub-micron particles from an air stream. The electrohydrodynamic spray ("e-spray") system charges tiny droplets that attract and hold particles from the air and are themselves attracted to a collecting plate, carrying the particles with them. A single milliliter of the fine mist created by the spray nozzles can produce a filtering surface area of about one square meter.

Because the filtering mechanism is a liquid, the e-spray system constantly cleans its own collecting plate so that particulates don´t build up and insulate the system; efficiency is maintained for long periods on the order of months.

Filtering efficiency is equivalent to HEPA filters (from .1-.3 microns). However, because the filtering medium is made up of tiny droplets, at any time 98% of the airstream is actually unobstructed. As a consequence, the e-spray system does not produce the backpressure of a HEPA filter. Power requirements and noise production are similarly reduced.

The e-spray system was designed with home use in mind, but it can scale up to factory size or down to the size of a personal respirator. Engineers estimate that such a respirator would require no more power than two AA batteries. Possible applications are wide-ranging, and include medical, automotive, and aircraft applications.

The system, its nozzles, and the special liquid were all engineered together, and are all covered by patents.

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