In recent weeks, Kähler, who heads the Institute of Fluid Mechanics and Aerodynamics at the university, has been testing a mobile room air filter. The device has two wheels and is as big as a refrigerator. It isn’t completely silent in normal operation, but it’s quiet enough. And it is capable of filtering out up to 99.995 percent of even tiny aerosol particles with a diameter of 0.0003 millimeters. The physicist believes these devices could be the solution for the safe operation of restaurants, shops, offices and schools, especially in winter when ventilation becomes even more difficult.
The filtration device halved the concentration of the aerosols in question after only six minutes of operation in Kähler’s 80-square-meter (860 square foot) test room. The device also uses heat to destroy the trapped viruses, while at the same time fighting pests like bacteria or mold. “If you keep this system running constantly,” the physicist says, “no one will be able to create an aerosol concentration at an infectious level in a room.”
These aren’t hi-tech devices, and they’re easy to make. “The air purifiers are comprised of a tin cabinet and a fan,” says Kähler. They use a class F7 prefilter for the larger impurities, while a special high efficiency particulate air (HEPA) filter of the quality class H13 or H14 must be installed to protect against viruses.
Currently, the high-performance filters are used in operating rooms, clean rooms and also in aircraft cabins. They were developed in the United States back in the 1940s as part of the Manhattan Project to build the first atomic bomb. The filters were designed to remove radioactive particles from the air, but they were soon deployed in gas masks and later in industrial settings.
Kähler estimates that the cost of the room air cleaner he experimented with is around 3,500 euros. The H14 HEPA filter alone costs 450 euros. Yet Kähler also expects that prices will fall rapidly with higher unit volumes. Kähler has published his test report in German on his institute’s website, and he’s hoping it will spark a debate about the future of the devices.
“We won’t always be the lucky ones,” he says, with a view to the infection numbers, which are currently rising in Germany, but still relatively low. He says that technical solutions will be necessary for dealing with the coronavirus for as long as there is no safe, effective and generally accepted vaccine.
H13 and H14 HEPA filters haven’t been used in office or residential buildings yet and they can’t be retrofitted, because they would alter the mode of operation of existing ventilation systems too significantly.