PureAir – Development of long-term stable filters with low pressure loss based on electrically conductive tufted structures for use in room air purifiers

Conventional filter-based room air purifiers have so far mainly used either electret filters (low pressure loss, but low long-term stability) or electrostatic separators (resource-saving because washable, but often ozone-emitting). The aim of the project was to develop a filtration solution that combines high separation efficiency with low pressure drop with ozone-free, long-term stability and washability. This was to be achieved with a three-dimensional tufted pile structure made of electrically conductive fibres. By applying an electrical voltage to the material, the electrostatic deposition is to be permanently maintained and due to the small distances between the fibres, particles of low electrical mobility can also be separated.

In initial preliminary tests to determine the purely mechanical separation efficiency, insulating microfiber yarns and conductive rows of stainless steel yarns on a standard primary backing proved to be the most suitable. With these, it was possible to achieve comparable separation efficiencies and pressure losses as in conventional filter media for air purifiers. In the next step, the filter media were electrically contacted in a specially developed filter media test bench. Sufficient insulation between the adjacent conductive rows proved to be a significant challenge. However, systematic parameter variation ultimately led to design parameters at which voltages of up to 4.5 kV could be applied to the filter medium without flashovers. As a result, it was possible to increase the minimum separation efficiency from about 30% to over 60%. The structure was further optimized by varying parameters such as gauge, stitch density, pile height, pile structure and combination of insulating and conductive pile yarns.

From the most suitable filter material, a full-format filter for a room air purifier was finally assembled. However, on this one again problems occurred with the electrical insulation between the conductive rows, which could not be solved during the duration of the project.

Thus, the general feasibility of the project approach on the scale of filter media was successfully demonstrated, but the transfer to a complete filter in the room air purifier is still pending. In some samples, there was a sudden strong increase in separation efficiency above a certain applied voltage, which is probably due to a controlled corona discharge in the filter medium. The behavior could not yet be reproducibly controlled, but offers an interesting starting point for further developments.

Figure: Tufted filter structure with electrically conductive filter elements

Funding program and project number

IGF 21783 N2

The final report is available in the TFI publication 2024/125.

Duration

01.04.2021 - 30.09.2023