On-line measurement of paperboard air permeability using aerostatic bearings

On-line measurement of paperboard air permeability during the manufacturing process allows process control with a shorter feedback loop than measuring off-line in the lab after the process. Measuring the permeability with airflow-based instruments has been only possible with stationary sample pieces, as the sample needs to be clamped against the instrument to create a seal between the instrument and the sample. The proposed method is based on creating a pressure difference across the paperboard and measuring the air flow rate through the web. The control volumes of the developed device are sealed against the paperboard with porous aerostatic bearings, in contrast to conventional solutions utilizing contact seals such as O-rings. Using aerostatic non-contact seals, paperboard web can travel through the measurement device at a high speed. The accurate measurement of the flow through the paperboard requires measurement of the seal leakage flowrate and the chamber inlet and exhaust flow rates. Thus, the air feeds are instrumented with flow meters, and the seal is designed so that it divides the flow to the volume and the ambient in a deterministic way. The present study focuses on stationary performance of the proposed method and does not consider the effects originating from the web traveling through the device. The device is validated in stationary conditions against a reference measurement, that uses a device based on O-rings to seal against the paperboard. Presented comparison of the studied non-contact seal to the reference contact seal allows determining the accuracy of the air permeability measurement method, including the methods to mitigate the effects of the added air flow from the aerostatic bearings.