INVESTIGATION OF THE SURFACE TREATMENT AND THE DUST CLOGGING BEHAVIOUR OF CLEANABLE NEEDLE FELTS BY IMAGE ANALYSIS

摘要

Needle felts are the today most used filter media for cleanable bag house filters. To prevent the penetration of dust particles in deeper layers of the felt, the needle felt usually will be surface treated. This surface treatment results in an increased particle separation above the outer surface of the filter medium. According to the different kinds of surface treatment there don’t exist parameters which characterize the effect of surface treatment on the porosity change, which is important for the particle penetration and a long life time of the filter. The aim of the work is to establish characteristic parameters by which the effect of the surface treatment on the porosity can be described quantitatively. Therefore the porosity situation near the outer surface of the needle felt is detected by a microscope, a CCD-camera and an image analysis software. Using this analysis method, the two-dimensional top-layer porosity (surface porosity) and the pore depth distribution of the fibre layer nearby the external surface of the needle felt will be defined. A so-called model pore for a surface treated needle felt can be introduced by connecting these two parameters. Besides using the model pore for characterizing the surface treatment of a needle felt the residual dust clogging of the needle felt can be also investigated. First results, gained from a VDI 3926 test apparatus with different surface treated needle felts are presented and showed that a running-in period exists during that an inter acting particle layer forms below the surface of the filter me dium. This period lasts approximately up to the 10~th filtration cycle number, dependent on the test and material condition. In this period particles will be deposited mostly by depth filtration. There after this layer has a protection effect for on coming particles and it can be presumed that transportation of particles through the filter medium will take place more or less by pull down and replacement effects.