Woven and knit fabrics are two-dimensional filter bag media in which yarns overlap one another. Due to the overlapping of yarns, the size of the pore, or the opening between the yarns, is quite large; which can be seen by holding up a piece of these filter fabrics into the light.
The only area which these fabrics allow air to pass through are the visible holes. The other areas are void, therefore, decreasing the available pore surface area.
- This increases the filtration system’s air-to-cloth ratio because there is less square footage of cloth to let air pass through.
- This also increases the velocity of the air passing through the fabric.
- If the particulate is small, then the pressure differential increases drastically.
When the fabric is cleaned, by shaking or pulse action, some of the particles will be removed from the pores. However, some particles remain.
Over a longer period of time, particles will fill up the pores and cleaning the filter bags will be of no help. At this stage, the pressure differential will remain permanently high and the filters will have to be replaced.
The disadvantages of using woven or knit fabrics when filtering fine particles (such as coal) are:
- Rapid rise in pressure drop
- High emissions
- Short service life