Felted filters are made by needle punching fibers onto a woven backing called a scrim. The fibers are randomly placed as opposed to the definite repeated pattern of the woven filter. The felts are attached to the scrim by chemical, heat, resin, or stitch-bonding methods.
To collect fine particles, the felted filters depend to a lesser degree on the initial dust deposits than do woven filters. The felted filters are generally 2 to 3 times thicker than woven filters. Each individual randomly oriented fiber acts as a target for particle capture by impaction and interception. Small particles can be collected on the outer surface of the filter (Figure 1).
Figure 1. Felted fabric filter
Felted filters are usually used in pulse-jet baghouses. A pulse-jet baghouse generally filters more air per cloth area (higher air-to-cloth ratio) than a shaker or reverse-air unit. Felted bags should not be used in high humidity situations, especially if the particles are hygroscopic (these particles have an affinity to absorb moisture and thus become sticky). Clogging or blinding could result in such situations.
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