Fabric Qualities

For all applications of filter bags there are two main categories of fabrics used in construction: woven and non-woven materials. Woven fabrics are constructed of yarn assembled into definite patterns. Non-woven filters may be made of felt composition or a porous membrane.

Of the woven fabrics used in filtering, there are several common weaves. The first is the plain weave, which provides the best filtering but clogs the
quickest. The other common weaves, twill and sateen, provide better permeability but less effective filtering.

The woven fabrics themselves do not provide a great amount of filtration, but rather the accumulated dust particles on the filter will capture most particulate from the air forced through the bags. This mass of accumulated particulate on the fabric is referred to as a “filter cake” which captures much of the dust. However, due to its nature the woven filters have a tendency to plug easily and need to be cleaned as often as every 15 minutes. These filters are usually used in low energy filtering systems such as reverse air or shaking cleaners.

The first typed of non-woven filters, felt, are made by loose fibers being attached to a woven backing called a scrim. The felt filters are generally at least twice as thick as a purely woven filter. Rather than depending on initial particle deposits to capture the majority of the dust in the air, the individual strand fibers help to capture and defer the unwanted particulate. Depending on the application there are several different fibers used in felted bags, including the natural cotton and wool fibers and the numerous synthetics. The natural fibers are inexpensive and effective in low temperature situations but the synthetic fibers can take higher heats, and can better withstand the erosion of particulate matter and chemicals.

Membrane filters are made by stretching a porous material, such as polyflourocarbon, and bonding it to a scrim. The membrane systems are generally used in flues to handle moisture and other filtering problems.
While there are several classes of non-woven filter fabrics some specific fibers used include cotton, wool, polypropylene, nylon, Orlon, Dacron, Nomex, Teflon, Fiberglas and P84. Because of its resistance to high heat, pressure drops and moisture, most flue membrane filters are made of polyflourocarbon.

No matter the fabric, almost all filters undergo a pretreatment process to increase life expectancy and efficiency. The natural fibers are generally preshrunk to avoid any size changes when subjected to the heat of use. Both natural and synthetic fibers, however, are subjected to other processes such as: calendering, napping, singeing, glazing and coating. These processes help to prolong the life of a filter and cause higher efficiency.

At HTIF, we're happy to answer your questions regarding filter bags and fabrics. E-mail us at customerservice@htif.com or call us 1-866-702-HTIF (4843).

Finishes

Scouring: A process which removes compounds added to fabrics when they are manufactured. Scouring must be done prior to heat setting of fabrics.

Heat Setting: Makes a fabric stable up to a predetermined temperature.

Calendar: Compresses the fabric into a smooth finish.

Napping: Produces a "fuzzy" texture on the fabric. This process facilitates the collection of fine particles.

Glazing: A process that melts and presses the fivers of a textile to produce a smooth glass-like surface. This process facilitates dust release.

Singeing: During this procecss the fabric is passed close to an open flame. The process melts the ends of any fibers protruding from the fabric. This process provides a fabric which is pebbled but quite smooth to facilitate dust release.

Siliconizing: A wet process where silicone oil in a water mixture is padded on to the fabric, then the fabric is dried or "cured." The oil makes the surface of the filter tacky to help build a residual dust cake which facilitates the filtering process. The process also makes the fabric somewhat waterproof.

Flame Retardency: A process which provides a finish on a fabric which inhibits combustion in high temperature filter applications. This process should not be confused whith flame proofing which cannot be accomplished with synthetic organic fabrics.