Surface vs. Depth Filtration

Needlefelt filter bags begin with polyester or polypropylene fibers compressed into rolls of high-density sheets. This material is then punctured with barbed needles, producing a pore structure with tangles of fibers that provide a non-direct "torturous path". Fluid entrained with particles follow the path of least resistance with finer particles caught deeper in the pores and larger particles closer to the surface. As the pores become clogged, the denser areas of the media retain particles until the system reaches its maximum recommended differential pressure (typically 20 – 30 PSI).

Meltblown fibers are manufactured by atomizing raw polymers with hot air. These fibers can be engineered for size and density by controlling the ratio of the polymer flow rate and pressure against the temperature of the atomizing air. Absolute rated depth filtration filter bags are created from multiple meltblown layers to form an ever-increasing density, allowing for stratification of retained particles throughout the thickness of the filter bag. Some designs feature as many as 7 distinct layers of density.

Fiber Migration: This refers to the potential for the fibers making up the filter bag to break loose and go downstream. To prevent this in needlefelt material, the external surface is typically "singed" – heated to bond the fibers together. The meltblown process naturally bonds fibers together, making downstream migration negligible.

Nominally rated needlefelt designs are the least expensive and provide a minimal amount of depth filtration. Meltblown filter media will cost more due to the complexity of its multiple layers of density and thickness enabling it to hold more particles.