The physical structure of filter bags cane placed into three categories, Mesh, Needle-felt and Melt-blown. The manufacturing technique for each category determines the materials of construction that can be used.

Needle-felt filter bags are generally the least expensive style and are the most common with mesh styles being a little more expensive mostly due to the materials compatible with the required manufacturing process. Melt-blown and multi-layered filter bags are the most expensive but also provide higher efficiencies.

In addition to the material and manufacturing process, filter bags have a sealing ring designed to prevent particulates from bypassing the material. The sealing ring thus provides a seal between the filter bag and the vessel. The most common sealing ring designs are a "Snap Ring" and a "Plastic Crush Seal Ring".

A snap-ring is a metal or plastic ring sewn within the opening of the filter bag, which aligns with a groove within the filter bag housing. Based upon the tolerance of the groove and ring, the filter bag is held firmly in-place yet can be removed by hand for change-outs; filter bags have handles to grab so they can be lifted from the vessel.

A crush-seal ring is a geometrical shaped plastic ring that also has an outer diameter that "snaps" into the support basket groove. Sealing is further enhanced by downwards pressure onto the top of the sealing ring, which creates an outward force and improving the seal between the ring and sealing grove. The downwards pressure is achieved by the filter vessel cover, either directly or via a hold-down ring to transmit the downwards force of a closed chamber cover to the top of the filter bag.

Nominal Ratings
Nominal filter ratings are an arbitrary (nominal) value, indicating a particulate size range at which the filter removes some percentage. Nominal ratings vary from manufacturer to manufacturer and cannot be used to compare filters among manufacturers. Processing or application conditions such as operating pressure, concentration of contaminant and particle type have a significant effect on the retention efficiency of the nominally-rated filters.

Absolute Ratings and High Removal Efficiency
Absolute filter ratings are a value associated with a filter, that represents the size of the smallest particle completely retained. Complete retention is within the experimental uncertainty of a standard test method consistent with the intended filter use. Among the test conditions that must be specified are particle size, challenge pressure, concentration, and detection method used to identify the contaminant.