Bag filters are used for filtering liquids, and are the most commonly used filters in the industry.
A filter bag is put in a filter housing. The bag is composed of polyester, polypropylene or nylon. Bag filters made of nylon provide perform surface filtration, where water flows from the inside to the outside through the 'bag'. Dirt particles are retained by the filter material, and sticking in the bag. Filtration from 5μ up to 2000μ.
The nylon mesh filter can be easily cleaned, and the bag can be re-used (limited). This is the big advantage of this type of filter bags. The bag filters manufactured from polypropylene provide depth filtration: water flows from inside to outside through the felt, where the dirt particles remain in the needle felt. The thickness of the needle felt determines the filter fineness. Needle Felt Filter can filter up to 1μ.
Finer efficiencies can be achieved by using special filter bags with several layers (absorption of the gelatinous soiling or absorption of oils, absolute filtration <1μ,...).