A bag filter is a device for separating solid particles in suspension from a gas stream. It does not eliminate contamination by volatile compounds.

Bag filters are mainly used in industrial facilities as an alternative to electrostatic precipitators.

Bag filters are continuous self-cleaning filters that use self-supporting bag filter elements capable of retaining submicron particles carried by the air or gas stream. The main quality of this equipment is the enormous simplicity of design and work and the great versatility in its use.

A bag filter unit consists of one or more insulated compartments containing rows of fabric bags, in the form of round tubes or sleeves. The dust transported by the air mass passes, by suction or depression, along the area of ​​the sleeves and then radially through the fabric, the latter retaining the dust particles on the outside. The filtered air, once inside the sleeves, passes to the upper clean air chamber (plenum) through the venturi nozzles and from there to the outside by means of the aspirator (fan). A manometer indicates the differential pressure between the clean air chamber and the dirty air chamber, completely separated from each other by the perforated plate (mirror plate) in which the cage-sleeve assembly is installed, only connected through the filter fabric. The filter is operated cyclically, alternating between relatively long filtering periods and short cleaning periods. During cleaning, dust that has accumulated on the sleeves is removed from the fabric area and deposited in a hopper for later disposal.

Bag filters collect particles of sizes ranging from submicrons to several hundred microns in diameter, with efficiencies generally in excess of 99 or 99.9%. The layer of dust or dusk cake formed on the fabric is the main reason for this high efficiency. The powder cake is a barrier with tortuous pores that trap particles as they travel through the cake. Most of the energy used to operate the system appears as pressure drop across the bags and associated parts and ductwork. Typical system pressure drop values ​​range from about 5 to 6 inches water column. Bag filters are used where high particle collection efficiency is required. Limitations are imposed by the characteristics of the gas (temperature, humidity and corrosivity) and by the characteristics of the particles (mainly adhesiveness), which affect the fabric or its operation and which cannot be taken into account economically.[1]​.

The performance of bag filters is determined, among other factors, by the fabric selected, the frequency and cleaning method, and the characteristics of the particles. The most important design parameter is the air-to-cloth ratio (the amount of gas in cubic feet per minute that penetrates one square foot of cloth) and the operating parameter of interest is usually the pressure drop across the filter system. The main operating characteristic of bag filters that distinguishes them from other gas filters is the ability to periodically renew the filtration surface through cleaning and not be discarded after a significant layer of dust accumulates on the surface like common furnace filters: high efficiency particulate air filters - HEPA, high efficiency air filters - HEAF, and automotive induction air filters.