[2] Baghouses came into widespread use in the late 1970s after the invention of high-temperature fabrics (for use in the filter media) capable of withstanding temperatures over 350 °F (177 °C).
[3] Unlike electrostatic precipitators, where performance may vary significantly depending on process and electrical conditions, functioning baghouses typically have a particulate collection efficiency of 99% or better, even when particle size is very small.
Most baghouses use long, cylindrical bags (or tubes) made of woven or felted fabric as a filter medium.
For applications where there is relatively low dust loading and gas temperatures are 250 °F (121 °C) or less, pleated, nonwoven cartridges are sometimes used as filtering media instead of bags.
The gas is drawn through the bags, either on the inside or the outside depending on cleaning method, and a layer of dust accumulates on the filter media surface until air can no longer move through it.
The use of chemically inert limestone (calcium carbonate) is most common as it increases efficiency of dust collection (including fly ash) via formation of what is called a dustcake or coating on the surface of the filter media.
Dirty gas enters the bottom of the baghouse and passes through the filter, and the dust collects on the inside surface of the bags.
Vibration produced by a motor-driven shaft and cam creates waves in the bags to shake off the dust cake.
The pressure makes the bags collapse partially, causing the dust cake to crack and fall into the hopper below.
However, to prevent total collapse and fabric chafing during the cleaning cycle, rigid rings are sewn into the bags at intervals.
The pulse-jet baghouse was invented by MikroPul (currently part of the Nederman group and still a major supplier of filtration solutions) in the 1950s.
A digital sequential timer turns on the solenoid valve at set intervals to inject air into the blow pipe and clean the filters.
Each compartment is periodically closed off from the incoming dirty gas stream, cleaned, and then brought back online.
Continuously cleaned baghouses are designed to prevent complete shutdown during bag maintenance and failures to the primary system.
The blast of compressed air must be powerful enough to ensure that the shock wave will travel the entire length of the bag and fracture the dust cake.
The horns, which generate high intensity sound waves at the low end of the ultrasonic spectrum, are turned on just before or at the start of the cleaning cycle to help break the bonds between particles on the filter media surface and aid in dust removal.
This method can be visualized by reminding users of putting a floor covering rug on a clothes line and beating the dust out of it.
This beating action accomplishes the same desired effect of creating a force that dislodges the particulates as the cage moves.
The chemical composition, moisture, acid dew point, and particle loading and size distribution of the gas stream are essential factors as well.
Pressure drop, filter drag, air-to-cloth ratio, and collection efficiency are essential factors in the design of a baghouse.
Fabric filter bags are oval or round tubes, typically 15–30 feet (4.6–9.1 m) long and 5 to 12 inches (130 to 300 mm) in diameter, made of woven or felted material.
Felted filters contain randomly placed fibers supported by a woven backing material (scrim).
Reverse air bags have anti-collapse rings sewn into them to prevent pancaking when cleaning energy is applied.
Some baghouses use pleated cartridge filters,[13] similar to what is found in home air filtration systems.
This allows much greater surface area for higher flow at the cost of additional complexity in manufacture and cleaning.