Baghouse Filter Basics

There are three types of baghouse, categorized by their discharge method, described below. All three operate under the same principle, pull vacuum through a filter media. The filter media will accumulate solid particles until a dust cake forms. Once a dust cake has formed, given the proper media and conditions, the discharged air or gas will be particle free, within acceptable tolerance. The filter bag itself does not perform the filtration in a dust collector. Either the initial dust cake, or a precoat captures fine particles.

The exception is with the use of an applied precoat to the filter media, either a membrane or a microdenier cap. These are fabric upgrades designed to capture particles as small as .5 micron on their surface, preventing both particle escape and particle entrenchment. Think of a nonstick pan, a membrane is the same coating and a microdenier cap mimics its properties. Particles sit above their surface rather than getting stuck in them or passing right through.

When a pressure drop limits the amount of air passing through the filter bags, discharge, by the methods described below, knocks dust from the bags and into a hopper under the collector for disposal or storage.

Shaker Discharge

Shaker dust collectors are the original baghouse. The earliest designs, still used heavily in smaller saw mills is a single large bag that fits over a 55 gallon drum. The bags can be 10’ to 20’ long depending on available overhead space. The shaking mechanism for this is typically a rope attached to a pulley on the top end of the bag. When it’s time for discharge, an operator pulls the rope, shaking the bag and breaking apart the cake that has formed inside the bag.

In order to reduce the dust collector’s footprint, envelope style collectors were developed. Rectangular multi-pocket fabric bags collect dust on the outside, their dimensions supported by wire frames or by porous foam to prevent collapse. To discharge an arm knocks the bags or shakes them, releasing dust to a hopper below.

Common in cement plants, tubular shaker collectors fabric bags, typically a polyester sateen weave (for each style of shaker described), with a diameter ranging from 3”-12” and length from 2’-14’. Compartments of these bags can hold a couple or hundreds of bags. Multiple compartment can be tied into the same dust stream to clean parts while other parts continue filtration. Dust cake accumulates inside the bag. The bottom attached to the cell plate by snap band, bolted flange, or hose clamp. The top end will have a hanger hook, fabric loop, or a grommet attached to a tensioner. A shaker arm above the bags creates the motion for cake discharge.

Pulse Jet

Developed in the 1960s, pulse jet dust collectors build a dust cake on the outside of fabric filters, and a blowpipe above the bags in the clean air plenum delivers a jet of air to each bag in a row ranging between 6-10 tubes. A venturi seated within the open end directs the pulse into the bag to achieve an even cake discharge. Each fabric bag has a wire cage inside to prevent its collapse during filtration while the bag is under vacuum. The top of each bag connects to its cage and cell plate by a snap band, a bolt on flange, or a hose clamp. The bottom of the bag is typically a flat disc bottom, sometimes with a wear strip to prevent abrasion from one bag rubbing against another and forming a hole.