How Does a Baghouse Work?

Baghouse dust collectors are a type of fabric filter air-material separator employed for particulate removal from manufacturing and other industrial operations to keep dust and solid particulates from entering the workplace or being released to the atmosphere. In effect, baghouses are industrial-scale fabric filter systems used as air pollution control devices. Among dust collection systems, baghouses are the most widely used. The reasons why are twofold: baghouses are both highly efficient and cost-effective. Baghouse systems are engineered to collect, capture, and separate dust and particulates from the air. That said, how do baghouses work?

Baghouses utilize fabric filter bags or pleated filters arranged in rows and mounted vertically in a sheet metal housing. They are designed to receive dusty gas from fugitive or process sources, capture the particulates, and then exhaust clean air. Typically via an induced draft blower, the dusty gas stream is drawn into the baghouse through a duct system. The gas stream then passes through the filters, while particles remain on the filter media surface, thus separating the particulates from the air. Over time, as the dust begins to build up and form a filter cake on the filter surface, various types of cleaning systems are used to remove the dust from the filters.

How the dust is collected and how the baghouse is cleaned depends on the type of baghouse. The three most common baghouse cleaning systems are pulse-jet, shaker, and reverse-air styles. The main differences between these kinds of baghouses are described below.

Pulse Jet Baghouse
The pulse-jet or reverse-pulse baghouse is ubiquitous in industrial dust collection. These industry workhorses boast no moving parts in their basic configurations and allow collection efficiencies greater than 99.9%. In the pulse jet dust collector, the dusty airstream usually enters from below the bags or filters and is pulled upwards through the filters where dust is captured on the exterior surface and clean gas passes through to the clean air plenum side for exhaust. When fabric filters are used, the bags are supported by internal wire cages. The pulse-jet cleaning cycle is controlled by a solid-state timer which sequentially pilots “pulses” of compressed air in the reverse direction of filtering, into blow pipes mounted above each row of filters. When activated, the reverse pulse air pulses travel down the length of each bag in the given row, causing a ripple effect, dislodging and releasing the caked dust into the hopper at the base. A main advantage of the pulse jet baghouse is that it does not have to be taken off-line to clean the filters. With the dust removal programmed for regular or on-demand intervals, the system offers more complete cleaning and reconditioning of the filter bags than the other systems. Operating costs are minimal outside of the cost for compressed air for cleaning.

Shaker Baghouse
A shaker baghouse, as the name suggests, mechanically disposes of collected dust by shaking it out of the bags into a collection hopper at the bottom of the baghouse. In a shaker baghouse, filter bags are hung and tensioned from the top of the filter housing and attached with the bottom open, to the tube sheet. No internal cages are used for shaker bags. The airstream enters from below the bags and is pulled upwards through the interior of the bags where the dust collects. The airstream passes through the filter bags trapping the contaminants on the dusty side. Clean gas is exhausted near the top of the collector. To clean a shaker baghouse, which normally uses woven filter bags, the airflow through the system must be periodically shut down, also referred to as being taken off-line, while mechanical shake-cleaning is employed. After the dust cake is released from the bags, it falls into a hopper at the bottom of the baghouse for removal.

Shaker style baghouses are generally uncomplicated and are commonly employed where compressed air is not available. However, limitations exist for the application of shaker baghouses in that they cannot operate and clean continuously without periodic shut-down. Also, particle collection efficiencies are generally lower than other types of baghouses that use felted or pleated filter media.

Reverse Air Baghouse
There are two styles of reverse air baghouses. More common nowadays, especially with fibrous dusts such as those from grain or wood processing, is a round reverse air collector style wherein dust collects on the exterior of the bags or filters. For cleaning, reverse air generated by the equipment fan or medium pressure blower is directed by a rotating arm over the bags, blowing reverse air into them to remove dust. This type of reverse air baghouse generates a lower cleaning air pressure than the compressed air pulses of a pulse-jet. It is known as a low-pressure high-volume cleaning system and cleans while on-line. This gentler cleaning method decreases wear and tear on the bags and saves on the cost of compressed air. The trade-off for the cost of compressed air, however, is the horsepower requirements and maintenance costs of the reverse air blower and moving parts.

An older type of baghouse, also known in the industry as reverse air, is often seen with a rectangular housing and multi-compartments. Like a shaker unit, in this style, dust collects on the interior of the bags as the airflow rises. These types of reverse air baghouses have to be taken offline for cleaning, thus are often divided into compartments so one section at a time can be cleaned. Generally, an entire compartment is pressurized with a system fan in the reverse direction of filtration. Once activated, the pressure from the reverse airflow causes the filter bags to slightly collapse. These types of reverse air bags have sewn-in rigid rings that allow them to flex but not collapse completely or “pancake” during the cleaning cycle. Due to the off-line cleaning requirement, this type of reverse air baghouse must be oversized to account for a portion of it to be off-line during cleaning, making this style less cost-effective than a pulse-jet.