Baghouses, also called fabric dust collectors or fabric filters, are air pollution control devices designed to use fabric filter tubes, envelopes, or cartridges to capture or separate dust and other particulate matter (PM). Their applications range from small household workplaces to large industrial facilities such as coal-fired power plants and cement plants.

Compared to other types of air pollution control (APC) equipment, baghouses are incredibly versatile and can be engineered for almost any dust producing application by varying size and bag types. They are very efficient when properly maintained and are also rugged enough to handle rough applications. However, they typically require a lot of maintenance and a relatively dry environment to operate effectively. Their use is also limited to certain operating temperatures and chemical conditions.

Baghouse Collector Operation

Baghouses consist of filter media (bags) suspended inside a housing or casing. Fans on the outside of the housing blow the dirty or polluted air through the filters, capturing the suspended particulate matter and solids on the bags and pushing clean air through the outlet. While filtering, a baghouse bag allows the formation of a layer of particulate matter on its surface, called a dust cake. This dust cake continues to build until the thickness reaches a level where flow is sufficiently restricted; at this point, the bags are cleaned. Cleaning can be done during operation or offline depending on the type of baghouse.

Diagram of baghouse. Red arrow indicates dirty air, blue arrow indicates clean (filtered) air.

Image Credit: Neundorfer, Inc.

As air is filtered through the baghouse, the dust cake on the bag filters continually thickens. For most bag fabrics (those without a membrane coating), the cake is what does most of the filtering of the particulate matter in the air stream. A thicker dust cake increases both collection efficiency and pressure drop as the pathways through the bag become finer and also more restrictive. Cleaning mechanisms must find the right balance for this tradeoff - too thorough or frequent cleaning results in a lower collection efficiency and possibly reduce bag life, but insufficient cleaning will cause excessive energy requirements for blower fans (i.e. high pressure drops).

Baghouse Design

Although the design of baghouses is typically the responsibility of the manufacturer, an understanding of the most important design criteria is helpful for making an informed selection.

The air-to-cloth ratio, also known as the superficial filtering velocity (in units of ft/min), is the most important criteria for baghouse design. It is defined as the amount of air entering the baghouse divided by the total surface area of the filter fabric in the baghouse. This ratio determines the airflow capacity of the baghouse, and must be optimized to balance the size of the baghouse (capital costs) with the pressure drop (operating costs).

The differential pressure, or pressure drop, is a measure of the resistance to gas flow in the system. Baghouses with higher pressure drops require higher powered fans to move air through the system, resulting in increased energy costs. The total differential pressure is the sum of individual pressure drops due to the fabric, particulate layer (dust cake), and baghouse structure. An abnormally high pressure drop in a baghouse can be caused by a number of factors relating to poor design or setup, including:

• Excessive air-to-cloth ratio
• Particulate adhesion caused by excessive moisture in the system
• Blinded filter bags due to insufficient cleaning energy

Cleaning Mechanism

Baghouses are primarily classified based on the methods they use for bag cleaning.

Types

There are three different types of baghouse cleaning mechanisms; each offers its own advantages for different applications.

Reverse Air

Reverse air (R/A) baghouses use continuous streams of low pressure air to remove collected solids. Bags are cleaned by backwashing (reversing the air flow) within a chamber after shutting off the dirty gas flow and isolating the compartment. The recommended air to cloth ratio for these baghouses is between 1.75:1 and 2.5:1.

Advantages

• R/A baghouses are typically compartmentalized, allowing sections to be cleaned without shutting off the whole system.
• Cleaning action is very gentle, which lengthens bag life.

• Preferred for high temperatures due to gentle cleaning action.

Disadvantages

• Cleaning air must be filtered.

• Provides no effective means for removing residual dust buildup.

• Requires more maintenance than other types due to dust re-entrainment on the bags.

Shaker<

Shaker baghouses use mechanical shaking or vibrating actions to dislodge the filter cake. Bag bottoms are secured to a plate and their tops are connected to horizontal beams. These beams, driven manually or by a motor, vibrate to produce waves in the bags which shake off particulate matter. The recommended air to cloth ratio for these baghouses is between 2.0:1 and 2.5:1.

Advantages

• Design and operation simplicity.
• Can be compartmentalized to allow sections to be cleaned without shutting off the whole system.

Disadvantages

• Cannot operate in high temperatures.
• More energy and time intensive than other cleaning methods.
• Small amounts of positive pressure inside the bag can significantly reduce collection efficiency.
• Large footprint and space requirements, and requires a large number of bags.

Pulse-Jet

Pulse-jet (P/J) or reverse-jet baghouses use compressed streams of high pressure air to remove particulate matter. During cleaning, brief (0.1 second) pulses of air are pushed through the bag, dislodging solids which collect in a hopper below. The recommended air to cloth ratio for these baghouses is between 3.25:1 and 4.0:1.

Advantages

• Cleaning mechanism allows P/J baghouses to be cleaned while the system is online.
• More complete cleaning than shaker or reverse air baghouses, lengthening bag life.

• Operates at lower pressure drops and with lower space requirements.

Disadvantages

• Requires the use of dry compressed air.
• Requires special fabrics for higher temperatures.

• Cannot tolerate high moisture levels or humidity in exhaust gases. 

Selection tip: Some baghouses incorporate combinations of the above methods (e.g. shaker with reverse air assist). Others may utilize sonic horn technology, which uses high intensity sound waves to provide additional vibrational energy for dislodging particles.

Cleaning Sequence

When considering a baghouse's cleaning mechanism, the cleaning sequence is a particularly important factor. It determines when and how often the cleaning takes place in the system.

• Intermittent cleaning requires the fan/process to be stopped at intervals while the bags are cleaned. This sequence is used for single compartment baghouses, usually shaker types.
• Continuous offline cleaning involves taking individual compartments offline in turn to clean, meaning the overall process is not shut down during cleaning. This sequence is used with multiple-compartment reverse air or pulse-jet baghouses.
• Continuous online cleaning allows the process flow to continue during cleaning. This fully automated sequence is typically used for pulse-jet baghouses.

Bag Materials

The bag material or fabric media is an important part of baghouse design and selection, as it determines the life and effectiveness of the filter bag. Fabric filter media must be compatible both physically and chemically with the gas stream and system conditions. Selection of the correct bag material incorporates these factors:

Bags can consist of one or a number of different materials. This chart from Filter Media Services, LLC. provides an overview of the properties of some of these materials used in bag construction.

In addition to the material type, whether the fabric or material is woven will affect what systems the bag is suitable for.

• Nonwoven materials consist of randomly placed fibers supported and attached to a woven backing. This strong construction is required for high energy cleaning techniques like pulse jets and aggressive shakers.
• Woven materials have fibers wound in uniform, repeating patterns. This construction is used for low energy cleaning methods such as reverse air and lower-intensity shakers. The weave space affects the strength of the fabric and the permeability/capture efficiency of the filter.

Performance Specifications

The most important performance specifications to consider when selecting a baghouse are the airflow rating and the minimum particle size.

• Airflow or volumetric flow rate is the acceptable flow rate or range of flow rates of the gas stream through the baghouse, measured in cubic feet per minute (cfm). An increase in gas flow rates causes an increase in operating pressure drop and air-to-cloth ratio. These increases require the baghouse to work more strenuously, resulting in more frequent cleanings and high particle velocity, two factors that shorten bag life
• Minimum particle size indicates the minimum diameter of particulate matter that the baghouse is capable of filtering, measured in micrometers. This rating effectively defines the filtering capability of the baghouse, which should be able to sufficiently meet the needs of the system.

Other important baghouse specifications to consider include:

• Gas temperature - fabrics are designed to operate within a certain range of temperature. Fluctuation outside of these limits even for a small period of time, can weaken, damage, or ruin the bags.
• Pressure drop - Baghouses operate most effectively within a certain pressure drop range. This spectrum is based on a specific gas volumetric flow rate.
• Opacity - Opacity measures the quantity of light scattering that occurs as a result of the particles in a gas stream. Opacity is not an exact measurement of the concentration of particles; however, it is a good indicator of the amount of dust leaving the baghouse.

Applications

Baghouses may be specifically designed to meet the needs of certain industries or applications. Some applications and media types include:

• Abrasives - baghouse fabrics are designed to withstand and capture abrasive particles.
• Acid gases - baghouse fabrics are coated with powdered lime or similar substance to absorb acid gas.
• Coolant and oil mists - unit is capable of filtering coolant smoke and mist from metal finishing and forming processes, and machining oil mists.
• Explosive media - unit is capable of filtering explosive dusts, mists, and/or fumes.
• Fine powders - unit is capable of filtering fine powders such as carbon black, talc, pigments, oxides, and plastic compounding dusts.
• Mercury - baghouse fabrics are coated with powdered activated carbon to absorb mercury or other difficult to capture air pollutants.
• Toxic media - unit is capable of filtering toxic materials such as dust, mist, fume, or smoke from the air.
• Welding fumes - unit is designed specifically for the collection of welding fumes or dust; these may include flux recovery systems.

References

Neundorfer - Baghouse / Fabric Filter Knowledge Base

Image Credit: Serifilco, Ltd. | United Air Specialists