Introduction
Baghouse dust collectors capture the particulate in an air stream by forcing the airflow through filter bags. A baghouse works by taking the inlet dust-laden air and initially reducing the velocity to drop out larger particles. The baghouse then filters the remainder of the particles by passing the air through a fabric bag. Separation occurs by the particles colliding and attaching to the filter fabric and subsequently building upon themselves, creating a dust cake. Since the dust has been deposited on the outside of the bag, when the dust cake is removed from the bag or cleaned, it falls by gravity into the collection hopper located below the bag section. Collected dust is then removed from the collector through a hopper valve.
General knowledge about baghouse collectors
Baghouse collectors are generally designed and sized to operate with a differential pressure between 4 and 6 inches wg. These collectors can achieve air cleaning efficiencies of more than 99.97 percent (high-efficiency particulate air, or HEPA) for fine particles. The fabric bags can be made from cotton, synthetic materials, or glass fiber.
The type of fabric bag used depends on the type of collector and application. For most applications involving ambient temperature, a cotton bag is the most economical. However, in a corrosive or high-temperature environment, a bag material other than cotton should be employed. Since bags must be changed periodically, fabric collector designs that facilitate bag changes should be chosen. Designs where the bags can be changed from outside the collector are preferred.
Baghouse systems can also be designed for economic optimization. For a given emission control problem, factors such as the overall pressure drop, filtration cleaning cycle, and total filtration surface area can be addressed simultaneously. Caputo and Pacifico [2000] provide a useful model, particularly for operations in the preliminary design phase. Bulk density of the material requires special engineering attention.
The effect that upward velocity (interstitial velocity) can have on the operation of a dust collector can be enormous. Materials with low bulk density (< 30 pounds per cubic feet) must have specialized designs.
In these cases, collector designs must be modified to accommodate lower interstitial velocities. Typical modifications include wider bag-to-bag spacing, shorter-length bags, or high side inlets.
Particle size distribution plays a key role in determining the air to cloth ratio and filter bag selection. The air to cloth ratio is a measure of the actual volume of gas or air per minute per unit area of bag (ft3 air per minute/ft2 of bag area), and can be expressed mathematically as follows:
Air to cloth ratio = Q / A
where Q = quantity of gas (air) in actual cubic feet per minute (acfm), and A = area of the filter cloth or total number of bags in ft2.
It is generally understood that the finer the dust, the lower the air to cloth ratio needed. Proper bag or cartridge selection based on the material to be collected is fundamental to a successful system. The article, “Fine Filtration Fabric Options Designed for Better Dust Control and to Meet PM2.5 Standards” [Martin 1999], provides a useful fabric characteristics and capabilities chart, matching fabric type to operating conditions. Another recommended resource is the article, “Pick the Right Baghouse Material” [Mycock 1999], which includes a chart detailing properties of textile fabrics for filtration.
Inlet loading refers to the amount of dust arriving at the inlet of the dust collector. It is typically expressed in pounds per minute (lbs/min) or pounds per hour (lbs/hour) and converted into grain loading expressed in grains per cubic foot (gr/cf) of airflow.
The grain loading within an air stream is dependent on many factors, which include the number of dust sources serviced by the dust collection system, the types of dust sources (e.g., crushers, screens, etc.), the dust emissions from these individual sources, and the capture effectiveness of the dust collection system at each source.
The amount of dust emitted by each source is impacted by a number of parameters, including the particle size distribution (dustiness) of the material being handled in the process, the moisture content, and the throughput rate.
The porosity of the filter media (cloth) will determine the amount of air that can be drawn through before the static pressure becomes too high for the collector housing or the fan capacity. By estimating the inlet loading, one can also determine the grains of dust striking each square foot of filter media per unit time.
The air to cloth ratio is also an indirect measure of the average velocity of the air moving toward the bags. Inlet loading directly affects the air to cloth ratio. The greater the inlet loading, the lower the air to cloth ratio should be. High inlet loading results in more dust being retained on the filter media and causes higher pressure drops. Air to cloth ratios below 4:1 are considered to be low, from 4:1 to 7:1 moderate, and above 7:1 high. By lowering the air to cloth ratio, the filter has more filter area to distribute the dust, thereby helping to keep the pressure drop lower on the residual dust cake.
There are basically two methods to reduce the air to cloth ratio—lowering the cfm of air or increasing the filter cloth area. However, in overall dust collection system design, changing the airflow volume can be impractical. Therefore, increasing the filter cloth area is more common.
In addition to the air to cloth ratio, inlet loading can also affect the method used for cleaning the bags. Since virtually all filter materials work better with a consistent filter cake to optimize collection efficiency, varying the duration of the cleaning cycle, in conjunction with the rest period between cycles, will allow the operator to maintain a consistent filter cake. This is manifested by a low variation in pressure drop across the filter media. This approach is called “on-demand” cleaning. Bag cleaning is initiated at a predetermined high pressure drop and stops when the pressure drop reaches a predetermined low set point. This method ensures that the bags always have a sufficient amount of dust cake. Experience and manufacturers’ recommendations are the best means of determining the optimum cleaning cycle for each system.
There are three techniques used with baghouse collectors to clean the dust cake from the filter media. These techniques are accomplished by mechanical shaker collectors, reverse air collectors, and pulse jet collectors.
Conclusion
In summary, baghouse dust collectors are highly effective systems for capturing airborne particulates, achieving efficiencies of over 99.97%. Their performance depends on factors like air-to-cloth ratio, filter media selection, and proper cleaning techniques, which must be tailored to the specific dust characteristics and operational conditions. Optimizing design elements such as bag spacing, material choice, and cleaning cycles ensures efficient operation and longevity. Understanding the interplay of inlet loading, dust properties, and filtration requirements is critical for achieving reliable and economical dust control. With careful planning, baghouse collectors can be a cornerstone of clean air solutions in industrial environments.
Reference
NIOSH Mining Program Report of Investigations, « Dust Control Handbook for Industrial Minerals Mining and Processing», Second Edition
