Bag filters in industrial processes are typically used to filter air, liquids or gases. The filter media selection criterion is Liquid Filter Bag usually temperature, but it is also important to consider the chemical characteristics of the gas stream.
Needle felts, woven monofilament meshes and meltblown are the most common materials used. They are available in sewn or welded constructions and in a range of different pore sizes.
Particle Size
Particle size is one of the most important factors to consider when selecting filter bags. Particles have a variety of sizes, from large to very fine. The size of the particles affects how they move through the gas and how likely they are to collide with the fibers in the bag. This interplay between the fibers and particles is what determines filtration efficiency.
For spherical particles, the particle size can be characterized by a single number: the diameter (or radius). For non-spherical particles, a three-dimensional size function is needed to describe them. Several different types of shape-dependent size functions have been developed. One commonly used is a volume based spherical equivalent diameter, which calculates the diameter of a sphere that has the same physical property as the particle being studied.
As the size of a particle increases, the likelihood of its collision with the fibers decreases due to Brownian diffusion. The collection efficiency of the fibers decreases until it reaches a minimum value.
When a particle enters the field of view of the filter, it encounters the surface tension of the fibers, which pulls it towards the center of the bag. The force of the interaction is proportional to the square of the radius of curvature of the surface of the particle.
A smaller particle has a greater surface area, which means it interacts with more fibers and has a higher chance of causing damage to the bag. A larger particle, on the other hand, will only come into contact with a few fibers at most.
Baghouse filters can be designed with either inside or outside bag filtration. Choosing the right design for your application will depend on the needs of your process. Inside bag filtration is the most common option, as it can be more cost effective and offers good performance in most applications. Outside bag filtration is also an option, but it requires more frequent filter change outs and may result in lower solids holding capacity.
The type of fabric and finish that is selected to make the bags for your system will have a significant impact on the lifespan of the filter bags. There are many different fabrics and finishes available, so it is crucial to choose a finish that can operate in the environment you will be operating in. If the wrong finish is chosen, it could lead to premature failure of the bags and a loss in filtration efficiency.
Temperature
Choosing the correct filter bag media for your application is one of the most important steps in maintaining consistent and high filtration performance. The pore size, finish, and fiber type of your choice can significantly affect how well it performs in the harsh conditions of the industrial gas or liquid stream.
There are a wide variety of fabric filter bags that can be used in industrial applications including woven polypropylene, polyethylene, nomex, polyester, ppps, teflon, and fiberglass. These fabrics are available in a variety of filter ratings ranging from 0.5 micron to 100 microns and are able to remove particles from liquids, gases, and steam at the molecular level.
Woven mono- and multi-filament filter cloths are the most common choice for baghouse applications. However, nonwoven materials have gained a foothold in this market and are now available in a number of different forms including needle felts, various forms of carded nonwovens, meltblown polypropylene and spunbonded webs.
Typical filter fabrics are designed to operate within a specific temperature range and any deviation from this limit, even for a brief period of time, can weaken or damage the fabric. Temperature fluctuations can also cause changes in gas opacity and pressure drop which will affect the performance of the system.
Woven fabrics are most commonly utilized in flue gas environments because they have the greatest resistance to high temperatures and chemical degradation. Other choices include Ryton which has good acid and alkaline resistance, as well as PTFE or Teflon fabric, which is extremely resistant to chemicals but has poor abrasion properties.
These fabrics are typically installed inside of a metal cage or basket to retain them in place and prevent them from blowing out during operation. Because of the potential high differential pressure that can develop across the baghouse, these materials are also usually installed with anti-collapse rings and mounted on wire cages or a support structure.
Conventional sewn bag filters are severely limited when dealing with very hot gas environments. Ceramic filter candles and elements have been developed that are self-supporting and do not require a support cage. They are rated to work in temperature ranges up to 900degC and are virtually chemically inert. This makes them an excellent option for chemically aggressive processes.
Pressure Drop
The design and material selection of your filter bags and dust collector is important for maintaining high filtration efficiency, low pressure drop, and consistent performance. To avoid reduced airflow and clogged filters, consider the use of larger bags or an alternative bag design that can provide increased filter surface area. The Micronics Engineered Filtration Group offers a wide variety of filter bag and cartridge solutions, including woven nylon, synthetic/organic, and lightweight felts to collect fume, light inlet grain loading, powders, plasma table dust, and more. Choose from a variety of pore sizes, from 100 to 425 microns.
Typically, a fan or blower either pressures (pushes) or vacuums (sucks) air through the filter bags' media to either side. This causes the filter media to compress and compact the accumulated dust particles into what is known as a "dust cake." In order for this deposited material to remain soluble, it is essential to season the filters with lower flow over time or inject a pre-coat powder prior to operation to help establish a dust cake.
Regular inspection and replacement of the filter bag media is critical to prevent leakage and ensure compliance with air quality standards. The frequency of bag change-out depends on the dust concentration in the gas stream, which can be monitored with a Dust Monitor. The monitoring system will alert the operator when the dust concentration begins to increase, suggesting a need for replacement of the filter media.
When selecting the correct filter media for your application, it is important to compare the filter media's chemical resistance and temperature capabilities with the process conditions to which it will be exposed. Once the proper media is selected, a comparison of the actual internal can and interstitial velocity ranges with the required outlet dust emission requirements to meet the air permit is revisited to determine if special filter media surface treatments or PTFE membranes are necessary to achieve the desired performance.
Polyester is a cost-effective and versatile filter media suitable for many different applications. It has good abrasion resistance and can be treated with several coatings and finishes to improve performance in specific applications. For instance, adding a PTFE layer to the face of polyester filter bags can double their life and reduce maintenance costs.
Material
The material or filter fabric that is used in a baghouse is the key element to its performance. Essentially the baghouse uses a fan or blower to pressure (push) or vacuum (suck) air across a series of filter bags that have an inside dirty side and an outside clean side. The dirty side intercepts and filters the dust, and the clean side contacts and compacts the filtered dust into a dust cake. The baghouse then periodically cleans the accumulated dust off of the filter bag media with reverse airflow called backwashing or pulse jet cleaning.
Filter bag fabrics are available in a wide range of materials from high-performance PTFE to low cost conventional sewn felt. Generally, the material selected depends on a combination of process, temperature and chemical resistance requirements as well as baghouse operating conditions. The fabric also needs to have good flex abrasion resistance and be capable of withstanding the flow of hot gases.
Conventional sewn felt is usually made of needled or nonwoven melt blown or spun bonded fibers. Typical fibers include polyolefin, polyester, and nylon. They come in a variety of pore sizes and ratings from 1 to over 12 microns. Other options are metalized antistatic, woven cellulose, and a variety of composite and hybrid materials.
Various software programs exist that can help with the selection of the best material for a particular application. GRANTA EduPack is a free tool for students that helps them understand the principles of material selection, while the commercial products GRANTA Selector and JMatPro allow engineers and design professionals to make accurate calculations and comparisons.
The most important factors in selecting the right filter fabric for a specific application are the particle size and temperature, followed by the required filtration efficiency. In general, higher efficiencies require lower pressure drops and a smaller pore size, while larger pore sizes and higher temperatures allow for better contaminant retention. Often a balance between the two is achieved by using a hybrid bag solution with both conventional and pleated filters to handle different particle sizes and efficiencies.