Selecting Filter Bag Media and Optimizing Filtration Performance

Posted by seomypassion12 on August 24th, 2023

The proper selection of filter bag media can significantly reduce total project costs by minimizing waste disposal, production down-time and maintenance expenses. Choosing the right filter bags can minimize dust collection resistance and maximize filtration performance.

The first step in selecting the best filter bag is to understand your industrial environment. This includes temperature and chemical compatibility.
Fabric Material

Bags are manufactured from a variety of fabric materials and fabrics can influence filtration performance. The most common is polyester, which is an economical couvreur eure et loircouvreur chartres choice for dry temperature dust removal applications. Polyester is resistant to many chemicals and abrasion and provides good resistance to dry heat degradation. However, it does not perform well with hydrolytic degradation. Polyester is a viable alternative to Nomex(r) or Teflon(r) bags for dry temperatures and may be more cost effective than these alternatives for certain conditions.

Spun-bonded polyester is the most popular choice for filter bags due to its low cost, chemical resistance and abrasion performance. It is available in both needled and non-needled versions. The choice of a needled version depends on the type and size of dust to be removed. Non-needled polyester is made with high-density fiber and can be glazed to improve mechanical stability and permeability. Filter pore size and surface area are enhanced by glazing, which also improves dust cake release and abrasion resistance.

Polytetrafluoroethylene (PTFE) is a highly inert material that is ideal for applications that require an extremely high level of chemical resistance. It is also nonflammable and can be used in a wide range of temperature applications. PTFE is very abrasion resistant, with a maximum of 100 grams per square inch, and can be glazed for improved durability. It is not recommended for wet environments or high pH applications, but it is an excellent option for carbon black reactors, coal-fired boilers and power plants.

Various factors affect the filtration efficiency of any filter bag material including pore size and surface area, the dimensional stability of the fibers and how the fibers are bonded together. Modulus and tenacity are also important parameters in determining how well the fabric performs. The modulus of a fiber describes how flexible or ridged it is while the tenacity measures its breaking strength. The greater the modulus or tenacity, the better the material performs. Additionally, the direction of the fibers in a fabric will also impact filtration efficiency.
Fabric Construction

In order to achieve high filtration performance, the fabric used in bag filter media must be made of a durable material. This is a key factor in how long a filter will last and how well it can resist the forces of dust collection. Filter bags are made of a variety of different materials, including nylon, polypropylene, polyester, porous PTFE film and other fluoropolymers, cellulose, viscose, microfibers, aramid and woven felts. These fabrics are woven into various shapes and sizes, which determine the pore size of the filter. The pore size determines how much dust can pass through the bag before it needs to be cleaned or replaced.

Woven fabrics make up the largest portion of filter bags used in industrial applications. The most common types of woven filters are cotton, polyester and synthetic fibers made from polyolefin or other manufactured fibers. Woven fabrics are composed of two threads, called warp and weft. The warp threads extend vertically across the filter and the weft threads are woven diagonally to each other. The properties of the fabric depend on the way the yarns are woven together.

The construction of the filter fabric is also important. For example, woven cotton filters have good strength and permeability for most applications. They are used in a wide range of industrial, woodworking and chemical applications. They have excellent energy absorption and abrasion resistance. Cotton filters are available in a broad range of thicknesses and widths to accommodate a variety of flow rates.

Woven polyester and polypropylene filters have a higher cost than cotton but provide superior durability, abrasion resistance and chemical compatibility. They are often used in applications where the fabric is exposed to acidic and alkaline chemicals or high temperatures. These types of bags are available in a wide range of diameters, widths and weights to accommodate a wide variety of flows.

Nonwoven fabrics have become increasingly popular for use in filter bags. They have the ability to form into a specific shape and size and are characterized by having a very low bulk density. Nonwovens are usually made from polypropylene, polyester or polyolefin and may be modified by impregnation with a variety of additives to enhance their filtration characteristics.
Temperature

The temperature of the gas being filtered can greatly affect the efficiency of a filter bag. As a rule, the higher the temperature, the more difficult it is for contaminants to adhere to the surface of the filter. This is because the chemical properties of a contaminant are affected by its temperature.

For this reason, it’s important to select the right fabric based on the operating temperature of the dust collector and the application. When operating at temperatures below 500 degF, cotton is a common choice. It is strong, abrasion resistant, and works well in most applications. Cotton’s abrasion resistance is improved by having it glazed or singed.

As the temperature increases, polyester becomes a good option for filter bags. Polyester is not only durable, but it has excellent abrasion resistance and chemical stability. It can also withstand high temperatures. Polyester’s abrasion resistance is better than most synthetic fabrics, with the exception of Nomex and Teflon. It does not work well with moist heat though and is prone to hydrolytic degradation in some conditions.

Felt filter bags are constructed from a combination of fiber diameters, weights and thicknesses to create an economical depth filtration medium. They are typically glazed or singed to reduce abrasion and migration of the fibers. The glazed or singed construction of these bags also improves their mechanical stability and reduces dust cake formation.

Woven, nonwoven and specialty fabrics are all used in baghouses today. When selecting the fabric for your application, consider factors like; the chemistry of the gas stream, the gas temperature range, EPA emissions requirements, and the percentages of sticky material in the gas stream. Once these criteria are identified, the corresponding selection chart can be consulted.

In addition to selecting the correct fabric for the application, it is also necessary to evaluate the internal can and interstitial velocities and their relation to bag size. This will allow the selection of a suitable air to cloth ratio and filter bag length. This will result in achieving the required outlet emission limits in compliance with the air permit.
Chemicals

Choosing the right bag filter is critical for any chemical processing operation. Inefficient filtration can have a domino effect in the plant and cause costly downtime. Regardless of the size of the process line, proper filter selection can dramatically impact efficiency and quality.

Filter bags for liquid filtration operate on the same basic principle as gas bag filters, but with the flow in-to-out rather than out-to-in. They typically use a simple filter housing with a basket for holding the bag in place during operation. Liquid bag filters are commonly found in industrial applications that require a finer degree of particle filtration.

Liquid bag filters are available in a wide range of filter material ratings from 1-1200 microns, including nylon, polypropylene, polyester, aromatic polyamides, phenolic resins, porous PTFE film and other fluoropolymers, glass-reinforced PVC and woven felt. Generally, the higher the micron rating of a liquid bag filter, the finer the level of filtration.

Woven filter bags are made of either a woven mono- or multi-filament fabric or needlefelt, and can further be divided into two categories: glazed or singed. A glazed finish smooths the surface of the bag to reduce fiber migration and improve dust cake release. The glazed finish also helps to resist water and oil penetration.

Singed and glazed bag filter materials offer better durability, especially for high temperatures and harsh chemical environments. They are ideal for use with low energy cleaning systems like shaker or reverse air baghouses.

Felt filters are a popular choice in a wide range of industrial applications due to their excellent dirt holding capacity and broad chemical compatibility. They are generally available in a variety of mesh sizes, and are typically treated to have a glazed or singed surface. These coatings provide a smoother, uniform surface for easier dust cake release and improved mechanical stability.

The type of bag filter that is best for a specific application will depend on a number of factors, including chemical resistance, temperature, vapor and dust control, cost and filter life. Using a high-quality GORE membrane or a hybrid bag of a woven felt and a singed, glazed PTFE element can double the filter life and significantly reduce maintenance costs in hot gas applications. For example, a producer of titanium dioxide installed these hybrid bags in their production process. This led to significant energy savings, decreased emissions and an increase in the amount of salable product that could be produced.