Designing High-Temperature Baghouse Systems

High-temperature airstreams require many substantial design changes. Regardless of whether it is used to control atmospheric pollution, eliminate process contamination, or add profits by recovering product, a baghouse system without the proper filters, collector design, gaskets, etc. will suffer from premature failure, increased emissions, and loss of product control. All of these outcomes result in a higher cost for the operators and less efficiency in creating products and services.

The following are a few items to keep in mind when reviewing the performance of your baghouse system with high-temperature airstreams.

What Exactly Classifies as a “High Temperature Baghouse”?

Engineering an adequate baghouse system is often a challenge. What complicates the situation is that many processes generate dust and other fine particulates suspended in hot gases as part of a normal process flow. These applications are on dust filter media and the vessels that contain them.

For example, baghouses operating in the region between 275°F (135°C) and 1500°F (816°C) are generally considered “high temperature” baghouses. At these temperatures, filter media limitations can be classified as follows:

✦ Level I is 275°F to 400°F (135°C to 205°C)
✦ Level II is 400°F to 500°F (205°C to 260°C)
✦ Level III is 500°F to 1500°F (260°C to 816°C).

Common applications that include high-temperature airstreams such as this include cement kilns, industrial dryers, steel mills, coal-fired power plants, etc.

Some of the specific problems that can occur if a high-temperature baghouse is not designed correctly include:

Moisture in the baghouse and resultant discharge challenges filter failure if the inlet gas temp is allowed to drop below the dew point and condenses.
Early filter failure due to incorrect filter media selection.
Incorrect fits and resultant air leakage, specifically between filters and sealing components (tube sheets) due to thermal expansion at high temperatures.
Corrosion, fatigue, and other temperature-related impacts on baghouse materials.

Options For Cooling the Airstream Before The Baghouse

In many plants, a viable solution to the problems associated with a high-temperature airstream is to install cooling systems upstream from the baghouse. Such treatment options vary greatly, in both design and cost. Some treatment options include heat exchangers, coolers, and quenchers. Sometimes, introducing bleed air into the system is sufficient to cool the gas stream below the design temperatures.

Cooling the dust-laden gases before entry into the baghouse is often the only way to make it possible to use a baghouse for filtration purposes. The cost of operating additional cooling devices should be calculated based on energy usage, maintenance requirements, etc. These costs are often negated by the reduced wear and improved operation of the baghouse due to the lower temperatures (e.g. less heat attack on the filters, less thermal contraction/expansion wear on the collector itself, etc.).

However, it should be noted that over-cooling the inlet gas below the dew point may lead to condensation and moisture in the baghouse, which will cause major problems with filter life and efficiency, difficulty in getting collected dust through the hopper and discharge system, etc. For this reason, engineers should carefully review the inlet gas composition and temperature and design the system to maintain the right temperature, not just the lowest possible temperature.

Selecting The Proper High Temp Filter Media

Baghouse filters damaged by a spike in high temperatures

Choosing the right filter media for your application’s temperature will avoid premature failure from thermal degradation

Choice of media is essential in high temperature applications. High temperature filters themselves often cost as much if not more than the entire baghouse, so protecting the filter media from damage is of vital importance. High temperature operation increases the filter medium’s susceptibility to premature failure from thermal degradation, chemical attack or both, as temperature and chemistry often go hand in hand.

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Aramid baghouse filters (trade name Nomex) is widely used because of its resistance to relatively high temperatures and to abrasion.

General applications for Aramid felt includes highly abrasive dust and chemical applications with high temperatures

In recently years a wide range of different fabrics have been developed to meet the needs of high temperature applications. The most popular are polyester, polypropylene, acrylic, for the lower range, aramid, P84, Ryton, fiberglass, Teflon, for the mid-range (up to 500°F), and ceramic, and sintered metal limited ultra-high range (up to 1500°F). In addition to the base fabric material, various treatments and coatings can improve the media’s performance. These include singeing or glazing the material, applying coatings such as silicone or PTFE to improve resistance to acid and alkali substances or improving fine particulate collection and dust cake release.

Filter Media Types Infographic

Would you like to know what filter and treatment would work best for your application? Download this infographic for free!

It is also necessary, depending on the kind of baghouse in use, to choose the correct media weight to withstand the baghouse cleaning cycles. A reverse-air baghouse will usually require only a 14 oz./yard fiberglass media, while in the same conditions, a pulse-jet baghouse will need a heavier 22 oz./yard fabric due to the greater strain on the filter fabric during its cleaning cycle compared to a reverse-air type.

Continuous operating temperature of 500°F (260°C) is the limit for traditional fabrics. After 500°F, the only options are to use specialty materials such as sintered metal or ceramics. These materials, while capable of handling temperatures of up to 1500°F (816°C), are much more expensive than traditional fabrics and require special design considerations to ensure they fit and function correctly in their challenging high-temp environment.

The key factors that should influence final media selection are operating temperature (both continuous and peak), abrasiveness of the particulate, chemical makeup of the gas stream, and moisture content.

Baghouse Collector Design Considerations

Even the best filters will be useless if the baghouse itself is not designed for operating in a high temperature application.

One area of concern is wear caused by regular expansion and contraction due to temperature variations. One common solution is to use a circular design. Many baghouse engineers feel this design is superior because most of the unit will expand and contract uniformly outward/inward together. This is however only partially effective since there are usually a number of “hotspots” in the collector where temperatures exceed those in other areas, thus causing uneven expansion/contraction regardless of the shape of the collector.

Insulating the ductwork coming into the baghouse will maintain the temperature above the dew point, avoiding condensation

Additional construction ideas to keep in mind include: using only heavy-duty materials and construction methods, including stiffening elements to mitigate the effects of thermal expansion/contraction, insulate to reduce heat loss and condensation (specifically the baghouse hopper, dirty side housing, and inlet ductwork), and use diaphragm and solenoid valves with copper or stainless-steel piping and Viton diaphragms rated for use in high temp applications.

Maximizing Baghouse Filter Efficiency

Once the system is up and running, maintaining best operating practices can increase your filter life substantially, while also lowering energy usage, increasing collection efficiency and reducing total emissions.

Using stainless steel cages will prevent damage from acidic or alkali gases from corroding the metal

Concern should be taken during startup/shutdown procedures to minimize the effects of crossing through the dew point, which can lead to increased condensation and cause early bag failure. Often this will include preheating the baghouse before introducing the dust-laden process gases to be filtered, and avoid unnecessary shutdowns.

Another good design practice for high temperature systems is to use stronger bag cages with more vertical wires to provide extra support for the filters. If the gas stream includes high levels of acidic or alkali gases, it is often wise to use stainless steel cages.

Concluding Thoughts

Designing a baghouse dust collection system for use in a high temperature application is no easy task. Proper care and attention must be taken to ensure the proper filter media is selected and that the system itself is engineered and constructed to withstand the effects of high temperature applications.

This guide is by no means an exhaustive list of all the things that must be considered when designing a high temp system. But this information is a good place to start when considering the design of a new system, or looking for ways to improve the operation of existing systems.

When selecting a baghouse supplier/engineering firm like Baghouse.com, work closely with them to ensure all of these considerations are taken into account. By doing so, higher efficiencies, less energy usage, and increased service life will be yours.