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A baghouse born again – part two

World Cement,

Developments in pulse jet technology

When looking at possible upgrades for a pulse jet baghouse, we need to consider the latest developments, which include, but are not limited to the following possibilities:

  • Snapband filter bag connection. This type of filter bag is installed with no tools and has a leak-free and reliable tubesheet to bag connection. This may not appear to be a new development to those familiar with modern fabric filter designs, but this particular baghouse used a clamped bag with a screw-in cast venturi that requires a separate felt ring gasket to avoid leaks.
  • PTFE Membrane filter media. This type of filter media has a very thin, microporous film on the filtration surface, with openings so small that it’s impermeable to water molecules – and to submicron dust. This film also has non-stick properties that allow filtered dust to be discharged very efficiently. PTFE membrane filter bags generally provide lower DP and a significant performance improvement on systems that filter very fine or humid dust.
  • Pleated filters. This technology offers increased capacity by packing two or three times the filtration area into the same space as a filter bag. This could be a solution, but has some disadvantages and is very costly, particularly on high temperature systems.
  • Longer filter bags, or long bag technology. Practically all modern process baghouses use what is known as long bag technology, with filter bag lengths up to 10 m (30 ft). Long filter bags provide proportionally larger filtration areas, but their resulting interstitial (rising) velocities would limit their capacity. To solve this problem, a percentage of the incoming flow can be introduced laterally into the array of filter bags, proportionally reducing the vertical velocity component.
  • Inlet flow distribution baffles, or diffusers. These help provide more uniform flowrate into all the filter bags. Numerous designs have been developed, but ladder vane baffles are a simple and robust design that has been adopted by numerous manufacturers. Different types of flow distribution systems are an integral part of modern long-bag pulse jet baghouses to properly split and direct inlet gasses.

Various other possibilities were also considered and discussed, such as a new control system, enlarging the heat exchanger hopper for increased particle dropout, larger compartment inlets, double-diaphragm pulse valves and a complete pulsing system based on larger diameter hardware for more effective pulsing.

Selection of upgrades

Some upgrades can be selected without much of an engineering evaluation. From the list above, we have:

  • A snapband bag upgrade provides a more reliable bag to tubesheet seal and easier filter bag installation. New bags, cages and tubesheets are required to implement this upgrade, but the significant benefits made this an easy decision to include in the refurbish project.
  • A PTFE membrane filter media upgrade was eliminated because its main advantages do not apply to clinker cooler venting, which handles coarse particles and has practically no moisture in the filtered gasses.
  • Inlet baffles provide a very inexpensive way to improve gas flow distribution inside each compartment, avoiding concentrated high flow velocities and resulting abrasion. This particular baghouse had different baffles in several compartments, some of which were severely worn out. It was decided that ladder vane baffles made of abrasion resistant plate at each compartment would be a part of the refurbish project.

Other items on the menu of options were discussed and decided upon. Among them, a new control system was not necessary, as it had been upgraded in recent years. A larger pulsing system also was eliminated, as the existing one was considered acceptable for pulsing the relatively short filter bags. New replacement blowpipes were added to the list because of wear and tear on the existing ones. New dampers also were added, which Ash Grove purchased separately.

To increase or not to increase capacity

To determine if pleated filters or longer bag adaptations were viable or required, simple flow velocity calculations were made, including compartment inlet velocity, can velocity, interstitial velocity and filtration velocity. The last one is better known as air-to-cloth ratio, or A/C ratio.

The numbers revealed that the baghouse was indeed sized adequately to handle the specified flow, so the very expensive, high temperature pleated filter alternative was eliminated.

Another concept used to expand capacity in walk-in plenum baghouses consists of positioning the new tubesheet higher in the plenum, extending the filter bag lengths and using two-piece cages. This idea was presented, but quickly discarded because it would require a much higher investment, yet calculations showed that it was simply not necessary for the specified flowrate.

The calculations and site discussions revealed a new area of opportunity. It was noted that the existing filter bags failed mostly at the very bottom. Part of this was due to uneven flows, to be partially addressed through the new inlet baffles, but the numbers also revealed that the A/C ratio was fairly conservative, while the can velocity and the interstitial velocity were one and the same.

Can velocity and interstitial velocities differentiated

Can velocity is commonly confused with interstitial velocity, and with good reason. These are practically the same when the filter bags extend down to the hopper level.

Can velocity is the vertical flow velocity above the hopper level, but before reaching the bottom of the bags, while interstitial velocity is the vertical flow velocity once the flow is past the bottom of the filter bags. Interstitial velocity is obviously higher due to a much smaller cross-sectional area once the filter bags are considered.

Having dead space under the bag array provides a low can velocity, essentially creating an internal dropout chamber that helps distribute and minimise horizontal flows that can cause abrasion problems at the bottom of the bags. Experience shows that baghouses with bags some distance away from the hopper have extended filter bag life, and the calculations showed that creating this low velocity zone was feasible and beneficial.

This is part two of a three-part article written for World Cement’s May issue and abridged for the website. Subscribers can read the full May issue by signing in, and can also catch up on-the-go via our new app for Apple and Android. Non-subscribers can access a preview of the April 2016 issue here.

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