Pulse-jet cleaning baghouses are a common form of dust collection on applications in cement plants throughout the world. Clinker coolers, finish mills, raw mills, coal mills, silos, transfer points, truck load-outs, bagging stations, and pneumatic conveying systems are only a few of the many areas where pulse-jet dust collectors are in operation.

Even though this design of baghouse cleaning has many advantages, there are also common problems associated with these dust collection systems used in cement plant applications. Potential challenges include:

* Inlet abrasion of filter bags;

* Compressed air usage and availability;

* Emissions caused by poor filtering efficiency of traditional felted media; and

* Insufficient airflow or pick-up draft.

Many of these problems are caused by aggressive designs that may have been adequate for past operating conditions, but have since changed. Since the baghouse is already constructed, there are limits to how many filters can be installed, based on the physical size of the baghouse.

In the past, correction of these common problems was limited to major capital expenses either to correct inherent design flaws or to add cloth area. During the past few years, the use of pleated filter elements has allowed cement plants to look at alternative methods of improving the operational performance of existing systems.

Application background North Texas Cement Co., with its sole plant located in Midlothian, Texas, was having problems associated with one of the three pulse-jet baghouses that ventilate its clinker cooler system. These systems are designated as numbers 1, 2, and 3. All three baghouses contain 864, 5-in.-diam x 8-ft-long Nomex felt filter bags. Each unit has nine compartments (each with 96 filter bags per cage) and is rated by design at 32,800 acfm with a designed air-to-media ratio of 3.63-to-1.

During an engineering evaluation visit by BHA in April 1998, North Texas Cement personnel discussed problems they were having with unit No. 3. The unit had extremely poor bag life due to inlet, high-velocity abrasion, which would abrade the bottom of the 8-ft-long filter bags. It was reported that an average of 15 to 20 bags were changed out every seven to 14 days due to this condition. This resulted in frequent and expensive labor, as well as reduced air volume during the recurring filter bag changeouts.

After reviewing the system design specifications and operational performance of this unit, it was noted that one distinct difference between the three collectors was that unit No. 3 had pyramid design hoppers versus a trough design hopper under units 1 and 2. This was a result of initial building design on unit No. 3 that would not allow construction of trough design hoppers due to structural support configurations in the area where the baghouse was erected.

The pyramid design hoppers are shallow (short) and the inlet duct work was causing the abrasive dust to enter the hopper, resulting in a high-velocity wear area on the wall opposite the inlet. A thorough inspection confirmed that there were very apparent abrasion "zones" within the compartment causing the shortened bag life.

Several inlet baffle designs were installed, but due to the shallow hopper design, minimal improvements were on the bottom-bag abrasion. The focus then turned to concentrate on the two options available to improve the system's performance:

1. Reconfigure the entire inlet duct work arrangement with the potential to install larger pyramid hoppers. Not only was this a significant capital expense creating substantial downtime, but it was questionable if the re-design could be accomplished in the area where the baghouse was located.

2. Install high-temperature pleated filter element technology to move the filtration units out of the direct abrasion zone. This would not only assist in achieving additional filter life, reducing emissions, and reducing labor costs, but would also provide a potential energy savings due to decreased compressed air usage for pulse cleaning, and the potential for higher air volumes by lowering overall differential pressure drop.

Based on the information discussed, North Texas Cement enlisted BHA to further investigate the potential installation of high-temperature PFEs in unit No. 3 to determine their effectiveness in this system.

High-temperature pleated filter elements in operation Low-temperature (below a maximum operating temperature of 265 degrees F, or 129 degrees C) pleated filter elements have been used successfully in cement applications over the past decade. Not until the past two or three years has there been a media available to handle temperatures of normal clinker cooler dust collectors of up to 375 degrees F (191 degrees C). Recent technical advancements in the use of pleatable high-temperature media and advanced construction methods now allow the potential to use this technology in clinker cooler vent dust collection systems.

High-temperature pleated filter elements are now available to replace traditional filter bag-and-cage assemblies, and are customized to fit virtually any style of dust collector system by modifications to the number of pleats, pleat depth, and lengths to 2 meters. The media is pleated, which provides an increase in filtration area (100% to 200%) versus traditional filter bags. They also can operate at significantly lower differential pressure drops than traditional filter bags.

A review of the potential use of high-temperature PFEs in unit No. 3 by the installation of a 1-meter-long element with 45 pleats is summarized in Table 1

As seen in Table 1, the high-temperature PFEs would provide a substantially larger drop-out area under the filters since they are only 1 meter (about 40 in.) in length, compared with the standard filter bags at 96 in., and eliminate the abrasion zone seen on the traditional filter bags. Due to the additional surface area provided by using pleated media, the total filtration area was increased by about 52%, thus assisting in the reduction of overall differential pressure drop and system efficiency.

In December 1998, North Texas Cement installed the recommended PFEs. On start-up, the fan damper was positioned from 100% open to only 30% to achieve the required 32,800 acfm. The plant also installed air pressure regulators to control the compressed air for pulse cleaning. Air pressure was set at 60 psi.

After nine months of operation, the plant reported the operational data as listed in Table 2.

Plant personnel also have reported overall system pressure reduction along with lower clinker cooler gas temperatures. As noted above, energy costs of compressed air can be reduced by 40% (in fact, the baghouse did not pulse until after one week of operation), labor costs were substantially reduced, and air volume remained consistent through the compartment.

During subsequent engineering visits by BHA, data continued to be collected with impressive operational performance to date. Dust monitoring equipment also confirms that no alarms have registered since the installation of the pleated filter elements.

A thorough evaluation of every particular plant application is needed before a recommendation can be made on how effective pleated filter elements will be when put into operation. In this particular application, the pleated filter elements were able to improve the system performance.