Since the 1990 Clean Air Act amendments were passed into law, the cement industry has had to make a number of modifications to ensure that its manufacturing facilities comply with emissions regulations. And, the regulations keep getting tougher. The Environmental Protection Agency's (EPA) impending Maximum Achievable Control Technology (MACT), PM 10 and PM 2.5 standards will require installation of additional controls by companies with any emission sources emitting more than 10 tons per year (tpy) of a single hazardous air pollutant (HAP) as defined under the standards or 25 tpy of combined HAPs. Further, New Source Performance Standards for particulate matter emissions are requiring new controls in a number of plant applications and transfer facilities.

For many manufacturing facilities, the new equipment needed to maintain compliance has added maintenance issues and production constraints. Additionally, equipment that was installed only 10 years ago to satisfy regulations may no longer be adequate as new standards are put in place. Does this mean that more new equipment will be needed?

Not necessarily. New filter elements are emerging that can help companies cost-effectively retrofit existing equipment to meet increasingly stringent emissions standards. When specified and used correctly, these elements can also help reduce energy use and maintenance requirements, while increasing operational efficiencies.

OPTIMIZING BAGHOUSE COLLECTORS

Historically, filter bags constructed of 16-oz. polyester felt have been the primary media used in baghouse systems for filtering process dust in temperatures up to 275°F. However, polyester felt bags were designed to meet less stringent regulations and, consequently, may not meet future emissions standards set by EPA and other government organizations.

Due to the abrasive nature of some materials, bags have to be changed rather frequently — every six months is common. Additionally, they often limit the actual cubic feet per minute airflow of the collector, making it difficult for manufacturers to achieve their material handling and production goals.

In many cases, these filter bags can be replaced with 100% nonwoven, spunbond, pleated filter elements to produce greater dust collection efficiencies. In side-by-side stack efficiency testing, the pleated filters were 58% more efficient than 16-oz. polyester felt bags in reducing emissions (see Table 1). The pleated elements can also increase the collector's airflow by lowering the delta P — the level of resistance in a system measured in inches — across the filter (see Table 2). For manufacturers looking to save energy, a 3- or 4-in. drop in delta P can save approximately $9,198 annually (operating 24 hours a day, seven days a week, 365 days a year @ $0.07 kw). Units with larger motors can save even more on energy costs (see Table 3).

TABLE 1. STACK EMISSION COMPARISON

m	16-oz. Polyester Felt Bags	100% Non-Woven Spun Bond Polyester Elements
Dust size	5 microns	5 microns
Air-to-cloth ratio	5:1	5:1
Grain Loading	30 g/acf	30 g/acf
Temperature	275 °F	275 °F
Emission	0.0060	0.0025

TABLE 2. ACFM COMPARISON*

	16 oz. Polyester Felt Bags	100% Non-Woven Spun Bond Polyester Elements
Filter elements	720 bags	720 40-inch elements
ACFM	100,000	117,000
Air-to-cloth ratio	7.10:1	5.26:1
Unit square feet	14.14	26.39
Operating delta P	7.00	3.88
Motor	300 hp	300 hp
System delta P	@15 in.	@11 in.
Brake hp	291.50	291.50
*Based on a 100,000 ACFM baghouse using a backward inclined fan.

TABLE 3. ENERGY COMPARISON*

	16 oz. Polyester Felt Bags	100% Non-Woven Spun Bond Polyester Elements
Filter elements	720 bags	720 40-inch elements
ACFM	100,000	100,000
Air-to-cloth ratio	7.10:1	5.26:1
Unit square feet	14.14	26.39
Operating delta P	7.00	3.88
Motor	300 hp	300 hp
System delta P	@15 in.	@11 in.
Brake hp	291.50	224.40
Total savings at $0.05kw= $19,732 per year
*Based on a 100,000 ACFM baghouse using a backward inclined fan.

Furthermore, the pleated elements are designed to resist wear from abrasive materials, enabling them to last two to three times longer than felt bags in the same environment. Also easier to change than felt bags, pleated elements require only 25% to 35% of the time typically needed for conventional bag and cage replacement. Accordingly, their use enables manufacturers to significantly reduce maintenance costs (see Table 4).

TABLE 4. MAINTENANCE COMPARISON

Change-out Time
720 re-bagging = 80 man hours
80 man hours @$45.00/hr = $3,600.00
720 re-bagging to pleated filter elements = 20 man hours
20 man hours @ $45.00/hr - $900.00
Change-out labor savings = $2,700.00

Overall, pleated elements can optimize both operational efficiency and throughput of existing baghouse systems. In addition to helping manufacturers meet more stringent emission standards, the pleated filter elements can provide thousands of dollars in savings compared to bag filters (see Table 5).

TABLE 5. SUMMARY OF TOTAL SAVINGS

3-year energy savings @ $19,732/yr $59,196.00

2-year change -out labor savings@ $3,600.00* $7,200.00

2 sets of bags @ $5,040.00* $10,080.00

1 set of cages @ $20.00* $14,400.00

Total Savings $90,916.00

*Assumes 1-year bag life versus 3-year pleated element life

Unlike many conventional filter elements, pleated elements do not require a filter cake to reach their peak efficiency, which leaves more of the product in the process. The elements are also typically 60 in. shorter than conventional filter bags, increasing the drop-out box by 60 to 90 in. Depending on the product's weight per cubic foot and the collector's opening parameters, 28% to 41% fewer particles reach the element due to the shorter design. This type of load reduction provides longer element life; since the elements do not have to be pulsed as frequently, lower compressed air usage is needed. The elements also feature wider pleat spacing and shallower pleats than conventional cartridges, making cleaning easier and eliminating bridging in the baghouse system.

ELEMENT DESIGNS

The elements can be specified in four different designs to meet a variety of filtration needs. Top-load pleated bags can be built to fit 4.625- to 8.75-in.-diameter tubesheet holes and can handle temperatures up to 500°F. Bottom-load pleated bags come in sizes to fit 4.625- and 5.75-in. elements and are constructed to meet temperatures to 200°F. In both the top- and bottom-load designs, a sonic-welded seam, combined with open helix cores, facilitates cleaning and helps ensure element integrity.

For applications that require multiple tubesheet fits from a single element, all-urethane, heavy-duty, top-load pleated bags are suitable. These elements can be used in temperatures up to 200°F and, at the end of their useful life, can be completely incinerated. This design offers operators of older collectors a cost- and labor-efficient way to meet increasingly stringent collection efficiency requirements.

Finally, for bottom-load filters operating in temperatures from 200° to 500°F, an all-metal, bottom-load unit can be specified. The seal at the interface with the bag cup is preferable to flexible tops or conventional bags because it can be securely tightened without the risk of cracking or fabric impingement. The elements' all-metal construction is combined with a 65% open helix core and mechanical interlocked bottom. The pleat retainers have riveted overlaps and are pleat-anchored, virtually eliminating band separation.

HIGH-TEMPERATURE APPLICATIONS

The type of fiber used in the element can be selected to accommodate a particular setting. For applications up to 375°F requiring high strength, abrasion resistance, dimensional stability and superior dust-release properties, aramid fibers are most effective. Extremely fine particle filtration at temperatures up to 375°F can be achieved with aramid fibers in combination with a PTFE membrane.

For alkaline, mineral and organic acid or solvent applications at temperatures up to 375°F, a polyphenylene sulfide (PS) fiber with superior dust-release properties is preferable. When combined with the efficiency of a PTFE membrane, PS can provide excellent filtration in high-temperature chemical applications.

Fiberglass elements are suitable for high-temperature applications (up to 500°F). Currently in field-testing, these elements are also designed to provide chemical resistance and low moisture absorbency.

Tom Anderson is the national sales manager for Midwesco/TDC, Portland, Ore. He can be contacted at 800/805-5694; fax: 503/977-0176; or e-mail: ANDERSONT@tdcfilter.com

For many facilities, the new equipment needed to maintain compliance has added maintenance issues and production constraints. Plus, equipment installed only 10 years ago to satisfy regulations may no longer be adequate as new standards are put in place.