Cement Plant Dust Collection

Although cement dust is relatively coarse and has optimum agglomeration properties, cement plants have some of the most demanding applications in the dust collection market.

First, all cement dust is dense and very abrasive. Even in the relatively less demanding application such as bag loading and bag unloading, these properties are important.

For many years, on the relatively light loading applications, shaker collectors were the preferred dust collectors. In fact the envelope collectors were very effective. The release of cement dust from shaker collectors was generally not a problem. In cylindrical bag collectors, the dust collected on the inside of the bag. The fastest moving air with its dust load was at the opening in the bag. There was a potential for wear near the bag entrance. If a bag developed a hole, the dust coming out of the tear would quickly abrade the surrounding bags. It seldom was possible to detect leaks until several bags, or even all the bags were damaged.

To filter a continuous process, off-line cleaning was necessary. On larger units, the collectors, in parallel sections or modules, were isolated and cleaned while maintaining flow through the remaining sections. On cement mills and other high load applications the collectors were usually preceded by cyclone pre-cleaners and the collectors often had dropout boxes or settling chambers. On these applications, shaker filters ran at filter ratios below 2:1. Air horns were often added to shaker collectors to improve cleaning and increase dust holding capacity.

In the late 60's the Fuller Corporation introduced their version of the pulse collector. This compartmented collector had large diaphragm valves that discharged into the clean air plenum. The burst of air agitated the filter bags, producing a thorough cleaning. More dust collected on a unit of filter area to allow handling of heavier dust loads. The name of this collector was "Plenum Pulse”. It was able to handle the heavier load for applications such as raw mills and finish mills. The first installations vented clinker cooler vent systems. A measure of the efficacy of this cleaning system was that the clean air plenum was built with heavier gauge metal because of weld failure with 12 gauge construction. While this collector was touted to be a pulse-jet collector, it was not. The compressed air did not create a reverse air jet. The collector operated more like a shaker than a reverse jet design. It produced more energy to clean the bags than existing shaker designs. This was especially necessary on the heavier loading processes.

Another effort to handle this high loading was the combination of shakers and reverse air cleaning systems, provided by "Norblo" a company that was based in Cleveland Ohio.

However in the late sixties, MiKroPulverizer (later renamed MikroPul) and their then licensee, FlexKleen, introduced the reverse pulsejet collectors. They were first applied to vent the raw and finish mills in the cement production process. The reverse jet collector was developed by MikroPul for their Pulverizers. These were similar to cement mills and were able to handle the heavier loads of the cement mills without pre-cleaners.

These collectors ran quite well, with filter lives exceeding three to five years. Then in 1969, when MikroPul's patent was challenged and declared invalid in court, many competitors copied their design. To counter the imitators, they made a major change in their design. The bag length went from six to ten feet. The pulse pipe orifice area was increased by the same ratio. Unfortunately, the venturi diameter was not changed. The velocity of the cleaning jet increased. It went from 15, 000 fpm to 25,000 fpm. Nobody recognized that the venturi velocity is proportional to the velocity of the dust leaving the bag towards adjoining bags during cleaning. In a dust like cement, it causes partial blinding and abrasive wear on adjoining rows during the cleaning pulse. Average pressure drops increased from 3 to 5.5 " wc. Bag lives were reduced by 50-60%. Compressed air usage went from about 0.5 SCFM per 1000 cfm of filtered air to 1.2 SCFM per 1000 cfm of filtered air. To counter these effects, the industry made some basic changes in selecting pulsejet collectors. The changes treated the symptom rather than the cause.

• Filter ratios were drastically reduced. Bag life was increased and pressure drop was decreased to the 5 inch w.c. range.
• Pressure actuated cleaning systems were introduced. This kept the abrading and blinding of bags from cleaning pulses to a minimum. (The cleaning frequency and air consumption per 1000 cfm of filter air remained much higher than with the old six foot long bag designs.)

The development of the ULTRA-FLOW advanced technology design was able to remedy all the shortcomings of the 10 foot bag design. These designs will be discussed in future papers of this series.

Generally, the operators have chosen very conservative air to cloth ratios. Cartridge collectors are quite effective on less demanding applications.

Application Engineering Data Filter ratio - Cartridges
The belief was that more filter media (and associated filter ratio) made the selection more conservative. This idea is firmly entrenched in the cement Industry. This is generally true with pulse jet fabric collectors with high velocity cleaning jets in that it extends bag life. The fact is that the opposite is true with cartridges because of bridging across the pleats if the media is not cleaned. As dust accumulates in the valley of the pleat, it bridges. During the cleaning cycle the cleaning air looks for the easiest path from the inside to the outside of the filter cartridge. That path is above the bridge.
What contributes even more to this phenomenon is the fact that a certain volume of reverse air can only clean a certain amount of media. Because the cartridge may contain huge amounts of media that cannot be cleaned, the media not cleaned will plug. In most designs running at filter ratios of less than two, an operating cartridge may contain 10 to 40 pounds of dust. Table 1 illustrates the area of media that can be cleaned with various orifices and /or converging diverging supersonic nozzles.
Table 1
Orifice / Nozzle dia = area of media cleaned
0.250 in. = 5.5 / 8.8 sq.ft.
0.312 in. = 8.6 / 13.75 sq.ft.
0.375 in. = 12.3 / 19.8 sq.ft.
0.500 in. = 22 / 35 sq.ft.
0.750 in. = 49.5 / 79 sq.ft.
1.000 in. = 88 / 132 sq.ft.
1.500 in. = 198 / 376 sq.ft.
There are new advanced technology cartridge style dust collectors available today which incorporate wide pleat spacing, vertical cartridges and supersonic nozzles.

Dust Collector Selection
The best designs for a fabric media pulse jet collector on these applications are those offered by ULTRA-FLOW with low jet velocities and higher filter ratios The characteristics of these designs are listed below:

1. Average velocity at bag opening = 10,000 feet per minute
2. Bag opening (no venturi) = 4" diameter
3. Jet volume = 740 CFM
4. Bag diameter and length = 4 inches x 96 inches
5. Bag area = 10 sq. ft.
6. Filter volume rating per bag = 190 CFM
7. Nominal filter ratio = 20 FPM
8. Average pressure drop = 2 1/2 inches water column
9. Average Air Consumption = 1/2 SCFM/1000 CFM of flow
10. Average dust penetration at 5 gr./cu. ft. load = 0.0005 gr. / cu. ft.

Conclusions: The best dust collection choices are a fabric collector with low jet velocities and a high inlet. The filter ratio is dependent on what the customer will accept, but ratios of over 18:1 are easy and reliable. Some modifications must be made to the inlet baffle because of abrasion.

Find out about Ultra-Flow ... Advanced Technology Baghouse Dust Collectors