Big Energy Savings & Maintenance Convenience Driving Trend Toward Multiple Small-Collector Systems for Sugar Cullet Collection

by Dale E. Tompkins, glass industry application engineer — Kirk & Blum

Ineffective cullet dust collection can drag down a glass plant's performance and productivity. If not properly handled, cullet dust (or "sugar cullet") developed from crushing operations can drift and settle on finished product, scratching surfaces and profits. Large central cullet dust collection systems — traditionally designed around one or two large baghouse units — were a high-maintenance, high-cost necessary evil used to control the nuisance dust. Usually located outside a plant and fed by massive trunklines, these energy-draining collectors require up to 200-hp fans running continuously regardless of air supply requirements. Maintenance on baghouses is cumbersome and time consuming, and requires the entire dust collection system to be shut down while work is done. Inherently inflexible, central collection systems also hamper future expansion, can be cost-prohibitive to expand and often require costly, major reworking to accommodate inevitable process changes.

Drawbacks such as these are driving the trend toward modular, multi-collector system designs using numerous small collectors located close to dust pickup points. Multi-collector systems, using maintenance-friendly cartridge-style collectors instead of traditional baghouse collectors, provide:

• greatly decreased energy cost due to lower horsepower requirements;
• reduced system maintenance downtime;
• smaller duct length and diameter;
• flexibility for future system expansion; and
• better overall dust control.

On a recent rebuild for a large building products manufacturer in Laurinburg, NC, Kirk & Blum, designer, fabricator and installer of process ventilation systems, engineered a modular dust collection system for the flat glass producer to handle increased dust collection requirements which would have overloaded the existing large baghouses. Since completion, the amount of fugitive sugar cullet has been negligible, the plant is getting effective dust collection even while system maintenance is performed, and plant is realizing $71,855 annual energy savings.

Case in Point: New crushers, more dust, modular systems cuts fan hp 38%

In response to costly and time consuming cullet conveyor system breakdowns, the glass plant installed a new-technology glass crushing system. Cullet, developed by crushing flawed as well as thickness- and color-transition sheet, falls onto conveyors located in a pit below the main floor for recycling back to the furnace. With the manufacturer's old cullet conveying system, pieces of glass simply fell through hoppers, crushing hoppers, edge trim hoppers or mechanical crushing hoppers directly onto the conveyor's rubber belts. Large falling cullet caused belt damage and occasional hopper back-up, leading to costly and time-consuming maintenance shutdowns.

To remedy this situation, the manufacturer purchased new technology crushers, crusher hoppers and vibratory pan crushing conveyors to reduce the size of cullet falling on the conveyors. The new system produces smaller-sized
pieces of glass, reducing conveyor belt damage, but also creating a greater amount of sugar cullet. An unwanted ingredient composed of sand, soda ash, limestone, gypsum and other ingredients, sugar cullet poses no environmental or OSHA problem, but is considered a nuisance dust. It clings to conveyor belts, causes dust piles and clouds at belt wipe brushes and beater bars, and can scratch finished product if allowed to escape out of the pit and drift onto the plant's main floor.

Modular System Design

Deviating from traditional central-collector system design, K&B engineered a modular dust collection system around six small cartridge-style collectors placed in the plant close to dust pickup points, and one large unit located outside the plant because of floor layout restrictions. Placing collectors close to pickup points decreased duct diameter and length as well as horsepower requirements for the fans. In most cases, fans on the small collectors were placed on the collector roof or sidewall to save floor space. Total horsepower requirement for the seven motors used for this rebuild was 325hp — 200hp less than if all collectors would have been located outside. This provides the plant with an annual energy cost savings of $71,885, covering the dust collection system cost in eight years, and realizing money-in-pocket savings from that point on.

Unlike airhandling systems for woodworking or mist removal applications,
thick-gage duct was used because of the abrasive nature of cullet particles and
dust. Many sheet metal contractors shy away from heavy gages to keep bids
low or because they simply cannot handle the thicker sheetmetal. However, skimping on duct gage to initially save a few pennies ends up costing more in
the long run with repeated duct removal-and-replacement maintenance operations. Sufficient abrasion resistance — especially for elbows — is critical for long-lived system performance for glass industry dust handling applications. Typical duct thicknesses for this project were:

• 14 gage-minimum straight sections;
• small elbows of 1/4" abrasion resistant plate;
• large elbows of 10 ga. abrasion resistant plate.

Cartridge Convenience

Cartridge-style collectors were chosen by K&B because of their reduced size, ease of maintenance and lower overall cost versus large bags. Cartridge filters have a protective metal wrap which shields against pieces of cullet that may be picked up by the system. The filter element is further protected by a metal baffle located above each row of cartridges within the collector, which deflects incoming cullet away from the cartridges.

Smaller cartridge-style collectors are more accessible and easier to work on than large baghouses. For example, horizontally mounted cartridges in the largest collector in this system (96 cartridges) can be changed by one person in one or two hours, with small four and eight-cartridge collectors taking one person only five minutes to change. Bag replacement in a baghouse requires two or more workers at least one full day, leading to increased maintenance costs and prolonged system downtime. Individual collectors in modular systems can be shut down and repaired while the other collectors remain online, providing constant dust handling.

Cartridge change is also a much cleaner operation. In the event material (More)
discharge points are not emptied on schedule, cartridge collector hoppers —
being smaller — are easier to clean out, and cartridges are also less likely to be damaged by such neglect. To remove dust collected on the outside of the cartridge while the collector is online, each cartridge row is alternately pulsed with compressed air, saving on compressor usage versus some baghouses which continuously send air through bags to remove material coating the bags.

Modular Benefits

Modular collector systems are ideal where dust collection is not always needed for pick up points far down the line or crushers that do not run constantly. Multiple unit collectors can be tied into conveyors and run only when needed, resulting in additional energy savings. For situations where glass can travel down multiple legs but not down two adjacent legs at the same time, diverter valves or butterfly dampers isolate the leg needing air, thus lowering the overall system air volume requirements.

In nearly every case, dust collection needs will change with future system expansion. With large-collector systems, it is likely that the entire system will need extensive reworking. With smaller modular systems, fans and collectors can be easily added or relocated. And if duct is to be scrapped, it is more cost-effective to throw away smaller diameters as opposed to massive sections of trunkline. Smaller-diameter duct is also more apt to be reused for other applications.

Cost/Value

Price is often the driving factor behind such systems. Typically, sheet
metal work is thrown into a mechanical package that includes civil, electrical and piping work. It is most often considered a minor item to be handled by the
lowest bidding sub-contractor regardless of experience or engineering capabilities. In this case, the glass manufacturer had the foresight to focus not
only on initial costs, but on the total cost savings it would realize having a system engineered by K&B to provide years of low-maintenance service, be more conducive to future growth and expansion, and require less energy to run.

Cartridge Versus Bag Replacement

Bag change in a typical baghouse is a time-consuming and messy operation, often taking a two-person team an entire day to complete. First the bag and its support cage are removed and taken outside the collector. The bag — in lengths to 12' and completely coated in sugar cullet — is ripped off the cage and disposed of, and a new bag installed over the cage. The bag is then brought back into the baghouse and reinstalled.

Cartridge replacement is much easier and cleaner. The cartridges are mounted horizontally, two-deep in tubes within the compact collector. Unscrewing a large wingnut allows the tube cap and cartridges to be removed. Cartridges literally slide out thanks to the collector design in which tubes are angled upward in the back. Installing cartridges, cap and wingnut completes the job. The largest collector for this rebuild has 96 cartridges, and takes one person one to two hours to change cartridges. Small four- and eight-cartridge collectors take one person a mere five minutes for changeout.