Central Dust Collection Complicates Multi-Cabinet Systems

Large plants often rely on one central system to pipe compressed air to all air tools and blast equipment, because doing so can be more efficient than operating compressors at several locations within the plant. This example often leads plant engineers to believe that a central dust collector can provide the same efficiencies. The difference: a compressor keeps a large reserve tank pressurized, cycling on only when air demand in the plant takes the reserve below a predetermined pressure; but a dust collector has no vacuum reserve. It must run continuously, whether it’s supporting all the blast cabinets in the plant or just one.

For applications where all the cabinets do not routinely run at once, using dedicated dust collectors for each cabinet reduces energy and media consumption and increases overall efficiency.

When It Causes Problems

The explanation: If the collector is sized to exhaust dust and air from 10 cabinets and only one is running, it will pull too much air through that one cabinet. This can carry good media into the collector and spoil the working mix. In shot peening applications, the inability to maintain the proper working mix can cause wide fluctuations in peening intensity and coverage.

Insufficient inlet openings will leave dust and media to build up in the cabinet sump. An open door in one or more of the other cabinets reduces the amount of air drawn through the operating cabinet(s), thereby reducing the amount of media conveyed from the sump to the reclaimer. And, every time doors open or close on any other cabinet, airflow through the other cabinets will fluctuate, affecting media cleaning.

Having several blast cabinets tied to one dust collector creates other problems as well. (1) Keeping the vacuum properly balanced may require a complex system of baffles, dampers, air make-ups, ductwork, and auxiliary blowers. Keeping everything properly tuned can turn into a full-time job. (2) When the central dust collector needs repair or maintenance, all blasting stops.

When It Does Work

For some applications, a central dust collection system does make sense. If the plant already has a busy blast room with a dust collector, connecting one or two small cabinets to hand the occasional small parts run can increase a plant’s capacity at the cost of the cabinets only. Because the blast room dust collector will draw much more air than the cabinet reclaimers can handle, it’s necessary to install a high-quality damper in the duct work to control the flow of air. If a plant operates several automated or manual cabinets more or less continuously, a central collector can save on the initial purchase, save space, and conserve energy.

A final word of caution – never connect blast equipment to a dust collector serving welding or grinding equipment. Some types of dust from blasting may ignite if exposed to sparks or hot slag particles.

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David  Dorrow

Mineral Aggregates Inc.

Back when I was a youthful dad, I took my two young daughters down to the creek behind our house to teach them how to fish. When we opened the tackle box, their jaws dropped as their eyes scanned the many lures neatly spread out on the bottom of the box. Big and small, hard and soft, some lures were heavy, to fish on the bottom, or light, to float on the surface. My girls asked, “Daddy, which one is the best?” With a wink, I said, “They are all the best! It just depends on what fish you’re trying to catch, the day’s conditions, and where you’re going to fish.”

Author with large mouth bass caught with the best lure for the “job” Photos courtesy of the author.

If you asked me a similar question about picking the best abrasive product for a job, I would give a similar answer. “It depends.” It all comes down to project parameters, surface conditions, and your expected outcome. Before selecting the “best” abrasive, you must answer several questions about the surface preparation project.

• What is the current surface condition: adhering paint, a brittle coating that is peeling, or mill scale?

• What are the goals and expectations after blasting? For example, is the surface being prepared for a new coating or cleaned to create a uniform visual finish that will be left uncoated?

• Will the surface profile need to meet the specification for the coating system or is it more important that the abrasive blast at fast cleaning rates?

Before selecting the best abrasive for a project, you must understand the characteristics of abrasives and how they affect the resulting finish. Like fishing lures, abrasives come in many sizes, hardnesses, shapes, and densities. Each abrasive’s characteristics will affect the blast cleaning process and final results.

Particle Size

Fine iron silicate abrasive (copper slag) under magnification

The size of the abrasive particles affects both the productivity and surface profile. Decreasing abrasive particle size can dramatically increase cleaning rate, with more particles impacting the surface per unit time when compared with the use of a coarser abrasive. However, increasing abrasive size may be necessary to remove heavy coatings and scale. The general rule is: “Use the smallest size abrasive particle that will do the job.”

Coarse abrasives generally leave a deeper and less uniform profile than finer abrasives. If a low profile is required, choose a finer abrasive. Conversely, use a coarse abrasive for a heavier profile. The normal tendency is to use a very coarse abrasive because it will knock off the paint, rust, scale, and other debris. But a coarse abrasive sacrifices coverage or cleaning rate. If a smaller abrasive will work just as well, use it because it will greatly increase cleaning rate.

Hardness

Extra fine staurolite abrasive under magnification

It is generally believed that the harder the abrasive is, the better it will perform. Very hard abrasives, however, tend to shatter on impact, expending most of the energy in particle disintegration and dust generation. A softer abrasive will transmit all of its energy to the surface, clean faster, and minimize dust. Similar to selecting abrasive size, select the minimum abrasive hardness that will effectively do the job. Hard abrasives will remove all coatings and surface contaminants and impart a profile on the surface. Very soft abrasives such as walnut shells and corn cobs are used to remove oil and grease and sometimes paint, leaving the existing substrate intact. In this case, the abrasive should break down on impact, carrying away the contaminants. If the intent is to remove only the coating and not disturb the substrate, use a slightly harder, friable abrasive such as soda ash, dry ice, or plastic pellets.

Shape

Productivity test, conducted under third-party guidance, on heavily pitted and rusted steel structure at a steel mill. Round and angular abrasives were tested for productivity and usage.

If the steel surface has a soft, pliable coating to be removed, an angular abrasive will be more productive than a rounded abrasive. Conversely, if the surface has a hard, brittle coating or mill scale, a rounded abrasive is preferred to pop off the coating or scale rather than pick away at it with a grit-like abrasive. An angular abrasive creates an angular and generally not very uniform surface profile. You will get a less angular but more peened surface if you use a rounded particle, such as steel shot or a heavy mineral sand (e.g., staurolite and olivine, which are naturally occurring and low in free silica). Both surfaces are acceptable, but different in appearance. Generally, angular particles work best when removing soft, pliable coatings, whereas shot or rounded particles are more effective in removing hard, brittle coatings (often aged) and mill scale. A mixture of both particle shapes is recommended for some jobs. Angular abrasives are generally used for coatings and rust removal; rounded sands are often used to remove mill scale from bare steel.

Density

Abrasive density can have a major impact on productivity. Generally, the higher the density, the better the productivity. Higher density particles impart more energy to the surface and, therefore, do more work. Application rate is the amount of abrasive required to achieve the level of cleanliness required. Generally, the denser the abrasive, the faster it will clean; thus, the denser abrasive will have a lower application rate compared to a less dense abrasive. As the mass or specific gravity of a particle increases, so does the amount of work being done by the particle. Therefore, if you change from a sand abrasive with a specific gravity of 2 to a garnet abrasive with a specific gravity of 4, you should expect an increase in cleaning rate because the higher specific gravity particle will do more work. If all else is held constant (such as nozzle pressure and particle size), productivity will increase with the garnet.

Preliminary blast cleaning trials using several different abrasive products with different sizes usually can determine the most productive abrasive product for the surface conditions. This is especially true for large surface preparation projects where productivity improvements can translate into huge increases in profits. To perform a productivity test, mark a section of the surface into grids, blast the section, and measure the area cleaned. Calculate the time required to blast the area and the amount of abrasive used. This will give you all the metrics required to calculate your surface preparation costs.

As with fishing lures, most people already think they know their “best” abrasive because they have been using it for years. However, to be a true professional, you must be willing to experiment, change, and adapt to the surrounding conditions. Surface conditions have a major influence on the type, shape, size, density, and hardness of abrasives, and you must to be knowledgeable in the art to pick the best.

And don’t forget: Abrasive blasting and abrasives are subject to regulations for environmental and worker protection. Regardless of the abrasive and cleaning method for a job, you must comply with all relevant regulations.