Comparing Screeners for Bulk Material Handling

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Choosing the right screener for your process and material is important to ensure your manufacturing system runs efficiently with as little downtime as possible. In this article, we’ll explore several types of screeners and the applications for which they’re most suited. Screeners we’ll discuss include gyratory screens, flip-flow screens, trommel screens, grizzly screens, finger screens, vibratory screens, and disc screens.

First, we must discuss why screening is necessary. As obvious as it is to some, there are many manufacturers who forego screening because they do not want the capital expense. Of course, the expense is one of the reasons why manufacturers install screeners: without a screen, foreign materials and/or oversized materials remain in the material stream that can (or will) damage equipment downstream. We had a customer accidentally run a 4×4 through our SMART Conveyor™, for instance, because they had no screening in place. The result was unscheduled downtime, a damaged conveyor gearmotor, and a very expensive 4×4.

Manufacturers also screen to remove oversized materials that will degrade their product quality. Pellet manufacturers and MDF producers, for example, require small, consistent particles. Particles above their target range are unacceptable in the finished product.

For boiler operators, screening is necessary to achieve an efficient burn. Inconsistent material size creates variations in the burn, which decreases the boiler’s efficiency. Fines, for example, will flash off when they reach the boiler, thereby creating a short-lived spike in the temperature. The inconsistency in the temperature affects steam production and the processes in which it’s used (kilns, veneer drying, etc.). Foreign or incorrectly sized materials can also foul up the boiler and increase the number of clinkers.

In chipping/grinding/shredding, screening not only prevents damage to the machine but can increase its efficiency by removing material under the target size. Say you have a chipper rated for 3 tons per hour, but you have ½ a ton per hour of fines going through the machine. In this scenario, you’ve decreased the efficiency of the chipper by a sixth. If the fines were removed, the machine could process more wood or be tuned for the lower volume and consume less power.

There are also legal reasons for screening. The NFPA states that manufacturers shall screen their material before it reaches processing equipment. The purpose of this code is to ensure that objects are removed that could create sparks downstream, thereby igniting a fire or explosion.

There are other reasons to screen feedstock, but suffice it to say, screening is not worth skipping.

As for which screener you should use at your facility, it depends on your material, budget, and what you hope to achieve with your machine.

Gyratory Screens

Screener Motion: Horizontal Gyrations

Purpose: Secondary Screening

Gyratory screens consist of a box that oscillates horizontally and has at least one screen. Multiple screen decks may be utilized. In such configurations, the screens are mounted above each other, the box is suspended, and material is fed into the screen at the top. The screens consist of punched plates or woven mesh, depending on the material being screened.

Gyratory screens are a newer technology than vibratory screens and, in some applications, have replaced them. The advantages of gyratory screens are that their footprint can be smaller than other types of screens, and the oscillation frequency and slope are lower than in vibrating screens. The lower frequency means it is generally easier for technicians to tune the machines to reduce vibrations in surrounding equipment and support structures. The lower slope also increases stay time on the screen, thereby helping ensure a quality screen. The oscillating motion, too, helps ensure a quality screen by spreading out the material as it travels over the screens. This motion also makes these screeners advantageous when handling fragile materials, as they do not bounce or thrust the material upward, as do vibratory conveyors.

Gyratory screeners do tend to be more expensive, however. They’re also more complex than vibratory screeners, though not necessarily more difficult to maintain. They can also have issues with the material getting stuck in the holes of the screener and blocking them (i.e., blinding). In circumstances where blinding occurs frequently, gyratory screens are sometimes equipped with balls that bounce against the underside of the screens to knock material loose.

Flip-Flow or Flip-Flop Screen

Screener Motion: Bouncing

Purpose: Primary Screening

Flip-flow screens consist of multiple, perforated, plastic screens, which the machine stretches and relaxes. As the plastic stretches, the hole size increases, and smaller material falls through. At the same time, larger materials bounce upward and move further down the screen. As the plastic relaxes, the hole size decreases, and the material stays in place, though particles smaller than the holes may still fall through the screen.

Flip-flow screens excel at breaking up material. It’s for this reason that some operations in northern climates prefer them, as the machines can help break up frozen materials. Blinding also isn’t a typical issue with these machines.

Flip-flows are not suitable for fragile materials. These screeners are costly and require more power than comparative screeners. They’re also quite heavy and will usually require their own support to isolate the vibrations.

Trommel Screens/Drum Screens

Screener Motion: Rotation

Purpose: Primary Screening

Trommels consist of long, tube-shaped screens that rotate at a low angle. As the trommel rotates, material rides up the wall (sometimes assisted by lifters) and subsequently falls back onto the bottom as it tumbles down the machine. This action breaks up the material and moves it through the machine.

Like flip-flow conveyors, trommels do a good job breaking up material, but they’re not as loud, and technicians do not have to deal with vibrations or tuning. Depending on their construction, trommels can also stand up to abuse well. They also cost less than other screening systems.

Trommels do take up more real estate than other screeners, though, and they require frequent cleaning, as blinding and plugging are common issues. They’re also less efficient than other types of screens in terms of the volume they handle, as only a small portion of the screen’s surface is utilized at any given time. Furthermore, they’re prone to damage when heavy objects get inside.

Grizzly Screens

Screener Motion: Vibration/None

Purpose: Prescreening/Rough Screening

In their simplest form, grizzly screens are nothing more than a passive set of parallel bars welded at a steep angle. Material is dumped onto the screen, which removes grossly oversized material. These are commonly utilized before trommels.

More complex grizzly screens consist of vibrating bars sloped at a shallow angle. The vibratory motion lifts the material and moves it down the screen.

Grizzly screens are often built on-site, though many professionally designed grizzlies also exist. Vibratory grizzly screens have the added benefit of longer stay time, which creates a better screen. The vibrations also help ensure the material does not get stuck on the screen.

Grizzly screens are often used in tandem with primary screens to remove grossly oversized objects.

Finger Screens

Screener Motion: Vibration

Purpose: Primary Screening

Finger screens consist of long, narrow rods, which are tapered at the end to prevent material from getting stuck on them. The rods are arranged in steps over which material cascades. Fingers can be more closely set at the top of the screener to remove smaller materials, while those at the bottom wider to screen out larger materials. Some finger screens are designed to jostle material up and down as they cascade over the fingers.

Fingers screens are often set in vibratory conveyors and used to separate materials in the C&D industry. They’re also employed in some hog fuel operations. Waste companies like these conveyors because they have no rotating parts around which materials can wrap.

Some finger screens are little more than vibrating grizzly screens. The terms grizzly screen and finger screen are sometimes blurred.

Vibrating Screens & Vibrating Conveyors

Screener Motion: Vibration

Purpose: Primary or Secondary Screening

Vibrating screens consist of an inclined box with one or more screens; either the box or screens are set on springs and are shaken via a counterweight motor. The motor creates vertical oscillations, which move and distribute the material across the screens. As with all oscillating and vibrating equipment, technicians must tune the screen’s supports to match the frequency of the oscillations.

Vibrating conveyors employ the same technology to transport material and are often equipped with screens.

Vibrating screens are quite good at breaking up frozen material, and they are commonly used to screen biomass. They will not break up material as violently as flip-flow screens or trommel screens, however, which is why they are not as common in waste-processing and recycling plants.

Like oscillating screens, vibrating screens can have issues with blinding. They can be noisier than oscillating screens, too, and require more maintenance.

Vibrating conveyors and vibrating screens are available through BE&E.

Disc Screens / Scalping Screens / Roller Screens

Screener Motion: Rotation

Purpose: Primary Screening

Disc screens consist of shafts and profiled discs, which jostle and size material in high-volume applications. Accept sizing in disc screens is determined by the space between the shafts (the slot length) and the space between the discs (the interface opening). Discs on these machines look very different from one application to the next, as they are designed for the specific materials the screen will handle.

Disc screens are usually employed to scalp, i.e., remove overs, in high-volume applications. However, they’re also sometimes used to remove material entering a hog/chipper/grinder/shredder so the machines won’t lose efficiency by processing material under their target size. Due to the volumes they handle and the fact that they’re primary screeners, they’re built (or should be built) heavy to withstand abuse and the contaminants to which they will be exposed.

Disc screens can be designed for various volumetric requirements, from small box units to building-sized machines. They’re employed to screen both many kinds of material.

BE&E manufactures disc screens that are dust-tight when used in conjunction with our SMART Conveyors™. Our screener is belt driven and requires no tensioning. Being belt-driven also removes any chain maintenance.

Learn more about our solutions for screening biomass.

 

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