In today’s ever changing aggregates market, screening efficiency and performance are being challenged as the screens are being required to do more screening in the same footprint due to a demand in smaller aggregate. A more balanced market was required in the past, with much more material being required for base (1 ¼”-). Today’s market is predominantly geared toward the asphalt and concrete markets with specialty mixes. The concrete industry is also requiring a smaller product, as more of it is being pumped, requiring a smaller aggregate size. So how do we get more out of our screens?
More Screening Surface Needed
These current product mixes of smaller aggregate material (1”-1/4”) require much more open area within the screen to obtain the desired results. The problems caused by this are twofold:
- The bed depths of the screens are sometimes twice the acceptable bed depth, creating excessive wear and too much carryover of in-spec product;
- Screen decks can become blinded over due to the excessive bed depth and moisture of the material.
Some solutions to these problems are to monitor your bed depth and ensure that it is not four times higher than the opening of the screen cloth.
Monitoring your crusher setting will also greatly help in keeping material in spec. Daily inspections of your crusher setting (and sometimes multiple in abrasive applications) will keep the crusher at its optimal setting and aid in proper screening.
Many operators are reluctant to run a screen uphill, but this extra retention time on the screen can greatly increase your throughput and decrease any carryover of undersized material. Running a screen uphill allows the material more time to find an opening to pass through at a time when the particle is positioned in the correct spot. As long as material is not building up in the crusher feedbox and the screen is performing, then running a screen uphill is a viable option.
Stroke and Speed of Screen Utilized for Size of Material
The stroke and speed of the screen are crucial to ensure that the size of the material being screened has enough g-force or speed of revolution to be screened adequately. The speeds can vary as much as 1,000 rpm for ¼” material to 600 rpm for 6” material.
Routinely check your stroke and rpm to ensure, especially when changing product sizes. Contact your manufacturer to determine the correct speed and stroke for the material you want to screen.
Screen Media Selection
The selection of screen media is extremely important when deciding what is best for your screening needs. There are three types of screen media that should be considered:
- Wire cloth - This is the most widely accepted screen media in the industry. It is the easiest system to change out and provides more open area to allow for your screen to run as efficiently as possible. Wire cloth comes in various diameters, but remember: The larger the diameter of the wire, the more open area you lose to screen.
- Urethane media - This has become extremely popular in the last 15-20 years and is very effective in wet and abrasive applications. In dry and fine screening applications, urethane may not be your best option. Urethane media has much less open area than wire cloth and the economic advantages of the media lasting longer must outweigh the lost output of tons per hour.
- Hybrid or flex type screen media - This has also become popular in high moisture applications where fine screening is needed. Blinding is when the screen becomes clogged between the openings and creates a solid area where no screening occurs. This media is not woven together but is held together by urethane or rubber strips that allow it to flex and eliminate the blinding effect. In fine, wet applications, the increase in production can be significant.