In this post, we’ll describe how to choose the woven wire screen that is optimal for you, including choosing the best method for determining screen opening size, wire diameter and attachments, as well as best practices for vibrating screen maintenance.
Screening / Screener Basics
A screener is also called a separator or sifter, and its purpose is to separate dry, free-flowing materials of various particle sizes through vibration. The vibrating action separates the particles, discharging the smaller unwanted materials through “holes” of specified shape and diameter in the screen and retaining the larger particles for further processing. The screens are attached to a frame within an assembly, called a screen deck.
They scalp material, remove fines and grade materials. For any of these uses, choosing the right screen is the difference between achieving high quality, efficient separation performance and inefficient, poor performance.
Choosing Your Screen Type
The opening size is the number one criteria for selecting screens because it impacts the separation quality of the screening process. However, when choosing the opening size, you must first know what the specifications are for the final product, and that depends on whether you choose to use the screener for scalping, fines removal or grading:
- Scalping. In the scalping process, the screener consists of a single screen and “takes a little off the top” — it removes a small amount of large particles from the surface of a material that’s almost entirely within a particle size spec (or on-size). Typically, the material that enters the screener contains 95 percent or more on-size particles. Work backward from the final product spec to arrive at the proper screen opening size. For example, if the sieve has a 250-micron opening size, then you should select a process screen with an opening that does not exceed 250 microns. Recommended practice is to choose a screen that has an opening size smaller than the opening of the specified test sieve because of variations in the wire-weaving process (called weaving tolerances).
- Fines Removal. Choosing a screen is more complicated when removing fine particles, especially if the screener has to remove very fine particles. Due to the inherent inefficiencies of screeners, it is not possible to remove all fine particles, so it is important to allow tolerances when creating specs for fines-removal. For example, a reasonable spec for allowable fines in the final product is five percent maximum, which means that no more than five percent of the sample should pass through the test sieve during evaluation. To achieve the five percent maximum – choose a market-grade screen with an opening size somewhat larger than the desired particle size spec. This will enable near-size fine particles to pass through the screen and make it more likely that the final product will meet allowable fines tolerance. A cautionary note: If the opening size is too large, large numbers of on-size particles may start passing through the screen and discharging with the fines.
- Grading. In grading applications, also called sizing or classifying, the screener separates matter into distinct discharge streams via the use of multiple stacked screen decks. Each stream has its own particle size distribution, and typically one or more of the discharge streams constitute the final product, which usually has a spec limiting the amount of oversize or fine particles. For grading applications, choosing the right screen opening size for a replacement screen is especially critical because any change in the opening size can affect not just one, but multiple final products. Different opening sizes will affect the particle size distributions of the final products, which in turn affect fines content, product yields and production rates. Therefore, it’s best to choose the screen opening size based on the results of screening tests conducted at the screener manufacturer’s laboratory.
Choosing The Best Wire Diameter
Screener capacity, screen flexibility and screen durability are important considerations when determining which wire diameter is best for your screen:
- Screener capacity. Selecting the correct wire diameter has a major impact on your screener’s capacity. It affects the screen’s open-area percentage and therefore, how quickly material is screened. Typically, the larger open-area percentage will yield higher depending on the capacity. Let’s look at two screens with the same dimensions and opening size: an 80-mesh large wire diameter market-grade screen with a 178-micron opening size, and a 94-mesh small wire diameter TBC screen with a 180-micron opening size. The first example has a 31.4 percent open area, while the second has a 45 percent open area. So, the 94-mesh TBC screen with a smaller wire diameter has 43 percent more opening than the 80-mesh market-grade screen for the same screen dimensions, resulting in a much higher screening capacity.
- Screen flexibility. A smaller diameter screen also tends to be more flexible, which makes it more resistant to blinding. Blinding occurs when particles that are slightly larger than the screen opening become stuck in the screen. This phenomenon mostly occurs in sizing applications involving materials containing high percentages of near-size particles. Screens with larger diameter wires are more likely to trap a particle stuck in the opening, making it more difficult to dislodge. A smaller diameter screen tends to yield a little so its natural movement can easier dislodge the particle.
- Screen durability. Despite the capacity and flexibility advantages of smaller diameter wire, there is a downside. They are less durable, which translates into short service life in some applications that handle larger, heavier material loads or abrasive materials. In these situations, it might be more practical to select a large wire diameter screen, but if there are concerns about blinding problems, the best option might be a screen with a smaller wire diameter, although you’ll need to replace it more frequently.