By Justin Marriott
Product Manager - Key Filters
In one of Parkinson Technologies’ previous articles, we discussed the use of reverse Dutch twill weave (RDW) filter belts used in ribbon-style continuous belt screen changers. We discussed the four basic types, how they differ, and how they evolved into the RDW style which offers the combination of strength and durability required for continuous belt screen changers. However, there are other factors to consider when choosing screen. The RDW weave pattern is one of them.
RDW is most commonly produced in three types of patterns: Straight Twill, Broken Twill, and Chevron Twill. Each of these patterns generally will have the same porosity for a given mesh size, but will have variations in the resulting torque that is applied to the screen. During the weaving process, pressure is applied to the wires increasing the torque in the screen, this additional torque will pull the screen to one direction or the other, introducing some skew. Skew in a screen ribbon could potentially cause problems in a continuous belt screen changer by binding and not allowing the screen to pass through freely. The difference between the weave patterns is as follows:
- Straight Twill: The least expensive to produce it alternates the over-under (2 or 3 wires) in a regular or repeated manner, this gives the appearance of a straight diagonal weave. Torque will be the highest in this weave resulting in more skew compared to broken or chevron patterns.
- Broken Twill: Reverses the over-under pattern within a defined distance (example a 50mm broken twill reverses the pattern every 50mm); this reduces material torque when weaving over straight twill. Torque will be neutralized each time the pattern is reversed which will tend to track the ribbon one way then back to the other reducing the overall skew in a single direction. This is the most commonly stocked pattern for Key Filters continuous screen applications because it effectively balances performance and economy.
- Chevron Twill: Uses a repeated reversal in the pattern (every 5 or 10 wires) which gives the very noticeable “herringbone” pattern. The pull on the wires when weaving is constantly neutralized by the reversal essentially leaving neutral torque throughout the weave. This will not have any pull to one side and will result in a straight ribbon. This type of pattern will be the most expensive to produce, and while the mesh size is guaranteed, there is a possibility that when the herringbone pattern intersects you can have a larger micron size at that intersection point then you would in the rest of screen.
As you can see, the screen will behave differently depending on the weave pattern design. Choosing the right pattern is important for any melt filtration application. However, a high percentage of applications will often use the Broken Twill pattern which Parkinson Technologies readily stocks.
The weave pattern is not the only factor when choosing screen. In our next article, we’ll discuss mesh size and how pressure and the particles to be removed need to also be considered when selecting the correct size.