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Filtration is a key element in fluid systems because it removes unwanted particles from the liquid in order to maintain the integrity of the product and keep the system clean. The degree of required filtration depends on the sensitive nature of the process. If debris can cause defects or damage, then filtration needs to be increased.
In sensitive applications, such as papermaking, a general rule of thumb is the farther along in the process you get, the more expensive the filtration becomes. This is a result of the need for finer retention sizes and more robust cleaning methods as particles and debris are smaller in size.
As the cost increases, the motivation to incorporate the right type of filtration technology increases as well.
Many papermaking applications require a surface treatment for paper, where they apply a coating. If the coating contains particles or fibers, which they normally do, surface defects may occur and reduce the quality of the end product. (Read more about filtering coatings: "Blinding and Blow Outs. When A Good Self Cleaning Filter Goes Bad")
Much of the debris in the coating process is made up of fibers, which is difficult to filter and remove due to their long shape and flexible nature. The correct filtration element should be able to screen round particles, trap long fibers without tangling them, be sturdy in the process, and be easy to clean.
The available filter media choices include fabric mesh, wire mesh, perforated screens, and slotted wedge wire elements. The mesh options prove to be troublesome as they allow the fibers in the fluid to “staple” themselves to the media material during backwashing, or reversing the fluid flow in order to clean the filter media. This phenomenon occurs when a fiber lies across two wires, held there by the entering flow, and remains there as the flow reverses for cleaning. Since the wires have small diameters, the fibers may be long enough to wrap around multiple wires with the flow reversal, causing them to bend on both ends like a staple bends around paper. Ironic, isn’t it? The same can go for perforation too.
This leaves slotted wedge wire elements as the optimal choice. The construction of this filter media is made up of wires with triangular cross sections that are supported by metal braces. These excel with fibers because then a fiber drapes over the wedge face, it is very difficult for the ends to tangle on anything else. When the backwash occurs, the fibers should be free to detach from the filter and make their way down the drain. This design makes filters much easier to clean, reduces the pressure drop caused by clogged filter media, and ensures that more fibers are removed from the system.
It goes to show that you can play favorites when it comes to selecting filtration equipment for the paper industry!
Searching for the right filtration technology for your application? Ask us about it! We gladly provide technical assistance to businesses in Wisconsin and upper Michigan.