By Mark Willingham, Purolator Advanced Filtration/Martin Kurz (MKI)/Purolator EFP
With a combination of process viscosities as high as 10,000 poise, temperatures up to 600┬░ F and operating pressures of up to 5,000 psi, the filtration of molten polymers represents one of the most challenging applications for filter manufacturers and users. Adding to this challenge is the fact that most polymer filtration applications require relatively fine levels of filtration, often as low as 10.
Though this application is unique in many ways, the same balancing act of filter life, quality of the product being filtered and filter acquisition cost/change out cost still applies. So, careful consideration must be given to filter media selection as the lowest cost filter media rarely results in the longest filter life or highest quality end product. It is also important to consider the filter change out costs, as these are among the highest costs of all filtration applications.
This article explores the basic types of woven wire cloth and how they are most often used in polymer filtration, with a special focus on Multipor™ woven wire cloth.
Historical and current use of woven wire cloth
Over the years, woven wire cloth has been the leading filtration media for polymer film, fiber, virgin and recycled resins, and specialty resin applications. It’s comparatively low cost and abundant availability, along with the ease of converting this cloth into a variety of shapes and configurations, have contributed to its universal appeal. Along with those attributes, woven wire cloth has a proven ability to remove unwanted particles in virtually all polymer filtration applications. Due to the wide variety of wire cloth weave patterns and micron ratings; custom solutions can be configured to meet virtually any polymer filtration need. There is hardly a polymer processing plant in the world, which is not using some type of filter containing woven wire cloth.
Among the many applications in polymer processing, wire cloth is used extensively in the following configurations and equipment:
- Flat, multi-layer screen packs used in polymer screen changers as well as spin packs in polymer fiber plants. These polymer screen changers are used in blown polymer film, bulk resin, and recycled resin applications, and can serve as the primary filter or a pre-filter. These are typically filtering in the 25-250 range.
- Cylindrical filter sleeves are used primarily in bulk virgin polymer and polymer film plants. These typically filter in the 50-75 range in film plants, and 50- 500 in bulk polymer plants.
- Pleated candle filter elements, which often reach differential pressures as high as 2,000 psi, and are cleaned and reused dozens of cycles before being discarded. These are typically filtering in the 15-50 range.
- Leaf discs, which are used primarily in polymer film applications and are also cleaned and reused dozens of times. These are typically filtering in the 10-40 range.
Types of woven wire cloth
As many articles related to woven wire cloth have been contributed to this publication over the years, the following is a brief summary of the properties, features and benefits of the most commonly used wire cloths below in Table 1.
This broad array of weaves and their respective properties and performance enable filtration experts to create the ideal wire cloth design for each application. For example, if the priority of the end user is finished product quality at any cost, a sacrifice can be made in the filter life to achieve this. Conversely, if filter life is paramount, some compromise in the level of filtration, and thus the end product quality can be dialed in.
Multipor Dutch Weave
A relative new-comer to the world of woven wire cloth, Multipor weave was created to provide performance levels not available in other types of wire cloth. Characteristics and benefits of the Multipor weave include:
- Weft wires are very small in diameter compared to warp wires, resulting in very high porosity and low initial pressure drop. This is illustrated in the below SEM photos comparing Multipor mesh to a Dutch twill weave of the same micron rating.
- Smallest part of the pore is at the surface, between weft wires, creating a surface type media.
- High filtration efficiencies compared to traditional Dutch weaves.
- Available micron ratings of 15 – 90 .
The pore geometry of the Multipor weave differs from conventional Dutch weaves, resulting in particles being trapped on the surface between the weft wires of the weave. This facilitates the cleaning of the Multipor media, whether it is during the back-flush mode in advanced screen changers or during external filter candle element cleaning to remove contaminants. More information on this below.
The higher porosity of the Multipor mesh leads to greater dirt holding capacities, as illustrated in the chart below. In multi-pass testing using ISO coarse and fine test dust, Multipor media delivered up to ~ 10% higher dirt holding capacities than 165 x 800 standard Dutch weaves. Depending on the particle size distribution present in the actual application, this ~ 10% increase in dirt holding capacity could translate into a ~ 10% longer on-stream life. This feature is of particular importance when Multipor media is used in spin packs, as the cost to service a spin pack is typically 10x the cost of the filter media itself.
In the area of filtration efficiency, Multipor mesh demonstrated significantly higher removal efficiencies across a broad range of particle sizes in a standard ISO multi-pass test. The chart below also includes a comparison to non-woven fiber metal felt (with the Purolator Porofelt┬« trade name). Nonwoven fiber metal felt such as this is commonly accepted among users as having the highest dirt holding capacity and efficiency, though it does carry a price premium over Multipor wire cloth.
Cleaning of Multipor wire cloth filter elements
As with any other filter media used in polymer service, the first step is to remove the polymer. Depending on the polymer type, this is accomplished with a solvent soak, oven, salt bath, steam/hydrolysis/oxidation device, or a combination of these. Whichever method is used, it must be compatible with the alloy-typically 304L/316L stainless steel – and temperatures above 800┬░ F are usually not required or recommended.
When used in pleated or cylindrical polymer filter elements, Multipor media provides exceptional clean-ability. As noted above, particles are trapped on the surface of the Multipor media and are easily removed by water spray (inside to out, and outside to in), supplemented with ultrasonic cleaning if required.
Multipor wire cloth filter elements can be tested in the same manner as other polymer filter elements, typically using a standard bubble point and open air flow tests. Due to the higher porosity and lower pressure drop of Multipor media, the pressure drop exhibited during the open air flow tests may be lower than comparable micron ratings of other weave types.
Sintering of Multipor wire cloth
To provide additional strength and pore size stability, a sintered version of Multipor media is also available. Here, 2-3 layers of support wire cloth are permanently bonded to the Multipor media to provide the strength of a sintered wire cloth laminate and maintain pore size stability across a wide range of pressures. This media is available in cylindrical or pleated filter elements.
Multipor wire cloth media provides a unique combination of high porosity with a corresponding low pressure drop as well as high efficiency and dirt holding capacity. This blend of performance attributes makes it exceptionally suitable for use in polymer filtration service, in filter elements as well as screen packs.
For more information contact:
Purolator Advanced Filtration