Selecting the right filter media for a process can be tricky, according to a recent blog post published by Micronics’ Engineered Filtration Group. Choosing the right filter fabric is a matter of balancing factors that can include the type of filter, cake release, cleanability, particle size, particle size distribution, particle shape, chemistry/temperature, and overall load.
The type of machine determines the overall strength needed for the filter media. Tensile strength, elongation properties, flexibility, and durability of the cloth are all important considerations in filter cloth selection. Different types of filters hold and pull the fabric in different ways. Some fabrics may work well with one type of equipment, but not with another because of how it stretches, bends, and holds up to stress in the application.
Consider how the cake is discharged from the filter. A dry cake may release differently from a sticky or slimy cake. Every type of cloth has different cake release characteristics. Some cakes may need to be released with help from a scraper blade. If a scraper blade is used to release the cake, the cloth may not need to have high cake release properties but will need to be stronger to handle abrasion from the scraper blade.
How clean does the filter need to be? Some processes may require the cloth to be cleaned after each cycle or during the process. How is the fabric cleaned, and what is used to clean it? A spray bar may be used to clean cloth on a filter but may also create additional abrasion and wear on a cloth.
Particle size is important when selecting filter cloth. Different fabrics are equipped to handle different particle sizes. If the cloth’s pores are too big, particles will pass through without being caught in the filter. If they are too small, the fabric will blind and prevent slurry from passing through. The cake that forms on the cloth often performs much of the filtering. The fabric must have the proper pore size to build a cake but still allow liquid or air to flow through. The fabric may have a membrane adhered to it in order to handle ultrafine particles.
Just as particle size is essential when selecting the right fabric, so is particle size distribution. In some applications, the particles are all about the same size. In others, large particles are mixed with fine particles. Understanding the distribution of particle size helps find the ideal cloth to use as filter media.
If particle size varies, the material can be processed multiple times in order to filter efficiently. The first pass will catch the largest particles on the cloth but not finer particles. The second pass will allow medium-sized particles to build a thicker cake on top of the largest particles. Additional passes catch and build up finer particles until optimal filtration efficiency is achieved. This is called recycling versus single-pass filtration.
Particle shape is important. They can be long and stringy, round, jagged, flat, or irregular. The shape of the particles will affect how the cake builds, how it adheres to the cloth, and how much wear is put on the filter cloth.
Different types of cloth are equipped to handle different chemical and temperature conditions. Most fabrics have a maximum operating temperature. Exceeding that temperature will damage the cloth and affect filtration efficiency. Depending on what the fabric is made of, it also may be susceptible to chemical damage. Some types of cloth have an ideal pH range. Often pH and temperature need to be considered in conjunction to ensure the cloth can handle the operating conditions.
Consider how much material is being processed. How thick does the cake need to be? How heavy is the cake? If the cloth must support all the weight of the cake, the fabric must be strong enough to handle the load of the material being processed.
