회사 소식 What Is a String Wound Filter Cartridge and How Does It Work?

A string wound filter cartridge, also called a wound filter cartridge, is a typical depth-type filtration element used for the removal of suspended solids from liquids. It is widely applied in industrial water treatment, chemical processing, power generation, plating, oil and gas, food and beverage pre-filtration, and general process liquid filtration.

Compared with surface-type filter elements, a string wound cartridge is designed to retain particles not only on the outer surface, but also within the internal depth of the wound yarn structure. This makes it especially suitable for applications where the liquid contains sand, rust, sediment, scale, colloidal particles, or other suspended impurities with a relatively high particle load.

A string wound filter cartridge is mainly composed of two parts: the filter yarn and the central support core.

The filter yarn forms the active filtration layer. Common yarn materials include polypropylene, cotton, glass fiber, and polyester. The selection of yarn material depends on the chemical properties of the fluid, operating temperature, filtration target, and cleanliness requirement.

The central core provides mechanical support and maintains cartridge stability during operation. Common core materials include polypropylene, stainless steel, and galvanized steel. Polypropylene cores are commonly used for general water and chemical filtration. Stainless steel cores are often selected for higher temperature or higher mechanical strength requirements.

During manufacturing, the yarn is wound around the core under controlled tension and a specific winding angle. The winding pattern forms a multilayer three-dimensional filtration structure. In a well-designed wound cartridge, the outer layers usually have relatively larger flow channels, while the inner layers become progressively denser. This graded structure allows larger particles to be captured in the outer region and finer particles to be retained deeper inside the cartridge.

This is the main difference between a wound filter cartridge and a simple screen or surface filter.

The working mechanism of a string wound filter cartridge is based on depth filtration.

When liquid enters the filter housing, it normally flows from the outside of the cartridge toward the inner core. As the liquid passes through the wound yarn layers, suspended particles are removed through several mechanisms, including mechanical interception, pore restriction, particle bridging, and deposition within the depth of the filter matrix.

Large particles are usually trapped near the outer layers. Smaller particles may move deeper into the cartridge before being retained by narrower channels or by accumulated particle layers. As filtration continues, the retained particles gradually increase the flow resistance, leading to a rise in differential pressure.

A properly selected wound filter cartridge should provide a balance between particle retention, dirt-holding capacity, flow rate, and pressure drop. If the micron rating is too fine for the inlet particle load, the cartridge may block too quickly. If the micron rating is too coarse, downstream equipment or final filters may not be adequately protected.

Most string wound filter cartridges are considered nominal-rated filters. This means that the stated micron rating represents a general particle removal level under specific test or operating conditions, rather than an absolute guarantee that all particles larger than the stated size will be removed.

Common nominal ratings include 1 μm, 3 μm, 5 μm, 10 μm, 20 μm, 30 μm, 50 μm, 75 μm, and 100 μm.

In practical industrial applications, the actual filtration performance is influenced by many factors, such as particle shape, particle concentration, liquid viscosity, flow rate, operating pressure, temperature, and the physical structure of the wound yarn. Therefore, filter selection should not rely only on the micron rating. The complete working condition should be evaluated before choosing the cartridge.

Material compatibility is one of the most important factors in selecting a string wound filter cartridge.

Polypropylene wound cartridges are widely used because of their good chemical resistance, low cost, and suitability for many water and general chemical applications. They are commonly selected for industrial water treatment, chemical pre-filtration, plating solution filtration, and wastewater pretreatment.

Cotton wound cartridges may be used in some low-temperature applications where natural fiber media are acceptable. However, chemical compatibility and potential fiber release should be considered carefully.

Glass fiber wound cartridges are often used in higher temperature applications or where certain chemical resistance properties are required. They can provide good thermal stability, but the fluid compatibility should still be confirmed.

Polyester wound cartridges can be used in selected industrial liquids where polyester offers suitable mechanical and chemical performance.

The core material should also be selected according to the process condition. For ordinary water and many chemical liquids, a PP core is often sufficient. For higher temperature, higher pressure, or more demanding mechanical conditions, a stainless steel core may be more suitable.

String wound filter cartridges are commonly used as pre-filtration elements. Their main function is to remove coarse and medium-sized suspended solids before the liquid reaches pumps, valves, membranes, ion exchange resins, activated carbon beds, or final precision filters.

Typical applications include:

• Industrial water pretreatment, including removal of sediment, rust, sand, and pipe scale.
• Chemical process filtration, where suspended particles must be reduced before the next production step.
• Plating solution filtration, where metallic particles and process residues need to be controlled.
• Power plant water filtration, including condensate, cooling water, and auxiliary water systems.
• Food and beverage pre-filtration, where coarse impurities need to be removed before finer filtration stages.
• Oilfield and general industrial liquid filtration, where robust and economical pre-filtration is required.

In many systems, wound cartridges are not used alone. They are often combined with melt-blown filters, pleated filters, bag filters, or membrane filters to form a staged filtration system.

The main advantage of a string wound filter cartridge is its depth-loading capacity. Because particles can be retained throughout the filter depth, the cartridge can handle higher impurity loads than many simple surface filters.

Another advantage is cost efficiency. For many pre-filtration applications, using a wound filter cartridge can reduce the load on downstream fine filters, extend final filter service life, and lower overall replacement cost.

String wound cartridges also have relatively good mechanical strength when properly manufactured. The wound structure, combined with a suitable central core, allows the cartridge to maintain shape and stability under normal operating conditions.

In addition, the product can be customized in terms of length, micron rating, yarn material, core material, outer diameter, and end configuration, making it adaptable to many industrial filter housings.

Although wound filter cartridges are reliable and economical, they are not suitable for every filtration requirement.

For applications requiring high precision, absolute-rated filtration, bacterial retention, or very low extractables, pleated membrane cartridges or high-purity filters may be more appropriate.

For fluids with very high viscosity, high solid concentration, or rapidly changing particle load, the flow rate and pressure drop should be carefully evaluated. Improper selection may lead to short service life or unstable filtration performance.

For sensitive applications such as food, beverage, electronics, or pharmaceutical pre-filtration, material cleanliness, fiber release risk, and compatibility should be confirmed before use.

Therefore, the best filtration solution should be selected based on actual operating data, including fluid type, particle characteristics, required cleanliness, flow rate, pressure, temperature, and replacement cost target.

String wound filter cartridges and pleated filter cartridges are both widely used in industrial filtration, but they serve different purposes.

A string wound filter cartridge is a depth filter. It is suitable for liquids with relatively high suspended solids and is commonly used as a pre-filter.

A pleated filter cartridge has a larger effective filtration area and more stable filtration accuracy. It is usually used where lower pressure drop, higher flow rate, finer filtration, or final particle control is required.

In a well-designed filtration system, these two types of cartridges can be used together. A wound filter cartridge can be installed at the first stage to remove coarse particles and reduce the particle load. A pleated filter cartridge can then be installed downstream for finer and more stable filtration. This staged design helps improve system reliability and reduce total operating cost.

Pullner provides string wound filter cartridges for industrial water treatment, chemical processing, plating, power plants, food and beverage pre-filtration, and general industrial liquid filtration.

Available options include different yarn materials, core materials, micron ratings, lengths, and customized dimensions. Pullner can support filter selection based on the customer’s working conditions, existing filter samples, drawings, housing dimensions, or replacement requirements.

For customers facing frequent cartridge blockage, unstable pressure rise, high downstream filter consumption, or excessive filtration cost, Pullner can help analyze the filtration system and recommend a more suitable pre-filtration arrangement.

A string wound filter cartridge is a mature and practical depth filtration product. Its multilayer wound structure provides good dirt-holding capacity, stable particle removal, and cost-effective performance in industrial pre-filtration applications.

However, effective filtration depends not only on the nominal micron rating, but also on material compatibility, pore structure, flow condition, particle load, and system design. For this reason, filter selection should be based on real operating conditions rather than cartridge size or micron rating alone.

Pullner is committed to providing reliable and technically appropriate string wound filter cartridge solutions for global industrial customers.

To learn more about Pullner wound filter cartridges and industrial filtration solutions, please visit www.pullnerfilter.com.