Optimizing Filtration: Understanding How Liquid Viscosity Affects
Filter Bag and Cartridge Applications

Impact of Liquid Viscosity on Filtration

Viscosity plays a fundamental role in liquid filtration as it affects how the liquid flows through the filter media. Liquids with higher viscosity have thicker and slower flows at a given pressure, while low-viscosity liquids exhibit thinner and faster flows at that same pressure. This difference in viscosity significantly influences liquid velocity and differential pressure across the filter media.

Liquid Velocity & Differential Pressure

Liquid Velocity: In the context of liquid filtration, liquid velocity refers to the speed at which the liquid flows through the filter media. High-viscosity liquids experience reduced flow velocities due to their inherent resistance. As these liquids traverse the filter media, they must overcome the small pores and interstitial spaces, resulting in slower movement and lower liquid velocities.

This is problematic for both filter bag and filter cartridge filtration because it can contribute to uneven flow distribution across the media. It can be off-set somewhat by operating at a higher pressure and thus filter cartridges might be a better choice due to their inherent ability to handle higher differential pressures.

There are filter bag designs, such as Eaton’s MAX-LOAD, which have comparable differential pressure ratings but also offer the advantage of higher flow rate capacities and increased surface area in support of less frequent media replacement and an overall simpler housing design.

Differential Pressure: Differential pressure is the pressure difference between the inlet and outlet sides of the filter. High-viscosity liquids tend to generate higher differential pressures for a given flow rate due to their increased resistance while passing through the filter media.

Filter Cartridges: Ideal for High Viscosity

The high viscosity applications in which filter cartridges are commonly used have a couple of characteristics besides higher differential pressure tolerance that favor filter cartridge designs: lower flow rates and necessity to retain finer solids.

Single filter cartridge systems are limited in flow rate to approximately 1 GPM per inch of length and the longest filter cartridges offered through Eaton are 40” long. If the flow rate is significantly higher than 40 GPM, it might be more cost effective to use a single filter bag housing design compared to a multiple filter cartridge design in terms of initial cost and ongoing cost related to media replacement.

Filter bag designs that have similar maximum differential pressure capabilities as filter cartridges are limited in their filtration efficiency and size of particles which can be retained. Filter cartridges tend to be more efficient and there are many designs for submicron levels of filtration. Therefore, if the flow rates are on the higher end and a nominal >1 micron particle retention is required, filter bags might be the most cost-effective solution whereas filter cartridges provide more options for higher efficiency retention of finer particle sizes and are especially cost effective for <40 GPM flow rate applications.

Advantages of Filter Bags in Versatile Applications

Larger Surface Area: Filter bags generally have a larger surface area compared to individual filter cartridges. This attribute makes them highly effective for filtering low to moderate viscosity liquids.

Cost-Effectiveness: Filter bags are typically less costly for high flow rate applications because filter cartridge systems often require multiple cartridges to handle higher flow rates which can be accommodated by a single filter bag. This is especially true for applications which require nominal filtration efficiency of particles >1 micron as the cost for complex filter bags with absolute rated efficiency are quite expensive.

Specialized Accessories & Complexity

Specialized Accessories: Filter bag systems offer various accessories, such as displacement balloons and magnetic inserts, which can enhance filtration performance. Displacement balloons reduce the liquid retained within the filter bag and inadvertently help maintain a higher velocity across the filter bag surface.

Magnetic separators are very powerful magnetic assemblies that reside within the center of filter bags and they significantly increase the retention efficiency of ferrous particles. There is no equivalent magnetic separation for industrial filter cartridges.

Complexity and Cost: Higher flow rate applications can be accommodated with a smaller filter vessel size and use fewer filter units when using a filter bag housing. The inherent higher capacity of filter bags results in less sealing points within the vessel; the simplification of design lowers the cost of the vessel and contributes to less downtime for replacing clogged media.

Conclusion

Selecting the right filter bag or filter cartridge style depends on the viscosity of the liquid and the specific requirements of the filtration process. Filter cartridges excel in handling higher viscosity liquids with elevated differential pressures and finer solids. On the other hand, filter bags offer larger surface areas and cost-effectiveness for low to moderate viscosity liquids.