Application guidance for dust filtration, liquid filtration, and HVAC filter selection.

Liquid Filter Bag Ratings: Microns, Mesh and Retention Efficiency

2023-08-24

Two liquid filter bags labeled “10 micron” are not necessarily interchangeable. The label does not tell you the full retention efficiency, test conditions, flow capacity or sealing arrangement. A useful specification connects the particle-removal requirement with a documented bag construction and the operating conditions.

What does a micron rating tell you?

A micron, or micrometer, is a unit of length. In a filter specification, it describes a particle size or opening size—but the meaning depends on how the rating was established.

Ask: at that particle size, what percentage is retained, under which test method and conditions? A number without that information is an incomplete basis for comparing products.

“Nominal” and “absolute” should not be treated as universal percentages. Request the manufacturer's definition and efficiency data for the exact product. Neither the word “absolute” nor a small micron number should be used as a claim that every particle will be removed in every process. Eaton's published ACCUGAF data, for example, reports defined efficiencies for particular products rather than a blanket 100% removal claim. [1]

Mesh count is not the same as opening size

Mesh count commonly describes the number of openings or repeats per linear inch—not per square inch. The opening also depends on the thread or wire diameter and the weave. Two materials with the same mesh count can therefore have different openings. [2]

For a mesh bag, ask for the specified opening in microns, material and construction. Avoid relying on a generic mesh-to-micron chart without checking the actual cloth.

For felt and other depth-media bags, the retention behavior cannot be described simply as a uniform sieve opening. Compare the supplier's filtration data rather than assuming that a mesh bag and a felt bag with the same label behave identically. Eaton, for example, explicitly identifies its multifilament polyester mesh bags as nominally rated. [3]

Choose the rating for the process objective

Start with the purpose: protecting equipment, reducing visible particles, recovering solids or meeting a defined downstream quality requirement. Identify the troublesome particles and how success will be measured.

Then review:

Fluid identity, concentration and operating temperature.
Viscosity at the actual operating temperature.
Required flow and the available pressure budget.
Solids loading, particle characteristics and current bag life.
Compatibility of the media, seams, ring and housing seals.

A finer bag is not automatically a better solution. It may load too quickly or impose an unacceptable pressure drop. Where contamination is variable, a controlled trial with agreed sampling and acceptance criteria is more informative than selecting by micron number alone.

For a microplastic-removal objective, define the particle sizes, shapes and polymers of interest, then agree on an analytical method and acceptance criteria. A bag's micron rating alone does not validate microplastic removal. Testing should address the actual liquid, operating conditions and sampling controls; results should state the measured size range and method limits.

Check sealing as well as the media

Particles can reach the outlet if the bag does not seat correctly, even when the media is suitable. Confirm the ring design, housing seat, support basket, bag length and installation method. Eaton recommends an insertion tool for relevant bag products to help achieve correct alignment in the basket. [3]

If outlet quality deteriorates, investigate installation and damage alongside the selected rating. Do not assume that changing to a smaller micron size will correct bypass.

Use equipment-specific change-out criteria

Record the clean differential pressure at the operating flow, then monitor the trend. Set change-out criteria using the instructions and limits for the actual bag, support basket and housing—not a universal pressure value copied from an article.

Before opening a housing, follow its approved shutdown, isolation, pressure-relief and safe-handling procedure. Do not open or adjust the closure under pressure. Fluid hazards and retained temperature also matter. Use the housing manufacturer's operating manual as the controlling instruction. [4]

What to include in an RFQ

Send the fluid and duty details above, the desired retention requirement, current bag part number, dimensions, ring photographs, housing model, quantity and observed problem. Request the proposed rating definition, available test evidence and material limits.

A well-defined inquiry makes it easier to compare offers on a like-for-like basis—and avoids buying a bag that matches the label but not the application.