Filtration is an important process in many industries such as chemical processing, food and beverages, pharmaceuticals, and water treatment. The micron rating of the filter bags being used remains one of the most pivotal performance factors in any filtration system. Understanding micron ratings is a crucial part in the selection of the most appropriate filter bag for any application.

This guide will explain what micron ratings are, the difference in nominal versus absolute ratings, and how to make informed decisions based on your filtration requirements.

What is a Micron?

Quite simply, a micron, short for micrometre, is a unit of measurement that is equal to one-millionth of a meter.

To give perspective:

  • The diameter of a human hair is about 70 microns.
  • Most bacteria are between 0.2 and 2 microns in length.
  • A red blood cell is approximately 6 to 8 microns in diameter.

Micron rating, in regard to filter bags, can be considered to be the size of the smallest particle that each filter can capture effectively. A lower micron rating means it can trap finer particles, while higher micron ratings suit filtering larger particles.

How Micron Ratings Work in Filter Bags

Its micron rating is an important aspect to indicate filter bag performance. The micron rating chosen would depend on the application and the size of the particles to be filtered.

For example, a filter bag rated at 10 microns would capture the particles that are 10 microns in diameter or bigger in size.

Typical Micron Ratings for Filter Bags:

1 to 5 microns: Ideal for applications in  requiring fine filtration, such as in the purification of water, activated carbon removal or processing fluids in the pharmaceutical industry.

10 to 50 microns: Optimal for filtering out small sized particles, like fine sediment.

50 microns and above: These are used in areas where filtration of the medium to larger sized particles is required. Generally speaking, visible particles are larger than 50 micron in size. Applications including the treatment of industrial wastewater and filtration of paints and coatings.

Importantly, filter bag performance depends not just on the micron rating of the bag, but also  whether the filter has a nominal or absolute rating.

Nominal Micron Ratings vs. Absolute Micron Ratings

When it comes to selecting filter bags, the differentiation between nominal and absolute ratings is very important. The two terms are used to describe the efficiency of filters to capture particles.

Nominal Micron Rating

A nominal micron rating is the filter’s ability to catch a certain percentage of the particles at or above the rated micron size. Typically, a nominal filter bag will catch anywhere from 60% to >90% of the particles that are equal to or larger than its rated micron size.

For example, a filter bag may have a nominal rating of 10 microns, but may only remove 80% of particles 10 microns or larger, allowing 20%of those particles to pass through.

Best for: The nominal micron-rated filter bags are used where absolute precision is not required, making them suitable for applications like pre-filtration or for general Industrial use. Normally, these filters are cheaper than their absolute-rated counterparts, and are recommended in cases where some particle retention variability is acceptable.

Absolute Micron Rating

By contrast, an absolute micron rating indicates that the filter can trap 99.9% or more of the particles which are equal to or larger than the size of micron specified. Filters having an absolute micron rating are much more precise in retention.

For example, a filter bag rated at 10 micron absolute captures >99.9 % of the particles both at 10 micron and above to ensure very best filtration efficiency.

Ideal for: Absolute micron-rated filter bags are indispensable in many industries to achieve the highest degree possible of purity and consistency, be it for pharmaceuticals, fine chemicalsfood processing, or final-stage filtration. Whilst more expensive, absolute filters yield totally consistent results with no compromise in filtration quality.

Choosing the Right Micron Rating for Your Application

The choice between nominal and absolute micron rated filter bags depends on several factors to determine which can best meet your filtration needs.

Filtration Requirements

First, the question is: do you require high efficiency filtration, or is some variability acceptable? For example, if the application is critical, such as pharmaceutical manufacturing or beverage processing, an appropriate choice would be an absolute micron-rated filter.

If some degree of filtration flexibility is acceptable such as that of pre-filtration or general industrial use, then a nominal filter could prove to be a more feasible choice.

Cost Considerations

Absolute filters are more expensive compared to nominal ones. Here at Allied Filter Systems, we look at the total cost of filtration. This means we don’t focus solely on the cost of the filter bag, but on the overall cost benefits of the application. Filter lifetime will be affected by whether a nominal or absolute filter is being used. As a consequence, factors including labour, change out frequency, overheads, maintenance, downtime, storage and disposal costs need to be considered. A nominal filter bag might seem to be a cheaper solution and have a longer life, but an absolute filter media might be more efficient and therefore have a shorter life. If filtration with nominal filter bags is insufficient and product needs to be reworked or even rejected, then an absolute rated filter bag may actually work out to be more cost effective in the long run. Ultimately depends on the goal and the application.

Flow Rate

Another factor is the influence of micron ratings on flow rate. The lower the micron rating a filter has, the more it restricts the flow rate due to it having finer pores. Thus if a high flow rate is to be maintained, there needs to be some sort of compromise between precision in filtration and acceptable flow.

Particle Size

Knowing the size of the particles that you want to filter helps in choosing the micron rating. If you were dealing with fine particles for example in water purification, the micron rating would have to be smaller. With larger sized particles, it would require higher micron ratings.

Conclusion

In conclusion, understanding micron ratings is essential for selecting the correct filter bag for your specific application. Matching the micron rating to the particle size to be removed is essential. Whether you need a nominal or absolute micron rating depends on the precision of filtration required, as well as other factors such as cost and flow rate. By evaluating these elements, you can ensure the most effective filtration system for your needs.

Understanding Micron Ratings for Filter Bags
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