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In this series on filtration I have not mentioned much about filter ratings other than to say that most filters are rated based on the minimum size of particles they will capture. It is important to know that a filter rated at 5 microns, for example, may not prevent the passage of all particles with a dimension greater than 5 microns. Most particles are not nice, round things and may slip through by turning their narrow side toward the opening. It’s also important to know that filters also have ratings based on “nominal” and “absolute” filtration. No filter is 100% efficient. A filter with a “nominal” rating of 10 microns will capture only a percentage of particles over 10 microns. To remove all particles over 10 microns would require re-circulating the same liquid through the filter many times until the percentages converge on 100%. If 60% of the target particles are removed on the first pass then, theoretically, 60% of the remaining 40% of the particles that weren’t removed on the first pass will be removed on the second pass meaning that 76% of the particles over 10 microns have now been removed. Each pass brings the number closer and closer to 100%. An engineer would say it will never actually get there – the philosopher would say that it will get close enough!
Filters and filter media rated for “absolute” filtration have a much higher efficiency of particle removal. But again, “absolute” is a relative term. There may always be that particle that manages to squeeze through at the opportune moment. Filters with an “absolute” rating, therefore, may carry efficiency numbers like 99% or 99.9% but I have never seen one rated at 100%.
At the beginning of the series on filtration I mentioned that a lot of what applies to the filtration of liquids also applies to the filtration of air and other gasses. In cleaning, the cleanliness of the air used for drying is very important. Dirty air, of course, will deposit particles on cleaned parts negating the benefit of cleaning. The ultimate in air filters is the HEPA filter. HEPA stands for High Efficiency Particulate-Air. But again, HEPA filters vary in efficieny just as liquid filters. A lot of filters that claim to be “HEPA” may be able to capture a percentage of sub-micron sized particles but fall short of what a true, properly designed and installed HEPA filter will be able to deliver in the way of clean air. Since the air in the dryer is the last thing that will see the part, paying attention to detail in chosing and maintaining a proper HEPA filtration system in the dryer is extremely important in critical cleaning applications.
The series on filtration has been a lot of fun! It made me do some thinking and research to assure that I accurately represented the subject. As I’ve heard many times, it is impossible to “over-filter” in a cleaning system. The problem is that filtration is expensive and in most cases maintenance intensive. A balance must be struck between what is enough filtration and what is economically feasible in a particular application. Hopefully this series will help you, as a person interested and involved in cleaning, ask intelligent questions about filtration.
Certainly, there is a lot more to know about filtration than I have been able to cover in this series. If you have questions or would like to challenge or correct anything you’ve read please feel free to comment in the comments section. I know it’s confusing but if you want to comment, and there is no previous comment, all you have to do is click on the “no comments” and a window will open to allow you to make your comment.
We’re coming up on a weekend so I have all weekend to decide where to go with Monday’s blog topic. There are so many things to talk about. If you have interest in a particular subject, just let me know. If I know anything about it, I’ll share what I know with you. If I don’t, I’ll either find out or share what I don’t know with you. Just click on the “comments” button. Have a great weekend.
– FJF –