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Air Filters...Chapter XXIV

     
     Food for thought (grab your flame-throwers):
     
     Scenario:
      
     We have two filters that have about the same surface area available 
     for filtering. Both rely on a maze of fibers to trap particulates from 
     the airstream. Both are dry (no oil). One filter flows more than the 
     other. Let's call this filter 'Flow'. How does 'Flow' wind up being 
     the less restrictive of the two? Answer, it filters less. There are no 
     smoke or mirrors here (remember, the surface area is the same). It has 
     to filter less. Either 'Flow' has a wider path between the fibers for 
     air to flow or the path is shorter for particulates to follow. Perhaps 
     both. Either way there is less filtration using 'Flow'. Is there any 
     room for debate here?
     
     We'll fix 'Flow's' lack of filtration problem and just coat the fibers 
     with oil. That decreases air throughput slightly, but filtration is 
     increased immensely. Essentially all of the larger particulates are 
     trapped by their sheer size and now a lot more of the smaller 
     particles stick to the oily fibers as they go thru the fiber maze. We 
     could use a finer fiber for 'Flow' to filter even better, but 
     capillary action of the oil might just totally fill the air gaps. Bad 
     idea. This would be very flow restrictive. Still, 'Flow' is less 
     restrictive than the other filter. Unless you believe in magic, this 
     means that 'Flow' is still filtering less of the smallest particulates 
     (can you tell I like the word 'particulates'?). 
     
     Is this a problem? Maybe. Is 'Flow' a bad filter? No. Life is a 
     compromise and so are air filters. With surface area fixed, then the 
     better flowing filter (good ol' 'Flow') is the right filter, if the 
     application calls for maximum airflow. As in - Race use or performance 
     street use in relatively clean ambient air. If maximum filtration is 
     all important, then 'Flow' might not be the best choice. A filter that 
     has 'Filter' as a first, middle, and last name is the right choice if 
     maximum filtering is necessary. What comes to my mind? Dusty 
     environments or ALL vehicles that are planned (praying, crystals, 
     luck, smoke and mirrors might help here) on being driven for hundreds 
     of thousands of miles.
     
     Bottom line: Air filtration is a tradeoff. When you go to one extreme, 
     max airflow or max filtration, there has to be a resultant price to 
     pay (less filtration or less airflow).
     
     ------------------
     Two more thought: someone attacked the fact that 'socks' weren't used, 
     as a prefilter, for all of the filters in the mining engineer's 
     findings (from the FAQ). A prefilter keeps the big stuff (I didn't use 
     the 'p' word) from prematurely clogging the filter. Usually, 
     prefilters have no effect on filtration of the smallest particulates 
     (I had to use it one more time) ;^)
     -------------------
     
     I'll step down now from my soapbox and put on my flameproof suit. At 
     least try to tone down the emotion, if you have to send any return 
     'arrows'.
     
     Barry Ritchey
     mbritch@domain.elided
     CCA#134069
     '325iX