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Jason, See if you can get the specific technical data on any filter you are considering. You should look for rated scfm at some pressure (usually 100 psig) with some pressure drop noted. I would pick something that has a rating equal to or greater than your maximum anticipated scfm usage rate with a pressure drop of say 5 or 6 psig (or less) at about 100 psig. Install it upstream, on the high pressure side, of your air regulator. The pressure drop is less for the same flow if the line pressure is greater. Those two things are more important than the actual fitting size. As CNKS says, a few undersized fittings are not going to cause a problem - it's the lengthy hose or piping runs that are too small. If you can get something with an acceptable rating that has 1/4" openings, then you should be OK in reducing down to 1/4, going through the filter and then increasing back up to whatever. What happens when a compressible fluid like air passes a short restriction is that the velocity increases and the pressure drops. However, downstream of the restriction, the velocity will decrease because of the now larger available flow area and there will be some (not fully, but a high percentage) of the pressure drop recovered. The permanent pressure loss due to the short restriction is significantly less than you would otherwise expect. There is still some loss over larger fittings but it is relatively small and only important in marginal cases. Here's an example: My compressor cycles between 140 and 170 psig. It has enough capacity to keep up with my full sized gun on a continuous basis, even at the highest pressure. My tank pressure therefore never gets lower than 140 psig during normal use. I take about a 10 psig drop between my tank and my wall regulator (filter bank upstream of regulator), most of which is across the filter bank. That's when my full sized gun is in use. My regulator is typically set at 90 psig or less downstream pressure to yield 50 psig or so at the gun. I have mostly 3/8" fittings in my filter bank but am positive that 1/4" ones would work OK too (providing the filters themselves were the same). Since I have an excess of 40 psig available (140-10-90) for additional pressure drop, the change to 1/4" from 3/8" on the fittings would be unlikely to create a problem. For just this reason, I think the emphasis on special full-flow, low restriction fittings is over- hyped. They could make a difference but only in the most marginal of installations and are otherwise not worth the money. Rod
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