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Fuel Polishing again.
After 30 years of screwing around with this stuff I cant still give a
an accurate technical reason .... my 'opinion' is the 'regime' of particle depositionIn and the formation of 'filter cake' ..... on a pressure filtration the deposition begins mostly on the upper surface or at least within 5% depth of the surface, while with vacuum filtration the deposition is essentially INSIDE the matrix of the media. Being inside the matrix causes higher internal velocities which drive the particles deeper and deeper into the matrix ... causing an exponential decrease in service life. ..... its the same for depth as well as membrane filtration. The quandy is that the fluids are incompressible and shouldnt make any difference due to the direction of motive pressure .... but in practice it does, it always does. article , Steven Shelikoff wrote: On Thu, 08 Jan 2004 02:51:46 GMT, Rich Hampel wrote: NOPE! In pressure mode, the filter will also act as a 'coalescer' (bringing similar surface tension fluids together to make larger and larger sized particles) and such particles will settle out into a 'drop-out-pot' ..... or usually into the bottom of the filter bowl (bowl pointing downwards). The smaller the retention size of the filter media the more efficient the coalescing efficiency. On the very bottom of the filter bowl, add a pigtail of oil compatible transluscent plastic tube (Tygon, etc.) with a cock on the end .... when you see water in the pigtail simply drain the bowl. Thats the same way a racor with an integral clear plastic knock-out-pot works. If you are regularly polishing the fuel the coalesced emulsions will be removed/trapped in the inverted filter bowl .... that why you put the dip tube for the recirc system at the VERY bottom of the tank and a drain cock on the inverted filter bowl. Dont want air leaks or fuel oil leaks ------ dont use compression fittings, use flared or better fittings. Pump should have a SCREEN (preferably integral) for protection to prevent damage by *huge* particles that would tear the rubber impeller or wobble plate. I say again, if you want long service life and efficient filtration employ PRESSURE filtration, especially on a recirculation system. The ONLY reason I can think of why fuel systems in boats use vacuum filtration is ........... the engine manufactures supply the lift pump and 'guard' filter - and puts it on the engine ...... and not on the tank (where it SHOULD BE). Cheap and dirty solution, easier for the boat builder - less wiring, less design, less effort, .... All this begs the question, why does the filter media care whether it's in "pressure" mode or "vacuum" mode? Sure, the plumbing and filter cases care. But the media only sees a pressure differential across it. What's the difference to the media if the there is 14psi (atmospheric pressure) on one side and, say, 10 psi (a 4 psi vacuum drawing fuel across the media) on the other side vs. 18 psi (4 psi pressure pushing fuel across the media) on one side and 14 psi (atmospheric) on the other? IOW, even if the pump is past the filter drawing fuel through it, the filter is still in "pressure" mode because it's really the atmospheric pressure pushing fuel through the filter. Steve |
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