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Why Do Filters Retain Any Air?


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I think we had a thread recently, in which a fuel filter that was half full of air was important.

 

I've just filled my fuel tank with safety foam.  Why it's 'safety' I don't know - if the tank is breached it'll all pour out anyway.     It will inhibit slosh, but Continental race scrutineers insist on it, so I'm told, so I'm fitting it.

I had to cut the foam to size, and bits may have been scraped off, stuffing it in through the access hatch, so I bought a cheap fuel filter from Halfords and fitted that between tank and fuel pump - which has it's own filters, but why clog those up with bits of foam?   I could remove the pre-filter and chuck it away later.

 

The filter works fine.   I've circulated a half tankful of fuel through it, to calibrate the new fuel gauge, and flow back into the jerry can via the pump is fast.    But the filter is three-quarters full of air!

It's a cylinder about 2" wide by 3" inches long, with a pleated paper (?) filter inside, with about a quarter inch clearance at each end, and the fuel inside goes up to the lower end of the filter only.   It's mostl;y full of air!    I have positioned it carefully, pointing up in the direction marked on the filter.   The plastic case of the filter is not crystal-clear, but I think the lower (in) connector connects to the cavity, and the upper, (out) connector comes from inside the filter.    But why doesn't the pump draw all the air out?

 

Please see pic.

 

John

 

PS I tried using a 200 ml oil syringe to draw out the air - it still didn't, and I ruined the syringe!  The fuel made the rubber seals swell so much, I couldn't get the plunger back down the barrel!

post-690-0-54517500-1497948664_thumb.jpg

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                              in

                   _____|  |______

                   |         |  |            |

                   |         |  |            |

                   |         |  |   A       |

               C  |~~~~|~|~~~~~~|

                   |                         |

                   |          B            |

                   |                         |

                   --------|  |---------|

                             |  |

                           out

A = Air

B = Fuel

 

I think as soon as fuel reaches C and your sucking on "out" you start creating a vacuum in A which is way more resistance than pulling fuel through the the "in", unless the fuel tank feeding the "in" was sealed in which case I think it would be a race to see if the pump was strong enough to either collapse the tank, the filter bowl, or burn itself out trying!

 

Alan 

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OTU,

Think the arrangement in the filter is more like this:

post-690-0-31613900-1497953793_thumb.png

 

Orange shows where the fuel sits.   Clearly, it can be sucked into the bottom of the filter itself, but the resistance to flow through the filter of air would be less, so why isn't the air sucked into the pump?

 

Unless - the filter paper is 'wetted' with fuel, so that surface tension increases flow resistance through the paper??  Then fuel would flow more easily than air.

 

With this arrangement, only the lower part of the filter is working, but I can't think of a way to easily fill the filter and expel all air before connecting it to the pump.  If you could connect it by a long hose and hold the filter below the tank, 'out' end up, it would fill, but then you'd have a mile of hose to tuck away!

 

Any ideas?

John

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OTU,

Think the arrangement in the filter is more like this:

attachicon.gifFuel filter diagram.png

 

Orange shows where the fuel sits.   Clearly, it can be sucked into the bottom of the filter itself, but the resistance to flow through the filter of air would be less, so why isn't the air sucked into the pump?

 

 

 

I believe because it doesn't matter what orientation you put that filter in as it always starts with some air in it, if you suck on the out it has two choices either pull fuel through the in and the filter, or start sucking the air which will start creating a vacuum. In fact when you turn the pump on it does initially start sucking the air but almost instantly the vacuum created rapidly exceeds the effort required to pull fuel through the in and the filter.

 

Alan

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I don't understand that.

 

The pump will lower the pressure at the 'out' port, then the atmospheric pressure and any head due to height of the tank above the filter will push fuel in.   That's what I meant when I said to lower the filter below tank level to fill it before fitting.

 

The pressure inside the filter canister on fuel and air is the same.   It can't affect one preferentially, unless there is a resistance to flow that is greater on one than the other.    I would expect a gas to have less flow resistance than a liquid, unless surface tension came into effect.

 

John

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Ok I don't understand either  :yes:

 

It would be interesting to put a bucket of water on a table put a length of hose in down to the filter and then out and up to higher than the bucket and suck, I think you may be right as long as the filter is below the level of the bucket, but I "may" be correct if it is above the level of the water.

 

Of course it could just be magic!

 

Alan

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Hello John

                 I do not know the answer for sure but! There must be some sort of tube or sleeve inside the paper element and it depends on were the outlet holes are? probably to allow for it to be fitted any way up.(well that,s my two penny worth!) 

 

Cut an old one open! (if I was at home I would do that as I have several of these as I bought a big bag cheap a few years ago)

 

Roger

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