On Fri, 24 Nov 2006 18:08:07 GMT, Rich Hampel
wrote:

Well first of all filters are not screen doors. The filter media in
fuel oil filters is a 'felt' made of microfibers bound together with a
waterproof resin.
Pressure feed and correct flow rate will establish that the flow is
very slow asw it crosses the face of the filter media. If designed
correctly the dirt will form a 'cake' ON the surface of the media ...
and the dirt will begin to filter out other dirt. If the velocities are
too great the cake will collapse and become very dense .... and the
filter will shut down/plug.
With vacuum filtration the cake forms IN the filter media, and there is
less space - because the filbers of the filter media are also found
there. Once the filter gets dirt IN the filter the fluid velocities
become higher and higher thus driving the dirt deeper into the media
due to the increased fluid velocity and quickly shuts down the fluid
flow because there are few flow paths still open.
In all filtration the larger the upstream surface area, the slower the
fluid velocity, the lower the viscosity of the fluid .... all make for
'happy' long lasting filters.
When selecting a filter always choose the LARGEST you can fit or afford
.... it will save you $$$$$$$ in the long term.

That didn't really answer the question, which is WHY does the "cake"
form IN the filter media for vacuum fed but ON the surface for
pressure fed? I would think that, all else being equal, it would work
the other way because the pump is operating more efficiently with
pressure fed, so the velocity would be higher, driving the dirt deeper
in the media. Other than that, I don't understand how the felt media
knows whether the pressure differential across it is caused by
pressure on one side or vacuum on the other. Theoretically, it
shouldn't matter. So I'm just trying to understand the physical "real
world" process that causes it not to behave according to theory.

Steve