On Thu, 23 Nov 2006 19:47:23 GMT, Rich Hampel
wrote:

Recirculation filters should be set up as PRESSURE FEED not vacuum feed
where the pump is at the END of the circuit ..... the filters will be
vastly more efficient versus on-stream service life due to the better
deposition of particles ON the filter surface than IN the filter when
in vacuum feed mode. The Walbro has an integral screen of about

When the filter media sees a pressure differential, how does it know
whether it's caused by sucking on one side or pushing on the other?
Does the filter media actually get compressed by the slightly (tiny)
higher pressure inside the filter housing when it's in pressure feed
vs. vacuum feed? Since the fibers are surrounded by the fluid, the
pressure on each fiber is pretty much equal all around. The only
difference being the difference between the front of the fiber and the
back, which only depends on the differential not the absolute
pressure. So the only way I can see pressure vs. vacuum makin a
difference is if the actual fibers get compressed and get smaller in
diameter by the higher pressure in the canister. But now I have to
understand how only a few PSI difference can cause any significant
deformation/compression of the media fibers.

The only other thing I can think of is maybe the fluid flow rate is
faster for pressure fed vs. vacuum because the pump may be operating
more efficiently that way. But then, assuming the flow rate is
different, the opposite situation would occur. I.e., particles would
be deposited IN the filter for the faster flow rate (pressure) vs. ON
the surface for the slower flow rate (vacuum)

Anyway, enough of my rambling. I'd just like to understand why
particles get deposited ON the filter surface for pressure fed and IN
the filter for vacuum fed.

Steve

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.

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

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

Possibly the Vac system is more constant, whereas the pumped system
pulsates, this may affect the level of turbulence causing the crap to
lie long ways across the filter, but with a more smooth flow (vac
system)the crap will align with the flow and penetrate deeper into the
filter media.

This is a blind guess, but may prompt someone who knows about these things.

I
Retention of particulate in/on a filter is an extremely complex entity
due to simultaneous and varied 'capture mechanisms'. that 'hold' the
particles in place: seiving - where the statistical pores of the filter
structure are smaller than the particle; inertial impaction - where the
particles leave the flow stream when the flow stream takes sharp bends;
aDsorption - where the particles are held to the filter media subtrate
by weak electronic bonding (van der waals forces); the formation of a
"cake" on the upper surface and into the upper 10-15% of the depth of
filter medium; Polarization of gel-forms, etc.

All filtration is 'particle specific' .... and depends exactly which
mechanism of capture 'predominates' for which type of particle you want
to remove. For fuel oil, etc. filtration where probably there are more
'deformable' particles or particles that can change shape under
increasing differential pressure and then are subject to extrusion
either through or partly through the media (settling deeper into the
media) thus 'blinding it' ..... vacuum filtration has historically
shown the least efficacy of on-stream service life versus pressure
filtration. Apparently vacuum feed filtration allows the deformables
and smaller than 'seiving' size particles to partly extrude more deeply
into the matrix, partly closees off the statistical pores which
increases the face velocity of the fluid in the cross sections of the
filter matrix. The increasing face velocity of the fluid through the
sections creates an untowards physical event (as per the standard
D'Arcys equation) derived to be: On stream life (T1/T2) being a
function of the ratio of face velocities to the 'n-th' power where 'n'
- approximately approaches to the 2/3 power). My 'guess' (after 35
years of observation, etc.) is that in vacuum filtration the capture
involves an *accelerating* particle; while pressure feed involves a
*decelerating* particle.

Simplistically and historically, vacuum draws the deformables and
smaller than the target 'seiving sized' particles deeper into the
matrix, shuts down the open flow paths quicker than in pressure feed
--- all apparently internal velocity dependent.
Filtration hydrodynamics is probably very similar to aerodynamics where
intuition and simple logic will always produce the wrong answer. I've
been deeply involved in ultra-purity filtration and 'separations'
engineering (and the physical chemistry of) for almost 35 years and
still dont know all the answers .... although I do know that vacuum
feed filtration will *always* have comparatively shorter service life
than pressure feed (for just about ALL filtrations) ... and for that
reason alone is good enough for me and most others to stay away from
vacuum feed filtration. There's probably a doctoral discertation
waiting for someone who can correctly figure this one out - many have
tried but none have ever been successful.

Like I posted earlier, filtration has nothing to do with 'screen doors'
and is an extremely variable complex entity at below the macroscopic
level. Dont attempt to 'rationalize' it as you will ultimately always
wind up with the wrong solution. It's really an 'art-form'.