Rich,

What do you recommend for a depth filter rather then the TP ot PT
filters?

Doug

"Rich Hampel" wrote in message
...
You are correct !!!
Most folks dont realize that a filter isnt a 'screen door' .... it
can pass particulate LARGER than the 'rating'.
:-)

In article , Steven Shelikoff
wrote:

On Fri, 05 Dec 2003 05:28:01 GMT, Rich Hampel
wrote:

In article , Steven Shelikoff
wrote:
Why not just always leave the polishing system in-line? Allow for a
bypass to change a filter or if it develops a vacuum leak, but other
then that, there's nothing wrong with always using "just polished"
fuel.

My 'read' from your statment is that perhaps you miss the functional
point of a recirculation filtration system - which is using a filter of
LARGE pore/rentention size to eventually wind up with a fluid with very
few very small particles.

A recirc filter only removes a paltry few percent of the 'target' size
particles (for example 5% of 2uM particles.). A large pore size filter
will have very little resistance to flow, will have more permeability
(open space), will have more ultimate 'dirt capacity'. If you pass the
fluid 20 times through the filter, you will remove approximately a
value approaching 100% of the target particles. With the same pump, a
LARGE filter is be able to pass a LARGE volume very quickly, whereas
2uM filter will take longer (due to resistance to flow - pump slows
down or starts to slip and fluid begins to bypass the vanes, etc. ).

Actually, I do understand that it's best to recirculate fuel many times
through the polishing system. That is what I do. But the engine has to
draw fuel from somewhere and I don't see why it's not better to draw
fuel from the outlet of the polishing system and always leave it on
while the engine is running (most of the flow from the system goes back
to the tank to be recirculated anyway) then to draw fuel directly from
the tank. Doing it this way allows for the most fuel passes through the
polishing system since it's running more often and because you get that
"one last time through" that you wouldn't get if you draw from the tank.

For single pass filtration (and without knowing the particle size
distribution) one typically needs a prefilter of the same surface area
(or dirt capacity) that is 5 times the size of the final filter.
eg.: 10uM followed by 2uM, where the prefilter is used to prolong the
life of the final filter. This is somewhat simplistic. When you
design a filtration system with a prefilter or multistage prefilters
one typically attempts to make ALL the filters fail at the exact same
time - so maximum debris is removed and the cost of change is minimized
- and you dont throw away filters that still have some capture ability
left in them.

Well, since we're talking about a polishing system and not single pass
filtration, the above may not be applicable. But for a recirculating
polishing system on our own boat where we do the filter changes
ourselves, to make it most cost effective I'd design it not so that all
of the filters fail at the exact same time but that the cost of
filtering of each stage in a multistage system is the same. That is, if
your second stage filter element costs 10x the first stage filter
element I'd want that second stage element to last 10x longer before it
needs to be replaced to be as cost effective.

If you monitor each stage with independent vacuum gauges you won't be
throwing away filters that still have some capture ability left in them.

Steve