Vacuums Suck At Filtration!

Collecting, filtering and dewatering of WVO SVO vegetable oil. For Biodiesel producers too.

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Vacuums Suck At Filtration!

Postby Burbarian » Fri Sep 12, 2008 5:57 pm

Assume your filter is strong enough to withstand a 250psi differential between input and output. With a vacuum, you can at best (with heroic effort) draw about -14.7psi(~30inHg) before the laws of physics prevent you from going any further. You cannot draw a vacuum greater than 100% vacuum. Hence, of the 250psi potential pressure differential you can only exploit at best ~1/16th of that capability.

If you have a 250psi capable pump and put it in the pressure side, it can push fuel through a filter that's got a 14.7psi backpressure quite easily. That same 250psi output pump placed on the suction side will only be limited to a maximum of ~14.7psi suction.

This is because a vacuum doesn't pull, it sucks. There is a difference. You don't pull gas or liquids, you pull solids. You can pull a cable up to its breaking strain. But there is no physical 'cable' between the pump and the gas or liquid. Instead, you are only creating a space with less pressure, and are therefore depending on ambient atmospheric pressure on the other side to do the pushing. Vacuums don't pull, they just push less, and since atmospheric pressure at sea level is 14.7psi, the atmosphere is pushing harder and pushes the gas or liquid to the pump. A pump in the vacuum side is equivalent to using atmospheric pressure as the pump on the pressure side. A pump with a maximum 14.7psi pressure rating.

As the filter clogs, flow resistance increases. With a vacuum, you run out of differential a lot sooner than if the filter were under pressure. With the fuel under pressure, you have considerably more margin to maintain adequate flow even with increasing resistance. Of course, even with the above example, you won't really get anywhere near 16x the filter life with an identical pump on the pressure side. The resistance is caused by a clogging filter. Assuming the same contaminant rate per unit volume of fuel passed, filter flow resistance increases at an accelerating rate. It is a nonlinear slope, and closer to an exponential curve. You can try to force more fuel through said filter, but realistically you'd be doing well to get near 2x the usable filter lifetime.

That being said, you can call me a hypocrite as I have my pump on the vacuum side. It sucks fuel through a CAT full metal jacket cartridge fuel filter. Fuel flows from the outside in, leaving the grit and crud on the outside of the filter. Then on purge, I backflow the filter, using the supply line as a return line. As clean pre-filtered fuel is now pressurized and pushes from inside the filter outwards, a significant fraction of the crud that has stuck to the outside gets blown or knocked off. Since you are only purging a quart or so of fuel, most of the crud stays in the filter housing, where it settles out overnight and winds up in the sediment bowl. This extends filter life considerably, as it is basically getting washed on a daily basis with clean fuel. Note however that the filter must have both inner and outer reinforcement. Most commercial filter elements are only reinforced on the inside, so backflushing them could cause the element to rupture.

Of course, the longest lasting filter of all, whether under pressure or vacuum, is one that has little to do. If you maintain putting very clean fuel into the system instead of relying on the onboard filter to do it for you, you will be much less likely to wind up on the side of the road trying to change a crud clogged filter. Hopefully it will not be during the middle of a rain or snow storm.
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