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Old 19-02-2012, 15:03   #1
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Fuel Filters in Series

I have had someone suggest that once you have one 2 micron fuel filter in the line to the engine, adding another 2 micron filter in series will not reduce the flow further. I am an electrical type and not very adept at fluid dynamics but it seems that putting one 2 micron filter in the line would reduce the flow by "X" and that putting a second 2 micron filter of the same type in series with the first would then result in a total flow reduction of "2X". Can someone tell me if I am correct, and point me to the fluid formula governing the topic?

Thanks,

Bob
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Old 19-02-2012, 15:23   #2
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Re: Fuel Filters in Series

Bob, it won't reduce the flow but might increase the pressure. However, if the fuel pump is on the engine and the filters between the tank and the engine, then air pressure is used to force fuel through the filters by creating a partial vacuum. The surface area of the filters is huge, doubt its going to be a problem for a small yacht engine.

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Old 19-02-2012, 15:25   #3
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Re: Fuel Filters in Series

Why would you want filters in series?? If one clogs, the other one will be blocked by the clogged one.

Best to plumb in parallel with valves so you can switch between the two should one become blocked. From experience, it's no fun to be cleaning out and changing a fuel filter while bouncing around in the ocean because it became clogged with algae or detritus from the fuel tank. Way better to just switch a valve to draw from the second filter and soldier on.

It would seem that if there is reduced flow from one filter, adding another in line of the same size will double the restriction.
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Old 19-02-2012, 15:37   #4
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Re: Fuel Filters in Series

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Originally Posted by Pete7 View Post
Bob, it won't reduce the flow but might increase the pressure. However, if the fuel pump is on the engine and the filters between the tank and the engine, then air pressure is used to force fuel through the filters by creating a partial vacuum. The surface area of the filters is huge, doubt its going to be a problem for a small yacht engine.

Pete
Pete,

Thanks for the explanation. The case I am referring to is where the pump is on the engine. If the pressure is kept constant at atmospheric is it still true that two series filters would have exactly the same flow as one? Sorry for the rudimentary questions, but this is not my area of expertise. My thought process was making an analogy to two resistors in series with a constant voltage.

Bob
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Old 19-02-2012, 15:51   #5
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Re: Fuel Filters in Series

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Pete,

Thanks for the explanation. The case I am referring to is where the pump is on the engine. If the pressure is kept constant at atmospheric is it still true that two series filters would have exactly the same flow as one? Sorry for the rudimentary questions, but this is not my area of expertise. My thought process was making an analogy to two resistors in series with a constant voltage.

Bob
No, adding filters will create additional resistance just like electrical resistors in series. The difference is that the pump might just put more effort into pumping, creating a deeper vacuum, which results in the same flow. This is not good, as it eats away at the margins normally used for filter elements that start to clog.

ciao!
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Old 19-02-2012, 16:18   #6
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Re: Fuel Filters in Series

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No, adding filters will create additional resistance just like electrical resistors in series. The difference is that the pump might just put more effort into pumping, creating a deeper vacuum, which results in the same flow. This is not good, as it eats away at the margins normally used for filter elements that start to clog. ciao! Nick.
Well yes and no. We use 1.5 litres an hour to move a 31ft yacht at 5 knots. If there was an air bubble in the fuel line you would be pushed to see it move the fuel flow is so slow even accepting some is returned unused.

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Old 19-02-2012, 16:20   #7
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Re: Fuel Filters in Series

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Originally Posted by rabend View Post
I have had someone suggest that once you have one 2 micron fuel filter in the line to the engine, adding another 2 micron filter in series will not reduce the flow further. I am an electrical type and not very adept at fluid dynamics but it seems that putting one 2 micron filter in the line would reduce the flow by "X" and that putting a second 2 micron filter of the same type in series with the first would then result in a total flow reduction of "2X". Can someone tell me if I am correct, and point me to the fluid formula governing the topic?

Thanks,

Bob
All you need is a large prefilter like Raccor with visible decanter bowl. The filter located near the lifting pump is a polishing filter, You could keep it for two years providing you replace the prefilter as required. Like one member say, having two prefilters in parrallel with proper isolating valve is the best.
Remember: the larger the prefilter, the better (just like a boat), as an example, if my engine was 50HP, I would size the prefilter for 100HP.
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Old 19-02-2012, 16:38   #8
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Re: Fuel Filters in Series

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Well yes and no. We use 1.5 litres an hour to move a 31ft yacht at 5 knots. If there was an air bubble in the fuel line you would be pushed to see it move the fuel flow is so slow even accepting some is returned unused.

Pete
Pete,

Are you taking into consideration that the fuel flow return to the tank is a multiple of the fuel actually burned, that the injector pump is cooled and lubricated by the fuel flow, and that additional restriction to flow may put a greater load on the lift pump? I am not sure I have that all calibrated.

Thanks,

Bob
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Old 19-02-2012, 16:52   #9
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Re: Fuel Filters in Series

I have opted for the Racors in parallel. If you have the ability to a) filter fuel down the deck fill to get the egregious muck and bug parts out, and b) you have a rig to polish the fuel into a daytank, into which the return line can be routed, you are guaranteeing that you have at any given time at least a day tank's worth of clean fuel you could put straight to the lift pump.

More than one Racor is, depending on its height above the tanks, going to make the lift pump work harder, perhaps to the point of affecting flow, if I am understanding your question.
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Old 19-02-2012, 17:12   #10
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Re: Fuel Filters in Series

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I have opted for the Racors in parallel. If you have the ability to a) filter fuel down the deck fill to get the egregious muck and bug parts out, and b) you have a rig to polish the fuel into a daytank, into which the return line can be routed, you are guaranteeing that you have at any given time at least a day tank's worth of clean fuel you could put straight to the lift pump.

More than one Racor is, depending on its height above the tanks, going to make the lift pump work harder, perhaps to the point of affecting flow, if I am understanding your question.
Thanks. Excellent advice! I agree that having two filters in parallel is the way to go. Unfortunately, due to the configuration of my boat, I don't have room for the second filter. I also like the idea of being able to switch filters quickly in an emergency.

Bob
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Old 20-02-2012, 06:31   #11
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Re: Fuel Filters in Series

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Pete,

Are you taking into consideration that the fuel flow return to the tank is a multiple of the fuel actually burned, that the injector pump is cooled and lubricated by the fuel flow, and that additional restriction to flow may put a greater load on the lift pump? I am not sure I have that all calibrated.

Thanks, Bob
Bob, yes but even so the amount of fuel flowing through the fuel line is tiny, well on a small yacht engine it is. A previous V8 5.7L petrol volvo was a completely different matter.

So will the resistance go up with two filters in sereis, yes, will the flow rate drop, nah.

If you can't manage two filters in parallel then sit a simple CAV water seperator in somewhere to catch water, larger lumps and any dirt. This is our set up and whilst Europe has good clean fuel we haven't had a problem an the water seperator catches any build up before it can reach the engine.

Pete
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Old 20-02-2012, 11:04   #12
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Re: Fuel Filters in Series

The CAV with a Fram or similar filter is a good idea, too. So is rerouting the fuel vents from the topsides to the cabin top, but that's more ambitious.
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Old 20-02-2012, 11:07   #13
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Re: Fuel Filters in Series

i have an electric assist pump to prime and i have small inline filters prior to that with a racor after the pump. works great. none are 2 micron--i believe they are 30 and 10 micron with the closest to engine being the finest filtration.
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Old 20-02-2012, 11:10   #14
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The CAV with a Fram or similar filter is a good idea, too. So is rerouting the fuel vents from the topsides to the cabin top, but that's more ambitious.
The fuel vents on a monohull should cross, i.e. the vent for the port tank shoul be at starboard and vice versa.

ciao!
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Old 20-02-2012, 11:33   #15
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Re: Fuel Filters in Series

I have used two racors in series before using a 10 micron in the first and a 5 or 2 in the second. My theory was that if i was having clogging problems, the 2 micron would clog real fast, so I protected it with the 10. On an older boat I would say parallel is best though, then you just switch the valve if you have clogging problems. On a newer boat or new tank, one Racor and the engine filter should be fine.
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