Pressure Drop in Filters

But if same amount of contaminant is passed from the filter rated as 3 micron and 10 micron, then there will be more pressure drop in 3 micron filter.

Is it correct?

Have several 3μm filters in parallel.

PWSlack is right in post 3. Increase the surface area and it will decrease the pressure drop.

I'll throw out another option that works for some scenarios. Is your pressure drop ok with the 3 micron filter initially but quickly blinds and the differential pressure goes up? Are you filtering solids from liquids? If yes to both then you may consider "body feeding" a "filter-aide".

You introduce a stream of filter-aide (often graded diatomaceous earth) into the liquid mix you are trying to filter. As the solids are caught by the filter you continuously introduce filter-aide. It is porous so the filter does not blind and a constantly growing new filter layer is being formed. You use filter-aide sized to act as the filter media as its layer grows.

This way instead of quickly blinding your filter you are continuously growing a new filter layer on top.

An interesting method. I would like some more information before I vote for the GA though.

How does it hold up in the long run? My experience is with jet fuel filtration media and pipeline fuels filtration. I would worry about sudden failure and the filter aide going downstream. Would this material settle in pipes, valves etc when flow stops? Would it get into differential pressure gauges?

Drew

The filter-aide is used in conjunction with filter media. The filter media does not allow the filter-aide to pass. The filter-aide is larger than the media's holes but lays down a porous bed so after it is deposited, things under a certain size do not go thru. The media below acts as a last resort to stop oversize.

With the diatomaceous earth I have used, I suspect some very small particles get thru but in many applications this is not an issue. This may not be the case with jet fuel. I suspect the suppliers of various filter-aides may have more inert and more consistent products than the ones I have used so far.

If the system sees large pressure variations or the pressure goes 0 or negative the built up filter bed will be disrupted. This could be bad unless you are doing it on purpose to back flush out the particles and restart the process.

As for how it holds up, you constantly monitor the differential pressure across the filter. When it gets to high you do an automatic back flush or take it apart and clean it out then start over.

Do I get a biscuit now?

Yup

With most of the fuel systems I have worked with we have periods of no flow between operations, so I doubt this would be effective for improving fuel filtration. On the pipeline where flow is more consistent we use a different type of filtration primarily designed to remove large particles and precipitate some water. Jet fuel destined for the airport is pushed through micronic and coalescers plus an additional filtration media before we let it to to the airport. The additional media consists of large cloth bags filled with dry granulated clay; this was new to me and I cannot recall if it was designed to absorb any final water or more particulate.

(dirty granulated clay filters)

These filter vessels are huge with lots of elements to keep pressure drop low.

Drew

Please do not consider this for any fuel that is going into something that I have to fly on - madness. The OP's last post was about filtering power fluid to NAS 3, not sure I would want to try this there either. As stated "in some scenarios", but I am not convinced.

I don't see that anyone was recommending the use of filteraide for filtering fuel.

The method I threw out is widely used in various processes (not madness) but obviously needs to be applied where it should be and in the correct manner. The original poster never even said what they were filtering out or what the carrier was.

There are thousands of properly characterized process options in the manufacturing ,and other, worlds. I see a huge benefit of CR4 is to help those with questions consider different scenarios and available methods. Drew K and I had kind of gotten off on some side points.

I was just throwing out an option where the original poster had not given details.

Thanks Dave, I only included the information I had because I feel this is a good forum to learn things. Even if my information was not exactly on topic it was interesting to me so I shared what I have and explored what else is out there. Never did I recommend using filter aide for fuel, just asked about it.

Besides many people here already answered the original topic's question; what is wrong with sharing knowledge now?

Drew

The principle is same as the normal pipe line flow. Think on that aspect.

3 micron filter means the spacing between wires is 3 micron (not actually, since other aspects come in, just assume)

10 micron has 10 micron spacing.

So the metal to passage ratio will be less in 3 mic filter.

The viscosity of fluid (even assuming perfectly clean oil) will generate a larger pressure drop. So what you do? increase the pipe size flow crossection. ie increase the filter section.

There are many ways (eg by increasing number of folds, increasing the filter cartridge diameter/length itself,...)

_{This is as already posted above (just explaining)}

Yes. Make the 3μ filter have more surface or cross-sectional area than the 10μ. Examples can probably be found in manufacturers' literature.

The point that people are making here is that "head loss" depends on more than one variable.

The finer filter presents less area to the flow which decreases velocity. Since head loss is directly proportional to the velocity squared, you will get a very large non-linear HL as the area is decrease by more closely spacing the filter fabrics.

This is offset by making the filter larger as V = Q/A where Q is the flow and A is the free area. For the moment, let's ignore boundary conditions.

As a very rough guide you would have to have approx 3.3x filters of 3 micron each to maintain the same pressure and flow rate as 1x 10micron filter. If you are able to manufacture a new filter then the 3micron filter would need to be 3.3 times as big as the 10micron filter to have the same pressure loss and flow rate.

So to have less pressure drop than a 10micron filter, you would need 4 or more equivalent 3micron filters sharing the flow.

Increase filter surface area by parallel filters or bigger size filters to minimise delta P. If the intial delta P is okay for process, but want to increase filter change interval, than use 10 micron filter before 3 micron filter in series. Hydralulic filter suppliers can provide the exact data about pressure drop at different flowes and dirt holding capacity of the filter. This information will be helpful for further calucaltions.

Hi,

You do not say what medium you are filtering and my comments arebased on hydrauliv fluid but the apply to other fluids as well. If you go to anyof the majorfiltermanufacturers catalogues you will see they give pressure drops based on Flow rate and temperature against micron rating. your NAS 3 is tight and would normally only apply to hydraulic servo systems but it is attainable, although the NAS standard has been upgraded the principles are the same. Try looking at Parker, Pall of MP Filtri for suitab;le filters and guidance. Google NAS standard and you will get to the revised standard.

Regards

Oliver Dunthorne