Filters in parallel or in series

[p3sx0s]

Hi Guys,

A sort of generic question:
If we were to use three identical filters, firstly with the two in parallel followed by the third in series, and then all three in series, which method would be the most effective in terms of impurities removal?(lets assume that the system you have is THF with carbon particles dispersed and try to keep it simple: The only parameter available is the flow rate, so, given that I use the same flow rate in the outlet of the systems (5 lt/min) for both systems, which one would get cleaner THF?

I'd appreciate if you could share your opinion

 

[ash9144]

A key point you fail to mention is mesh size. If all 3 are different sizes go through the largest mesh first the smallest mesh last. If they are all the same size then I would recommend two in parallel one as a spare to use when one of the first two plugs.

 

[p3sx0s]

Hi ash9144,

I mentioned that all 3 filters are identical,

Thanks for that,

Zicos

 

[aspearin1]

Big question. As you probably already know, filters are dynamic in terms of efficiency. The more the cake builds up, the more efficient it becomes at particle retention. This said, filters in series, you are building the cake on the first filter... and that is the filter doin essentiall ALL the work, it is pulling out the big... and the small. The only use for the other filters is to catch whatever larger particles pass by the first. But pretty much all the smaller particles that pass the first will pass the second and third.

For the filters in series, the two in front will take most all the load. The flux is cut in half, so efficiency increases slightly, but the cake is also divided between the two. This improves pressure drop, but reduces efficiency. If in the first scenerio (with the filters in series) you used 100% of the filter capacity, in this parallel scenerio, you now used two filters, but only half the capacity for each. Again, the third filter is relatively useless. The real reason for filters in parallel is to reduce the pressure across a single filter. Suppliers prefer multi-round housings rather than parallel piping because it: A) makes them more money and B) evenly distributes the filter load when compared to parallel plumbing legs that do not have identical pressure drops. (i.e. the filter in the shorter leg of piping gets most of the work)

In short, if you want to increase efficiency, you can have a dynamic system, where the "caked" filter gets transfered to the third position in series after a cake builds up... OR... follow ash9144's recommendations, which is industry standard.

Hope this helps. Was this more of a philisophical question (i.e. class assignment), or are you looking for a practical solution?

Aaron A. Spearin
ASQ CSSBB
Engineering Six-S'$

http://www.Engineering6ss.com

"The only constant in life is change." -Bruce Lee

 

[p3sx0s]

Dear aspearin1,

thanks a lot for your views, it does help, and no this is not an assignment, it is a real project i want to sort out...we already have a system with 2 parallel and one in series and thinking about converting to all 3 in series...

Regards

 

[p3sx0s]

oh, and something else...

Does it make sense if we assume an electric analog (thinking of the filters as resistors)where, in series the total resistor would be 3R whereas in two parallel and one in series, the total resistance is 1.5R?

what do you think of that?

 

[epoisses]

True.
Note though that this does not take into account that delta P across a filter changes over time as aspearin described. Filters in series will disturb the downstream and/or the upstream unit in the sense that it upsets the pressures. Filters in parallel (OK unless you put a third one in series...) will do that much less as long as you change them diligently one at a time.

 

[roker]

hello,

two should be in series and the third at stand by,
after the cake is built on yhe first one (dp control) switch between the first and the second (by piping design) so the second one with the cake will catch the smaller particles. The stand by filter will replace the first one while the second one will be cleaned.
All by dp control.

regards,
roker

 

[aspearin1]

It sounds alot like what roker is saying. That third one in series sounds more like it is to protect from upstream disruptions when changing between the two in parallel. So essentially, at any given time, you only have 2 operating in series, with one stand-by. This makes sense.... As far as "resistance" the dp for the in series compounds. The dp in parallel, is cut by the number of legs added, but this is really because you are multiplying the flow area of your piping. i.e. each leg has 1/2 the flowrate if 2 lines are in parallel. Rather than changing your plumbing system, it's highly likely you can get the results by changing your filter time. Most filter users use the cheapest available filter, when a premium priced filter will give them much better cost of ownership. Whay type are you using? What is your application?

Aaron A. Spearin
ASQ CSSBB
Engineering Six-S'$

http://www.Engineering6ss.com

"The only constant in life is change." -Bruce Lee

 

[rbcoulter]

If removal efficiency were the only design parameter then putting all three in series would be best. As other pointed out; however, this may not be the optimal choice for other reasons.