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# How to Calculate the Blocking Effect of a Filter

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## How to Calculate the Blocking Effect of a Filter

(OP)
Hi all,

I have a problem that i can't seem to find an answer too.

The problem:
I have a vessel with a single inlet and a single outlet.
the inlet is connected to an air pressure line with Q=1.145 l/min and pressure of 1.5-2 bar (steady flow).
The outlet is "obstructed" by an Air filter (PTFE, Teflon) with filtration size of 0.2um.

In case of a steady state, i want to calculate the accumulated pressure (if at all), because of the filter "blocking" the outlet.

Any help is appreciated.

Best,
SD

### RE: How to Calculate the Blocking Effect of a Filter

The volume you put into any pipe will also have to come out of the pipe. (If you try to put more in than the capacity you will overflow backwards). Think liquid in a sieve with constant liquid level. The pressure against the sieve is constant but the velocity increases, https://sciencing.com/calculate-air-flow-rates-682...

### RE: How to Calculate the Blocking Effect of a Filter

(OP)

#### Quote (IRstuff)

?? Where was the 1.5 to 2 bar measured, then?
This value is set at the outlet of the pressure regulator valve (before connecting to the vessel).

#### Quote (gerhardl)

The volume you put into any pipe will also have to come out of the pipe. (If you try to put more in than the capacity you will overflow backwards). Think liquid in a sieve with constant liquid level. The pressure against the sieve is constant but the velocity increases, https://sciencing.com/calculate-air-flow-rates-682...
Thanks for the link, but it doesn't answer my question (or I don't understand my problem correctly).

If i remove the filter from the outlet, i will get a free flow and no pressure will be build up in the vessel.
But as soon as i place the filter it's fine mesh hinders the free flow, so there is a pressure build up in the vessel (most probably a bit lower than the inlet pressure as air can flow through the filter, but still higher than atmospheric).

If I'm looking at this problem correctly, without a filter the conditions at the outlet are straight forward to calculate, and with a large enough outlet diameter the pressure inside will be atmospheric.
If i block the outlet completely, the pressure in the vessel will equalize to the pressure at the inlet (1.5-2 bar).
But with the filter in place it's like partially blocking the outlet, effectively shrinking the outlet diameter.

So if my analysis of the problem is correct, i'm looking for a mathematical expression that will address this problem, of how much blockage the filter creates, which means I'll get a pressure build up in the vessel (this pressure build up is what i'm trying to find).

Same way as you have a correction factor of K (for bend loss), when the flow is not through a straight pipe.

### RE: How to Calculate the Blocking Effect of a Filter

You need the flow coefficient (Cv) for the filter. dP = (Q/Cv)2. Call the filter supplier, this is all proprietary info they should provide.

### RE: How to Calculate the Blocking Effect of a Filter

Seems to me there's nothing to calculate; the pressure is basically 1.5 to 2 bar, dictated by the pressure regulator. If the vessel pressure were higher, there would be no flow, since the pressure regulator prevents the pressure from getting higher

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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### RE: How to Calculate the Blocking Effect of a Filter

Helepolis,

That clarifies what you are after. You need to brush up on flow theory. Better ask the factory (as stated in another post) or by measuring the flow through the filter at a given pressure (1,5 to 2 babr?) https://en.wikipedia.org/wiki/Flow_coefficient

Once you have the cv you can calculate for other flow values.

### RE: How to Calculate the Blocking Effect of a Filter

For a reference you may want to peruse thru the "Handbook of Filtration" authored by The Eaton-Dikeman Company. This handbook basically deals with filter paper research and testing, however, it could provide you with additional insight into your research of the Air filter (PTFE, Teflon) with filtration size of 0.2um stated in your OP. Amazon list the price of that book under \$5.00

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