# Why does filter efficiency decrease with thickness?

I understand that the most important parameters for a filter are basically pressure drop and thickness, both of which affect efficiency. However why does thickness cause a change in efficiency?

I'm particularly interested in the use of nanofibers as filters, and why decreased diameter increases efficiency.

• When you say that thickness is a parameter, are you talking about fiber thickness or total thickness of filter? I am confused because you talk about the fiber diameter at the end of your question. Are you instead talking about opening size?
– hazzey
Nov 7, 2015 at 14:32
• Thickness of the total filter, although Im also interested in fiber diameter Nov 8, 2015 at 1:24
• Thicker filter will decrease the flow rate. It will almost certainly improve efficiency, that is purity of the filtered substance.
– SF.
Nov 16, 2015 at 9:41

More stuff in the way of the flow causes more back-pressure.

Think of a thicker filter as being two filters in series. For example, if a filter develops 0.1 PSI across it when 100 CFM air flow is going thru it, then two filters in the same 100 CFM air flow must together drop 0.2 PSI across them.

Depending on what exactly your criteria for efficiency are, more filters in series may decrease efficiency. If the first filter blocks all the particles you care about, then extra filters only cause more back pressure without any better filtering. For example, consider two window screens on the same window. The outer one already blocks all mosquitos, so the inner one doesn't make the overall screen perform better. It does however make it more difficult to see thru the overall screen. If you consider efficiency as mosquitos blocked per light blocked, then two screens is clearly less efficient than one screen.

• It seems like the efficiency of the filter is due to the spacing between the wire in the screen, using your metaphore. If this is true, why do the properties of the wires in the screen really matter? Nov 13, 2015 at 17:59

Filters can be evaluated by a few different parameters. "efficiency" is a broad term that doesnt lend to a better understanding in this application. Here are some better metrics to consider:

1. Maximum flowrate (same as pressure drop or efficency when considering a power loss from the pressure drop. Proportionally increases with filter surface area.)
2. Maximum pressure (before rupture)
3. Material (chemical resistance)
4. Storage (how much contamination the filter can hold; how quickly pressure drop increases)
5. Maximum diameter of particles that can flow through. (ex. 1 micron, 10 micron, 400 mesh)
6. Other technology/principles utilized (vortex, oil wetting, diatomaceous earth filter aid, self cleaning, gravity settling, scrubbing, etc)

In a cotton(or other fiber) wound filter cartridge there is additional thickness beyond what is necessary to archive the 1 micron maximum particle rating. This additional media increases the storage of the filter; giving it open surface area even as the filter is contaminated. If there was a single 1 micron membrane on the outer surface it would quickly plug with even the slightest contamination. So while the thicker filter may initially have a larger pressure drop for a given flow and surface area, it will last much longer than a thin filter that would plug quickly and have an even larger pressure drop.

Nano fibers are attractive for filtering because the bulk ratio of empty space to fibers is much greater for a desired particle size rating. The larger amount of empty space both reduces flow resistance(pressure drop) and increases the storage capacity of the filter. It may be easier to visualize at the extremes; you could make a 400 micron filter with 1 inch diameter machined rods placed side by side, but the max flow rate and storage would be very low compared to the same surface area of cotton fabric composed of small fibers.