AHU filter understanding

[geopat69]

Hi.

Hoping for a bit of advice regarding dust sizing for filter selection. It seems as though most specs and literature think of dust as roughly spherical in shape.

But In regard to fibrous dust (eg wool batts etc or husk factories) the fibres can be 0.1 micron diameter but the length can be 5 microns long.

Any ideas on what the govenrning dimension should be for ahu filter selection? Or does one have to convert the fibre to a equvalent sphere shape.

 

[nuuvox000]

So it's 50 times longer than it's width? If you were to design to the 0.1 micron diameter, it would have to be perfectly aligned (use the force, Luke) to make it through the filter opening; logically, I don't see that happening most of the time. Since filters aren't typically 100% efficient anyway, I would design to the 5 microns. I don't think you'd see any practical difference.

 

[MintJulep]

High aspect ratio fibers will tend to accumulate on the entering face of a filter.

None of the filter ratings will apply, because they are all based on spherical test dust.

A mesh screen or strainer is likely a better choice than a filter. Look at the lint "filter" of your clothes drier for example.

 

[geopat69]

Thanks for the responses so far. It is an unusual one.


But why a mesh filter? I would have thought it was too coarse! And l let fibres and other sperical dust thru.

It is an interesting observation of the fibres accumulating on the entering face. I would think...given the size, most could penetrate and the remainder caputured by diffussion (but within the filter medium). For example by ignoring the 5 micron length, and assuming a 0.1 micron sphere, diffusion would capture most particles... the answer lies in between. Strange that test dust is only spherical and does not really address fibre dust