Breathing filter design resources

[marcin_m]

I'm trying to get a rough idea of the optimum filter size & configuration for a face mask in different breathing conditions (sitting, walking, exercise etc.). I'm also trying to determine if it'd make sense to boost the airflow in the mask with a fan.

I'm struggling to find good resources on filter design and sizing calculations. Could anyone recommend any books/papers on the subject? It seems like in the industry you determine the required air velocity and flow rate (how?), and that simply gives you the filter size.

For a face mask, we're trying to minimise the breathing resistance (pressure drop across the filter). I'm trying to calculate how this depends on filter size and configuration.

Basically, the idea is to deterministically design a breathing apparatus, from the filter onfiguration (prefilter->HEPA->activated carbon), through filter size, to casing, sealing, manufacturing etc.

I'd appreciate any technical references that could serve as a starting point for this process.

I'm new to the forum, and I did not know which category this question best fits in. Please move it if there's a better place for it.

 

[itsmoked]

Filters always present resistance. Resistance is bad for your lungs, actually damaging them slowly. This is why in job related uses of respirators testing is done to confirm a person can handle the added burden of that resistance. If you want to add HEPA type resistance you simply MUST provide power assistance. Adding carbon etc. more so yet!

The almost ideal solution is something along these lines:

That's a 3M powered unit that runs about 12 hours on a charge. Unfortunately they charge and arm, leg, and first born for it. It's also ridiculously long lead-time. I'm slowly working on a 3D printable clone of it. It would pretty much completely solve all COVID hassles. Looks cool in my estimation too!

The filter elements are at the waist mounted blower with the battery minimizing head-born weight. The helmet only needs to sort-of seal because you need the air to turnover in it frequently and continuously so there is no tough sealing problem. It needs to leak like a sieve. This provides a cloud of sterile air in the user's face with no detectable breathing resistance. The filters can be reduced essentially to flat rectangular pieces of material. (inexpensive)

Keith Cress
kcress -

http://www.flaminsystems.com

 

[marcin_m]

Thanks for the reply Keith. Loose-fitting PAPRs are definitely the most comfortable to wear for porlonged time, but I'm not convinced that we can convince the general public to wear them - they're big, bulky, expensive, and putting them on is a hassle.

I'm thinking more along the lines of a tight-fitting half-face APR, with as low breathing resistance as possible. Possibly with a small fan to aid the wearer's lungs. 5 million people wear those for work in the US alone. My 3m 7500 series is quite comfortable to wear for 1-2 hours at a time.

I'm looking for references that would allow me to calculate the breathing resistance, lifetime etc., to make rational choices about my filters, and go directly to a manufacturer with a well-defined spec.

 

[DAVIDSTECKER]

Just a thought but if you are going to use a powered filter pack, maybe use the filters already available and do something like an air curtain that can be worn like a head band or a collar?
You could make the air flow adjustable to account for different conditions such as inside or outside to adjust for wind.
Another option maybe to pressurize a "Neck Gator"?
I'd also recommend using a common battery from someone like Ryobi, or other tool maker.

 

[itsmoked]

DAVIDSTECKER; Interesting. However "outdoors in the wind" would require no protection so and on/off sensor would cover that. Love the power tool battery suggestion. I'll take that! Though I'd use a good battery rather than a Ryobi cheap junk. I believe Milwaukee are the gold standard for tool batteries.


marcin_m; I get it... But I find putting regular APRs on to be a far bigger hassle and then I end up with marks all over my face for an hour after taking them off. Adding anything to them makes them heavier yet. You'd be adding a motor, blower (not a fan), and a battery. That all adds to the weight trying to pull one's face down and inertia that one's neck muscles have to accelerate and decelerate constantly. Might not matter for just a few hours but someone would find themselves using it all day.

Keith Cress
kcress -

http://www.flaminsystems.com