Goodness me, I did miss a lot of interesting topics while in hospital!!
Mostly it's me dabbling and tinkering with a bit of maths here and there... But it looks like progress will resume this weekend.
I have taken receipt of a pair of centrifugal fans. two days ago. I immediately tested them out. These are two miniature 5 cm wide, 5.0V brushless computer fans. The 5V rating is critical because it allows me to use USB power bank batteries for smartphones, which I think would be an easy and excellent battery pack.
Fan Specs:
Dimensions: 50mm x 50mm x 15mm
Rated Voltage: DC 5V
Connector:USB
Bearing Type: Sleeve bearing
Rated Current: 0.12±10%Amp
Noise: 25.7±10%dBA
Air flow: 2.91CFM±10%
Weight: 40g/pcs
At 2.9 ft
3/min this yields an airflow of about 82 L/min which seems very high, if I just take it at face value
I don't know if these give me 82 L /min, but I can tell you that the flow is several times higher than the maximum flow I get with the piston running at 4 cycles per second. So let's say 10 times, then it'd be about 60 L/min at least, and that certainly seems realistic. I put them to the test right away, because I need to know how much they can compress air. The problem being that if I want the system to be upgradeable to a high pressure air con system it needs to at least use the same kind of tubes that will hold the basketball needle, and that's without saying anything about whether it can build enough pressure to pass any air through the needle.
What you want is to build enough pressure in a stagnation chamber (pill bottle below) to overcome the drag of a fully developed flow in the vinyl tube. The larger the stagnation chamber the better to make sure the flow comes to rest and all kinetic energy in the flow is converted to pressure energy. As the flow velocity goes to zero, you maximize the pressure in the stagnation chamber (hence the name "stagnation chamber").

At low pressure and velocity say M < 0.3 the airflow behaves more like an incompressible liquid than a gas.
Hence we can compare to a developing flow of water in a pipe. Net viscous drag force is proportional to the length of tube

I expected that these would be poor compressors compared to the piston, because that is a thermodynamic fact. Vane rotor compressors such as turbine compressors are one whole step lower in efficiency than pistons, which is why a piston internal combustion engine is always more efficient than a turbine jet engine in converting fuel to kinetic/mechanical energy. The reason turbine engines are better in aircraft is that compared to piston engines, turbine engines are very light relative to the power they unleash. To match the power of a modern jet engine you need an absolute gigantic monster of an engine block, which is prohibitively heavy, and thus kills any and all thermodynamic efficiency you had with the piston cycles.

Similarly, in this case I found that while the piston engine is every efficient, the way that the piston is made gives you a heavy cumbersome mechanism with issues such as high friction forces, heat, reciprocating oscillations, etc. My rotating compressor pump is basically nothing more than a rotating piston, and made from leather and wood and items like that I found it ridiculously difficult to bring down friction and achieve good compression (leaky seals), unless I make the pump out of some highly polished metal. In other words, you waste most of your energy just overcoming friction with a geared electric motor, making the piston's "high thermodynamic efficiency" a moot point ( the overall compressor's thermodynamic efficiency is paired down to nothing, in other words).
As expected the centrifugal fans are very quiet and relatively frictionless. As I wrote I 'd expect the flow rate from the nozzle of the fan to be over 60 L/min - it's hard to measure, I'd need a little anemometer). So I'm throwing math away for a second and just doing things by tinkering. I know the "compressor" is very poor. What I found is that performance drops dramatically as you close the diameter of the air outlet. At the diameter of the needle, barely any air comes out. Not even close to the amount of air the bicycle pump can push through the needle and more like the fish tank air pump, I'd say.
The centrifugal fan is by definition a compressor, as it has the right geometry (a spiraling horn opening in the direction of the air, alternatively called an "involute casing") but it simply can't push air hard enough to pump it through the needle. So I tried "measuring" the flow without the needle, and I found that the 6 mm internal diameter of the vinyl tube with just a short length is probably giving me more than 26-40 L/ min (I'm guessing here - very unscientific, but I now have a feel for what 60-80L/min feels on my face). Even at 26-40 L/min that is already 4 times to 6.7 times the minimum respiration rate. So if I'm lucky (high end) the setup shown above with one single fan is already giving me the maximum respiration rate of a human being, Enough to run around the block.

As a compressor, the device is poor, but the problem is made a bit worse by the fact that the vanes are inclined in the opposite direction when compared to an actual centrifugal compressor direction. Centrifugal pumps can gave vanes that are perpendicular, inclined toward the direction of the flow (shown above) or inclined in reverse to the direction of the flow. Paradoxically, it is the reverse orientation of the vanes that gives you the highest thermodynamic efficiency and highest compression potential. The forward facing vanes give you the highest output velocity and the lowest thermodynamic efficiency. So these little buggers were designed to maximize speed and not pressure and the manufacturer doesn't care about efficiency. But there's nothing I can do other than 3D printing my own rotors. This will have to do.
It looks like these little centrifugal blowers/fans are just the thing I need. They're quiet enough and I trust I can use a battery. I'm going to try using a smartphone power bank and see how long it lasts. The goal would be to at least get one hour out of it, for high risk situations such as riding the bus in close proximity to others.
If I really wanted to 3D print my own, then I'd have to use all these nice maths which I did study in college, but I'm not sure if it;s even worth my time to dive into this subject. It just seems that every step I take turns into an engineering project. Being an engineer doesn't help that way

Ignorance is bliss.
https://en.wikipedia.org/wiki/Centrifugal_compressorThe thought has crossed my mind that I could use a metal canister, such as a thermos bottle, as a stagnation chamber and add and LED UV light inside the stagnation chamber to kill as many viriii as I can. Impossible to tell how effective that would be, but it's an interesting thought. Another even more outlandish idea is to find a UV laser and force the airflow to pass through a "laser gate" zapping the killer SARS bugs along their way.
Assuming that real world horrors don't stop me, I'll be working on this during the weekend...
Cheers!