Air flow figures in compressors...

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

Externet

Contributor
Messages
168
Reaction score
40
Location
Mideast U.S.
Hello all.
Something does not smell correct. Is the rated air flow for a compressor at the rated output pressure; or is it the flow suctioned at first stage inlet ? Or is it outlet flow not filling anything ? Can anyone clarify ?

My 3.5 CFM Kidde used to take about half an hour to fill a tank. 80/3.5 = 23 minutes; about right.

Now, 3.5 cubic feet is a considerable volume at atmospheric pressure = 100 litres ! That is half a '55gallon drum' of air to be pumped every minute !. And a brutal amount of air mass if at 3000psi.

Light, please !
 
Let me give it a twist with other view/wording...

71.2 cu ft of air = 2 cubic metres of air; which is 2 metres x 1 metre x 1 metre = a 'big' amount of air.
80 cubic feet of air is 2.25 cubic metres of air, which is 2.25 metres x 1 metre x 1 metre = a little bigger amount of air, like a big closet size of your house to grasp an idea of the room that much air occupies.

All that 2250 litres 'closet size' amount of air is compressed reducing its volume to ~12 litres to fit inside a scuba cylinder.

A compressor specs say 4.3 cubic feet per minute = 0.12 cubic metres per second = 120 litres per minute. Should take 80 / 4.3 = 18.6 minutes to fill.

Is that 120 litres each minute the flow coming out of the compressor outlet at which pressure ? On a 3mm internal diametre of a filling hose, to pass 120 litres every minute, the speed of air flow has to be brutal.

Or, is that the amount of atmospheric air suctioned by the compressor first stage ?

If that 4.3 cu ft/min compressor = 120 litres/min and the cylinder has a capacity of 12 litres ! ; that compressor would fill ten cylinders in a minute ! If that is outlet air volume per minute.

=====================================================================

2.25 cubic metres of 'closet size' amount of air are suctioned by the compressor and fitted in a 12 litre room inside the scuba cylinder.

12 / 2250 = 0.0053 = 0.53% The volume reduces to 0.53 % of the original.

Then, the air volume entering the scuba cylinder has to be 0.53% of the rated 4.3 cu ft/min = 0.53% of 120 litres/min = 0.63 litres/min. That sounds reasonable to flow into a 3mm internal diametre entering to fill the scuba cylinder.

Can you confirm my rationale ?

I have something distorted in my head about this... Perhaps because never paid attention before ?
 
Or, is that the amount of atmospheric air suctioned by the compressor first stage ?
Most compressors are marketed on the basis of "free air delivery" so its the volume pumped against no resistance.

Once you add resistance - so there is pressure in the receiving tank that output drops because compressors are not perfect machines and you get what is called "blowby" where a fraction of the gas moves past the rings and isnt compressed. How much blowby you get is a function of the initial build quality and wear and tear. At very high pressures you get very high blow by
 
https://www.shearwater.com/products/peregrine/

Back
Top Bottom