Bags and pistons - compression ratio?

[danl]

Again, a copy of a post I put on another board.

What is the ratio of air volume in a cylinder between the state when the air is compressed (maximum ballast) and when it is fully expanded (minimum ballast)? (New to this - does that sound right?)
The reason for asking is to understand how much energy is used for compression. Compressing only half the volume of the air would double the air pressure. If though the same volume decrease represented 4/5ths of the initial air volume in a smaller enclosure, the final pressure would be five times higher, requiring a lot more energy to compress. Is this a factor used in sizing a system? Or not a factor at all?

[anonymous]

Hi Dani,

Yes I see what

Hi Dani,

Yes I see what you mean.
Without doing the calculations, I would expect (for models) friction and other losses to be much greater than the energy used for compression.

Try working out the energy used by a motor running for several minutes at say an amp or two. Try comparing that with the energy used to compress say a litre or two of air up to a few atmospheres.

Excuse the mixed units and the fact that my thermodynamics is incredibly rusty for not doing the calculation!

David

[anonymous]

Hi Dani,
Just an addition to

Hi Dani,
Just an addition to my previous reply which refers mainly to pumped systems.
The limiting factor is more the pump itself. I doubt whether you can get pumps to deliver more than 1 or 2 atmospheres in any case.

With cylinder systems you start to get excessive friction or seal leakage again at a few atmospheres.

If you are also looking to size ballast tanks could I recommend Skip Assay's excellent suggestions. (In previous SCR's and by searching this site for Skip, I think.

David