Amount of air required to charge one cylinder using another

[R1chJC]

Hi All,

I have a cylinder at 270bar containing 250 litres of air at around 15Deg.

I need to fill a volume of 10m^3 to a pressure of 2bar from the above. What volume of air is required from the first cylinder? And what percentage loss is that?

I'm only after approximations.

My initial thought was using PV=mRT, but i'm struggling to connect the theory.

Any help would be great!

 

[R1chJC]

I should clarify, by air I mean atmospheric air with a density of 1.29 kg/m^-3.

 

[zeusfaber]

With a target pressure of 2 bar, the first question is going to have to be what the starting condition is in your destination. Teaching Granny to suck eggs here, but pressurising a ten cube tank that's already full of air at 1 bar is going to take a lot less air than blowing a ten cube ballast tank at 10m depth.

Assuming this is a HP Blow sort of problem, you're trying to work with 20 "Normal" cubic metres of air.

A 250l bottle will drop about 4 bar for each "Normal" Cubic metre of ideal gas you take out of it at constant temperature so, if you draw 20 cubes off slowly enough for the temperatures to stay roughly constant, you will end up dropping about 80 bar (30ish percent).

In practice, the pressure drop will be a bit greater for two reasons.

One is compressibility - above about 230 bar, air starts to deviate noticeably from ideal gas behaviour.

The other is cooling. As the air expands out the storage cylinder, both the escaping air and the residual air will cool. Once you close the valve, the air left in the cylinder will start to warm up and you will see some pressure recovery there. Depending on the design of your receiver, as the cold gas you've filled it with warms up, you may either see an overpressure or find that you start to vent off excess volume as the gas expands.

A.

 

[georgeverghese]

It is always a good habit to qualify your pressure values - use barg or bar abs and not just bar

Also, what is the initial pressure of the second cylinder ?