Compression of air

[radio1]

Can anyone help?

If I want to calculate the time required to pressurise a large cylinder with air, what formula do I use? I know the volumetric flow rate of the air and the volume of the cylinder! I assume the air compressibility comes into it and whether it is an adiabatic or isothermal compression!

Thanks, Andy Ryder

 

[pmover]

ok,
-determine mass of air in cylinder based on initial pressure and temp.
-determine final mass of air in cylinder based on final pressure and temp. final temp can be swagged by knowing initial pressure and final pressure. some heat will be rejected to surrounding atmosphere, but state assumptions.
-determine difference in cylinder air mass at final-initial conditions.
-determine mass flow rate into large cylinder. does the mass flow rate remain a constant or does it vary with cylinder pressure? something to ponder...
this should be sufficient to get a reasonably educated engr started to work the problem.
good luck!
-pmover

 

[sailoday28]

pmove has hit it on the head.
FOR THE ACADEMICS, consider the cylinder to be perfectly evacuated (negilible mass)and the inlet source to be air at a fixed Temperatrue. For a perfect gas, const specific heats, what is the final temp after compression? AND WITH PERFECT INSULATION THRU OUT

 

[israelkk]

As long as the pressure inside the cylinder is approximately less than 50% of the charging pressure the mass flow into the cylinder is constant (choked flow). Above it the mass flow rate decreases all the time because the inlet pressure of the cylinder increases. Therefore, you have to solve a time dependet differential equations for the choked zone and the non-choked zone.

If the inlet orifice to the cylinder is large enough it will be closed to an adiabatic process. However the gas temperature at the end of charge will be much higger at the charged pressure and when the cylinder will be colder the pressure will be smaller. Therefore, when you want exact pressure after the cylinder is cooled you have to use the gas weight in the cylinder as a measure when to stop charging the cylinder.

When we pressurise our vessels we weigh the vessel before and after charging to make sure we have the exact quantity of gas in the cylinder. From the weight/mass of the gas and the cylinder volume we calculate and know the pressure in a desired temperature.

Is this a real life problem or an academic assignment?

 

[sailoday28]

I am the only one that posed the academic question.AND pmover has answered the original question.

 

[zdas04]

Israelkk,
If you assume that the compressor is direct-coupled to the tank, then you will never have choked flow (compressor discharge will always be slightly greater than tank pressure).

This is one case where those pesky SCF beasties significantly simplify the calculation. If the cylinder starts with 0 psig then the initial volume of the tank is already in SCF (more or less). The volume of the tank at pressure and temperature is Vinitial*(Pacutal*Tstd*Zstd)/(Pstd*Tactual*Zactual). The compressor rating will be SCF/min so you divide the difference by the compressor capacity to get fill time.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The Plural of "anecdote" is not "data"

 

[israelkk]

zdas04

radio1 didn't say that he uses a direct connection with a compressor. In our system we have either a large vessel or many 42 liter vessels in parallel which are charged by a compressor/booster to high enough pressure and then we use this pressure to charge the smaller vessels to the desired pressure.

 

[israelkk]

zdas04

I assumed that radio1 just wanted the physics of constant volume charge from a constant pressure source.

 

[zdas04]

Israelkk,
Trying to do engineering with partial information is truly an excellent way to solve problems that don't really exist.

Oh well, between all of us we've answered most of the viable scenarios.

David