Principle of Operation
When gas contained in a cylinder at high pressure is allowed to flow to another cylinder containing gas at a lower pressure, the pressures will equalise to a value somewhere between the two initial pressures. The equilibrium pressure is affected by transfer rate as it will be influenced by temperature, but at constant temperature the equilibrium pressure is described by Dalton's law of partial pressures and Boyle's law for ideal gases.
The formula for the equilibrium pressure is:
- P3 = (P1×V1+P2×V2)/(V1+V2)
- where P1 and V1 are the initial pressure and volume of one cylinder
- P2 and V2 the initial pressure and volume of the other cylinder
- and P3 is the equilibrium pressure.
An example could be a 100 litre (internal volume) cylinder (V1) pressurised to 200 bar (P1) filling a 10 litre (internal volume) cylinder (V2) which was unpressurised (P2 = 1 bar) (resulting in both cylinder equalising to approximately 180 bar (P3). If another 100 litre cylinder pressurised this time to 250 bar were then used to "top-up" the 10 litre cylinder, both of these cylinders would equalise to about 240 bar. However, if the higher pressure 100 litre cylinder were used first, the 10 litre cylinder would equalise to about 225 bar and the lower pressure 100 litre cylinder could not be used to top it up. In a cascade storage system, several large cylinders are used to bring a small cylinder up to a desired pressure, by always using the supply cylinder with the lowest usable pressure first, then the cylinder with the next lowest pressure, and so on.
Read more about this topic: Cascade Storage System
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