Principle of operation
When gas in a cylinder at high pressure is allowed to flow to another cylinder containing gas at a lower pressure, the pressures will equalize 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 a 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-liter cylinder pressurized this time to 250 bar were then used to "top-up" the 10-liter cylinder, both of these cylinders would equalize to about 240 bar. However, if the higher pressure 100-liter cylinder were used first, the 10-liter cylinder would equalize to about 225 bar and the lower pressure 100-liter 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 the 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. In practice, the theoretical transfers can only be achieved if the gases are allowed to reach a temperature equilibrium before disconnection. This requires significant time, and a lower efficiency may be accepted to save time. Actual transfer can be calculated using the general gas equation of state if the temperature of the gas in the cylinder is accurately measured.Uses
Breathing sets
A breathing set cylinder may be filled to its working pressure by decanting from larger (often 50 liters) cylinders. (To make this easy the neck of the cylinder of theCompressed natural gas fueling
Cascade storage is used at compressed natural gas (CNG) fueling stations. Typically three CNG tanks will be used, and a vehicle will first be fueled from one of them, which will result in an incomplete fill, perhaps to 2000 psig for a 3000 psig tank. The second and third tanks will bring the vehicle's tank closer to 3000 psi. The station normally has a compressor, which refills the station's tanks, using natural gas from a utility line. This prevents accidentally overfilling the tank, which could happen with a system using a single fueling tank at a higher pressure than the target pressure for the vehicle.Hydrogen storage
In cascade storage systems for hydrogen storage, for example at hydrogen stations, fuel dispenser A draws hydrogen from tank A, while dispenser B draws fuel from hydrogen tank B. If dispenser A is over-utilized, tank A will become depleted before tank B. At this point dispenser A is switched to tank C. Tank C will then supply dispensers A and B and tank A until tank A is filled to the same pressure as tank B and the dispensers are disconnected, after which the control system will close the control valves to switch to its former state.Arrangement of system
References
External links
* {{Underwater diving, divsup Breathing gases Industrial breathing sets Diving support equipment Gas technologies Hydrogen storage Pressure vessels