6.7 One way to liquefy a gas is to flash it through a valve. However, this requires that the inlet conditions to the valve be at a state where an expansion will cause the gas to flash into saturated liquid and saturated vapor. Consider a situation where helium gas is at 10 bar and the desired pressure of the liquid is 1 bar. To produce liquid helium, the inlet state to the throttling process must lie between the two states, identified as a and b on the P–h diagram shown in Figure P6.7A. For an inlet pressure of 10 bar, this temperature range is 2.53 K

≤ T ≤ 7.60 K for helium (calculated using EES). One possible way to achieve this low temperature at the entrance to the valve is shown in Figure P6.7B.

This system is known as a liquefaction system. In this system, the helium gas is supplied at 10 bar, 18 K, at a flow rate of 4.6 g/s. This gas is cooled further