Throttling High-Pressure Water
| Water at high pressure expands adiabatically to a lower pressure (throttled) through a partially-closed valve. You can vary the inlet temperature T1 and outlet pressure P2. The exit stream is a vapor-liquid mixture, and the outlet temperature is the saturation temperature at the outlet pressure. An energy balance (ΔH = 0) determines the fractions of liquid and vapor in the exit stream, and they are shown in the bar graph. |
Details:
Water at high pressure undergoes Joule–Thomson expansion when passing through a throttling valve. This process is assumed to be adiabatic, so the enthalpy change is zero.
ΔH = 0, so H1 = H2, where H1 is the inlet enthalpy and H2 is the exit enthalpy.
When two phases are present, the saturated vapor enthalpy HV and the saturated liquid enthalpy HL are used to determine the fraction of the exit stream that is vapor. For the conditions chosen, liquid and vapor exit the throttle at saturation conditions, because HL < H2 < HV.
The steam quality q is calculated by:
H2 = qHV + (1-q)HL,
so that 
.
When only liquid is present q = 0 and H2 = HL.
Related Link: