As the steam passes through a pressure reducing valve or throttling device, its enthalpy does not change (isenthalpic expansion). Since the saturation enthalpy of steam is lower at low pressure, wet steam dries up; if the steam is already dry, it turns into superheated steam . This tool calculates the dryness or degree of superheat on the secondary side.
Formula
If h₁ is greater than the enthalpy of saturated steam on the secondary side, the steam is superheated; the temperature is found exactly by solving for the equation h(p₂, T) = h₁ in IAPWS-IF97 region 2 (the approximate cp formula is not used).
Example
When the steam is reduced from 10 bar(g) to 2 bar(g) at 95% dryness, the dryness increases to 97.95%. If the same process is done with dry saturated steam, the steam will be superheated at 2 bar(g) for approximately 25.5 °C (159 °C).
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