Degrees of Superheat

Superheat is simply how far the thermometer sits above the boiling point at the pressure the steam is actually at. Steam at 1 MPa absolute and 250 °C has 250 − 179.886 = 70.1 degrees of superheat. In trade units, 100 psig at 500 °F is 500 − 337.9 = 162 °F of superheat. The number is meaningless without the pressure beside it, which is the mistake I see most often on log sheets: somebody records 400 °F and calls it superheated without noting that the line is at 250 psig, where saturation is already 406 °F and the steam is in fact slightly wet.

Superheat buys two things and costs a third. It buys dryness, because a superheated main can lose heat through its lagging for a long way before it reaches saturation and starts making condensate, which is why long distribution runs and every steam turbine want it. It also buys turbine life, since expansion starting well above the saturation line ends with less moisture in the last stages. What it costs is heat transfer. A superheated vapour film has a poor film coefficient compared with condensing steam, so a process heater fed 150 degrees of superheat spends the first stretch of its surface doing almost nothing while the steam desuperheats, and the exchanger behaves as though it were undersized. That is exactly why plants run a desuperheating station upstream of process users while sending the same header to the turbine untouched. A negative answer here is worth taking seriously rather than shrugging at: it means the temperature is below saturation, so either the sample is wet or the pressure and temperature were taken at two different places in the system.