Isotherm (steam)
Definition
... a process at constant temperature, denoted by a horizontal line on a Ts diagram.
Where it occurs.
Whenever wet steam is encountered - this applies to condensation and boiling. In the theoretical Carnot Cycles, heat is added and rejected at constant temperature. The isotherm might well be used as a construction line - e.g. if a reheat temperature is specified.
Special features
Steam is not an ideal gas - it is possible to increase the enthalpy of steam without changing its temperature. This is particularly true of wet steam.
How to draw it
Locate the POI and press the isotherm button.
The theory
With regards to steam properties, see
page on the steam POI.
Exercises
Click on the graph to get a POI at about T=475K and plot an isotherm through it. Click along the isotherm to set new POIs. Note (1) no predictions are possible well to the left of saturated liquid (the "f-part" of the saturation curve) (2) a few pixels to the left of saturated liquid, pressures are very high and we are in IAWPS region 1 (subcooled water) (3) as we move from saturated liquid (f-side) to saturated vapour (g-side) dryness franction and enthalpy increas (in addition to entropy) but pressure is roughly constant. We are in IAWPS region 4. (4) as we move to the right of the g-side, in IAWPS region 2, pressure decreases.
Links
... to follow.