Saturation pressure of water against temperature

Learning zone

What the steepness costs

Read the chart from the pressure side and it explains the whole economics of a steam header. Getting water to 100 °C takes 1.014 bar absolute, which is essentially free. Getting it to 180 °C takes 10.03 bar, and every component from the boiler shell to the flange bolting has to be rated for it. The curve is convex, so each extra degree of steam temperature costs more pressure than the one before it, and the pressure is what the pressure vessel code charges for.

Read it from the temperature side and it is a boiling point chart. Water in a condenser at 0.07384 bar absolute boils at 40 °C, which is why a vacuum lets a turbine exhaust into something cold. Water in a 54.4 psig heating main condenses at 150 °C, which is the temperature the coil actually delivers heat at. The gauge is a thermometer, which is the single most useful thing this line has to say.

Where it stops

The top end of this curve is real physics and not a plotting choice: the critical point, 373.946 °C and 22.064 MPa, above which there is only one phase and nothing left to draw. The bottom end is a standard’s edge rather than nature’s. Water’s triple point is 0.01 °C and 611.657 Pa, and below it there is no liquid to be in equilibrium with, but IF97 extrapolates its own saturation equation a hundredth of a degree further down to 273.15 K, where it reads 611.213 Pa. That is the floor the engine accepts, so it is where this chart and every table on this site begin.