Determining Properties of Steam

One of the main skills that the aspiring professional engineer must acquire is the ability to determine the properties of steam. In practice, the pressure and temperature of steam can be found easily. The other useful properties of steam like entropy, enthalpy and specific volume must be found through the use of the (1) P-H Diagram, (2) Mollier Diagram and (3) Steam Tables.

A simple way to find the properties of steam given the temperature and pressure is to draw a simple P-H diagram. For example, assume water is at 14.7 psia and 60 F, we can draw a simple P-H diagram. It is known that at 14.7 PSIA (1 ATM), the boiling point is 212 F, thus the constant temperature line in blue can be drawn. Since the temperature of the water is 60 F, then the point must be located to the left along the horizontal constant pressure line. Note that since constant temperature lines are vertical in the sub-cooled liquid region, that the enthalpy of water at 14.7 PSIA, 60 F is equal to the enthalpy of saturated liquid water at 60 F (take vertical line down from 14.7 PSIA, 60 F to the intersection of the saturated liquid curve.

If water was given at a temperature of 212 F and a pressure of 14.7 PSIA, then it would be impossible to find the location. The point could be located anywhere at the intersection of the constant temperature line in blue and the horizontal constant pressure line, which is anywhere on and in between the saturated liquid and vapor curves.

The previous example is simple because it is near the standard boiling point. However, the same method can be used for varying temperatures and pressures. For example, water at 600 F and 300 PSIA. Start the sketch by drawing in the constant temperature line for 600 F. Then look up the steam tables as a function of temperature and find 600 F. In the table it shows that saturated steam at 600 F corresponds to a pressure of 1542.5 PSIA. It is also known that the point must lie on the constant temperature line and it must lie below the horizontal portion corresponding to a pressure of 1542.5 because the pressure is only 300 PSIA.