Understanding the Thermodynamics Steam Table

Thermal power plants used for the generation of electricity utilize water as the working fluid. Water is known as a "pure substance" because it always has the same chemical composition irrespective of its phase, be it water, ice, or steam. Because water can exist as different phases at different temperatures, there are differences in volume and other properties. Calculation of the properties at each phase and at various temperatures or pressures is very time consuming. Therefore the properties of steam such as the pressure, temperature, specific volume, enthalpy, and entropy are available in a tabular form, and they are known as the thermodynamics steam table.

For engineers, the calculations are simplified as the values are picked up from the steam table and do not have to be calculated by complex calculations each time. The steam tables and the graphical charts known as Mollier diagrams are used worldwide by engineering students and professional engineers.

The thermodynamics steam tables contain the following tables:

1. Saturated water and steam temperature tables: In these tables for every temperature the absolute pressure, specific volume for saturated water and saturated steam, specific enthalpy for saturated water and saturated steam and specific entropy for saturated and saturated steam are given.
2. Saturated water and steam pressure tables: In these tables for every pressure the temperature, specific volume for saturated water and saturated steam, specific enthalpy for saturated water and saturated steam and specific entropy for saturated and saturated steam is given.
3. Specific volume of superheated steam: In this table for every pressure the saturation temperature and the specific volume at various temperatures are given.
4. Enthalpy of superheated steam: In this table for every pressure the saturation temperature and the enthalpy of superheated steam at various temperatures are given.
5. Entropy of superheated steam: In this table for every pressure the saturation temperature and the entropy of superheated steam at various temperatures are given.
6. Specific volume of superheated steam: In this table at every absolute pressure the specific volume of supercritical steam is given.
7. Enthalpy of supercritical steam: In this table at every absolute pressure the enthalpy of supercritical steam is given.
8. Entropy of supercritical steam: In this table at every absolute pressure the entropy of supercritical steam is given.

In steam tables the properties of the dry steam are listed and for the wet steam the properties may be calculated from the steam tables of the dry and saturated steam.

For values that are not listed exactly in the tables, the value between two figures can be obtained by linear interpolation. Interpolation is a mathematical tool by which, depending on the interval between two variables, a value in between can be calculated.

The steam table shown above is a saturated water and steam table. As all the other tables are used on the same principle we will only discuss this one. For an absolute pressure of 9 bars, the saturation temperature is 175.4 C. It means that at a temperature of 175.4 and above C all of the steam will be saturated. Of course, any temperature above this will be super heating of the steam.

It must be noted however that at 175.4 C, depending on the latent heat supplied for vaporization, the steam can have any dryness faction.

v_g is the specific volume of steam, h_f is the specific enthalpy of water, h_g is the specific enthalpy of steam, s_f is the specific entropy of water, and s_g is the specific entropy of steam.

We will now become familiar with this formula:

h = h_f + xL

where x is dryness fraction and L = h_g - h_f

By the above formula, if we know the dryness fraction of steam, we can calculate the enthalpy of wet steam, and its value would lie between that of the saturated water and saturated steam.

For example if the dryness fraction is 0.8 for steam at 9 bars absolute pressure in bars.

Referring to the steam table above, h_f = 743 kJ/Kg, L = 2031 kJ/Kg,

h = 743 + 0.8 x 2031 = 2367.8 kJ/Kg

This is a simple sample calculation; for more complex ones, please refer to your book on thermodynamics, but the essence is the same.