How to Use the Steam Table
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.
vg is the specific volume of steam, hf is the specific enthalpy of water, hg is the specific enthalpy of steam, sf is the specific entropy of water, and sg is the specific entropy of steam.
We will now become familiar with this formula:
h = hf + xL
where x is dryness fraction and L = hg - hf
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, hf = 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.