Tabulated property data for water are available and are referred to as steam tables. These tables are presented for vaporisation process and superheating process separately.
Features of steam tables:
- We normally assume the specific heat of liquid water to be constant and hence we do not use compressed high or sub-cool liquid property data tables. For the vaporisation process the steam tables give two different tables. The first table known as temperature table list in the first column the temperatures starting from 0.01oC and the highest temperature value listed is 374.15oC which is the critical temperature.
- The second column gives us saturation pressure for each and every temperature listed.
- The next two columns gives the specific volume of the saturated liquid. vf (m2/kg) and specific volume of dry saturated vapour, vg (m3/kg). Specific enthalpy is listed in the next three columns.
- These are hf, hfg and hg namely, enthalpy of saturated liquid, enthalpy of vaporisation (latent heat) and enthalpy of dry saturated vapour, their units being kJ/kg. The next three columns give sf, sfg and sg namely, specific entropy of saturated liquid, vaporisation and dry saturated vapour. The units of entropy listed are kJ/kg k.
- The second type of table known as saturated water-steam (pressure) table lists in the first column the pressures starting from 0.667 kPa till the critical pressure of 22.03 MPa.
- The second column gives the saturation temperature in oC corresponding to each pressure.
- The heat two columns give the values of vf and vg namely the specific volume of the saturated liquid and dry saturated vapour for each pressure. Specific enthalpy is lifted in the next three columns in kJ/kg.
- These are hf, hfg, and hg namely, enthalpy of saturated liquid enthalpy of vaporization (latent heat) and enthalpy of dry saturated vapour.
- The next three columns list specific entropy (kJ/kg k) sf, sfg and sg namely that of saturated liquid, vaporisation and dry saturated vapour.
When water – water vapour is in equilibrium state with any quality x, the mixture properties can be obtained using Eq
by taking listed properties from saturated water steam tables. In case the internal energy u needs to be calculated then the same is obtained using the relation
u = h − pv
When we deal with superheated vapour the pressure and temperature are independent. Therefore, for each pressure separately tables are made. These tables start from the saturation temperature corresponding to the given pressure till about 1200oC is reached. The property values list for each pressure are the specific volume (m3/kg), specific enthalpy (kJ/kg k) and specific entropy (kJ/kg k). While using tables if we observe that for a particular pressure or temperature property values are not listed, then we can interpolate the property values by fixing the pressure or temperature in between two values with in which the given value lie.