Steam Tables (IAPWS-IF97)


by: Shashank_Aggarwal
These tables give the thermodynamic properties for those states where the liquid and vapour phases can exist together in equilibrium (as shown in image below). Pressure and temperature cannot be varied independently in the two phase region and because of the shape of the curve it is convenient to use temperature as the independent variable at low temperatures up to 100 deg C.



IProperties of Pure Substances
IIBrief Knowledge on Steam Table

The steam tables used in the thermodynamics course are a set abstracted from more complete data published by the National Engineering Laboratory. They are designed to illustrate one convenient method for presenting the thermodynamic properties of a pure substance in equilibrium, when simple analytical relationships are not available. Where greater accuracy is required, the original tables should be used. It should be emphasised that there is little data on the properties of the solid phase since this is not normally of great importance in systems studied in engineering thermodynamics. Note that the units and notation are described inside the front cover. Saturated water and steam

  
 
These tables give the thermodynamic properties for those states where the liquid and vapour phases can exist together in equilibrium (as shown in image below). Pressure and temperature cannot be varied independently in the two phase region and because of the shape of the curve it is convenient to use temperature as the independent variable at low temperatures up to 100 deg C. For higher temperatures pressure is a more convenient variable.It is found that all states in this region can be represented in terms of the properties of saturated liquid and saturated vapour (subscripts f and g respectively). The difference between the values for water and steam is given the double subscript fg. Saturated liquid is defined as liquid, at a given pressure or temperature, for which any increase in its internal energy, enthalpy or volume must be accompanied by the formation of some vapour. Similarly saturated vapour (often called dry saturated vapour to emphasise the absence of any liquid). States intermediate between saturated water and saturated steam are often referred to as wet steam.
States intermediate between saturated liquid and vapour are conveniently defined in terms of a property, called the dryness fraction or quality, which is the raito of the mass of vapour to the total mass.
 

Thus dryness fraction x = mg/(mf + mg) The use of the dryness fraction is illustrated by considering some steam at pressure p and a specific volume v which lies between the values vf and vg listed under pressure p. Dryness fraction x = (v-vf)/(vg-vf)Enthalpy = hf + x(hg -hf)= hf + x(hfg) (A)= hg - (1-x)hf (B)=xhg + (1-x)hf These equivalent expressions can each be used with advantage in certain situations, expressions A and B being particularly appropriate for low and high values of the dryness fraction, respectively. Superheated vapour The data for superheated steam is presented using pressure and temperature as the independent variables for a limited number of pressures up to 1000 bar and certain temperatures up to 800 deg C. In general sufficient data is given to allow linear interpolation without too great an error although the data for high temperatures and pressures may not be entirely satisfactory. Where no data are recorded in these tables, it signifies that the state corresponding to this pressure and temperature is outside the superheated region, where values are required for interpolation in these regions use the values for dry saturated steam.
Compressed liquid 

Compressed water is water at any temperature t above the freezing point and a pressure p, greater than the saturation pressure Psat corresponding to the temperature t. it is found for compressed water that the variations in the extensive properties with change of pressure are small compared with the variations due to changes of temperature. In addition, lines of constant temperature are very nearly parallel straight lines. Therefore, it is convenient to tabulate, for different pressures, the changes in the extensive properties from their saturation values, corresponding to the temperature t. The table contains values of v - vf, h - hf, and s - sf for a few values of pressure and temperature and linear interpolation is normally sufficiently accurate.Note: It is important to realise that the values listed show changes from the values vf etc. corresponding to temperature t and pressure Psat, and not the difference from the values vf etc. corresponding to pressure p and temperature t'. (From the image above). Triple point 
Under certain fixed conditions of pressure and temperature, the solid, liquid and vapour phases can exist together in equilibrium, the states for which such equilibrium exists are known as triple point states. If two extensive properties of a substance are plotted against each other, the triple point states define an area on the diagram, and a simple geometrical counstruciton allows the masses of the different phases to be determined. Thus on the u-v diagram (below) states S, L and V represent solid, liquid and vapour phases which can exist in equilibrium.
 

States lying on a line such as SB have a mass ratio of vapour to liquid given by the ratio of the lengths LB to BV. Hence state M has the following mass ratios:Mass of vapour/mass of solid = SC/CVMass of liquid/mass of solid = SA/ALMass of vapour/mass of liquid = LB/BV Knowing the values rs, rL and rv of extensive property r at points S, L and V respectively, the value at M is calculated as follows: M x r = msrs + mLrL + mvrv Where ms is the mass of the solid phase present, etc. and m is the sum of ms, mL and mv