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 = m_{g}/(m_{f} + m_{g}) 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 v_{f} and v_{g} listed under pressure p. Dryness fraction x = (vv_{f})/(v_{g}v_{f})Enthalpy = h_{f} + x(h_{g} h_{f})= h_{f} + x(h_{fg}) (A)= h_{g}  (1x)h_{f} (B)=xh_{g} + (1x)h_{f} 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 P_{sat}
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  v_{f}, h  h_{f}, and s  s_{f} 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 v_{f} etc. corresponding to temperature t and pressure P_{sat}, and not the difference from the values v_{f} 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 uv 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 r_{s}, r_{L} and r_{v} of extensive property r at points S, L and V respectively, the value at M is calculated as follows: M x r = m_{s}r_{s} + m_{L}r_{L} + m_{v}r_{v} Where m_{s} is the mass of the solid phase present, etc. and m is the sum of m_{s}, m_{L} and m_{v}.
