The steam engine.
The purpose of this assignment is
a. To learn how to obtain actual thermophysical data using NIST’s website.
b. To represent the data in a meaningful manner, such that you can convince yourself that
in the liquid phase important properties such as the density, the enthalpy or the entropy
do not depend very much on pressure, and are nearly completely controlled by the
temperature. For water vapor this is not at all the case, it does not behave as an ideal gas.
c. To get some notion of the phase diagram for steam.
d. To use the data to model a steam engine.
Part A. Getting the data.
1. Open this website: http://webbook.nist.gov/chemistry/
2. Go to the section Models and Tools and select “Thermophysical Properties of
Fluid System”.
3. Under species of interest Water should be selected. Modify the units selected such
that you have the temperature in Kelvin, the pressure in bar, density in kg/m3 and
energy in kJ/kg.
4. For the first part of this assignment we will need data at constant pressure, so
select Isobaric properties and press continue.
You can now get data for a whole range of properties but you have to specify the pressure
for which you want data, and the temperature range. Note that there are limitations. E.g.
you will not be able to get fluid data below the triple point. Getting one or two data sets is
not a problem, but generating a lot of them is a bit of a hassle. Most of the work has been
done for you, and you will be able to find the data sets ready for use in Origin here.
However, to get some practice, you will have to generate three of them yourself, namely,
for 0.1 bar, 10 bar and 100 bar.
5. Enter the desired pressure, a lowest temperature of 280 K, a maximum
temperature of 1200 K and a temperature increment of 5 K, and hit Press for
Data.
You should now see a page like this. Under the graph you can select what you plot on the
x- and y-axes. You always wanted to know the sound speed as a function of enthalpy.
This is your chance!
6. To get the data in some useful form you should Download the data as tabdelimited text file. When you do this a web page with tabulated data opens.
How to get this into Origin? You have several options. I would recommend the last one.
It has several advantages, among them the fact that you end up with generally usable data
files. These are some options:
1) Save the page and use origin’s import wizard to import the saved file.
In this case this works reasonably well, but sometimes additional columns are generated.
So this requires little work, but the most attention.
2) Select the data on the web site and copy. Now paste this directly into an Origin
worksheet. Quick, but headers now end up in the first row instead of the column header.
Everything will work fine, but if you have a lot of data, not being able to see the headers
when you want to plot something makes things more difficult.
3) The safest way to get data from a web page to a program such as Origin is to select the
data (in this case use select all) and copy it. Paste it into a Notepad file and save it. Origin
can open this file directly, and will put the headers in the right locations. (Make sure that
when you open the file you tell Origin that this is ASCII data, not some text.)
Part B. Producing graphical representations
(20 points)
7. Collect the data files that you found here
and the three that you generated yourself
in a single directory, and start Origin.
Your first task is to make a graph of the
density versus temperature, for pressures
ranging from 0.1 bar to 350 bar. It
should look like this 
Include the graph in your report.
NM
8. Make two more graphs, for the enthalpy
versus temperature and the entropy
versus temperature. Clearly indicate in your graphs where the liquid and vapor
phases are and label the curves with the corresponding pressures.
The next step is to construct a T-s diagram that contains the coexistence curve. The first
thing is to collect the data, so back to the NIST website. Where you previously selected
Isobaric properties, you now select Saturation properties. For a T-s diagram we want
temperature increments.
Part C. making phase diagrams
(30 points)
9. In the same way you used to get your isobaric data files, get the coexistence data
file. Use a temperature range from 280 K to 645 K and again increments of 5 K.
This file contains two sets of data, for the liquid phase (l) and for the vapor phase (v) with
the temperature being the independent variable for both sets.
10. To get some idea of what data you have make a graph that shows simultaneously
the density of the vapor and the density of the liquid as a function of temperature.
Do the same for the viscosity and the enthalpy. The message should be clear.
When you approach the critical point, the properties of the liquid and the vapor
become the same, and above the critical temperature there is no distinction
between liquid and vapor.
Although most properties of coexisting phases are different, e.g., water vapor at room
temperature has a much lower density than the coexisting liquid, the fact that they coexist
in the same container must mean that they exist at the same pressure and temperature.
You will see that your data file has only one pressure column. The coexistence pressure
is only a function of temperature.
11. Plot the coexistence curve in the P-v plane. Note that over the temperature range
for which you have data the pressure varies by some four orders of magnitude.
Select an appropriate mode for your axes.
Now to construct the T-s diagram with a
coexistence curve. You want s to be the
independent (x) variable, and the best thing
to do is to take the data file and make two
new files containing just svapor and T and
sliquid and T. While you are at it
12. With these two files you can make a
T-s diagram like this 
S (kJ/kg K)
13. Add the isobars for P = 0.1 bar and
P = 100 bar in the T-s diagram that you just made.
Part D. Calculations for a Rankine cycle
(30 points)
Now consider a power cycle in which liquid water is compressed (isentropically) from
0.1 bar to 100 bar (1  2) This is followed by heat addition at constant pressure to a
temperature of 1100 K (2 3). The vapor is then isentropically expanded in a turbine,
back to a pressure of 0.1 bar (3  4). (This is called the Rankine cycle.)
14. Indicate this cycle in your T-s diagram (mark the thermodynamic path, label the
points 1 – 4).
15. Now calculate the net work that this steam cycle delivers. Keep in mind that the
expansion in the turbine happens at constant entropy.
Part E. For the ambitious
(20 points)
16. Finally, assume that the maximum temperature remains 1100 K, but that the
maximum pressure can be as high as 350 bar. Indicate in the T-s diagram how you
could increase the output considerably, and compute the change in efficiency that
is achieved.