Modern Engineering Thermodynamics

4.14 A Thermodynamic Problem Solving Technique 131
Start by reading the
problem statement
1) Identify the system
1a) Draw a sketch of the system
1b) Label the sketch with data
from the problem statement
2) List the problem
unknown(s) on the
system sketch
3) Identify the type of
system
4) Identify the “process”
connecting the states or
stations
5) Write the basic equations
and write any necessary
auxiliary equations
6) Algebraically solve for
the unknown(s)
3a) Is it a closed system?
Identify the system “States”
3b) Is it a open system?
Identify the inlet and outlet
“Stations”
5a) Basic equations:
> Conservation of mass
> Conservation of
energy (1st law)
> Second law
5b) Auxiliary equations:
> Heat modes
> Work modes
> Mass flow
> Equations of state, etc.
7) Calculate the value(s)
for the unknown(s)
8) Check all algebra,
calculations, and units
Another problem?
End
FIGURE 4.21
Flowchart for solving thermodynamic problems.
No
Yes
Read next problem statement
analysis and algebraic manipulations are complete, you make the necessary calculations (paying close attention
to units and significant figures) to obtain the desired results.
The steps to be followed are shown in Figure 4.21, and each step is discussed in detail next.
Begin by carefully reading the problem statement completely through.
Step 1. Make a sketch of the system or device described in the problem statement and determine the
material (air, steam, liquid water, etc.) with which you are working. Then, carefully define the part(s) you
choose to analyze by inserting a dashed line to identify the system boundary.
Step 2. Identify the problem’s unknown(s) by rereading the problem statement and picking out all the
things you are supposed to determine. Write them on your system sketch.
Step 3. Determine whether it is a closed system or an open system. If your system is closed, identify as
many of the state properties as you can. Most problems have only two states (initial and final), but some
also have intermediate states with which you have to contend. To keep the numerical values and units of
the state properties straight, list each one under a “state” heading.