PROPERTY DIAGRAMS AND STEAM TABLES
Example 4: Use of the T-s diagram
Carbon dioxide is used in a particular process in which the pressure is 100 psia and the
temperature is 100°F. What is the enthalpy value of the gas?
From the carbon dioxide diagram, Figure A-4 of Appendix A, locate the pressure of 100
psia. Follow that line until reaching a temperature of 100°F. The intersection of the two
lines gives an enthalpy that is equal to h = 316 Btu/lbm.
Once the various states have been fixed for the particular process the substance has passed
through (for example, going from a saturated liquid state to a compressed liquid state across a
pump), energy exchanges may be determined as was shown in Example 1. The energy exchanges
are never 100 percent efficient, as already discussed. The degree of efficiency obtained by the
system depends upon the process through which the system has passed. Generally, the efficiency
of a component depends upon how much friction exists in the flow of the substance, the pressure
drops within the system, the inlet and outlet temperatures, and various other factors. The
properties affecting the efficiency of the system are determined by use of the charts and diagrams
mentioned in this section.
When power cycles are utilized for large systems, the efficiency of each component should be
maximized in order to have the highest possible overall efficiency for the system. Each
component affects the system efficiency in a different manner. To maximize efficiency, the
practical approach to large systems is to have multistage expansion with reheat between stages
and regenerators in the system where applicable.