Modern Engineering Thermodynamics

198 CHAPTER 6: First Law Open System Applications
2. The general open system energy rate balance (ERB) equation:
_Q − _W +∑ _m h+ V 2
/2g c + gZ/g c − ∑ _m h+ V 2
/2g c + gZ/g c
inlet
outlet
(6.4)
= d dt
U + mV 2 /2g c + mgZ/g c
system
3. The modified open system energy rate balance equation:
_Q − _W + _m½h in − h out + ðV 2 in − V2 out Þ/ð2g cÞ + ðZ in − Z out Þðg/g c ÞŠ = 0 (6.12)
Other equations are developed in this chapter that apply to specific geometries and boundary conditions. These
equations are best studied in the context of the specific examples in which they were developed. The secret to
mastering this material is to become competent at developing the equations that fit your specific problem by
starting from the basic energy and auxiliary equations and applying the relevant boundary conditions (adiabatic,
negligible kinetic or potential energies, and so forth). Consequently, not all the equations developed in this
chapter are listed here so that you are encouraged to learn how to use the basic equations by studying the example
problems and solutions presented.
The example problems discussed in this chapter are not meant to cover all the possible aspects of open
system energy analysis. They were chosen to illustrate the problem solving technique, thermodynamic
table usage, and how to make basic assumptions about process variables. You must learn how to successfully
apply a generalized problem solution technique, such as the one used in this chapter and illustrated in
the flowchart of Figure 4.21. More problem solving skills are gained by doing the problems at the end of
this chapter.
Table 6.5 lists some of the new technical thermodynamic terms introduced in this chapter and earlier chapters.
These terms are used without further explanation in the chapters that follow. It is recommended that the student
learn their definitions before proceeding to the next chapter.
Table 6.5 Glossary of Technical Terms Introduced in Chapter 6 and Earlier Chapters
EB
The energy balance
ERB
The energy rate balance
1Q 2 and Q _ Heat transfer and heat transfer rate
1W 2 and W _ Work and work rate (power)
Aergonic
No work
Reversible work
No losses (i.e., no friction, heat transfer, etc.)
Work efficiency
A measure of the losses within a machine
Coefficient of performance (COP) The name we give energy conversion efficiency when it is more than 100%
Conduction heat transfer
The heat transport of energy that obeys Fourier’s law
Convection heat transfer
The heat transport of energy that obeys Newton’s law of cooling
Radiation heat transfer
The heat transport of energy that obeys the Stefan-Boltzmann law
Adiabatic
No heat transfer
Steady state
A thermodynamic state that is constant in time
Steady flow
A state wherein the mass of an open system is constant in time
Flow stream
Where mass crosses a system boundary
Station
A data monitoring point on a flow stream
MEB
The modified energy balance
MERB
The modified energy rate balance
Pressure energy
Sometimes called the pressure head, _mpv
Nozzle
A device for converting pressure energy into kinetic energy
Diffuser
A device for converting kinetic energy into pressure energy
Heat exchanger
A device for promoting heat transfer from one fluid to another