Modern Engineering Thermodynamics

318 CHAPTER 9: Second Law Open System Applications
incompressible liquid or an ideal gas of your choice when the
vessel is filled either adiabatically or isothermally (again, your
choice). Input all necessary information in proper units. Allow
the user to work in either Engineering English or SI units.
Create and solve problems
Engineering education tends to focus on the process of solving
problems. It ignores teaching the process of formulating solvable
problems. However, working engineers are never given a well-phrased
problem statement to solve. Instead, they need to react to situational
information and organize it into a structure that can then be solved
using the methods learned in college.
These “Create and Solve” problems are designed to help you
learn how to formulate solvable thermodynamics problems from
engineering data. Since you provide the numerical values for some of
the variables, these problems do not have unique solutions. Their
solutions depend on the assumptions you need to make and how
you set them up to create a solvable problem.
87. You are a new engineer at a small paper making company. The
company’s chief engineer wants to install a new drum dryer that
produces heat at a rate of 500,000 Btu/h with a drum surface
temperature of 175ºF. The factory has a steam boiler and can
produce saturated vapor at any desired pressure and flow rate.
Write and solve a thermodynamics problem that satisfies the
chief engineer’s needs. Choose a steam mass flow rate and inlet
conditions, then determine the exit quality and the entropy
production rate of the dryer.
88.* Your boss wants to know how much power is required to
compress 15.0 kg/min of air (an ideal gas) in an insulated
compressor from 0.100 MPa, 20.0°C to 1.50 MPa, in a steady
state, steady flow process. Write and solve a thermodynamics
problem that provides her with the answer plus the final
temperature of the air.
89. Your new job at a domestic refrigerator manufacturing company
involves the design of expansion valves. The liquid refrigerant
passes through the expansion valve (also called a throttle valve),
where its pressure abruptly decreases, causing flash evaporation
of 35% of the liquid. The resulting mixture of liquid and vapor,
at a lower temperature and pressure, then travels through the
refrigerator’s evaporator coil and is completely vaporized by
cooling the warm air of the space being refrigerated. The
resulting refrigerant vapor returns to the refrigerator’s
compressor inlet to complete the thermodynamic cycle. For a
new refrigerator design, saturated liquid Refrigerant 134a enters
the expansion valve at 80.0°F and exits at 10.0°F. Your
supervisor needs you to determine the following items: (a) The
increase in entropy per lbm of R-134a flowing through the
expansion valve. (b) The entropy production rate per unit mass
flow rate of R-134a flowing through the expansion valve.
(c) The average Joule-Thomson coefficient for this process. Write
and solve a thermodynamics problem to provide the answers.
90.* The corporate vice president of the company that employs you
needs to know what the entropy production and the heat
transfer rates are in a newly designed valve that has been
designed to control the flow of saturated liquid ammonia. At
the normal flow rate 0.55 kg/s, the minor loss coefficient of the
valve is 18.3, and the inlet and outlet oil temperatures are 3.00
and 19.0°C, respectively. The surface temperature of the valve is
constant at 20.0°C. Under normal conditions, the flow velocity
through the valve is constant at 7.50 m/s. Write and solve a
thermodynamics problem that gives your corporate vice
president the answers to his questions.