Modern Engineering Thermodynamics

Problems 589
under these conditions when the compressor isentropic
efficiency is 80.0%.
64* A Joule-Thomson expansion refrigeration system is being
considered for use in a meat-storage facility. The working fluid is
to be carbon dioxide that is to be expanded from 20.0
atmospheres at 20.0°C to 1.00 atm. The system is to have a
compressor with an isentropic efficiency of 75.0%. Determine
the cooling temperature and the coefficient of performance of
this system.
65. A newly formulated gas is found to have a Joule-Thomson
coefficient of 0.500°F/psi. If this gas expands from 70.0°F at
100. psia to atmospheric pressure, determine the exhaust
temperature and the coefficient of performance of this system if
the compressor has an isentropic efficiency of 88.0%.
66.* A highly toxic gas is discovered to have a negative Joule-Thomson
coefficient of −8.00°C/atmosphere. Since this coefficient is
negative, the gas heats rather than cools on expansion, and thus it
can be made into a heat pump. Determine
a. The exhaust temperature as this gas expands from 20.0°C at
100. atm to 1.00 atm.
b. The coefficient of performance of this unit as a heat pump if
it is compressed with an isentropic efficiency of 65.0%.
67* Determine the irreversibility rate produced by an adiabatic
compressor that compresses 0.150 kg/s of refrigerant R-134a
from 0.00°C at 0.100 MPa to 80.0°C at 0.500 MPa. The local
environmental temperature is 20.0°C.
68.* Refrigerant R-22 flows through a condenser at a rate of 2.70 kg/s.
It enters the condenser as a superheated vapor at 1.00 MPa
and 50.0°C and exits as a saturated liquid at 25.0°C. The average
temperature of the condenser is 30°C and the local
environmental temperature is 20.0°C. Determine the
irreversibility rate for this condenser.
69.* Find the irreversibility rate of an adiabatic expansion valve
that reduces 0.0660 kg/s of refrigerant R-22 from a saturated liquid
at 25.0°C to a liquid-vapor mixture with a quality of 15.0% at
0.00°C. The local environmental temperature is 20.0°C.
Design Problems
The following are open-ended design problems. The objective is to
carry out a preliminary thermal design as indicated. A detailed design
with working drawings is not expected unless otherwise specified.
These problems have no specific answers, so each student’s design is
unique.
70. Design a small vapor-compression cycle refrigeration system
that can serve as an experimental apparatus for a junior or
senior mechanical engineering laboratory course. The system
must be instrumented with the proper pressure, temperature,
and mass flow transducers, so that its coefficient of
performance can be accurately determined. The system should
have at least one variable parameter (such as refrigerant
mass flow rate) to provide a range of performance to study.
You may wish to start by modifying the components of an
existing domestic refrigerator. Either construct the apparatus
yourself or else provide sufficiently accurate drawings (or
sketches) and instructions that it can be made by an
engineering technician.
71. Carry out the preliminary thermal design of a vaporcompression
cycle heat pump that uses a solar collector as the
heat source. Use Refrigerant-134a as the working fluid, and
assume an average solar flux of 496 Btu per square foot per day
in December (the worst case) with a yearly average solar flux of
1260 Btu per square foot per day. The heat pump must provide
400. × 10 3 Btu per day to a house during December and average
200. × 10 3 Btu per day during the year. Be sure to determine the
following items in your analysis:
a. The required collector surface area for worst case and average
conditions. (Is either too large for an average roof?)
b. The resulting system coefficient of performance.
c. The required mass flow rate of R-134a.
Note: Assume a solar flux (sunshine) period of 8 to 10 hours
per day.
72. Design a domestic heating system that uses a heat pump to
extract heat from the earth to heat a house. The evaporator is to
be made of long lengths of plastic pipe buried below the frost
line at a constant temperature of 50.0°F. The heat pump system
must provide 200. × 10 3 Btu/h to the house at 70.0°F. Specify
the refrigerant; the length, diameter, and type of plastic to be
used for the evaporator piping; the mass flow rate of the
refrigerant; and the compressor efficiency. Also determine the
pumping losses (i.e., the pressure drop) in the evaporator.
Estimate or compute an appropriate temperature differential
between the outside and the inside of the condenser and the
evaporator.
73. Design a small, laboratory-scale, reversed Brayton cycle
air conditioning system that can serve as an experimental
apparatus for a junior or senior mechanical engineering
laboratory course. The system must be instrumented with the
proper pressure, temperature, and mass flow transducers so
that its coefficient of performance can be accurately
determined. The system should have at least one variable
parameter to provide a range of performance to study. You
may wish to start by modifying an automotive turbocharger to
provide the turbine and compressor stages. Either construct
the apparatus yourself or else provide sufficiently accurate and
detailed drawings and instructions that it can be made by an
engineering technician.
74. Design an inexpensive reversed Brayton cycle air conditioning
system for an automobile using air as the working fluid.
Convert one of the engine’s cylinders into an air compressor or
add a separate compressor driven off the fan belt. Determine the
amount of cooling required (in tons) when the outside air
temperature is 100.°F and the inside temperature is maintained
at 70.0°F. Size and locate the components on the automobile.
Estimate the unit’s coefficient of performance, its input power
requirement, and manufacturing cost. The final unit must add
no more than $200.00 to the cost of the automobile to the
consumer.
Computer Problems
The following computer programming assignments are designed to
be carried out on a personal computer using a spreadsheet, equation
solver, or programming language. They may be used as part of a
weekly homework assignment.