ME 322 – Mechanical Engineering Thermodynamics
Final Exam
Spring 2017
You can get started on the exam as soon as
you have it. Only those that show up for class
Friday will get the Dr. Dan Dollars.
Please read the following statement:
Article II, Section 1 of the University of Idaho Student Code of Conduct states,
Cheating on classroom or outside assignments, examinations, or tests is a violation of this code.
Plagiarism, falsification of academic records, and the acquisition or use of test materials without faculty
authorization are considered forms of academic dishonesty and, as such, are violations of this code.
Because academic honesty and integrity are core values at a university, the faculty finds that even one
incident of academic dishonesty seriously and critically endangers the essential operation of the
university and may merit expulsion.
I have read and understand the above statement.
___________________________________________________
Signature
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Date
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Printed Name (70 Points)
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Dr. Dan Dollars Collected
EXAM INSTRUCTIONS – PLEASE READ THIS CAREFULLY
You will have several days to complete this exam. You may use your notes, the online
course resources, your computer (EES, Google, etc.) and pretty much any non-human
resource you can find. You will also be allowed the use of “Dr. Dan Dollars” as outlined in
the guide posted on our course schedule.
Some problems will be open-ended calculations where you need to provide numerical
answers. For these problems, make sure to attach all of your calculations (or EES
code/output) attached (in order) to the back of the exam.
For multiple choice questions, circle the correct answer(s). You also need to provide
details about where your answer came from.
There will be a maximum of 170 points on this exam (including extra points from “Dr.
Dan Dollars.” Of that 170 point max, 70 points are for putting your name on this page.
Part 1: Engineering Calculations – each numbered problem is worth 12.5 points
Write your answer in the space provided, but attach your EES code to the back of the exam
You are designing a HVAC system for your house that is in a warm, humid climate. Air entering the HVAC system
is at a dry bulb temperature of 95°F and 80% relative humidity. You would like the air circulating through the
building to be similar to typical ambient conditions in Moscow, Idaho during the early summer, which is about a dry
bulb temperature of 70°F and 50% relative humidity. You will use a cooling process (to remove moisture from the
incoming air), then a heating process (to get the air to the desired temperature and humidity ratio). The incoming air
flow rate to the HVAC system is 1500 cubic feet per minute. Answer the following questions:
1) Use the ASHRAE English Unit psychrometric chart (a copy can be printed from our BbLearn site, under
the “Homework Solutions” menu) to draw the process. Attach your chart behind this page, and clearly draw
and label the:
a. Start point
b. End point
c. Path taken for these two processes.
Attach your chart on the next page
2) Calculate the mass flow rate of water condensing out of the air stream (in lbm/hr)?
3) The cooling unit uses a VRC cycle with a COPR of 5.2. The unit will run for 6 hours each day. Calculate
the following:
a. How much power (kW) will the compressor require for this cooling process?
b. Electrical energy where the house is located costs $0.10 per kW-hr. How much money ($/month)
will it cost for the cooling process over 30 days?
4) The heating unit is a condensing natural gas furnace with an efficiency of 92%. Like the VCR unit, this will
also run for 6 hours each day. Calculate the following:
a. Rate of heat (Btu/hr) necessary to be added to the heating system?
b. Keeping in mind the furnace efficiency, what mass flow (kg/hr) of natural gas (with heating value
of 43,000 kJ/kg) will be required to produce the heat rate for the question above?
c. Natural gas where the house is located costs $0.12 per cubic meter. The density of natural gas is
0.8 kg/m3. How much money ($/month) will it cost for the heating process over 30 days?
5) You decide that the cost of cooling/heating is too expensive. Using the same cooling unit, you are just
going to cool the house down to 70 °F and 100% relative humidity for 6 hours each day.
a. How much money ($/month) would it cost over 30 days?
b. How much money ($/month) would that save you compared to the analysis from questions 3-4?
6) You are working with a methane-fired boiler that needs to put a heat input of 20 x 106 Btu/hr in the steam
cycle. The boiler has a 95% conversion efficiency (from chemical energy to boiler heat). For improved
emissions, you are burning with 110% theoretical air. Methane, CH4, has a higher heating value of 23,900
Btu/lbm, a lower heating value of 21,700 Btu/lbm, and a molar mass of 16.04 lbm/lbmol. Exhaust
temperature leaves the boiler at 125 °F. Calculate the following:
a. Balanced chemical equation for the actual reaction (not stoichiometric coefficients)
b. Dew point temperature (°F) of the products of combustion.
Hint: Use this to decide if you should use the HHV or LHV of the fuel
c. Mass flow of air (in lbm/hr, at standard pressure and temperature) consumed in the combustion
reaction
7) You decide you are replace the 72 hp flathead engine for your 1949 International Pickup with a modern
LSx engine. The engine you are looking at is a four-stroke gasoline spark ignition 8-cylinder engine that
has a total displacement volume of 454 cubic inches. The compression ratio is 9.0:1, and peak power is
produced at 6800 rpm. Use ‘air_ha’ as the working fluid in an EES model to predict the power output of
this engine in several configurations. Note: The temperature of state 3 will be out of range. This is okay for
your calculation of power output.
a. Using 87 Octane fuel, the change in specific internal energy of the fluid in the cylinder (modeled
as air_ha) between states 2 and 3 will be 700 Btu/lbm. Inlet conditions for the engine are P[1] =
14.7 psia, and T[1] = 80 °F. Calculate the power (hp) that this engine will produce.
b. For an additional $12,000 you can purchase a supercharged version of this same engine. The
supercharged engine has the same specifications, except that P[1] = 24.7 psia, T[1] = 200 °F, and
when using 93 octane fuel the change in specific internal energy of the fluid in the cylinder
(modeled as air_ha) between states 2 and 3 will be 850 Btu/lbm. Calculate the power (hp) for this
engine.
c. For an additional $4000 you can add an intercooler for the supercharged engine. All the
specifications are the same as in part b, except now T[1] = 100 °F. Calculate the power (hp) for
this engine.
8) You have taken a summer internship at an old steam power plant. They have been running a very standard
Rankine steam cycle with superheat (like in Lecture 24). However, they have decided to upgrade their
cycle with either reheat or regeneration with an open feedwater heater (lectures 25 and 26). The turbine was
purchased before you started, so you can’t change the pressure between the high and low pressure turbines.
The key information for both cycles are: P[1] = 1600 psia
T[1] = 1100 °F mdot = 1.2*106 lbm/hr
The isentropic efficiency of each turbine is 0.85, and of each pump is 0.90. Pressure between the high and
low pressure turbines is 250 psia. Pressure leaving each low pressure turbine is 1 psia. Quality entering
each pump is zero. For the reheat option, temperature after reheat would be 900 °F.
a. For the reheat and regeneration options, calculate the net thermal efficiency of the cycle, and the
net power produced.
Thermal Efficiency (%)
Net Power (MW)
Reheat
Regeneration
b.
If the heat energy for the reheat portion came from ‘waste heat’ in the stack that you did not have
to pay for, recalculate the thermal efficiency for the reheat option.
c.
Circle below which option you would recommend to your manager, and discuss why you give this
recommendation. Things to consider would be: cost, efficiency, power output, complexity, etc.
Reheat
Regeneration