Some thermo exam type questions Some fairly straight forward

Some thermo exam type questions
Some fairly straight forward problems (i.e. not quite short answer, but no significant series of
calculations / table look ups required)
I-1) Feed to a steam turbine is at 1200 psi and 900 F. It leaves the turbine at 10 psia. The
turbine isentropic efficiency is 80% What are the temperature and enthalpy of the steam leaving
the turbine.
I-2) An ideal gas with constant cp of 0.3 kJ/kg K is taken from a state at 300 C, 300 kPa to a
state at 500 C and 600 kPa. What is the entropy change for the process.
I-3) Methane is burned with a stoichiometric amount of pure oxygen at 1 atm and 25 C. What
is the dew point of the exhaust from the burner?
I-4) A gas mixture that is 80 mole % ethane and 20 mole % methane is compressed from 300 K
and 9 MPa to 425 K and 20 MPa. What is the enthalpy change.
I-5) In a refrigeration cycle, R134a is compressed from 4 psia and 60F to 200 psi. The
compressor has an isentropic efficiency of 0.8. What is the temperature of the compressed
refrigerant.
I-6) 10 kmol of hydrogen at 25 C and 101.3 kPa is mixed with 5 kmol of oxygen, also at 25 C
and 101.3 kPa. What is the entropy change of this process.
I-7) Air enters an evaporative cooler at 14.7 psia, 93 F, and 30% RH and exits saturated. What
is the exit air temperature?
I-8) What is the (thermodynamic) purpose of extracting steam from the turbine and supplying it
to an open feed water heater in a Rankine cycle?
Problems requiring a bit more work:
II-1) Refrigerant 134a is expanded adiabatically in an expansion valve from 700 kPa and 25 C
to 160 kPa. For a environmental conditions of 100 kPa and 20 C, determine:
a) The work potential for R134a at the inlet to the expansion valve;
b) The exergy destruction during the process.
II-2) An air standard Diesel cycle has a compression ratio of 16 and a cutoff ratio of 2. At the
beginning of the cycle, air is at 95 kPa and 27C. Considering that the heat capacity of air varies
with temperature, determine
a) the temperature after the heat addition process
b) the mean effective pressure.
II-3 A steam plant operates on the normal Rankine cycle. The pressure levels are 10 kPa and
6000 kPa and steam leaves the turbine as saturated vapor. The pump efficiency is 70% and the
turbine efficiency is 75%. Determine the thermal efficiency of the plant.
II-4 i-Octane (iso-octane, C8H18) is burned with 50% excess air at atmospheric pressure. What
is the adiabatic flame temperature for this process.
II-5 Estimate the Joule-Thompson coefficient of steam at 3 MPa and 300 C
ME 4320 –Thermodynamics II
Final Exam
Spring 2012
Closed book portion 3 questions worth 5 points each
The following three questions refer to the Figure 1, below, which depicts a piston / cylinder
arrangement being heated by an external source. The top of the cylinder is open to the
atmosphere (assumed to be at constant pressure. The piston makes a tight, frictionless seal with
the cylinder and contains only water vapor.
Answer the three questions below, placing you answer in the space next to the question number.
Circled, underlined, or otherwise marked answers will be ignored – only answers in the
space next to the question number will be graded. So, obviously, you must turn this sheet in.
Patm
H2O Vapor
Heated Frictionless Piston Containing Water Vapor
Figure 1
1. _____ The density of the H2O will: (a) Increase (b) Remain constant (c) Decrease
(d) Insufficient information
2. _____ The pressure of the H2O will: (a) Increase (b) Remain constant (c) Decrease.
(d) Insufficient information
3. _____ The energy of the H2O will: (a) Increase (b) Remain constant (c) Decrease.
(d) Insufficient information
ME 4320 –Thermodynamics II
Final Exam
Spring 2012
Open book portion – 5 questions of varying point value.
Problem 4 -- 20 points
A two stage cascade refrigeration system is shown in Figure 1. The refrigerant is R 134A
(HFC 134A). The system consists of two ideal vapor-compression cycles with heat
exchange between the condenser of the lower temperature cycle and the evaporator of the
higher temperature cycle. As indicated in Figure 1, the higher temperature cycle operates
between 0.7 MPa and0. 35 MPa, while the lower temperature cycle operates between
0.35 MPa and 0.12MPa. If the flow rate of refrigerant in the hotter cycle is 0.05 kg/s,
determine:
a) What is the flow rate of refrigerant in the lower temperature cycle?
b) What is the rate of heat removal from the refrigerated unit. This is
Qc in Figure 1?
c) What is the power needed to each of the compressors (C1, C2)?
d) What is the overall COP
Cascade Refrigeration System
Figure 1
ME 4320 –Thermodynamics II
Final Exam
Spring 2012
Problem 5 -- 20 points
Liquid hydrazine (N2H4) is injected in a jet combustion chamber at 400 K and burned
with 100% excess air that enters at 700 K. The combustion chamber is jacketed with
water. The combustion products leave the jet exhaust at 900K. In the test, if 50 kmol /hr
are burned and water at 25 C enters the cooling water jacket at a rate of 100 kg/min and
1 atm, what is the temperature and phase state (liquid, vapor, or wet vapor) at exit of the
jacket. The reaction is:
N2H2 (l) + O2 (g)  N2(g) + 2 H2O (g)
The heat of formation of hydrazine is:
H 0f  44.7 J / mol
The heat capacity of liquid hydrazine may be assumed constant at 139 J/mol K
5 point bonus: What is the dew point temperature of the exhaust gas?
Problem 6 -- 15 points
Temperatures taken around a forced draft cooling tower were as follows:
Inlet
Exit
Air
85 F
90 F
Water
102 F
89 F
The wet bulb temperature of the entering air is 77 F. Assuming that the air leaving the tower
is saturated, determine:
a) The humidity of the entering air
b) The mass of dry air (lbm) through the tower per pound of water into the tower
c) The fraction of the entering water that is vaporized in passing through the tower
Problem 7 -- 15 points
Liquid water enters an adiabatic piping system at 15 C at a rate of 3 kg/s. It is observed
that the water temperature rises in the pipe by 0.3 C due to friction. If the environmental
temperature is also 15 C, compute the rate of exergy destruction in the pipe.
ME 4320 –Thermodynamics II
Final Exam
Spring 2012
Problem 8 -- 15 points
Ws
Feed
Ws
P=5 MPa
Exhaust
Extract
We have the following data for the above steam turbine system:
Feed: P = 8 MPa; T = 550 C; Flow = 300 kg/hr
Exhaust: P = 0.8 MPa
Extract: 15% of feed flow (at 5 MPa)
Turbine efficiency: Each unit has an efficiency of 80%
How much work (kJ/hr) is generated by each turbine?