ENGG1500 Engineering Thermodynamics Semester Two Final Examinations, 2017
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School of Chemical Engineering
EXAMINATION
Semester Two Final Examinations, 2017
ENGG1500 Engineering Thermodynamics
QUESTION 1 (10 marks)
A 50 litre piston cylinder contains nitrogen at -20 oC and 1.7 bar. The cylinder is then
compressed until the temperature reaches 15 oC and the pressure 4 bar.
(a) (5.5 marks) During this process, 5 kJ of work was done on the nitrogen. How much heat was added or removed from the nitrogen during this process (in kJ)? As part of your solution, sketch a diagram of the process indicating the system and direction of all relevant mass and/or energy flows. List all your assumptions.
(b) (4.5 marks) From 15 oC and 4 bar, the piston is expanded adiabatically and reversibly until the pressure reaches 2 bar . How much boundary work (in kJ) is done in this process? List any additional assumptions (if any).
QUESTION 2 (7 marks)
6 kg/s of argon enters a diffuser with a temperature of 150 oC and a pressure of 0.2 MPa.
It leaves the diffuser a temperature of 200 oC and a pressure of 0.22 MPa . The inlet cross
sectional area is 0.01 m2 . What is the outlet velocity? As part of your solution, sketch a
diagram of the process indicating the system and direction of all relevant mass and/or
energy flows. List all your assumptions.
QUESTION 3 (5 marks)
Please answer the following in a few sentences by giving the answer and explaining why
that is the answer . Just stating the answer will not get any marks . Feel free to use
equations/diagrams if it helps your explanation.
(a) (3 marks) An ideal gas passes through a valve where the pressure is decreased from 1 MPa to 0.2 MPa . Will the temperature increase, decrease, or remain the same? Assume the velocity is the same at the inlet and the outlet of the valve.
(b) (2 marks) Which will have the larger change in specific entropy for an ideal gas maintained at constant temperature: (i) going from 1 bar to 2.5 bar or (ii) going from
Question 4 (6 marks)
Two streams of water are mixed together in a tank and leave as a single stream. Stream
1 enters the tank at 25 oC with a flowrate of 0.3 kg/s . Stream 2 enters at 800 oC with a
flowrate of 0.4 kg/s. Both streams are at a pressure of 1 MPa . What is the state and the
temperature of the outlet (stream 3)? Assume that stream 3 is also at 1 MPa . As part of
your solution, sketch a diagram of the process indicating the system and directions of all
relevant mass and/or energy flows. List all your assumptions.
QUESTION 5 (14 marks)
Consider an air conditioner with refrigerant R- 134a as the working fluid. It is cooling a
building in Darwin to maintain the internal temperature at a comfortable 23 oC, whilst the
outside is at a toasty 32 oC.
(a) (2 marks) Assuming this is an idealised vapour compression refrigeration cycle, draw the process on a P-h diagram (with P as the y-axis) . Use the same numbering for points in the cycle as above and include the vapour-liquid envelope on your diagram. Show where work and heat enters and leaves the cycle.
(b) (3.5 marks) The inlet to the compressor is a saturated vapour at 400 kPa. The outlet is at 1200 kPa and 70 oC . The refrigerant flow in the cycle is 0.05 kg/s . What is the power consumption of the compressor (in kW)? State your assumptions and number streams as per the diagram above so that your working is easy to follow.
(c) (6 marks) What is the coefficient of performance of this refrigeration system? State additional assumptions to part (b) and number streams as per the diagram above so that your working is easy to follow. If you could not answer part (b), assume the compressor power is 2 kW .
(d) (2.5 marks) What is the 2nd law efficiency of this air conditioner? If you could not answer part (c), assume the cycle has a COP of 5.
QUESTION 6 (18 marks)
A power station is using a steam reheat Rankine cycle to generate power. The cycle is
depicted below. A furnace at 1000 oC supplies heat to the boiler and the super heater . The
condenser exchanges heat with a river that is at 20 oC . The boiler operates at 15 MPa and
has saturated water vapour leaving (point 2) . The condenser operates at 20 kPa. The mass
flow rate in the cycle is 10 kg/s.
(a) (4 marks) What is the specific enthalpy (in kJ/kg) of the water at point 1 in the cycle if the pump has a power input of 200 kW? State any assumption used and number streams as per the diagram above so that your working is easy to follow.
(b) (3 marks) What is the rate of heat input to the boiler (in MW)? State any additional assumptions and number streams as per the diagram above so that your working is easy to follow.
(c) (9 marks) Steam enters the low pressure turbine at 700 oC and 4 MPa (point 5). The low pressure turbine generates 12.5 MW of power . What is the isentropic efficiency of the turbine? State any additional assumptions and number streams as per the diagram above so that your working is easy to follow.
(d) (2 marks) What is the highest possible thermal efficiency for a power cycle using this heat source and heat sink?
2023-05-20