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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Repeat Examinations 2010/
Module Title: Thermodynamics: Laws & Cycles
Module Code: MECH 7017
School: Mechanical and Process Engineering
Programme Title: Bachelor of Mechanical Engineering (Honours)
Programme Code: EMECH_8_Y
External Examiner(s): Prof. R. Clarke, Mr. J.J. Hayes Internal Examiner(s): Dr Keith McMullan
Instructions: Answer Question 1 and two other questions.
Duration: 2 Hours
Sitting: Autumn repeat 2011
Requirements for this examination: Steam tables Log book
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you are attempting the correct examination. If in doubt please contact an Invigilator.
(1) Describe using sketches how the Carnot cycle can be adapted to overcome equipment operational constraints, and achieve the highest efficiency in practice. (10 marks)
Explain the effect that raising the boiler and re-heat temperatures has on system performance. (5 marks)
A small steam turbo-generator produces 4MW of electricity. The efficiency of the plant expressed as electrical output/steam plant output is 80%. The steam generator conditions are 3MPa, 500degC and after expansion with isentropic efficiency 90% to 0.2MPa the steam is reheated to 500degC. There is a further expansion with isentropic efficiency 90% to the condenser pressure of 3kPa. Determine: the steam mass flow rate the cycle efficiency the state of the steam at exit from the LP turbine. (25 marks)
(2) Regarding real vapor compression refrigeration cycles, explain clearly why the refrigerant usually a) exits the evaporator superheated, and b) enters the throttle sub-cooled. Illustrate your answer with P-h and T-s diagrams. (10 marks)
A heat pump using R12 takes energy from the sea at an evaporating temperature of 0degC and delivers it to an hotel at a condenser pressure of 1.219MPa. The R12 is 20 degrees superheated at compressor entry and 20 degrees sub-cooled at condenser exit. The compression isentropic efficiency is 85%. The heating load is 30kW.
(a) Determine the COPhp of the cycle;
(b) If the price of electricity is 5c/kWh, determine the cost of running the plant continuously for 180 days and compare this with burning solid fuel for the same period. Assume solid fuels costs €65 per tonne, burns with 70% efficiency and has a calorific value 32MJ/kg (20 marks)
(3) Sketch an Indicator Diagram for a four-stroke cycle spark-ignition engine, annotating valve opening and closing positions and showing representative pressure values for each part of the cycle. (8 marks)
On a P-v diagram sketch this real cycle, and an air-standard Otto cycle. List the assumptions that have to be made when performing an air-standard analysis. (6 marks)
The compression-ratio of an air-standard Otto cycle is 10. At the beginning of the compression stroke, the pressure is 0.1MPa and temperature is 15degC. The heat-transfer to the air per cycle is 1800kJ/kg. Determine a) the pressure and temperature at the end of each process in the cycle b) the thermal efficiency (16 marks)
