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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Repeat Semester 2 Examinations 2011/
Module Title: Thermofluids 3
Module Code: INTR 7009
School: Mechanical and Process Engineering
Programme Title: Bachelor of Engineering in Mechanical Engineering Bachelor of Engineering (Honours ) in Sustainable Energy
Programme Code: EMECH_7_Y ESENT_8_Y
External Examiner(s): Mr. M. McKeon, Dr. E. Ahearne, Mr. P. Kenny, Mr C. Buckley Internal Examiner(s): Dr. Keith McMullan, Mr Michael P O’Mahony
Instructions: Answer question 1 and TWO other questions
Duration: 2 Hours
Sitting: Repeat Semester 2 2012
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) Sketch an ideal vapour compression refrigeration cycle on T-s and P-h diagrams. Indicate how a real cycle would differ from the ideal. (8 marks)
Why is the compressor/motor-assembly usually housed in a hermetically sealed housing? (4 marks)
Discuss the properties of refrigerants, and their resultant potential environmental impact. (6 marks)
A heat pump using R134a takes energy from the sea at an evaporating temperature of 0degC and delivers it to an industrial heating system at a condenser pressure of 11.6bar. The refrigerant is 10 degrees superheated at compressor entry and 10 degrees sub-cooled at condenser exit. The heating load is 30 kW.
(a) Determine the COPhp of the cycle
(b) If the price of electricity is 15c/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 cost €125 per tonne, burns with 70% efficiency and have a calorific value 32 MJ/kg. (22 marks)
(2) Derive an expression for the thermal resistance of a cylindrical layer to heat conduction. (8 marks)
With regard to insulated pipes, explain what is meant by the Mean Critical Thickness of Insulation ( 4 marks)
A cylindrical refrigeration chamber 0.9 m inside height is made from a layer of stainless steel 1 mm thick lagged on the outside with plastic foam 190 mm thick. The surface heat transfer coefficient on the inside is 50 W/m²K and on the outside 75 W/m²K. The inner diameter of the refrigerator is 0.7 m. The thermal conductivity of stainless steel is 41 W/mK and of plastic foam is 0.5 W/mK. Calculate the heat transfer rate from the chamber when the internal temperature is 190 K and the ambient temperature is 16degC. (Ignore the problems of the corners.) (18 marks)
(3) With regard to heat exchangers, explain the Log Mean Temperature Difference approach to their design. Use a sketch to aid your explanation. (10 marks)
Exhaust gases flowing through a tubular heat exchanger at 0.3 kg/s are cooled from 400degC to 120degC by water (Cp = 4.186 kJ/kgK) entering at 10degC. The specific heat capacity of the gases is 1.13 kJ/kgK. The overall heat transfer coefficient is 140 W/m²K. Calculate the surface area required for a water flow rate of 0.4 kg/s under conditions of:-
(a) parallel flow (b) counter flow (20 marks)
