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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Autumn Examinations 2010/
Module Title: Thermofluids 3
Module Code: INTR
School: Mechanical & Process Engineering
Programme Title: Bachelor of Engineering (Honours ) in Sustainable Energy
Programme Code: ESENT_8_Y
External Examiner(s): Prof. Eugene Coyle, Mr. Richard Linger Internal Examiner(s): Michael P O’Mahony, Dr Ger Kelly, Dr Keith McMullen
Instructions: Answer Question ONE and TWO other Questions
Duration: 2 hours
Sitting: Autumn 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 have received the correct examination paper. If in doubt please contact an Invigilator.
Q1.
(a) List FOUR desirable properties for a Refrigerant (4 marks)
(b) It is proposed to use a heat pump working on the ideal vapour-compression cycle for the purpose of heating the air supply to a building. The supply of heat is taken from a river at 7oC. Air is required to be delivered into the building at 1.013 bar and 32oC at a rate of 0.5 m^3 /s. The air is heated at constant pressure from 10oC as it passes over the condenser coils of the heat pump. The refrigerant is R 134a which is dry saturated leaving the evaporator; there is no undercooling in the condenser. A temperature difference of 17 K is necessary for the transfer of heat from the river to the refrigerant in the evaporator. The delivery pressure of the compressor is 11.595 bar. (i) Draw the P-h and T-s diagrams for this process (6 Marks) Calculate: (ii) the COPhp (18 Marks) (iii) The mass flow of Refrigerant (8 Marks) (iv) The motor power required to drive the compressor if the mechanical efficiency is 87% (4 Marks) Take Specific heat capacity of air(constant pressure) 1.01kJ/kgK, density of air 1.2kg/m^3
Q
(a) Explain the difference between a Venturi Meter and an Orifice plate (6 Marks)
(b) state an expression for the actual flow rate through a horizontal venturi meter, explaining the terms used in the expression (8 marks)
(c) A venture meter having a throat diameter of 150mm is installed in a horizontal 300mm diameter water main. Determine the difference in level of the mercury columns of a differential manometer attached to the venture meter if the discharge is 0.142m^3 /s Take C = 0.982, Density of mercury 13600kg/m^3 and water 1000kg/m^3 (16 Marks)
Q
(a) What does the effectiveness of a heat exchanger represent? Under what conditions is the effectiveness method preferred over the LMTD method in heat exchanger analysis (8 marks) (b) An enterprising home-owner decided he could reduce his heating bill for domestic hot water by running the cold water supply to his hot water cylinder through a heat-exchanger situated in the chimney of his central heating boiler. The arrangement he devised is shown at right.
Assuming an overall heat transfer coefficient of 25 W/m²K, cp water = 4.185 kJ/kgK, water = 1000kg/m^3 ; cp flue gas =1.1 kJ/kgK and given the data shown in the attached diagram
Determine the water outlet temperature from his heat-exchanger. (15 marks)
Would such a device be practical? Why? (7 marks)
