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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Autumn Examinations 2008/
Module Title: Thermofluids 3
Module Code: INTR 7009
School: Mechanical & Process Engineering
Programme Title: Bachelor of Engineering (Honours) in Sustainable Energy – Award Bachelor of Engineering in Mechanical Engineering - Award
Programme Code: ESENT_8_Y3, EMECH_7_Y
External Examiner(s): Dr. Eugene Coyle Internal Examiner(s): Dr Gerard Kelly
Instructions: Answer any Three questions (33 marks each)
Duration: 2 Hours
Sitting: Autumn 2009
Requirements for this examination:
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) 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. (7 Marks)
(b) A 4 kg/s product stream from a distillation column is to be cooled by a 3 kg/s water stream in a counterflow exchanger. The hot and cold stream inlet temperatures are 400 and 300 K, respectively, and the heat transfer area of the exchanger is 30 m^2.
(i.) Sketch the flow direction and known temperatures for this counter flow heat exchanger. (6 Marks)
(ii.) If the overall heat transfer coefficient is estimated to be 820 W/m^2 K, determine using the NTU effectiveness method and with the help of Figure Q1 at the end of this exam paper, the product and cooling stream outlet temperatures. The specific heat of the product stream can be taken to be 2500 J/kg K and that of the cold stream as 4180 J/kg K. (20 Marks)
Q4.
(a) Derive a relationship between the COP of a refrigerator and heat-pump operating at the same conditions (6 Marks)
(b) Sketch an ideal vapour-compression refrigeration cycle on P-v,T-s and P-h diagrams,and describe the processes that make up this cycle. (9 Marks)
(c) Refrigerant-134a is the working fluid in an ideal compression refrigeration cycle. The refrigerant leaves the evaporator -20o^ C and has a condenser pressure of 0.9 MPa. The mass flow rate is 3 kg/min. Find COPR , COPR, Carnot for same Tmax and Tmin (18 Marks)
Q5.
(a) Describe a venturi meter (with the aid of diagrams) and derive an expression for the determination of flowrate through an inclined meter in terms of height difference in a differential manometer connected to the meter, and the dimensions of the meter; define all terms used. (8 Marks)
(b) A venture meter with a throat diameter of 100mm is fitted in a vertical pipeline of 200mm diameter with oil of specific gravity 0.88 flowing upwards. Two pressure gauges are fitted at the tapping points one at the throat and the other at the inlet pipe 320 mm below the throat. The difference between the two pressure guage readings is 28 kN/m^2. Determine the flow rate through the meter. Assume Cd for the venturi meter is 0.96. (6 Marks)
(c) Determine the difference in level in the two limbs of a mercury manometer if it is connected at the tapping points and the connecting pipes are filled with the same oil. (6 Marks)
