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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 1 Examinations 2009/
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
Instructions: Answer THREE Questions
Duration: 2 hours
Sitting: Winter 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) The fuel supplied to a boiler has the following composition by mass: 85% C 13% H 2 2% O (^2)
The air supplied is 60% in excess of that theoretically required for complete combustion. If the temperature of the boiler house is 15 o^ C and the temperature of the flue gases is 330 o^ C, calculate the mass of the dry products per Kg of fuel burned, and the energy carried away by the dry products per hour when the fuel consumption is 1500 Kg/hr. Take specific heat capacity of Dry product as 1005J/KgK (20 Marks)
(b) Explain the importance of knowing CO 2 content of the flue gases. (13 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. (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 Q2 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)
Q.
An ammonia vapour-compression refrigerator operates between an evaporator pressure of 2.077bar and a condenser pressure of 12.37 bar. The following cycles are to be compared; in each case there is no under cooling in the condenser and isentropic compression can be assumed. (a) The vapour has a dryness fraction of 0.9 at entry to the compressor (b) The vapour is dry saturated at entry to the compressor (c) the vapour has 5K of superheat at entry to the compressor
In each case calculate the COP and the refrigerating effect per Kg.
Comment on your results. (33 Marks)
