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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 1 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): Dr Eugene Coyle, Mr Richard Linger Internal Examiner(s): Michael P O’Mahony
Instructions: Answer Question ONE and THREE other Questions
Duration: 2 hours
Sitting: Winter 2010
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) In the context of a refrigerant, explain the meaning of ODP and GWP (6 Marks)
(c) A cold storage plant is used to cool 9000 litres of milk per hour from 27oC to 4oC and the heat leakage into the plant is estimated to be 60kW. The refrigerant used is ammonia and the temperature required in the evaporator is -6oC. The compressor delivery pressure is 10.34bar and the condenser liquid is undercooled to 24oC before throttling. The plant has a brine circulating system and the rise in temperature of the brine is to be limited to 3K. Assuming that the vapour is dry saturated on leaving the evaporator and that the compression process is isentropic, calculate (i) The power input required taking the mechanical efficiency of the compressor as 90% (22 Marks) (ii) The rate at which the brine must be circulated in litres per second (8 Marks) For milk: specific heat capacity, 3.77kJ/kgK; density, 1030 kg/m^3 For Brine: specific heat capacity, 2.93 kJ/kgK; density 1190kg/m^3
Q
A steam boiler generates 5000kg/hr of superheated steam at 20 bar and 300oC from feed water at 55oC. If the boiler efficiency is 75%, calculate the fuel oil consumption in tonne/hr if the calorific value of the fuel oil is 44,000kJ/kg If the mass of flue gases is 27.4kg/kg of oil burned and has a temperature of 310oC, calculate the percentage of the fuel energy carried away. Air intake temp 18oC; specific heat capacity of flue gases 1.04kJ/kgK (20 marks)
Q
Exhaust gases flowing through a tubular heat exchanger at the rate of 0.3 kg/s are cooled from 400 to 120 °C by water initially at 10 °C. The specific heat capacities of exhaust gases and water may be taken as 1.13 and 4.19 kJ/kg K respectively, and the overall heat transfer coefficient from gases to water is 140 W/m^2 K.
(i.) Sketch the schematics depicting the temperature and flow through the heat exchanger for the parallel and counterflow heat exchanger. (2 marks) (ii.) Calculate the surface area required when the cooling water flow is 0.4 kg/s, for parallel-flow (9 marks) (iii.) Calculate the surface area required when the cooling water flow is 0.4 kg/s, for counter-flow. (9 marks)
