MECH4880 2016 S2.pdf - UNSW Australia SEMESTER 2 2016...

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UNSW Australia SEMESTER 2 2016 EXAMINATIONS MECH4880: REFRIGERATION AND AIR CONDITIONING 1. TIME ALLOWED- 3 hours 2. READING TIME -10 minutes 3. THIS EXAMINATION PAPER HAS 4 PAGES 4. TOTAL NUMBER OF QUESTIONS-3 5. TOTAL MARKS AVAILABLE-100 6. MARKS AVAILABLE FOR EACH QUESTION ARE SHOWN IN THE EXAMINATION PAPER 7. ALL ANSWERS MUST BE WRITTEN IN INK. EXCEPT WHERE THEY ARE EXPRESSLY REQUIRED, PENCILS MAY BE USED ONLY FOR DRAWING, SKETCHING OR GRAPHICAL WORK 8. THIS PAPER MAY BE RETAINED BY CANDIDATE 9. CANDIDATES MAY BRING TO THE EX.AMI NATION: A university approved calculator and drawing instruments. 10. THE FOLLOWING MATERIALS WILL BE PROVIDED Thermodynamic Properties of Fluids and other data Mathew and Rogers 5.1. AIRAH Psychrometric Chart 11. THE FOLLOWING AID IS ATTACHED TO THIS EXAMINATION PAPER: Thermodynamic tables for R22 Data: For water vapour, hv= 2501 + 1.83t kJ/kg For dry air, CPa = 1.005 kJ/kgK For moist air, CPm = 1.02 kJ/kgK Pv w = 0.622 p _ P, V Page 1 of4
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QUESTIONl (25 Marks) (a) Explain what happens to a refrigeration system in the event of an evaporator fan failure. (b) Explain the effect of moisture present in an air conditioning system. ( c) List six requirements of a good refrigerant and give a brief explanation for each one. ( d) The air conditions at the intake of an air compressor are 25°C, 60% RH and 101.3 kPa pressure. The air is compressed to 400 kPa and then sent to an intercooler. If the condensation of water vapour from the air is to be prevented, what is the lowest temperature to which the air can be cooled in the intercooler? (e) For moist air at 27 °C dry bulb temperature 16 °C dew point temperature and 101.325 kPa abs. pressure Determine using steam tables, i. Moisture content, w ii. Degree of Saturation, μ 111. Relative humidity, <I> 1v. Specific volume, v Page 2 of4
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QUESTION2 (35 Marks) A space in a commercial building has a winter sensible heat loss of 59 kW and a negligible latent heat load (latent losses to outside are made-up by latent gains within space). The space is to be maintained precisely at 24°C and 50% relative humidity. Due to the nature of the process, 100 percent outside air is required for ventilation. The outdoor air conditions can be taken as saturated air at 1 °c. The amount of ventilation air is 3300 Lis and the air is to be preheated, humidified with an adiabatic saturater to the desired humidity, and then reheated. The temperature out of the adiabatic saturator is to be maintained at 15°C DB. Determine the following: (a) temperature of the air entering the space to be heated, 0 c. (b) heat supplied to preheat coil, kW. (c) heat supplied to reheat coil, kW. (d) rate of humidification, g/s. Page 3 of 4
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QUESTION3 (40 Marks) CONDENSER 5 EVAPORATOR 1 3 Float control \ Compressors Flash chamber intercooler 6 2 7 EVAPORATOR2 In the R22 system shown in the figure above, one evaporator is to provide 30kW of refrigeration at -30°C and another evaporator is to provide 50kW at s 0 c.
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