ME-801 Refrigeration and Air Conditioning - May 2023 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aDiscuss any three natural methods of producing refrigeration.
   How to answer

   • Define natural refrigeration
   • Describe three methods e.g. night cooling, evaporative cooling, ice from natural sources / cold water
   • Principle of each method
   • Limitations / applications
2. 7 Marks
   bA refrigerating system produces 30 kg/hr of ice at $0^{\circ}C$ from water available at $25^{\circ}C$. Find the refrigerating effect per hour and tonnage capacity of unit. If it takes 1 kW of power, find COP of system.
   How to answer

   • Heat removed = sensible cooling of water 25C to 0C + latent heat of ice
   • Use $Q = m c \Delta T + m L$
   • Convert to kW and TR (1 TR = 3.517 kW)
   • COP = Refrigerating effect / Power input
3. 14 Marks
   The speed of an aircraft flying at an altitude of 8000 m where the ambient air is at 0.341 bar and 263 K temperature is 900 km/hr. The compression ratio of the air compressor is 5. The cabin pressure is 1.013 bar and temperature is $27^{\circ}C$. On the basis of 1 kg/sec of flow of air, determine (i) the power requirement of the aircraft for pressurization (ii) additional power requirement for refrigeration and (iii) refrigerating effect if $C_p$ for air = 1.005 kg/kgK and $\gamma = 1.4$.
   How to answer

   • Calculate ram temperature rise from aircraft speed
   • Apply isentropic compression with ratio 5 for cabin pressurization
   • Power for pressurization = $m C_p \Delta T$
   • Additional refrigeration power and refrigerating effect from cabin vs ambient
4. 7 Marks
   aShow the simple vapour compression refrigeration cycle on P-H chart and explain it.
   How to answer

   • Draw P-H diagram with saturation dome
   • Mark compression, condensation, expansion, evaporation processes
   • Explain each process with enthalpy change
   • Expression for COP
5. 7 Marks
   bDiscuss the effect of change in suction and discharge pressure on the performance of refrigeration cycle.
   How to answer

   • Effect of lowering suction pressure on RE and work
   • Effect of raising discharge pressure on power and COP
   • Volumetric efficiency and compressor work trends
   • P-H / T-S illustration
6. 7 Marks
   aExplain the working of a practical vapour absorption refrigeration cycle.
   How to answer

   • Components: generator, absorber, pump, condenser, evaporator, heat exchanger
   • Flow of ammonia-water / LiBr-water solution
   • Role of heat input replacing compressor work
   • Schematic diagram with explanation
7. 7 Marks
   bDiscuss advantages of vapour absorption cycle over vapour compression cycle.
   How to answer

   • Low operating cost using waste heat
   • No moving parts except pump - low noise/vibration
   • No compressor power, eco-friendly refrigerants
   • Comparison table vs VCRS
8. 7 Marks
   aDiscuss seven desirable property of an ideal refrigerant.
   How to answer

   • Low boiling point, high latent heat
   • Chemical stability, non-toxic, non-flammable
   • Low cost, leak detection, material compatibility
   • List seven properties with brief reason
9. 7 Marks
   bDiscuss property of Ammonia and R-22 as refrigerant.
   How to answer

   • Ammonia: properties, high latent heat, toxicity, steel compatibility
   • R-22: properties, low ODP vs CFC, applications
   • Comparison of pressure, temperature, safety
10. 14 Marks
    Define the following terms :

    1. i)Dry bulb temp.
    2. ii)Dew point temp.
    3. iii)Specific Humidity
    4. iv)Saturated Air
    5. v)Sensible Heating Process
    6. vi)Cooling and Dehumidification process
    How to answer

    • Define DBT, dew point, humidity ratio, saturation
    • Define sensible heating and cooling-dehumidification on psychrometric chart
    • Sketch processes on chart
    • Give examples/units
11. 14 Marks
    Air flowing at the rate of 100 $m^3/min$ at $40^{\circ}C$ DBT and 50% RH is mixed with another stream flowing at the rate of 20 $m^3/min$ at $26^{\circ}$ DBT and 50% RH. The mixture flows over a cooling coil whose ADP temp. is $10^{\circ}C$ and bypass factor is 0.2. Find DBT and RH of air leaving the coil. If this air is supplied to an air-conditioned space where DBT is $20^{\circ}C$ and RH is 50%, estimate room sensible heat factor and cooling load capacity of coil in TR.
    How to answer

    • Find mixture condition by mass/enthalpy balance using psychrometric chart
    • Apply bypass factor: $BF = (t_{out}-ADP)/(t_{mix}-ADP)$ to get off-coil DBT
    • Determine RH of leaving air from chart
    • Calculate RSHF and coil load in TR from enthalpy difference
12. 14 Marks
    Write short note on any three :

    1. a)Advantages of Air refrigeration
    2. b)Cascade system
    3. c)Steam jet refrigeration
    4. d)Calculation of summer air-conditioning load
    5. e)Eco-friendly refrigerants
    How to answer

    • Select any three topics
    • Give definition/principle with diagram where relevant
    • List advantages/applications
    • Conclude with significance