ME-801 Refrigeration and Air Conditioning - May 2024 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
   aExplain Boot-strap cycle and coefficient of performance or refrigerator.
   How to answer

   • Definition and purpose of Boot-strap air refrigeration cycle with schematic layout
   • Working of two compressors/heat exchangers and expansion turbine
   • Expression for COP of refrigerator in terms of temperatures/pressure ratio
   • P-H or T-S diagram and applications
2. 7 Marks
   bA Carnot refrigeration cycle absorbs heat at $270\,K$ and rejects it at $300\,K$.

   1. i)Calculate the coefficient of performance of this refrigeration cycle.
   2. ii)If the cycle is absorbing 1130 KJ/min at 270 K, how many KJ of work is required per second.
   3. iii)If the Carnot heat pump operates between the same temperatures as the above refrigeration cycle, what is the coefficient of performance.
   4. iv)How many kJ/min will the heat pump deliver at 300K it at absorbs 1130 kJ/min at 270K.
   How to answer

   • COP of Carnot refrigerator = $T_L/(T_H-T_L) = 9$
   • Work input from refrigeration effect $W = Q_L/COP$ per second
   • COP of Carnot heat pump = $T_H/(T_H-T_L) = 10$
   • Heat delivered $Q_H = Q_L+W$ in kJ/min
3. 7 Marks
   aExplain the P-H chart of theoretical vapour compression refrigeration system.
   How to answer

   • P-H axes and dome, constant entropy, temperature and quality lines
   • Marking of theoretical VCR processes: compression, condensation, expansion, evaporation
   • Effect of subcooling and superheating on diagram
   • COP expression from enthalpies
4. 7 Marks
   bA refrigeration machine using R-12 as refrigerant operates between the pressures $2.5\,bar$ and $9\,bar$. The compression is isentropic and there is no undercooling in the condenser. The vapour is in dry saturated condition at the beginning of the compression. Estimate the theoretical coefficient of performance. If the actual coefficient of performance is $0.65$ of theoretical value, calculate the net cooling produced per hour. The refrigerant flow is $5 kg$ per minute. Properties of refrigerant are: Take $C_p$ for superheated vapour at $9\,bars$ as $0.64\,kJ/kg\,K$.

   How to answer

   • Determine dry saturated state at 2.5 bar and isentropic compression to 9 bar
   • Use $s_g$ and $C_p$ to find superheated temperature and enthalpy $h_2$
   • Calculate theoretical COP = $(h_1-h_4)/(h_2-h_1)$
   • Calculate actual COP = 0.65 x theoretical and net cooling = mass flow x refrigerating effect
5. 7 Marks
   aExplain Practical vapour absorption system? Write down the properties of Ideal refrigerant-absorbent combination.
   How to answer

   • Components and working of practical vapour absorption system
   • Difference from theoretical/simple system with heat exchanger, analyzer, rectifier
   • Desirable properties: affinity, boiling point difference, stability, latent heat, safety
   • Examples of refrigerant-absorbent pairs
6. 7 Marks
   bDraw a neat diagram of aqua-ammonia absorption system and explain its working.
   How to answer

   • Neat labelled diagram of aqua-ammonia system with generator, analyzer, rectifier, condenser, evaporator, absorber, pump
   • Function of each component and circulation of ammonia and water
   • Working cycle steps and energy inputs
7. 7 Marks
   aExplain the classification of refrigerant. Write down the desirable properties of ideal refrigerant.
   How to answer

   • Classification by chemical nature: halocarbon, inorganic, hydrocarbon, HFC/HCFC
   • Classification by safety and working pressure
   • Thermodynamic, physical, chemical and safety desirable properties
   • Examples of each class
8. 7 Marks
   bWrite down the factors considered for the selection of refrigerant for a system. Explain R-12 and R-22 as a refrigerant.
   How to answer

   • Thermodynamic, transport, safety, economic and environmental selection factors
   • Properties and applications of R-12 (CCl2F2)
   • Properties and applications of R-22 (CHClF2)
   • Comparison of ODP, GWP and usage
9. 7 Marks
   aUsing Psychrometric chart, explain the following processes:

   1. i)Sensible heating and Sensible cooling process
   2. ii)Heating and Dehumidification process
   How to answer

   • Psychrometric chart axes and lines used
   • Sensible heating/cooling as horizontal constant humidity process
   • Heating and dehumidification path and coil process
   • Sketches of processes on chart
10. 7 Marks
    bIn a cooling application, moist air enters a refrigeration coil at the rate of $100\,kg$ of dry air per minute at $35^{\circ}C$ and $50\%\,RH$. The apparatus dew point of coil is $5^{\circ}C$ and by-pass factor is $0.15$. Determine the outlet state of moist air and cooling capacity of coil in TR.
    How to answer

    • Locate inlet state at $35^\circ C$, 50% RH and ADP at $5^\circ C$ on chart
    • Use bypass factor $BF=0.15$ to find outlet temperature/enthalpy
    • Determine outlet humidity, RH and dew point
    • Calculate cooling capacity = mass flow x enthalpy drop in TR
11. 7 Marks
    aExplain the working of steam jet refrigeration system.
    How to answer

    • Principle of flash cooling and ejector action
    • Components: boiler, steam nozzle, evaporator, condenser
    • Working cycle with motive steam and entrainment
    • Advantages, limitations and applications
12. 7 Marks
    bDraw the T-S and H-S diagram of a steam jet refrigeration system.
    How to answer

    • T-S diagram with motive steam, flash and compression processes
    • H-S (Mollier) diagram representation
    • Labelling of states through nozzle, diffuser and condenser
    • Interpretation of COP
13. 7 Marks
    aThe amount of air supplied to an air-conditioned hall is $300\,m^3/min$. The atmosphere conditions are $35^{\circ}C\,DBT$ and $55\%\,RH$. The required conditions are $20^{\circ}C\,DBT$ and $60\%\,RH$. Find out the sensible heat and latent heat removed from the air per minute. Also find the sensible heat factor for the system.
    How to answer

    • Locate outside $35^\circ C$ DBT 55% RH and inside $20^\circ C$ DBT 60% RH on chart
    • Find enthalpies, humidity ratios and specific volume
    • Calculate mass flow from $300 m^3/min$ and sensible/latent heat = $m(h)$ differences
    • Compute sensible heat factor = sensible/total
14. 7 Marks
    bA small office hall of 25 persons capacity is provided with summer air conditioning system with the following data. Outside conditions $= 34^{\circ}C\,DBT$ and $28^{\circ}C\,WBT$ Inside conditions $= 24^{\circ}C\,DBT$ and $50\%\,RH$ Volume of air supplied $= 0.4\,m^3/min/person$ Sensible heat load in room $= 125600\,kJ/h$ Latent heat load in room $= 42000\,kJ/h$ Find the sensible heat factor of the plant.
    How to answer

    • Calculate ventilation air mass flow from $0.4 m^3/min/person$ for 25 persons
    • Find ventilation sensible and latent loads from psychrometric difference
    • Add room loads to get total sensible and total loads
    • SHF of plant = total sensible/total
15. 14 Marks
    Write short note on any three :

    1. a)Grand sensible heat factor
    2. b)Cascade system
    3. c)Leak detection methods
    4. d)Production of dry ice
    How to answer

    • Brief definition and formula for grand sensible heat factor
    • Working and layout of cascade refrigeration system
    • Common leak detection methods with procedure
    • Methods of dry ice production with diagram