Question Paper: Refrigeration and Air Conditioning : Question Paper May 2015 - Mechanical Engineering (Semester 7) | Mumbai University (MU)
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Refrigeration and Air Conditioning - May 2015

Mechanical Engineering (Semester 7)

TOTAL MARKS: 100
TOTAL TIME: 3 HOURS
(1) Question 1 is compulsory.
(2) Attempt any four from the remaining questions.
(3) Assume data wherever required.
(4) Figures to the right indicate full marks.


1 (a) A Carnot refrigerator works between the temperature limits of -5°C and 23°C. Calculate the power required to run the refrigerator in case it produces 1000kg of ice per hour at -5°C. Surrounding temperature is 23°C. Assume reversible heat transfer, specific heat of ice as 2kJ/kg K and that of the water as 4.187 kJ/kg K and latent heat of freezing s 335 kJ.kg. (5 marks)


1 (b) What are pressure losses in air distribution ducts? Explain them in brief. (5 marks)


1 (c) What is multistage compression? State advantages and disadvantages of multistage compression. (5 marks)


1 (d) State merits and demerits of an air refrigeration system. (5 marks)


1 (e) Discuss why refrigerators are so selected that evaporator pressure and condenser pressures are greater than atmospheric pressure? (5 marks)


2 (a) A class room of 60 seating capacity has to be air conditioned. Outdoor conditions are 32°C DBT and 22°C WKT. Required comfort conditions are 22°C DBT and 55% RH. The quantity of outdoor air supplied is 0.5m3/min/student/ The comfort conditions are achieved first by chemically dehumidifying the air and then cooling by coil. Find (i) DBT of the air leaving dehumidifier. (ii) Capacity of the dehumidifier, (iii) Capacity of cooling coil in tons of refrigeration, iv) Required surface temperature of the cooling coil if the bypass factor of the cooling coil is 0.3. (10 marks)


2 (b) Differentiate Central District and Unitary air conditioning systems. (10 marks)


3 (a) Explain the working of two stage compression with water intercooler and subcooler used for vapour compression system. (10 marks)


3 (b) Draw a neat diagram of LiBr-Water absorption system and explain its working. List the major field of applications of this system. (10 marks)


4 (a) A vapour compression system using R12 works between -15°C and 35°C on the evaporate and condenser side respectively. Using P-h chart determine:
i) COP
ii) Mass flow rate of refrigerant per TR,
iii) Piston displacement per TR using volumetric efficiency of 80%.
iv) Heat rejected in the compressor per TR, and
v) Ideal COP.
(12 marks)


4 (b) Discuss desirable thermodynamic properties of refrigerants. Explain numbering system s of refrigerants with example. (8 marks)


5 (a) Following data is recorded for a reduced ambient air refrigeration system:

Ram air pressure and temperature  1.1 bar, 293 K
Pressure of air at exit of compressor  3.3 bar
Isentropic efficiency of compressor  80%
Effectiveness of heat exchanger  0.8
First cooling turbine exit pressure of
85% of internal efficiency
 0.8 bar
Cabin pressure and temperature  1.01 bar, 25°C
Isentropic efficiency of second cooling turbine  84%
Refrigerating load required  25 tons

Find:
i) Mass flow rate air in kg/min
ii) Compressor power
iii) COP (12 marks)


5 (b) What do you understand by Wet Compression? How it can be avoided? Explain any one method. (8 marks)


6 (a) Describe a psychrometric process where sensible heat removal and latent heat gain are same. (10 marks)


6 (b) A 12 cm long duct passes air at the of 1.3 m2/s. If the friction factor is 0.005 calculate the pressure drop in the following cases. i) When duct is circular of diameter 270mm. ii) when the duct is 270mm square section. (10 marks)


7 (a) How to estimate cooling load of a college library. Hence define RSHF, GSHF and explain how to draw RSHF and GSHF lines. Clearly state the assumptions made if any. (10 marks)


Write short notes on (any two):

7 (b) (i) Thermal insulation of air conditioning system. (5 marks)


7 (b) (ii) Global warning and refrigerants. (5 marks)


7 (b) (iii Dry ice manufacturing and its applications (5 marks)


7 (b) (iv) Evaporating cooling systems (5 marks)

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