Refrigeration & Air Conditioning : Question Paper May 2016 - Mechanical Engineering (Semester 8)

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Refrigeration & Air Conditioning - May 2016

Mechanical Engineering (Semester 8)

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.

Answer any four of the following:

1(a) Define Energy Efficiency Ratio, Write a note on star rating of BEE.(5 marks) 1(b) What are the advantages of air refrigeration for cooling of aircraft?(5 marks) 1(c) Write a note on designation of refrigerants.(5 marks) 1(d) Explain in brief an adiabatic saturation process. Represent the same on a Psychometric chart.(5 marks) 1(e) What is the function of a flash inter-color? Why it is not preferred for R-12 refrigerant.(5 marks) 1(f) Discuss the static regain method of duct design.(5 marks) 2(a) Derive an expression for COP of an ideal Bell Coleman cycle in terms of pressure ratio.(4 marks) 2(b) An airplane using 20 TR bootstrap air refrigeration system has ambient conditions of 0.9 bar and 15°C. Ram air pressure after isentropic compression is 1.1 bar. Main compressor exit pressure is 3.5 bar, and exit pressure of secondary compressor is 4.5 bar. The carbine is required to be maintained at 1 bar and 25°C. Isentropic efficiency of each compressor is 85% and that of cooling turbine is 90%. The effectiveness of both heat ex-changers is 60%. Find
(i) Mass flow rate of air passing through the carbine (ii) Power required, (iii) COP of the system. Assume C_p=1.01 kJ/kg K.(12 marks) 2(c) Discuss the effect of evaporator and condenser pressure on the performance of a Vapour Compression Refrigeration System.(4 marks) 3(a) Define bypass factor of a cooling coil. What are the factors affecting it?(4 marks) 3(b) Explain with a block diagram the working of a practical Ammonia Water vapor absorption refrigeration system.(6 marks) 3(c) A refrigeration machine using R-12 as refrigerant operates between the pressures 2.5 bar and 9 bar. The vapor entering the compressor is dry saturated and there is no sub-cooling in the condenser. If the capacity of the plant is 20 TR and the relative COP is 6.5%.
Determine: i) Actual COP, ii) Actual power input to the compressor.
Properties of the refrigerant are given below:

Pressure bar	Sat Temp ⁰C	Sp.Enthalpy, kJ/kg	Sp. Enthalpy, KJ/kg K
Liquid	Vapour	Vapour
9	36	456.4	585.3	4.74
2.5	-7	412.4	570.3	4.76

Take sp. Heat of refrigerant vapour as 0.67 kJ/kg-K(10 marks) 4(a) Explain with a neat sketch the function of a thermostatic expansion valve.(6 marks) 4(b) A sample of moist air has a dry bulb temperature of 25°C and a relative humidity of 50 percent. The barometric pressure is 740 mm of Hg. Without using Psychometric chart, calculate: 1) partial pressure of water vapor and dry air; 2) Dew point temp; 3) specific humidity of air; 4) Enthalpy of air per kg of dry air.(8 marks) 4(c) Explain with schematic the working of a refrigeration system with a single evaporator and two stage compression system, with a water inter-cooler and flash inter-cooler interposed between the stages. Represent the cycle on a P-h diagram.(6 marks) 5(a) Derive an expression for equivalent diameter of circular duct for rectangular duct for the same frictional loss per unit length when the quantity flowing through both the ducts is same.(6 marks) 5(b) The following data are given for the space to be air conditional:
Outside air conditions = 40°C DBT and 50% RH, Inside design conditions = 26°C DBT and 50% RH, Apparatus Dew Point = 10°C; By-pass factor of the cooling coil = 0.2
The return air from the room is mixed with the outside air before entry to the cooling coil in the ratio of 3 : 1 by mass. If 20 m³/min of fresh air is supplied find:
(i) Condition of air leaving the coil
(ii) Capacity of cooling coil in TR
(iii) Room sensible heat factor.(14 marks)

Write short notes on any four of the following

6(a) Types of cooling tower.(5 marks) 6(b) Methods of defrosting.(5 marks) 6(c) Thermoacoustic refrigeration.(5 marks) 6(d) Variable refrigerant flow systems.(5 marks) 6(e) Liquefaction of Natural Gas (LNG).(5 marks) 6(f) Deep sea air conditioning(5 marks)

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