JNTU
JNTU B.Tech III REFRIGERATION AND AIR CONDITIONING Previous Question Paper (July - 2023 - R16)
Note: i) Question paper consists of Part A, Part B. ii) Part A is compulsory, which carries 25 marks. In Part A, Answer all questions. iii) In Part B, Answer any one question from each unit. Each question carries 10 marks and may have a, b as sub questions.
Part A
Explain how the COP of a refrigeration system related to its energy efficiency?
Answer
The Coefficient of Performance (COP) is the ratio of the cooling effect produced to the net work input. A higher COP indicates that the system provides more cooling for every unit of electrical energy consumed, directly reflecting higher energy efficiency and lower operating costs.
Explain the concept of dense air refrigeration and discuss its advantages over open air refrigeration.
Answer
Dense air refrigeration operates with the air kept at a higher pressure than the surroundings throughout the cycle. Its primary advantage over open air systems is that the higher air density allows for a smaller mass flow rate to achieve the same cooling capacity, resulting in more compact equipment, smaller duct sizes, and higher volumetric efficiency.
What are the main disadvantages or limitations of vapor compression systems?
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What are the common measures to be considered to improve the actual cycle performance of vapor compression refrigeration systems?
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What are the environmental impacts of refrigerants on global warming and ozone depletion?
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List out various expansion devices used in refrigeration systems. Which device is used in domestic refrigeration system and why?
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Which components in VARS replace compressor in VCRS? Discuss.
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What materials are commonly used in thermoelectric refrigeration devices?
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In an air conditioning process 5 kJ/min heat is extracted from room. If the sensible heat factor is 0.7. Then find the latent heat load.
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Write the applications of heat pump.
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Part B
Dense air refrigeration machine operating on reverse Brayton or Bell coleman or Joule cycle operates between 3.4 and 17 bar. The temperature of air after the cooler is 15°C and after the refrigeration is 6°C, if the refrigeration capacity is 6 tonnes, determine
a) Temperature after compression and expansion.
b) Air circulated per min.
c) Work of compressor and expander.
d) COP.
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In an air craft refrigeration air enters the compressor at 0.2 MPa, 40°C and is compressed to 0.5 MPa with an isentropic efficiency of 82%. The air is then cooled to 55°C at constant pressure and is then expanded in a turbine to 0.1 MPa with an isentropic efficiency of 88%, the lower temperature air absorbs the cooling load of 5TR at a constant pressure before returning to the compressor, Assuming air to be an ideal gas. Find
COP
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Net power input.
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A vapour compression refrigeration machine, with Freon-12 as refrigerant, has a capacity of 20 tons of refrigeration operating between -28°C and 26°C. The refrigerant is sub-cooled to 4°C before entering the expansion valve and the vapour is superheated by 5°C before leaving the evaporator. The machine has a six-cylinder single-acting compressor with stroke equal to 1.25 times the bore. It has a clearance of 3% of the stroke volume. Determine:
a) Theoretical power required.
b) COP.
c) Volumetric efficiency.
d) Bore and stroke of cylinder.
The speed of the compressor is 1000 r.p.m.
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Draw the vapour compression refrigeration cycle on T-s diagram when the refrigerant is dry and saturated at the end of compression and find the expression for the COP in terms of
Temperature and entropies,
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Enthalpies.
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Explain the following in brief with neat sketches:
a) Automatic expansion valve
b) Water cooled condenser
c) Shell and coil type evaporator.
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Explain the working of hermetically sealed compressor and explain its advantages and limitations.
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Explain the working of following types of evaporators with neat sketches: (i) Flooded evaporator, (ii) Natural convection evaporator.
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For a vapor absorption refrigeration. Explain the properties required for choosing:
Ideal refrigerant,
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Ideal absorbent,
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Ideal refrigerant-absorbent combination.
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A steam ejector water vapor refrigeration system is supplied with motive dry saturated steam at 7 bar. It expands through a nozzle down to flash chamber pressure meant to chill water at 4°C. Taking nozzle efficiency = 0.95; entrainment efficiency = 0.75; and compression efficiency = 0.80, obtain
Amount of water to be evaporated and motive steam per tonne of cooling tonnage of the plant for 2 kg/s of evaporation of water.
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COP.
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A summer air conditioning system for a small office building is to be designed. The design is to be based on the following information:
Outside design condition: DBT-35°C, WBT-28°C
Inside design condition: DBT-26°C, 50% RH
Room sensible heat gain: 45 kW
Room latent heat gain: 9 kW
Ventilation air: 0.95 m3/s
A four row direct expansion refrigerant 134A coil with bypass factor of 0.2 is to be used. Analyze the problem on a psychometric chart and determine the following:
a) The room Apparatus Dew Point (ADP)
b) The temperature of the air leaving the coil
c) The total quality of air required (m3/s).
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Define various psychrometric properties.
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How do you calculate sensible and latent heat loads? Explain.
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