# Pune University BE Mechanical Cryogenic Engineering Question Papers

B.E. (Mechanical) CRYOGENIC ENGINEERING (2008 Pattern)

(Open Elective) (Sem. – II)

Time: 3 Hours]                                                                           [Max. Marks :100

Instructions to the candidates:

1)            Answer three questions from each section

2)             Answers to the two sections should be written separate books.

3)            Neat diagrams must be drawn wherever necessary.

4)             Figures to the right indicate full marks.

5)             Use of logarithmic tables, Mollier charts, electronic pocket calculator,P-h charts is allowed.

6)            Assume suitable data, if necessary.

SECTION – I

Q1) a) Define the term “Cryogenics”. Discuss the various applications of cryogenics.     

b)            Discuss the mechanical properties of materials at cryogenic temperatures.



c)            Discuss the various properties of the following cryogenic fluids : 

i)              Oxygen,                                ii) Argon,

OR

Q2) a) Discuss the phase diagram for super fluid helium.                           

b)            Explain the difference between ortho-hydrogen and para-hydrogen.

c)            Describe Walker’s chart of cryocooler classification.                       

Q3) a) What are the system performance parameters for gas liquefaction system? How these parameters are related?                                                                 

b)            Explain the Ideal cycle used for liquefaction of gases with neat sketch. 

c)            An ideal J-T refrigeration system operates between 300K and 70K using Nitrogen as working fluid. The gas is compressed from 50.7kPa to 15.2 MPa. Determine the coefficient of performance and figure of merit of this system.                  

Explain the Linde-Hampson cycle used for liquefaction of gases with neat sketch.          

Explain the Joule-Thompson coefficient. What do you mean by inversion temperature? Mention the inversion temperatures of few cryogenic fluids.                                          

Determine the minimum work required to provide 100 W of refrigeration at 20.4 K if the heat is rejected at (i) 77K and (ii) 300 K.



Explain the working of Simon helium-liquefaction system with suitable sketch.  

Determine the fraction of air liquefied in a simple Linde cycle if the inlet conditions on the warm side of the two-channel heat exchanger are 310 K and 20.2 MPa while the exit conditions are 303 K and 0.101 MPa.                                                                                               

Determine the liquid yield, the work per unit mass compressed, the work per unit mass liquefied, and the figure of merit for a simple Linde-Hampson system using Argon as the working fluid. The system operates between 101.3 kPa and 293 K at point 1 and 20.67 MPa at point 2. The system may be assumed reversible, except for the expansion through the expansion valve. Draw the neat sketch for the arrangement and represent the cycle on T-S chart.                          

OR

Explain the working of Claude liquefaction system with suitable sketch.



Air enters a Claude system at 1atm and 25°C and is compressed to 100atm at 100 atm and 240K, 50% of the main flow is diverted to the expander. The remainder flows through the heat exchangers and expanders through the expansion valve to 1 atm.

Adiabatic efficiency of expander = 80%, Mechanical efficiency of expander = 90%.

Overall efficiency of compressor = 75%, Effectiveness of heat exchangers = 98.5%

Determine :

i)              Liquid yield,

ii)           Expander work output per unit mass compressed,

iii)         Figure of merit.                                                                                          

Determine the fraction of flow diverted through a reversible and adiabatic expander of an ideal Claude refrigeration system operating under the following conditions : Working fluid is nitrogen compressed isothermally from 0.101 MPa and 300 K to 4.04 MPa; inlet to the expander is 4.04 MPa and 240 K; refrigeration effect is 81 kJ/kg. 

SECTION – II

With the help of schematic and T-S diagram, explain Philips Refrigerator. Also explain briefly the importance of refrigerator effectiveness.                                                       

A Carnot refrigerator operates between 300 K and 72 K. Determine the COP of the refrigerator. Please comment if low temperature source reduces to 10 K, what is the effect on COP of the cycle. Draw T-S diagram for the same.                                                                      

Write short note on the G-M refrigerator.                                                      

OR

A Philips refrigerator operates at 100 K with an ambient heat sink temperature of 300 K. The unit operates between a lower pressure level of 0.101 MPa and an upper pressure level of 2.02 MPa. If the working fluid is nitrogen and is assumed to behave as an ideal gas, determine the refrigeration effect per unit mass of nitrogen compressed, the coefficient of performance of the refrigerator and the figure of merit.  Write short note on the Vuilleumier refrigerator. 

Explain briefly the importance of refrigerator effectiveness.                     

A natural gas stream has a volumetric composition of 80% methane, 12% ethane, 3% propane, 1% butane and 4% nitrogen. Assuming that all the gases are in mixture follows the ideal gas equation of state; evaluate the ideal work of separation at 300K in kJ/kg of propane if

i)              all the gases are separated and

ii)           only the ethane and the propane are separated individually from the other gases that remain mixed.                                                                                                          

Define the Murphree efficiency and discuss the factors that affect it.  Estimate the equilibrium vapour and liquid compositions for a mixture of Oxygen and Argon at a pressure and temperature of 0.101 MPa and 86 K respectively assuming that the vapor acts as an ideal gas and the liquid behaves as a perfect solution.                                                                           

OR

3 A mixture of nitrogen and argon exists as a two-phase mixture at 506.6 kPa. The mole fraction of nitrogen in the gaseous phase is 0.7. Determine the temperature of the mixture and the mole fraction of nitrogen in the liquid phase, using distribution coefficients for the components.           

Show that y = ax + b is the form of equation of the operating line in the upper part of rectification column. State the assumptions made.  Determine the number of theoretical plates required to yield 96% nitrogen as top product stream and 93% of oxygen as bottom product stream. The feed composition is 79% N2 and 21% O2. The feed, bottom product and top product streams are saturated liquids. The desired flow rate of the bottom product is 30 kgmol/s. The heat removal at top of the column is 1150 kW. The column operates at a pressure of 1 atm. 

Discuss the importance of vacuum technology in cryogenics.  Compare various cryogenic insulation methods.                                                                                            

With the help of neat sketch explain the construction of Dewar Vessel. 

OR

With a neat sketch describe the operation of diffusion pump. Further discuss why the getters are used in cryogenics.                                                                                             

Write short note on Cryopumping.                                                                  

Discuss various temperature measurement techniques used in cryogenic applications.    

❖ ❖ ❖