Total No. of Questions 12] [Total No. of Printed Pages 7 [4062]-186 S.E. (Chemical) (Second Semester) EXAMINATION, 2011 HEAT TRANSFER (2008 PATTERN) Time : Three Hours Maximum Marks : 100 N.B. : (i) Answers to the two Sections should be written in separate answer-books. (ii) (iii) Neat diagrams must be drawn wherever necessary. Use of logarithmic tables, slide rule, Mollier charts, electronic pocket calculator and steam tables is allowed. (iv) Assume suitable data, if necessary. SECTION I 1. (a) Explain Newton s Law of Cooling. [4] Explain in detail Modes of Heat Transfer. [5] (c) A steam pipe 190 180 mm in dia. is covered with two layers of insulation. The thickness of the first layer is 40 mm and second layer is 60 mm. The thermal conductivity of pipe and insulating layers are 60, 0.16 and 0.09 kcal/m hr C respectively. The temperature of the inner surface of steam pipe is 400 C and that of the outer surface of insulation layer is 60 C. Determine the quantity of heat loss per meter length of steam pipe and layer contact temperature. [9] P.T.O.
Or 2. (a) By algebraic Rayleigh method of dimensional analysis derive the following relationship for Natural Convection heat transfer process (, e and f are numerical cont.) : e Nu = a éë Pr f Gr ùû. [9] Calculate the heat flow rate per unit length through a long tube of inner dia. 4 cm and outer dia. 8 cm, K = 0.59 W/mk. The inner wall of the tube is at 80 C saturated steam at 120 C is condensing on the outer surface of the tube h 0 = 6000 W/m 2 K. [9] 3. (a) Derive expression for rate of heat flow through composite cylinder constructed from different material. [8] An aluminium rod 35 mm in dia. and 120 mm long protrudes from a wall which is maintained at 550 K (277 C) into the environment maintained at 298 K (25 C). Estimate the heat lost by assuming that the rod end is insulated. Also find the fin efficiency and temperature at the end of the fin. [8] k = 250 W/m.K (for aluminium) h between the rod surface and environment = 20 W/m 2 K. [4062]-186 2
Or 4. (a) Derive expression of steady state heat transfer rate for the following cases when k is linear function of temperature i.e. k = k 0 (1 + T) : (i) Plane wall (ii) Hollow cylinder. [8] A steel pipe with an outside dia. of 115 mm and a wall thickness of 5 mm is covered with 50 mm thickness of 85% magnesia. The surface temperature on the inside of the pipe is 423 K (150 C) and that on the outside of insulation is 305 K (32 C). Calculate : [8] (i) The heat flow per meter of length (ii) The temperature at the outer surface of the steel pipe and (iii) The conductance of the pipe and insulation based on its inside surface area. k for steel = 48.08 W/mK k for insulation = 0.07 W/mK. 5. (a) Explain : [8] (i) Thermal boundary layer (ii) Velocity boundary layer. [4062]-186 3 P.T.O.
Air at the temperature of 525 K (252 C) flows over a flate plate 0.4 m wide and 2 m long at a velocity of 8 m/s. If the plate is to be maintained at 355 K (82 C). Calculate the rate of heat to be removed continuously from the plate. [8] Properties of air at the mean temperature are : Kinematic viscosity 3.90 10 4 m 2 /s Thermal conductivity 36.4 10 3 W/mK N Pr = 0.69. Or 6. (a) Distinguish between filmwise and dropwise condensation. Which of these two gives higher transfer coefficient? Why? [4] Explain critical heat flux in pool boiling. [4] (c) A 30 cm long glass plate is hung vertically in the air at 300 K (27 C). The plate is maintained at 356 K (77 C). Calculate the average heat transfer coefficient for natural and forced convection. [8] The properties of air at 325 K (52 C) are : = 18.41 10 6 m 2 /s, k = 28.15 10 3 W/(m.K), N Pr = 0.7, = 3.077 10 3 K 1. Take free stream velocity of air = 4 m/s. [4062]-186 4
SECTION II 7. (a) What are different laws of radiation? Explain any two laws. [10] A small object at 47 C is placed in a large furnace whose interior is maintained at 927 C using the following data calculate the rate of absorption by and emission of radiation from this object : [8] Absorptivity ( ) Temp. (T K) 0.78 320 0.67 600 0.55 1200 Or 8. (a) What is specular and diffuse reflection? Explain radiosity, Irradiation and radiation shields. [10] A thermosflask with evacuated space to reduce the heat losses having surfaces facing each other of emissivity 0.02. If contents of the flask are at 380 K and the ambient temperature is 298 K, compute the heat loss from the flask. If same effect is to be achieved by using insulating material of conductivity 0.04 W/m K, what would be the thickness required? [8] [4062]-186 5 P.T.O.
9. (a) How are heat exchanger classified? Explain. [8] It is required to cool 250 kg/hr of hot liquid with inlet temperature of 393 K using parallel flow arrangement 1000 kg/hr of cooling water is available for cooling purpose at a temperature of 283 K. Taking overall heat transfer coefficient of 1160 W/m 2 K and heat transfer surface area of 0.25 m 2, calculate the outlet temperature of liquid and water and the effectiveness of heat exchanger. [8] Specific heat of water = 4187 J/kg K Specific heat of liquid = 3350 J/kg K. Or 10. (a) Explain shell and tube heat exchanger in detail. [8] 20 kg/sec of water at 360 K entering a heat exchanger is to be cooled to 340 K by using cold water at 300 K flowing at rate of 25 kg/sec. If the overall heat transfer coefficient is 1500 W/m 2 K, calculate heat transfer area required in : (i) co-current flow concentric pipe heat exchanger, and (ii) countercurrent flow concentric pipe heat exchanger. [8] 11. (a) What is evaporation? Explain classification of evaporator. [8] An evaporator operating at atmospheric pressure is fed at the rate of 10,000 kg/hr of week liquor containing 4% caustic soda. Thick liquor leaving the evaporator contains 25% caustic soda. Find the capacity of the evaporator. [8] [4062]-186 6
Or 12. (a) What is multiple effect evaporator? Explain different feed arrangement in detail. [8] A single effect evaporator is used to concentrate 20,000 kg/ hr of solution having concentration of 5% salt to a concentration of 20% salt (by weight). Steam is fed to the evaporator at a pressure corresponding to saturation temperature of 399 K. The evaporator is operating at atmospheric pressure and boiling point rise is 7 K. Calculate heat load and economy. [8] Feed temperature = 298 K Specific heat of feed = 4.0 kj/kg K Latent heat of condensation of steam at 399 K = 2185 kj/kg Latent heat of vaporization of water at 373 K = 2257 kj/kg. [4062]-186 7 P.T.O.