12204 21314 3 Hours / 100 Marks Instructions – Seat No. (1) All Questions are Compulsory. (2) Answer each next main Question on a new page. (3) Illustrate your answers with neat sketches wherever necessary. (4) Figures to the right indicate full marks. (5) Assume suitable data, if necessary. (6) Use of Non-programmable Electronic Pocket Calculator is permissible. (7) Mobile Phone, Pager and any other Electronic Communication devices are not permissible in Examination Hall. Marks 1. a) Attempt any THREE of the following: i) Give three modes of heat transfer with example of each. ii) Define Dimensional analysis. Give it’s physical significance. iii) State: iv) 1) Kirchoff’s Law 2) Stefan-Boltzmann Law 12 Give any two equipments of each based on: 1) Sensible heat exchange 2) Latent heat exchange P.T.O. 12204 [2] Marks b) Attempt any ONE of the following: i) 6 A steam pipeline, 150/160 mm in diameter, is covered with a layer of insulating material of thickness 50 mm. The temperature inside the pipeline is 393 K and that of the outside surface of insulation is 313 K. Calculate the rate of heat loss per one meter length of pipeline. Data: Thermal conductivity for pipe is 50 W / (m. k) and for insulating material is 0.08 W / (m. k) ii) Calculate the critical radius of insulation for asbestos (K = 0.17 W / (m. k.) surrounding a pipe and exposed to room air at 293 K with h = 3 W / (m2. k) Calculate the heat loss from 473 K 50 mm diameter pipe when covered with the critical radius of insulation and without insulation. 2. Attempt any TWO of the following: a) Derive the expression for heat transfer through furnace wall made of three different materials in series. Assume, K1, K2 and K3 be the thermal conductivities of materials and x1, x2 and x3 be the respective thicknesses. Assume hot face and cold face temperature be T1 and T2 respectively. b) Calculate the inside heat transfer coefficient for fluid flowing at a rate of 300 cm3/sec. through a 20 mm inside diameter tube of H.E. Data: viscosity of flowing fluid = 0.8 ( N. S. ) / m2 density of flowing fluid = 1.1 gm / cm3 specific heat of fluid = 1.26 kJ / kg K. Thermal conductivity of fluid = 0.384 W / (m.k), Viscosity at wall temperature = 1.0 (N. S) / m2, length of H. E. = 5m. c) Explain in brief significance of heat transfer coefficient in boiling liquids w.r.es. to, natural convection, nucleate boiling, transition boiling, film boiling. 16 12204 [3] Marks 3. Attempt any FOUR of the following: 16 a) Hot oil at a rate of 1.2kg/s (Cp = 2083 J/kgk) flows through double pipe H.E. It enters at 633 K and leaves at 573 K. The cold Fluid enters at 303 K and leaves at 400 K. If the overall heat transfer coefficient is 500W/(m2 k) Calculate the heat transfer area for: i) Parallel flow ii) Counter current flow b) Define: i) Capacity ii) Economy of evaporator c) An evaporator operating at atm pressure (101.325KPa) is fed at the rate of 10000 kglh of weak liquor containing 4% caustic soda. Thick liquor leaving the evaporator contains 25% caustic soda. Find the capacity of the evaporator. d) Calculate the heat transfer area of 1 - 2 H. E. from the following data. Inlet and outlet temperature of hot fluid are 423 K and 353 K respectively Inlet and Outlet temperature of cold fluid are 303 K and 318 K respectively. Overall heat transfer coefficient = 4100 W/(m2k). Heat loss = 407 kW LMTD correction factor = 0.84. e) Define heat transfer coefficient and compare with thermal conductivity. P.T.O. 12204 [4] Marks 4. a) Attempt any THREE of the following: i) 12 Calculate overall heat transfer coefficient from the following data: Inside heat transfer coefficient = 5800 W / (m2 k) Outside heat transfer coefficient = 1750 W / (m2 k) Outside diameter of tube = 30 mm Inside diameter of tube = 20 mm K of metal wall = 46.52 W / (m. k) ii) Distinguish filmwise and dropwise condension. iii) Compare parallel flow with counter current flow in heat exchanger. iv) Water enters a two fluid H.E. at 328 K and leaves at 358 K. Hot gases enter at 578 K and leaves at 433 K. If the total heat transfer area is 500m2 and the overall heat transfer weff. is 700 W / (m2 k). Determine total heat transfer for: 1) Parallel flow 2) Counter - current flow of the two fluids. b) Attempt any ONE of the following: i) What are advantages of baffles in H.E.? ii) Write in brief for scrapped surface heat exchanger with diagram. 6 12204 [5] Marks 5. Attempt any TWO of the following: 16 a) Derive the relationship between overall heat transfer coefficient and individual heat transfer coefficient with diagram. b) In a double pipe counter current flow H.E. 10000 kg / h of an oil having a specific heat of 2095 J / kg K is cooled from 353 K to 323 K by 8000 kg / h of water entering at 298 K. Calculate the H. E. area for an overall heat transfer coefficient of 300 W / (m2 k). Take Cp for water as 4187 J / kg K. c) State the utility of forced circulation in evaporator and draw a diagram of a forced circulation evaporator with horizontal external heating element. 6. Attempt any FOUR of the following: a) State Fourier’s law for conduction? Give it’s mathematical statement. b) Give Dittus. Boelter equation for heating of liquids and define each term in it. c) Define absorptivity, reflectivity and transmissivity of a body and prove that “When any body is in thermal equilibrium with it’s surrounding it’s emissivity is equal to absorptivity.” d) Calculate the heat loss by radiation from an unlogged horizontal steam pipe, 50mm D.D. at 377 K to air at 283 K. Data: emissivity, e = 0.90. e) Distinguish between forward feed and backward feed evaporator. 16