a) Mention two modes of heat transfer with examples.
b) Draw a sketch and describe the principle of convection as a mode of heat transfer.
c) Define perfect Black body.
d) Name two heat transfer equipments where latent heat is exchanged.
Q.1 (b) Attempt any ONE of the following:
a) Derive rate equation for heat transfer through a thick walled cylinder.
b) Draw the diagram and describe the concept of optimum thickness of insulation with a neat diagram.
Q.2 Attempt any TWO of the following:
a) A furnace is insulated with 230 mm thick fire brick, 115 mm of insulating brick, 230 mm thickness of building brick. The inside temperature of furnace is 1213 K and outside temperature is 318 K. The thermal conductivities of fire brick, insulating brick, building bricks are 6.047, 0.581 and 2.33 W/m.K Find out
1) Heat loss per unit area.
2) Temperatures at interfaces
b) Water is flowing in a tube of 16 mm diameter at a velocity of 3 m/s. The temperature of tube is 297 K and the water enters at 353 K and leaves at 309 K.
Data: Properties of water 12204
1) Density of water = 984.1 kg/m3
2) Specific heat of water = 4.187 KJ/kg.k
3) Viscosity of water = 485 x 10 -6 Pa.s
4) Thermal conductivity of water = 0.657 W/m.K
Calculate the heat transfer coefficient.
c) Cold fluid is flowing through a double pipe heat exchanger at a rate of 15 m3/hr. It enters at 303 K and is to be heated to 328 K. Hot thermic fluid is available at the rate of 21 m3/hr. & at 383 k.
Data: 1) Specific heat of thermic fluid = 2.72 KJ/kg.K
2) Density of water = 1 gm/cm3
3) Density of thermic fluid = 0.95 gm/cm3
4) Specific heat of water = 4.187 KJ/kg.k
Find out the log mean mean temperature difference for counter current type of flow by the following steps:
i) Outlet temperature of hot fluid
ii) Temperature difference at two ends
iii) LMTD
Q.3 Attempt any FOUR of the following:
a) Draw a neat labelled diagram of 1-2 pass heat exchanger.
b) Mention any four characteristics of solutions to be considered before selecting the evaporator?
c) Differentiate evaporation and drying on two points.
d) What is dropwise condensation and filmwise condensation?
e) Mention any four dimensionless groups used in heat transfer and give significance of each group.
Q.4 (a) Attempt any THREE of the following:
a) Draw a neat diagram to indicate heat transfer from bulk of a hot fluid to bulk of cold fluid flowing across a metal surface and show the temperature profile.
b) Write down equation to calculate Nusselt number in laminar flow and explain all the terms.
c) Explain why heat transfer rate is more in dropwise condensation? Give two reasons.
d) Draw a graphical diagram indicating co-current and counter current heat exchange and give expression for LMTD in both the cases.
Q.4 (b) Attempt any ONE of the following:
a) Draw a neat diagram of a plate heat exchanger and show the types of flow in it. Give only one advantage of this type of heat exchanger.
b) What are surface extended heat exchangers? What is their specific application in chemical industry?
Q.5 Attempt any TWO of the following:
a) Derive a relation between overall and individual heat transfer coefficient in convection.
b) Water is to be heated from 298 K to 313 K at a rate of 30 kg/s. Hot water is available at 353 K at the rate of 24 kg/s for heating in a counter-current heat exchanger. Calculate the required heat transfer area if overall heat transfer coefficient is 1220 W/ m2K.
c) An evaporator at atmospheric pressure is fed at the rate of 10,000 kg/hr of 4% concentration of caustic soda. Thick liquor leaving evaporator contains 20% caustic soda. Find:
i) Capacity of evaporator.
ii) If 9000 kg of steam is fed. What will be the economy of an evaporator.
Q.6 Attempt any FOUR of the following:
a) Differentiate between Natural convection and Forced convection on the following points. i) rate of heat transfer ii) how the currents are generated?
b) What is Dittus-Boelter equation? Write it down and give it’s use in heat transfer.
c) State and explain Stefan Boltzmann law of radiation.
d) Explain the terms absoptivity, reflectivity.
e) How economy of an evaporator can be increased? Name methods and explain any one of them?
