R05220801-process-heat-transfer

Set No. 1

Code No: R05220801

II B.Tech Supplimentary Examinations, Aug/Sep 2008 PROCESS HEAT TRANSFER (Chemical Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. Derive steady state heat conduction equation for (a) Heat conduction across a flat plate (b) Heat conduction through composite cylindrical walls.

[8+8]

2. Write short notes on: (a) Log mean temperature difference (b) Fouling factors (c) Parallel flow heat exchangers (d) Counter flow heat exchangers.

[16]

3. For a turbulent flow in a tube with no change in temperature of the liquid and the tube wall, how will the heat transfer coefficient change if (a) the diameter of the tube is tripled (velocity of the flow is maintained constant by change in the liquid flow rate). (b) three times increase in the fluid velocity.

[16]

4. Water available at 24 0 C is heated by a heating element having 30 mm outside diameter and 0.4m length. The heating element is dipped in water. Surface temperature of heating element is 80 0 C. Properties of water at average temperature are: Density = 988 kg/m3 ; Viscosity = 0.55 × 10−3 N-s/m2 ; Specific heat = 4180 J/kg 0 C; thermal conductivity = 0.64 w/m 0 C; Pr = 3.592; Volume coefficient of expansion = 3.37 × 10−3 0 K −1 .Calculate the average value of heat transfer coefficient over the entire length of the eating element and total heat transferred to water by the heating element. [16] 5. Derive Zuber’s analytical expression for the peak heat flux in nucleate boiling. [16] 6. (a) Classify the heat exchangers according to flow type and explain the characteristics of each type. (b) What is the role of the baffles in a shell and tube heat exchanger? How does the presence of baffles effect the heat transfer and pumping power requirements? Explain. [16] 7. (a) Write briefly the methods of feeding in multiple effect evaporators. (b) Write on the capacity, economy, effect of liquid head and boiling point elevation with respect to multiple effect evaporators. [16] 1 of 2

Set No. 1

Code No: R05220801

8. Two very large parallel plates are maintained at uniform temperatures of 10000 K and 5000 K and have emissivities of 0.2 each. It is desired to reduce the net rate of radiation heat transfer between the two plates to one fifth by placing thin aluminium sheets with an emissivity of 0.2 on both sides between the plates. Determine the number of sheets that need to be inserted. [16] ⋆⋆⋆⋆⋆

2 of 2

Set No. 2

Code No: R05220801

II B.Tech Supplimentary Examinations, Aug/Sep 2008 PROCESS HEAT TRANSFER (Chemical Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. A one square meter 6mm thick steel furnace door (k = 30 w/m 0 k) is insulate on the inside by a 2 cm thick layer of ceramic fiber matting (k = 0.05 w/m 0 k) and a 10 cm thick layer of refractory brick (k = 1.0 w/m 0 k). If the temperature of the brick surface in the furnace is 700 0 C and the outside steel surface of the door is at 50 0 C, what is the heat loss by conduction through the door? If the maximum temperature of the ceramic fiber were limited to 500 0 C, what would its thickness have to be and what effect would this have on heat loss. [16] 2. (a) Explain a double pipe heat exchanger and a single pass tubular condenser with a labeled figures. (b) What is the limitation of double pipe exchanger. -

[10+6]

3. Liquid sodium is flowing through a 30 mm inside diameter pipe at a velocity of 3 m/s. Liquid sodium enters at 300 0 C. It is exposed to uniform heat flux at the pipe wall. Physical Properties of liquid sodium are: Density = 900 kg/m3 ; Specific heat = 1330 J/kg 0 K; Thermal conductivity = 81.5 w/m 0 K; viscosity = 0.455 × 10−3 Ns/m2 . Calculate the value convective heat transfer coefficient for liquid sodium. [16] 4. What electric power is required to maintain a 0.076 mm diameter 0.6 m long vertical wire at 400 0 K in an atmosphere of quiescent air at 300 0 K. The wire’s resistance is 0.0118 ohms per meter. Nu = 0.37. thermal conductivity = 0.03003 W/m 0 K; Kinematic viscosity = 20.76 × 10−6 m2 /s; Pr = 0.697 [16] 5. Derive Zuber’s analytical expression for the peak heat flux in nucleate boiling. [16] 6. Hot oil is to be cooled in a double tube counter flow heat exchanger. The copper inner tube has a diameter of 2cm and negligible thickness. The inner diameter of the outer tube is 3cm. Water flows through the tube at the rate of 0.5 kg/s and the oil through the shell at a rate of 0.8 kg/s. Taking the average temperature of water and the oil to be 45 0 C and 80 0 C respectively. Determine the overall heat transfer coefficient of this heat exchanger . Properties of water :Density = 990 kg/m3 ; Pr = 3.97; K = 0.637 W/m 0 C; Kinematic viscosity = 0.602 × 10−6 m2 /s. Properties of water :Density = 852 kg/m3 ; Pr = 490; K = 0.138 W/m 0 C; Kinematic viscosity = 37.5 × 10−6 m2 /s. [16] 7. 1000 kg/h of a dilute solution of sodium hydroxide containing 10% NaOH is to be concentrated to 40% NaOH by weight in a single effect evaporator. The feed 1 of 2

Set No. 2

Code No: R05220801

is available at 25 0 C. Boiling point of the solution may be considered as 100 0 C. Specific heat of dilute solution is 4180 J/kg 0 K; Latent heat of vaporization of water is 2239 kJ/kg; Saturated steam corresponding to 1.8 bar pressure and 117 0 C is available for heating purpose. Latent heat of condensation of steam is 2212 kJ/kg. If the overall heat transfer coefficient for the system is 850 W/m2 0 K. Calculate the quantity of water evaporated, steam consumed and steam economy and the surface area of the evaporator. [16] 8. Consider a 20 cm diameter spherical ball at 8000 K suspended in the air. Assuming that the ball closely approximates black body, determine (a) the total black body emissive power (b) the total amount of radiation emitted by the ball in 5 min and (c) the spectral black body emissive power at a wavelength of 3 µm. ⋆⋆⋆⋆⋆

2 of 2

[16]

Set No. 3

Code No: R05220801

II B.Tech Supplimentary Examinations, Aug/Sep 2008 PROCESS HEAT TRANSFER (Chemical Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) State and Explain Fourier’s law of heat conduction (b) Derive the equation for the rate of heat flow for conduction of heat through a composite flat wall (c) Derive the equation for the rate of heat flow for conduction of heat through a composite cylindrical wall. [4+6+6] 2. Kerosene is to be heated from 25 0 C to 50 0 C in a heat exchanger. Flow rate of kerosene is 0.38 kg/s. It flows through the tube having inside diameter of 30 mm and outside diameter of 34 mm. Water at 94 0 C is used for the heating purpose. Flow rate of water is 0.125 kg/s. Outer surface area of the heat exchanger is insulated. Properties of kerosene are: Density: 815 kg/m3 ; Cp water = 4180 J/kg 0 K; Cp Kerosene = 2090 J/kg 0K; Convective heat transfer coefficient on water side = 1350 w/m2 0 K; Convective heat transfer coefficient kerosene side = 730 w/m2 0 K. Thermal conductivity = 380 w/m 0 K. Calculate the length of heat exchanger in a parallel flow and counter flow arrangement. [16] 3. For a certain forced convection process, the following correlation applies: Nu = 0.031 (Re)0.75 (P r)0.3 .Work out the percentage change in the rate of heat flow per degree temperature difference when the original coolant is replaced by another fluid having viscosity equal to two thirds that of the original coolant. Assume that the other fluid variables and configuration remain the same. [16] 4. (a) Write the heat transfer correlations for free convection. (b) Differentiate between free convection and forced convection with examples. [8+8] 5. (a) Explain pool boiling (b) Write briefly on i. Critical temperature drop ii. nucleate boiling

[16]

6. (a) How is a heat exchanger cleaned? Give the answer with respect to the tube side shell side of a fixed head / floating head exchanger. (b) Explain the factors responsible for deterioration of performance of heat exchanger with time. [8+8] 7. (a) Define evaporator economy and capacity. 1 of 2

Set No. 3

Code No: R05220801 (b) Explain the agitated film evaporator.

[16]

8. A thin aluminium sheet with an emissivity of 0.15 on both sides is placed between two very large parallel plates, which are maintained at uniform temperatures 9000 K and 6500 K and have emissivities 0.5 and 0.8, respectively. Determine the net rate of the plates and compare the result with and without the shield. [16] ⋆⋆⋆⋆⋆

2 of 2

Set No. 4

Code No: R05220801

II B.Tech Supplimentary Examinations, Aug/Sep 2008 PROCESS HEAT TRANSFER (Chemical Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) Define thermal contact resistance. (b) The thermal contact conductance at the interface of two 1 cm thick aluminium plates is measured to be 11000 w/m2 0 C. Determine the thickness of the aluminium plate whose thermal resistance is equal to the thermal resistance of the interface between the plates. [16] 2. In a counter current double pipe heat exchanger, 0.4 kg/s of oil (Cp = 2.2 KJ/kg 0 K) is used to cool from 120 0 C using 1 kg/s of water which enters at 25 0 C. The heat exchanger has an overall heat transfer coefficient of 600 w/m2 0 K and a heat transfer area of 3 m2 . Calculate the exit temperature of the oil and cooling water. Also determine the total heat transfer rate. [16] 3. A ball of ice, 4cm in diameter, at 0 0 C is suspended in a dry air stream at 25 0 C which is flowing at a velocity of 2 m/s. Determine the initial rate of melting of ice. Assume that the shape of the ice ball remains spherical all the time. Data: Heat of fusion of ice = 334 KJ/kg ; Density of air = 1.248 kg/m3 ; thermal conductivity of air = 0.026w/m 0 C; Viscosity = 1.69∗ 10−5 kg/m.s.; Cp = 1.005 KJ/kg 0 C; Density of ice = 920 kg/m3 . Use the correlation: Nu = 2+[0.4(Re)1/2 +0.06(Re)2/3 ](P r)0.4 . [16] 4. Estimate the heat transfer from a 40 w incandescent bulb at 127 0 C to 27 0 C quiescent air. Approximate the bulb as a 50 mm diameter sphere. What percentage of the power is lost by free convection. Nu = 0.6 (Gr Pr)1/4 . Density = 994 kg/m3 ; thermal conductivity = 0.628 W/m 0 K; Kinematic viscosity = 0.658 × 10−6 m2 /s; Pr = 4.876 [16] 5. (a) How does film boiling differ from nucleate boiling.? Is the boiling heat flux necessarily higher in the stable film boiling regime than it is in the nucleate boiling regime? (b) Draw the boiling curve and identify the burn out point on the curve. Explain how burnout is caused. Why is the burn out point avoided in the design of boilers. [16] 6. In a shell and tube heat exchanger oil is to be heated from 25 0 C to 75 0 C by condensing steam at 110 0 C on the shell side. The outside diameter of tube is 42mm while the inside diameter is 38 mm. Due to fouling inside diameter of the tube is reduced to 34 mm. Oil velocity through tubes is 0.85 m/s. From the

1 of 2

Set No. 4

Code No: R05220801

previous experiments it is known that the oil side heat transfer coefficient varies with temperature as follows: Oil Temperature 0 C Oil side HT coefficient, W/m2 0 K

25 35 50 60 75 72 78 98 140 255

Specific heat of oil = 1900 J/kg 0 C; Density of oil = 900 kg/m3 . Resistance to heat transfer due to pipe wall and fouling on inner surface together = 0.00095 m2 k/w. Resistance to heat transfer due to condensate film on steam side may be neglected. Calculate the length of the tube bundle required. [16] 7. Write short notes on (a) circulation evaporators (b) Long tube evaporators (c) falling film evaporators (d) agitated film evaporators.

[16]

8. (a) What does the view factor represent? When is the view factor from a surface to itself is not zero. (b) How the view factor F12 is determined when the view factor F21 is available. (c) What is the summation rule and super position rule for view factors. [4+4+8] ⋆⋆⋆⋆⋆

2 of 2