6085303-r05220801processheattransfer

Code No: R05220801 Set No. 1

II B.Tech Supplimentary Examinations, Aug/Sep 2008PROCESS HEAT TRANSFER

(Chemical Engineering)Time: 3 hours Max Marks: 80

Answer any FIVE QuestionsAll Questions carry equal marks

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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 thetube wall, how will the heat transfer coefficient change if

(a) the diameter of the tube is tripled (velocity of the flow is maintained constantby change in the liquid flow rate).

(b) three times increase in the fluid velocity. [16]

4. Water available at 24 0C is heated by a heating element having 30 mm outsidediameter and 0.4m length. The heating element is dipped in water. Surface tem-perature of heating element is 80 0C. Properties of water at average temperatureare: Density = 988 kg/m3; Viscosity = 0.55 × 10−3 N-s/m2; Specific heat = 4180J/kg 0C; thermal conductivity = 0.64 w/m 0C; Pr = 3.592; Volume coefficient ofexpansion = 3.37 × 10−3 0

K−1.Calculate the average value of heat transfer coef-

ficient over the entire length of the eating element and total heat transferred towater 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 charac-teristics of each type.

(b) What is the role of the baffles in a shell and tube heat exchanger? How does thepresence 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 elevationwith respect to multiple effect evaporators. [16]

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

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1. A one square meter 6mm thick steel furnace door (k = 30 w/m 0k) is insulate onthe inside by a 2 cm thick layer of ceramic fiber matting (k = 0.05 w/m 0k) and a10 cm thick layer of refractory brick (k = 1.0 w/m 0k). If the temperature of thebrick surface in the furnace is 700 0C and the outside steel surface of the door isat 50 0C, what is the heat loss by conduction through the door? If the maximumtemperature of the ceramic fiber were limited to 500 0C, what would its thicknesshave 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 witha 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 3m/s. Liquid sodium enters at 300 0C. It is exposed to uniform heat flux at the pipewall. Physical Properties of liquid sodium are: Density = 900 kg/m3; Specific heat= 1330 J/kg 0K; Thermal conductivity = 81.5 w/m 0K; viscosity = 0.455×10−3N-s/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 verticalwire at 400 0K in an atmosphere of quiescent air at 300 0K. The wire’s resistanceis 0.0118 ohms per meter. Nu = 0.37. thermal conductivity = 0.03003 W/m 0K;Kinematic viscosity = 20.76× 10−6m2/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 copperinner tube has a diameter of 2cm and negligible thickness. The inner diameter ofthe outer tube is 3cm. Water flows through the tube at the rate of 0.5 kg/s and theoil through the shell at a rate of 0.8 kg/s. Taking the average temperature of waterand the oil to be 45 0C and 80 0C respectively. Determine the overall heat transfercoefficient of this heat exchanger . Properties of water :Density = 990 kg/m3; Pr =3.97; K = 0.637 W/m 0C; Kinematic viscosity = 0.602× 10−6 m2/s. Properties ofwater :Density = 852 kg/m3; Pr = 490; K = 0.138 W/m 0C; Kinematic viscosity= 37.5× 10−6 m2/s. [16]

7. 1000 kg/h of a dilute solution of sodium hydroxide containing 10% NaOH is tobe concentrated to 40% NaOH by weight in a single effect evaporator. The feed

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is available at 25 0C. Boiling point of the solution may be considered as 100 0C.Specific heat of dilute solution is 4180 J/kg 0K; Latent heat of vaporization of wateris 2239 kJ/kg; Saturated steam corresponding to 1.8 bar pressure and 117 0C isavailable 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 0K. Calculate thequantity of water evaporated, steam consumed and steam economy and the surfacearea of the evaporator. [16]

8. Consider a 20 cm diameter spherical ball at 8000K suspended in the air. Assumingthat 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. [16]

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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 acomposite flat wall

(c) Derive the equation for the rate of heat flow for conduction of heat through acomposite cylindrical wall. [4+6+6]

2. Kerosene is to be heated from 25 0C to 50 0C in a heat exchanger. Flow rate ofkerosene is 0.38 kg/s. It flows through the tube having inside diameter of 30 mmand outside diameter of 34 mm. Water at 94 0C is used for the heating purpose.Flow rate of water is 0.125 kg/s. Outer surface area of the heat exchanger isinsulated. Properties of kerosene are: Density: 815 kg/m3; Cp water = 4180 J/kg0K; Cp Kerosene = 2090 J/kg 0K; Convective heat transfer coefficient on water side= 1350 w/m2 0K; Convective heat transfer coefficient kerosene side = 730 w/m2

0K. Thermal conductivity = 380 w/m 0K. Calculate the length of heat exchangerin 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(Pr)0.3.Work out the percentage change in the rate of heat flow perdegree temperature difference when the original coolant is replaced by another fluidhaving viscosity equal to two thirds that of the original coolant. Assume that theother 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 tubeside shell side of a fixed head / floating head exchanger.

(b) Explain the factors responsible for deterioration of performance of heat ex-changer with time. [8+8]

7. (a) Define evaporator economy and capacity.

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(b) Explain the agitated film evaporator. [16]

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

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1. (a) Define thermal contact resistance.

(b) The thermal contact conductance at the interface of two 1 cm thick aluminiumplates is measured to be 11000 w/m2 0C. Determine the thickness of the alu-minium plate whose thermal resistance is equal to the thermal resistance ofthe interface between the plates. [16]

2. In a counter current double pipe heat exchanger, 0.4 kg/s of oil (Cp = 2.2 KJ/kg0K) is used to cool from 120 0C using 1 kg/s of water which enters at 25 0C. Theheat exchanger has an overall heat transfer coefficient of 600 w/m2 0K and a heattransfer 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 0C is suspended in a dry air stream at 25 0Cwhich 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 offusion of ice = 334 KJ/kg ; Density of air = 1.248 kg/m3; thermal conductivity ofair = 0.026w/m 0C; Viscosity = 1.69∗10−5 kg/m.s.; Cp = 1.005 KJ/kg 0C; Densityof ice = 920 kg/m3. Use the correlation: Nu = 2+[0.4(Re)1/2 +0.06(Re)2/3](Pr)0.4.

[16]

4. Estimate the heat transfer from a 40 w incandescent bulb at 127 0C to 27 0Cquiescent air. Approximate the bulb as a 50 mm diameter sphere. What percentageof the power is lost by free convection. Nu = 0.6 (Gr Pr)1/4. Density = 994 kg/m3;thermal conductivity = 0.628 W/m 0K; 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 fluxnecessarily higher in the stable film boiling regime than it is in the nucleateboiling regime?

(b) Draw the boiling curve and identify the burn out point on the curve. Explainhow burnout is caused. Why is the burn out point avoided in the design ofboilers. [16]

6. In a shell and tube heat exchanger oil is to be heated from 25 0C to 75 0C bycondensing steam at 110 0C on the shell side. The outside diameter of tube is42mm while the inside diameter is 38 mm. Due to fouling inside diameter of thetube is reduced to 34 mm. Oil velocity through tubes is 0.85 m/s. From the

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previous experiments it is known that the oil side heat transfer coefficient varieswith temperature as follows:

Oil Temperature 0C 25 35 50 60 75Oil side HT coefficient, W/m2 0K 72 78 98 140 255

Specific heat of oil = 1900 J/kg 0C; Density of oil = 900 kg/m3. Resistance to heattransfer 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 surfaceto 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]

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6085303-r05220801processheattransfer

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