Heat Transfer Jan2003 Or 220852

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Code No:220852 II.B.Tech II-Semester. Supplementary Examinations. January,2003 HEAT TRANSFER (Chemical Engineering) Time: 3 hours. Max.Marks:70 Answer any FIVE questions All questions carry equal marks. --1. Derive the equation for steady-state heat transfer through a spherical shell of inner radius r1 and outer radius r2 and compare the result with the solution obtained for a thick walled cylinder. 2. Oil is flowing through a 75mm ID iron pipe at 1m/sec. It is being heated by steam outside the pipe and the steam film co-efficient may be taken as 11 kW/m2 0C. At a particular point along the pipe, the oil is at 500C. Its density is 880 kg/m3, viscosity is 2.1 cp, thermal conductivity is 0.135 W/m0C and specific heat is 2.17 J/g0C. What is the overall heat transfer co-efficient at this point based on the inside area of the pipe. If the steam temperature is 120 0C, what is the heat flux at this point based on the outside area of the pipe. 3. A vertical tubular condenser is to be used to condense 2,100 kg/h of ethyl alcohol, which enters at atmospheric pressure. Cooling water is to flow through the tubes at an average temperature of 300C. The tubes are 31mm OD and 27mm ID. The water-side co-efficient is 2800 W/m2 0C. Fouling factors and the resistance of the tube wall may be neglected. If the available tubes are 3m long, how many tubes will be needed? Data: Boiling point of alcohol =78.40C Heat of vaporization = 856 J/g Density of liquid = 769 kg/ms 4. A radiant heating system is installed in the plaster ceiling of a room 18ft long by 18ft wide by 8ft high. The temperature of the concrete floor is maintained at 650F. Assume that no heat flows through the walls, which are coated with a reradiating material. The temperature of the air passing through the room is held at 650F. If the required heat supply to the floor is 3500 Btu/h. Calculate the necessary temperature at the ceiling surface. How much heat is transferred to the air in Btu/hr. Emissary of plaster is 0.93, absorptivity of concrete is 0.63. The convective heat transfer co-efficient between the ceiling and the air is given by hc=0.20(DT)1/4 Btu/hr ft2 oF. Contd….2

Code No:220852

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5. A tubular exchanger 889 ID contain 828,19mm OD tubes, 3.66m long on a 25mm square pitch. Standard 25% baffles are spaced 305mm apart. liquid benzene at a bulk temperature 600F is being heated at the rate of 1,00,000 lb/hr (45,360 kg/hr). If the outside surfaces of the tube are at 140oF, estimate the individual heat transfer coefficient of benzene. 6. A forced circulation evaporator is to concentrate 60,000 kg/hr of 44% NaOH to 65% using steam at 3 atm pressure. The feed temperature and the condensing temperature are both 400C. The density of the feed solution is 1,450 kg/m3. If the overall heat transfer co-efficient is 2000 W/m2 0C, calculate (a) the steam requirement in kg/hr (b) The heat transfer area required. 7.a) b)

At a low overall DT the boiling heat flux for a tube bundle is about 4 times that for a single tube, but at a high DT the flux is much lower than that for a single tube. Give a logical explaination of the difference. Explain the Logarithmic Mean Temperature .

8. Write short notes on any THREE: a) Pool boiling of saturated liquids b) Fourier’s law c) Types of vapor compression d) Performance of tubular equipments e) Graetz and Peclet numbers ************