TAMIL NADU AGRICULTURAL UNIVERSITYDEPARTMENT OF AGRICULTURAL PROCESSING

MID-SEMESTER EXAMINATIONAPE 142 HEAT AND MASS TRANSFER (2+1)

Date: 18.7.2001 Total marks : 20Time : 1 hr

PART A

* Answer any EIGHT questions only 8 x 0.5 = 4

Define the following (1to 6):

1. Fourier law of conduction2. Newton Rikhman’s law 3. Thermal conductivity4. Film coefficient of heat transfer5. Heat exchanger6. Explain shape factor

Give the SI unit for the following ( 7 to 10)

7. Thermal conductivity8. Heat transfer rate9. Thermal resistance10. Overall heat transfer coefficient

PART B

* Answer any SIX Questions only 6 x 1 = 6 1. What is Logarithmic Mean Temperature Difference2. What is good conductor. Give two examples 3. What is an insulator. Give two examples. 4. Explain the types of heat transfer5. What are the modes of heat transfer. Explain.6. Mention the applications of heat exchangers7. Explain the working principles of a heat exchanger. 8. Differentiate between the Film coefficient of heat transfer and overall heat transfer coefficient.

PART C

* Answer any FIVE of the following 5 x 2 = 10

1. Derive an expression for heat transfer rate through a composite cylinder with 2 layers. Use usual notation.2. Derive an expression for the Logarithmic Mean Temperature Difference for the flow in a counter flow heat exchanger.

3. Derive an expression for the heat transfer rate through a hollow concentric sphere using usual notations.4. How much water can be heated per hour in a double tube heat exchanger with counter flow if super heated steam enters the inner tube at 250C and leaves at 190 C with water entering the outer tube at 25 C and leaving at 140 C. The heat exchanger area is 0.2 sq.m. and the overall heat transfer coefficient from steam to water is 730 W/ m2K.

5. A furnace wall is made up of two layers of fire brick and red brick. The inner and outer surfaces are 840 C and 35 C respectively. The thermal conductivities of these layers are 1.16 and 0.87 W/m K, and the thickness are 22 and 11 cm. Find the interface temperature.

6. In determining the thermal conductivity of an insulating material, the temperatures were measured on both sides of a flat slab of 2.5 cm of the material and were 318 and 305K. The heat flux was measured as 25 W/m2 K. Calculate the thermal conductivity.