Lesson 7 - Radiation and Combined SAQ
This lesson allows you to test yourself on the material covered in the lesson on Principals of Heat Transfer – Radiation and Combined, and to determine the level to which you have met the learning outcomes.
Self-Assessment Questions
1. ** Calculate the heat loss due to radiation from the surface of a 100 hL copper kettle at 100°C to a room at 20°C. The surface area of the kettle is 27.5 m2 and the emissivity of copper is 0.03.
2. ** Calculate the heat loss from a pipe with hot wort flowing through it. The wort is at 45°C and the air around the pipe is 20°C. The pipe is 12 m long and made of stainless steel with OD of 25 mm and ID of 21 mm and a thermal conductivity of 20 Wm−1·K−1. The film heat transfer coefficients inside and outside the pipe are, respectively, 350 and 7 Wm-2 K-1.
3. ** Calculate the overall heat transfer coefficient through a multi-layer wall. From the outside moving in, the wall contains a 100 mm brick section, a 50 mm insulation section, a 15 mm cement section, and a 5 mm plaster finish section. The film heat transfer coefficients for the inside and outside walls are, respectively, 8 and 34 Wm-2 K-1. The thermal conductivities of the materials are listed in the table below.
|
Material |
Thermal Conductivity Wm-1 K-1 |
|
Brick |
0.84 |
|
Insulation |
0.035 |
|
Cement |
0.5 |
|
Plaster |
0.5 |
4. * The overall heat transfer coefficient for a new piping process is 400 Wm-2 K-1. Due to fouling, over time the effective heat transfer coefficient becomes 350 Wm-1 K-1. Calculate the magnitude of the fouling factor that has developed over a period of time.