Explain the Processes of Conduction, Convection and Radiation and Their Industrial Applications
Learning Outcomes
Explain the processes of conduction, convection and radiation and their industrial applications
Using industrial examples, calculate energy flow for given thermal conductivities and temperature gradients, and emissivities
Explain the differences between heat-transfer mechanisms in solids, liquids, gases and combinations of substances, in terms of molecular motion, bulk motion and surface properties in industrial processes
Thermal Energy Transfer
Scenario
You are communications officer for the British & Overseas Leading Laboratory of Cooling Kinetics Studies, an organisation which offers advice to businesses on matters relating to thermal energy transfers. The following are a series of enquiries which have been submitted to your office, and you must respond to them in an appropriate manner.
Task 1
"I have recently taken over a business based in a building which is painted black on the outside, and has single-glazed windows. The building is rather draughty and I have noticed that our heating bills are rather high, can you suggest ways in which we could become more economical?" I.M. Freezin, Warks.
Design a series of experiments, using conventional laboratory apparatus, to demonstrate to Mr Freezin the various ways in which he could reduce the loss of heat from his building. You should address the problems of heat loss by conduction, convection and radiation, and show how each process contributes to heat loss.
For each experiment, include a labelled diagram and your results presented in the most appropriate way.
Report on your findings, explaining carefully how the apparatus you have used represents the conditions in his business premises.
Task 2
"I run a poultry marketing business in Norfolk; members of our staff have been complaining for some time that the conditions in our refrigeration units are making them unwell. Could you please advise us of the best way to keep our workers warm?" Irma Gobbler, North Walsham.
Use the following data to calculate the rate of heat loss from the human body for the employees listed, and so make recommendations to
Ms Gobbler about the most sensible clothing for her employees whilst they are working in the freezers.
Human body skin temperature: 30 ºC Air temperature in freezer: -10ºC Area of skin: 0.8 m2
Thermal conductivity of cotton fabric: 0.03 Wm-1K-1
Thermal conductivity of woollen material: 0.07 Wm-1K-1
Thermal conductivity of kapok: 0.034 Wm-1K-1
Employee 1 wears a cotton shirt 0.5 mm thick
Employee 2 wears a woollen jumper 3 mm thick
Employee 3 wears a kapok padded jacket 6mm thick
Task 3
"Further to my previous query, it has also come to my attention that the temperature in our refrigeration unit rises quite considerably in the summer. I have received a quotation for a new cooling system which is considerably higher than I am willing to pay; can you suggest how I could increase the amount of insulation in the unit to keep the temperature down?" I. Gobbler, North Walsham.
The diagram below is a floor plan, of the location and construction of the refrigeration unit in the factory.
Using data from the diagram above, calculate:
1. The rate of heat transfer into the unit through the walls from the factory
2. The rate of heat transfer into the unit through the walls from outside
a. On a winter's day when the temperature is -5 ºC
b. On a summer's day when the temperature is 25 ºC
3. The thickness of insulation required to reduce the rate of heat transfer from outside on a summer's day to the value you calculated for a winter's day. Comment on your answer.
Task 4
"I manufacture X-ray tubes for local hospitals and have recently have received complaints that some of our tubes have been overheating, which is odd since they all operate at similar powers. Can you explain to me why this should be?" Xavier Raymond Snapper, Coventry.
Use the following data to explain to Mr Snapper which of his X-ray tubes is overheating and why.
You should support your answers with calculations which fill the gaps in the table below.
Stefan-Boltzmann constant = 5.7×10-8 Wm-2K-4
|
Anode Material/Surface
|
Operating Power /W
|
Surface Emissivity
|
Anode area/m2
|
Operating Temperature/K
|
|
Dull copper
|
|
0.78
|
1×10-3
|
1000
|
|
Shiny copper
|
800
|
0.03
|
1.2×10-3
|
|
|
Polished tungsten
|
|
0.04
|
5×10-4
|
4000
|
|
Aged tungsten
|
1000
|
0.35
|
6×10-4
|
|
|
Polished Molybdenum
|
650
|
0.15
|
1.4×10-3
|
|
Task 5
"We are about to take on a trainee in the testing laboratory, and need her to understand the science behind our work here. Could you prepare a briefing document for her?" U.R. Manager.
Refer back to the processes described in tasks 1-4 and explain each in detail, using ideas about molecular motion, bulk motion and surface properties, highlighting the differences of each in solids, liquids and gases.