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Heat Transfer By Design Lesson 4

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Heat Transfer By Design Lesson 4

  1. 2. Learning objectives <ul><li>What factors affect heat transfer from hot materials. </li></ul><ul><li>How heat losses can be reduced. </li></ul>
  2. 4. “ Energy can neither be created or destroyed, just changed in form.”
  3. 5. You need to remember that whenever energy is changed from one form to another, some energy is always ______. In what two forms is energy usually wasted? 1._________ 2._________ Sound Heat wasted If you can hear a device that is not designed to make noise then energy is wasted as sound. If a device gets warm and it is not designed to get warm then energy is wasted as heat.
  4. 6. Heat Transfer <ul><li>Heat always moves from a warmer place to a cooler place. </li></ul><ul><li>Hot objects in a cooler room will cool to room temperature. </li></ul><ul><li>Cold objects in a warmer room will heat up to room temperature. </li></ul>
  5. 7. Can you name some devices that waste energy as sound? _____________ _____________ _____________ _____________ _____________ _____________ _____________ _____________ Hairdryer Washing machine Car engine Computer Microwave Bunsen burner Wind turbine Dish washer
  6. 8. Can you name some devices that waste energy as heat? _____________ _____________ _____________ _____________ _____________ _____________ _____________ _____________ Computer Turbine Motor Transformer Any electrical device Television Radio Car engine
  7. 9. Wasted e_____ spreads out (d_______) into the s_________. This makes the energy harder to r____. Remember energy can not be c______ or d_______. What happens to the wasted energy you get whenever energy is changed from one form to another? This is why it is important to r_____ the amount of wasted energy there is. reated estroyed nergy issipated urroundings e-use educe
  8. 10. Heat Loss from Homes
  9. 11. Reducing Heat Loss From The Home <ul><li>How can Heat be lost at our homes? </li></ul><ul><li>These include: </li></ul><ul><ul><li>Roof </li></ul></ul><ul><ul><li>Walls </li></ul></ul><ul><ul><li>Gaps around doors and windows </li></ul></ul><ul><ul><li>Floors </li></ul></ul><ul><ul><li>Windows </li></ul></ul>
  10. 12. Reducing Heat Loss From The Home <ul><li>We can reduce heat loss from our homes by using different methods of insulation. </li></ul><ul><li>These include: </li></ul><ul><ul><li>Double glazing </li></ul></ul><ul><ul><li>Loft Insulation </li></ul></ul><ul><ul><li>Cavity Wall Insulation </li></ul></ul><ul><ul><li>Carpets, Curtains, Draught Excluders </li></ul></ul><ul><li>(You will need to know more detail about how these methods reduce heat loss) </li></ul>
  11. 14. The vacuum flask <ul><li>The vacuum prevents conduction and convection </li></ul><ul><li>The silvered surfaces reduces radiation </li></ul><ul><li>Cap and base are made of good insulators to reduce conduction </li></ul>
  12. 15. Double Glazed
  13. 16. Double Glazer <ul><li>Improved thermal insulation - reduces heat loss more effectively. </li></ul><ul><li>Promotes passive solar heat gain - free heat from the sun passes easily through the glass helping to warm your home. </li></ul><ul><li>Improved energy efficiency - reduced need for heating helps to save energy and protect the environment whilst saving you money on heating bills. </li></ul><ul><li>More comfortable rooms - improved thermal insulation means fewer cold spots and drafts near windows and doors. </li></ul><ul><li>Less condensation - the inner surface of the glass stays warmer reducing internal condensation and prolonging the life of your window. </li></ul>
  14. 17. Heat Transfer: Questions An engineer wishes to pump hot fluid from one place to another with as little loss of heat as is possible. a) What physical characteristics should the pipe have? (2) Narrow bore, shiny surface, low thermal conductivity, low heat capacity........
  15. 18. b) Should it necessarily be wide? c) Is the rate of flow of fluid important? No, it might increase the SA so much that more heat is lost - but that is unlikely in most cases . Yes, the less time it is in the pipe, the less energy it is likely to lose.
  16. 19. Heat Transfer: Radiation Reduce surface area and used highly polished, shiny surfaces. Evaporation Reduce exposed surface area - use a lid - float polystyrene balls on surface. Methods of decreasing heat transfer - insulation (continued) Note that generally the rate of cooling increases with increased excess temperature .
  17. 20. Heat Transfer: Conduction Leave a fluid (air gap) between surfaces - if the gap is “large” you might increase convection - introduce a vacuum jacket. Convection Reduce surface area and restrict the flow of the fluid - introduce a vacuum jacket.