Thermal studies U Vlaues

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Thermal studies U Vlaues

  1. 1. Tools: For now you will need…. Pen Paper Calculator Put bags under tables Context…
  2. 2. Salford City College Construction & Engineering Nationals In Construction
  3. 3. Construction science and materials After this next session is over you will be able to :• Calculate the "U" value of a concrete roof slab 150 mm (0.15m) thick, asphalted 30 mm (0.03m) for rainwater protection and the soffit faced with plasterboard 12 mm (0.012)thick on 50 mm softwood battens, the void filled with 50mm (0.05) Glass fibre quilting . • K Concrete = 1.0 W/m² °C K Asphalt = 0.25 W/m² °C K Plasterboard = 0.35 W/m² °C K Glass fibre Quilt = 0.035W/m² °C
  4. 4. You will also…..• Know about heat gains and losses in buildings Where is heat lost in a dwelling?...
  5. 5. Next – Heat Losses in Homes
  6. 6. Heat Losses In Homes Give an example of steps you can take to save energy …Coming Next – Construction Techniques
  7. 7. Construction Techniques DG
  8. 8. Double galzing… Next:Note taking… Next note taking …Factors affecting energy use
  9. 9. Name things which can increase or decrease energy usage …• climate: • Usage:• • the way that the property is Location used including space and• solar radiation heating requirements• Wind• degree of exposure • internal temp causes : • domestic appliances • desired internal temp.• Building • heating system and its Characteristics efficiency• shape and size The Total Heating Load… Coming Next – the Total Heating Load
  10. 10. The Total Heating Load RoofSolarGains Cooking Windows FuelSupply Electrical appliances Walls Ventilation Flue losses Boiler Hot water Floor Next slide…FHL + VHL = SHL
  11. 11. • Gains Total Heat Load • Losses • How is heat lost from a• What types of things heat a house up? building?•• Solar effect • Fabric Heat Loss• Cooking • Ventilation Heat Loss• Hot water • Specific Heat Loss• Electrical appliances • In what ways does heat flow? ...
  12. 12. THERMAL STUDIES• HEAT FLOW  Radiated heat flow through air such • The current Building Regulations carry a as the rays from the sun section relating to the  Convection as fluid, gas or liquid. It conservation of expands on heating becoming lighter energy and is forced upwards by cooler fluid taking its place and therefore creates • Someone has to know circulation how to add up the amount of heat loss  conduction by passing heat from one molecule to another, this is the source • So who does it and of most heat loss in buildings. how is it done?.. K…
  13. 13. What is the standard unit of THERMAL CONDUCTANCE? ("K" value)• In physics, thermal conductivity, k, is the property of a material that..• …indicates its ability to conduct heat. It appears primarily in Fouriers Law for heat conduction. Coming Next – values for common materials
  14. 14. • K values vary with good insulators having a value of around 0.003 W/m² °C and up to 400 W/m² °C for good conductors.• (Don’t try to remember these numbers they are set out in industry tables)….
  15. 15. Material K value (W/m2 0C ) K values• Aluminium alloy 160•• Asbestos cement sheet Asphalt roofing 0.4 0.8 for••• Brickwork Concrete (regular) Concrete (lightweight) 1.83 0.84 0.19 common•• Copper Corkboard 160 0.042 building materials:• Fibre insulating board 0.05• Glass 1.0• Glass wool (mat or fibre) 0.04•• Hardboard Mineral wool 0.13 0.039 adapted• Gypsum plaster 0.46•• Plasterboard Polystyrene (expanded) 0.16 0.033 from the••• Polystyrene (solid) Polyurethane foam 0.17 0.026 CIBSE PVC flooring 0.04 chartered institute of building service engineers• Carbon steel 150•• Sandstone Softwood timber 1.3 0.13 guide• Hardwood timber 0.15• Woodwool slab 0.085 Reciprocal of K is R…
  16. 16. Resistivity & Conductivity• K values are good but • How do you find so is the opposite of resistance? conductance,which is… • Resistivity =1/K value• Resistance • materials are not always• to show how a material supplied in one-metre resists heat a blocks, using the Resistivity (r) value can resistivity you can look be found. This is the at the thickness of the reciprocal (opposite) material and calculate of a K value the "resistance" (R) What if you have lots of Rs …
  17. 17. HEAT LOSS FROM BUILDINGS• Finally to find how much heat energy is being conducted we use a thing called the unit of "Thermal Conductance" used …• U = 1/total resistance expressed in Watts per metre² What does the U-value represent?…..
  18. 18. • The term ‘U’ represents overall thermal conductance from the outside to inside covering all modes of heat transfer.• What use is this in relation to the whole?• An average U-value can be calculated
  19. 19. • The U-Value is an important concept in building design.• It represents the air-to-air transmittance of an element (part or fabric). This refers to how well an element conducts heat from one side to the other, which makes it the reciprocal of its thermal resistance.• So if you calculate the thermal resistance of an element, we can simply invert it to obtain the U-Value..• U=? •U = 1/total resistance expressed in Watts per metre² Lets work some out…
  20. 20. Example • Calculate the "U" value of a brick wall, plastered one side with 20 mm background and 3 mm of hardwall gypsum finish, where "K" values for Brickwork = 0.55 W/m² °C, background = 0.6 W/m² °C, hardwall plaster = 0.9 W/m² °C. Material L (Thickness) K Value R=(1/k) x LBrickwork 0.103 0.55 0.187Background 0.020 0.6 0.033 Gypsum 0.003 0.9 0.003 Total Resistance 0.223 U value = 1/Total Resistance = 1/0.233 = 4.484 W/m² °C Coming Next – further examples…
  21. 21. • Calculate the "U" value of a concrete roof slab 150 mm (0.15m) thick, asphalted 30 mm (0.03) for rainwater protection and the soffit faced with plasterboard 12 mm (0.012)thick on 50 mm softwood battens, the void filled with 50mm (0.05) Glass fibre quilting . • K Concrete = 1.0 W/m² °C K Asphalt = 0.25 W/m² °C K Plasterboard = 0.35 W/m² °CExample K Glass fibre Quilt = 0.035W/m² °C • Work to 3 decimal places. Material L K R=(1/k) x L Concrete 0.15 1.0 0.15 Asphalt 0.03 0.25 0.12 Glass Fibre 0.05 0.035 1.426 Plasterboard 0.012 0.35 0.034 U value = 1/Total Total R Resistance = 0.58 W/m² °C 1.73 Coming Next – air layers & cavities
  22. 22. BOUNDARY STILL AIR LAYER & CAVITIES• In any construction • This boundary still air layer between the fabric and has an insulating quality the internal/external that can be included in the environment there will heat loss calculation. be a layer of air which• The exterior surfaces of is "trapped" by the buildings are usually surface roughness of rougher than the internal the materials used faces Coming Next – thermal properties of cavities
  23. 23. CAVITIES• cavities are used to keep heat in• To increase the width of a cavity will not necessarily produce any significant improvement in heat retention-meaning…• You only need a small gap Coming Next – standard values…
  24. 24. BOUNDARY STILL AIR LAYER & CAVITIES• Standard Internal Resistance value 0.123 W/m² °C• Standard External Resistance value 0.055 W/m² °C• Standard Cavity Resistance value 0.18 W/m² °C Coming Next – further standard values
  25. 25. HEAT AS A FORM OF ENERGY• Total heat loss per second = Area x "U" x Temperature difference.• Calculations to determine the total heat loss for a particular building for a particular period for a particular temperature difference can now be made This session completed…
  26. 26. • Average U value = total exposed surface area / total exposed surface area
  27. 27. Heat load calculator• http://www.shophmac.com/info-center/hvac-ca• STOP
  28. 28. Material L (Thickness) K Value R=(1/k) x LBrickwork 0.103 0.55 0.187Background 0.020 0.6 0.033 Gypsum 0.003 0.9 0.003 Total Resistance 0.223 U value = 1/Total Resistance = U value = 1/Total Resistance = 1/0.233 = 4.484 W/m² °C Coming Next – further examples…

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