Temperature and Heat Loss

1. Temperature and Heat Loss • The following topics are covered in this presentation: • Temperature and temperature scales • Transfer of heat • Thermal conductivity, resistivity and resistance • Total thermal resistance • U-value

2. Temperature and Heat Loss The term temperature is used to express the degree of hotness of a substance. It is measured by using a device called the thermometer. Mercury in glass and the electronic thermometers are widely used. Mercury thermometer Electronic Thermometer

3. Temperature and Heat Loss There are three temperature scales: Celsius (ºC), Fahrenheit (ºF) and Kelvin (K) ºC ºF K Boiling point of water 100 212 373 Freezing point of water 273 0 32 ‘Absolute zero’ -273 0 Only used in USA

4. Temperature and Heat Loss HEAT TRANSFER: Heat transfer from one substance to another or from one point to another in a substance may take place due to conduction, convection and radiation. Heat Conduction: As a material is heated, the atoms start to vibrate with the increased thermal energy. The vibrating atoms make the adjacent atoms to vibrate and the process continues. As a consequence, thermal energy (or heat) travels from one end of the material to the other.

5. Temperature and Heat Loss Convection:All fluids are heated by convection Heat The vessel shown contains water and as it is heated, the particles at the bottom of the vessel become hot and expand. On expansion the particles become lighter and rise. Cold water particles at the top are heavier than hot particles. They descend as the hot water particles rise, and the process continues.

6. Temperature and Heat Loss Radiation:Heat energy travels from one place to another without requiring a medium. Sun Earth Heat energy from the Sun is received by us due to radiation as there is no medium (air) beyond earth’s atmosphere.

7. Temperature and Heat Loss Thermal conductivity: Thermal conductivity or λ-value (also known as k-value) of a material is a measure of the rate at which heat is conducted through it under specified conditions. • Flow of heat through a material is directly proportional to: • surface area (A) • temperature difference between the opposite faces (θ2 – θ1) • time for which heat flows (t) • Flow of heat is indirectly proportional to the thickness of material (d) θ2 θ1 Heat Heat in, Q out A d Combining the above factors, the amount of heat flowing through a material, Q, is proportional to: (see next slide)

8. Temperature and Heat Loss Q   = where λ (or k) is the thermal conductivity of the material (continued)

9. Temperature and Heat Loss • To work out the unit of thermal conductivity, the symbols in the equation are replaced with appropriate units: • Quantity of heat, Q: Joules (J) • Thickness of material, d: m • Time during which heat flows, t: seconds (s) • Surface area of the material, A: m2 • Temperature difference, θ2 – θ1 = ºC or K

10. Temperature and Heat Loss Thermal conductivity (λ) is the rate at which heat is conductedthrough a material of unit measurements, to maintain the opposite faces of the materials at a temperature difference of 1 ºC. Thermal resistivity (r) is the reciprocal of thermal conductivity. Thermal resistance (R) takes into account the thickness of the material. where d is the thickness of the material in metres

11. Temperature and Heat Loss The Total Thermal Resistance of a material is a combination of material resistance and surface resistance Material resistance: All materials offer some resistance to the transmission of heat. Material resistance = d/λUnit: m2K/W Surface resistance: The surfaces of all building materials have irregularities which trap air. Stationary layers of air are formed which resist the transmission of heat. Building material Outside surface (Rso) Inside surface (Rsi)

12. Temperature and Heat Loss Airspace resistance (Rairspace): If there is a cavity in a component, the airspace resistance needs to be considered as well, for example in a double glazed window, cavity wall etc.The airspace resistance depends on the width of the cavity. Glass Airspace Double-glazed window

13. Temperature and Heat Loss The total thermal resistance of a component is given by: Rtotal= Rsi + Rmaterials + Rairspace + Rso Where Rtotal is the total thermal resistance Rsi is the thermal resistance of the inside surface Rmaterials is the thermal resistance of the materials Rairspace is the thermal resistance of the cavity Rso is the thermal resistance of the outside surface

14. Temperature and Heat Loss Thermal transmittance or the U-value indicates the amount of heat energy that will flow per second through one square metre of a building element when the temperature difference between the inside and outside surfaces is one Kelvin (or 1 ºC). The U-value of an element/component can be determined from the following relationship: Unit: W/m2K