Heat in Building PDF

1. HEAT CONDUCTION CONDUCTION is the process by which heat is transferred from the hotter end to the colder end of an object. Heat conduction in solid or liquid works by transferring the energy through the structure of atoms or molecules. Example: Metal and aluminum are very good thermal...

1. HEAT CONDUCTION CONDUCTION is the process by which heat is transferred from the hotter end to the colder end of an object. Heat conduction in solid or liquid works by transferring the energy through the structure of atoms or molecules. Example: Metal and aluminum are very good thermal conductors. Course: Heat in Building. Prepared by lecturer: 1 Leanghong LEU  THERMAL/HEAT INSULATION Thermal insulation reduces heat transfer between solid objects, fluids, or gases by introducing a barrier between them. Substances that do not conduct heat well are called INSULATOR. Example: Plastic, Fabric, or wood are very low and slow thermal conduction material. Course: Heat in Building. Prepared by lecturer: 2 Leanghong LEU  THERMAL CONDUCTIVITY (K) The ability of the object to conduct heat is known as its THERMAL CONDUCTIVITY and is denoted k. THERMAL CONDUCTIVITY OF MATERIAL describes how well the material conducts the heat. Heat conduction in solid or liquid works by transferring the energy through the structure of atoms or molecules. Course: Heat in Building. Prepared by lecturer: 3 Leanghong LEU  CONDUCTIVE HEAT EQUATION Conductive heat transfer can be expressed with q = (k / s) A dT = U A dT Course: Heat in Building. Prepared by lecturer: 4 Leanghong LEU  EXAMPLE OF CONDUCTIVE HEAT EQUATION EXAMPLE: Conductive Heat Transfer A plane wall is constructed of solid iron with thermal conductivity 70 W/moC. Thickness of the wall is 50 mm and surface length and width is 1 m by 1 m. The temperature is 90 oC on one side of the surface and 20 oC on the other. Course: Heat in Building. Prepared by lecturer: 5 Leanghong LEU  2. CONVECTION KEY QUESTION : Can the air flow carry the thermal energy/ Heat ??? Course: Heat in Building. Prepared by lecturer: 6 Leanghong LEU  CONVECTION CONVECTION is the transfer of heat by the motion of fluid (liquid, air, gas). Course: Heat in Building. Prepared by lecturer: 7 Leanghong LEU  CONVECTION ❖ Convective heat transfer can be Natural or free convection or Forced convection: Natural or free convection: Natural convection is caused by natural forces due to density differences caused by temperature variations in the fluid. At heating the density change in the boundary layer will cause the fluid to rise and be replaced by cooler fluid that also will heat and rise. This continues phenomena is called free or natural convection. Course: Heat in Building. Prepared by lecturer: 8 Leanghong LEU  FORCED CONVECTION Forced or assisted convection: Forced convection occurs when a fluid flow is induced by an external force, such as a pump, fan or a mixer. Both free and forced convection help to heat/cold the house and cool the engines machine. Course: Heat in Building. Prepared by lecturer: 9 Leanghong LEU CONVECTIVE HEAT TRANSFER EQUATION The heat transfer per unit surface through convection was first described by Newton and the relation is known as the Newton's Law of Cooling. The equation for convection can be expressed as: q = h A dT Course: Heat in Building. Prepared by lecturer: 10 Leanghong LEU CONVECTIVE HEAT TRANSFER EQUATION q Course: Heat in Building. Prepared by lecturer: 11 Leanghong LEU CONVECTIVE HEAT TRANSFER EQUATION ❖ Convective Heat Transfer Coefficients Convective heat transfer coefficients - hc - depend on the type of gas or liquid, Geometry, Flow. ❖ Typical convective heat transfer coefficients for some common fluid flow applications: Free Convection - air, gases and dry vapors : 0.5 - 1000 (W/(m2K) Free Convection - water and liquids: 50 - 3000 (W/(m2K) Forced Convection - air, gases and dry vapors: 10 - 1000 (W/(m2K) Forced Convection - water and liquids: 50 - 10000 (W/(m2K) Forced Convection - liquid metals: 5000 - 40000 (W/(m2K) Boiling Water : 3.000 - 100.000 (W/(m2K) Condensing Water Vapor: 5.000 - 100.000 (W/m2) Course: Heat in Building. Prepared by lecturer: 12 Leanghong LEU  EXAMPLE OF CONVECTIVE HEAT TRANSFER EQUATION ❖ Example : fluid flows over a plane surface 1 m by 1 m. The surface temperature is 50oC, the fluid temperature is 20oC and the convective heat transfer coefficient is 2000 W/m2oC. What is the convective heat transfer between the hotter surface and the colder? Course: Heat in Building. Prepared by lecturer: 13 Leanghong LEU EXAMPLE OF CONVECTIVE HEAT TRANSFER EQUATION Course: Heat in Building. Prepared by lecturer: 14 Leanghong LEU  EXAMPLE OF CONVECTIVE HEAT TRANSFER EQUATION Hot air at 80°oC is blown over a 2m by 4m flat surface at 30°oC. If the average convection heat transfer coefficient is 55 W/m2oC, determine the rate of heat transfer from the air to the plate, in kW. Course: Heat in Building. Prepared by lecturer: 15 Leanghong LEU  EXAMPLE and PROBLEM k k s s Course: Heat in Building. Prepared by lecturer: 16 Leanghong LEU EXAMPLE and PROBLEM k s Course: Heat in Building. Prepared by lecturer: 17 Leanghong LEU EXAMPLE OF CONVECTIVE HEAT TRANSFER EQUATION Course: Heat in Building. Prepared by lecturer: 18 Leanghong LEU EXAMPLE and PROBLEM Course: Heat in Building. Prepared by lecturer: 19 Leanghong LEU  3. RADIATION KEY QUESTION: How does the Sun’s heat reach the Earth ?? Course: Heat in Building. Prepared by lecturer: 20 Leanghong LEU  3. RADIATION The third form of heat is radiant heat. All parts of the electromagnetic spectrum transfer radiant energy Radiation is heat transfer by electromagnetic waves. The higher the temperature of an object, the more thermal radiation it gives off. Course: Heat in Building. Prepared by lecturer: 21 Leanghong LEU  3. RADIATION the fundamentals of heat transfer and solar radiation helps in understanding the underlying processes that take place in a building and its interaction with the external environment. To understand the process of heat conduction, convection and radiation occurring in a building, consider a wall having one surface exposed to solar radiation and the other surface facing a building Course: Heat in Building. Prepared by lecturer: 22 Leanghong LEU  INTERACTION OF RADIATION TO MATERIALS 1. Transmittance—the situation in which radiation passes through the material. 2. Absorptance—the situation in which radiation is converted into sensible heat within the material. 3. Reflectance—the situation in which radiation is reflected off the surface. 4. Emittance—the situation in which radiation is given off by the surface, thereby reducing the sensible heat content of the object. Polished metal surfaces have low emittance, while most other materials have high emittance. Course: Heat in Building. Prepared by lecturer: 23 Leanghong LEU INTERATION OF RADIATION TO MATERIALS Course: Heat in Building. Prepared by lecturer: 24 Leanghong LEU INTERATION OF RADIATION TO MATERIALS Course: Heat in Building. Prepared by lecturer: 25 Leanghong LEU  GREENHOUSE EFFECT BY RADIATION ❖ The greenhouse effect is due to the fact that the type of interaction that occurs between a material and radiant energy depends on the wavelength of that radiation Course: Heat in Building. Prepared by lecturer: 26 Leanghong LEU GREENHOUSE EFFECT BY RADIATION Course: Heat in Building. Prepared by lecturer: 27 Leanghong LEU  THERMAL MASS Thermal mass is the ability of a material to absorb, store and release heat. Thermal lag is the rate at which a material releases stored heat. For most common building materials, the higher the thermal mass, the longer the thermal lag. Materials with long thermal lag times (for example, brick and concrete) will absorb and release heat slowly Materials with short thermal lag times (for example, steel) will absorb and release heat quickly. Course: Heat in Building. Prepared by lecturer: 28 Leanghong LEU  Why is thermal mass important? ❖ When used correctly, materials with high thermal mass can significantly increase comfort and reduce energy use in your home. Thermal mass acts as a thermal battery to moderate internal temperatures by averaging out day−night (diurnal) extremes. Course: Heat in Building. Prepared by lecturer: 29 Leanghong LEU Why is thermal mass important? Course: Heat in Building. Prepared by lecturer: 30 Leanghong LEU Why is thermal mass important? Course: Heat in Building. Prepared by lecturer: 31 Leanghong LEU  KEY WORDS & KEY QUESTIONS 1. CONDUCTOR 2. INSULATOR & 3 Examples & 3 Examples 5. CONVECTION 3. THERMAL 4. HEAT CONDUCTION CONDUCTIVITY EQUATION 7. FORCED 8. HEAT CONVECTION CONVECTION EQUATION 9. RADIATION 10. INTERACTION OF RADIATION TO MATERIALS 11. GREENHOUSE EFFECT BY RADIATION 12. THERMAL MASS Course: Heat in Building. Prepared by lecturer: 32 Leanghong LEU