Building Science Lecture 2&3 Heat Transfer PDF

Building Science Basic Thermal process Woodward Avenue Environmental Laboratory and Administration Building, Hamilton ON (LEED Silver) McCallum Sather Architects Architectural Technology Humber College  “A typical...

Building Science Basic Thermal process Woodward Avenue Environmental Laboratory and Administration Building, Hamilton ON (LEED Silver) McCallum Sather Architects Architectural Technology Humber College  “A typical Canadian house consumes 50 to 70 % of its total Energy during the fall, winter & spring heating seasons. Retaining heat is the primary function of the building envelope. Heat flow through wall assembly is the basic principle of physics” Best practice guide CMHC & SCHL Architectural Technology Humber College  Acknowledgements Much of the material have been sourced from presentations by: - Professor John Straub, University of Waterloo - Professor Ted Kesik, University of Toronto - Professor John Timusk, University of Toronto - Professor Kim Pressnail, University of Toronto - Professor Terri Boake, University of Waterloo - Instructor Bruce Taylor, Humber College - Instructor Oruba Alwan, Humber College Building Science for Building Enclosures, John Straub and Eric Burnett Architectural Detailing, Edward Allen, Patrick Rand Building Science for a Cold Climate, Neil Hutcheon and Gus Hildegard Heating, Cooling, Lighting, Sustainable Design Methods for Architects, Norbert lechner. Best Practice Guide, Building technology CMHC. Canada PCI Architectural Precast Concrete, Bruce Taylor Contributing Editor Architectural Technology Humber College  Heat Transfer Architectural Technology Humber College  Thermal Comfort Historically, the main focus of shelter was on Thermal Comfort. Architectural Technology Humber College  Heat & Temperature There is a difference between Heat & Temperature They are related BUT are NOT the same. Heat Heat is energy Produced or transferred from one body, region, set of components, system to another Architectural Technology Humber College  Heat & Temperature Temperature Temperature is a measure of stored heat energy. Temperature is never transferred ONLY heat energy is transferred! Architectural Technology Humber College  Basic Physics of Heat Energy comes in different forms We are concerned with three energy forms: 1. Sensible heat 2. Latent heat 3. Radiant heat Architectural Technology Humber College  Form of Heat HEAT exists in three different forms: 1. Sensible Heat is the amount of energy released or absorbed by a chemical substance during a change of temperature The amount of sensible heat 100° 100° depends on: - the mass - temperature 50° 100° Can be measured by thermometer. Can be felt by skin In each case, the blocks on the right have twice sensible heat content as the blocks on the left. Architectural Technology Humber College  Form of Heat 2. Latent Heat Occurs during a change of state ( ice to water or water to vapor) Temperature does not change! It cannot be measured with a thermometer Architectural Technology Humber College  Form of Heat 3. Radiant Heat Is a form of electromagnetic radiation Transferred in the form of waves. Architectural Technology Humber College  Units of Heat and Temperature Units of heat & temperature Imperial System SI System Heat British thermal units BTU Joule (J) or calorie (Cal) Heat flow BTU/Hour (BTU/H) Watt (W) or Joule/second (J/s) Temperature Fahrenheit (°F) Celsius (C°) System of Units in Canada Architectural Technology Humber College  Heat Transfer Heat is transferred through the building envelope in three main ways: 1. Conduction 2. Convection 3. Radiation https://www.drenergysaver.com/insulation/how-insulation-works.html Architectural Technology Humber College  Heat Transfer Heat Forms Mechanisms 1. Conduction 1. Sensible heat 2. Convection 2. Latent heat 3. Radiation 3. Radiant heat Do not get confused! Architectural Technology Humber College  Conduction What are the scientific explanations of heat transfer? Conductive Heat Transfer Is the process that occurs when objects are in direct contact Only from hotter to colder object. https://www.pbslearningmedia.org/resource/conduction-heat- transfer-animation/unc-tv-science/ http://www.youtube.com/watch?v=UmaP988DXNE Architectural Technology Humber College  Conduction in Building Conduction occurs through a wall Transferring heat from inside to outside in cold climates Primarily by the direct contact of the different layers. Architectural Technology Humber College  Conduction in Building The speed of heat moving through building materials depends on: The materials present Contact area Heat resistance (R-value) Thickness The thicker the material = higher R value, and the greater its capacity to resist heat flow Architectural Technology Humber College  Conduction in Building Every material has different: ▪ Conductance (C) ▪ Thermal Conductivity coefficient (k) ▪ Thickness of materials (L) ▪ Temperature differences ( ∆T) ▪ Resistance (R) C= k / L Conductance is the rate of passing heat per unit area and temperature Architectural Technology Humber College  Conduction in Building - R value is the reciprocal of conductance R= 1/ C But in a composite wall, Ie: gypsum, insulation, wood sheeting, exterior brick cladding Rtotal= 1/ C gypsum + 1/C insulation + 1/C wood sheeting + 1/C brick cladding The Rt-value more correctly called the overall heat resistance coefficient, describes how well a building element resists the conduction of heat. Architectural Technology Humber College  Conduction in Building So how do we figure out heat loss through a wall? Heat excites the molecules They crash into other molecules They transfer their excitement to neighbouring molecules Now the question is: How fast each one of the materials used in wall will transfer heat? Architectural Technology Humber College  Temperature Gradient Measure temperature at each location in the wall assembly and then graph the temperatures Architectural Technology Humber College  Thermal Conductivity and Conductance of Building Materials Thermal Conductivity (k) Shows the ease that heat will flow through unit thickness and unit area of the material Building Science – Heat Transfer Thermal Conductance (C) indicates the ease that heat will flow through a material of a specified thickness as found on the chart Some one has figured out the C for a specific thickness  C = conductance k = W/m*m/m/ K k = coefficient of heat transfer k = W/mK L = thickness of material W = watts of energy m = area when squared K = difference in temperature from one side to the other Resistance ( R) is the inverse of Conductance = 1/C Building Science – Heat Transfer Time to figure out how to calculate temperature through a wall and the resistance to transfer of heat Outside temp = -5 Inside temp = +22 Sample selection of materials 12mm plywood 75mm expanded polystyrene 12 plywood Building Science – Heat Transfer Each material has different resistance to transfer of heat Need to calculate one at a time = Each material has a different k and has a different C Need to calculate one at a time Building Science – Heat Transfer Let us start with the insulation: From the chart, we are directed to use k or the coefficient of the specified material (it would take to much room to list every possible thickness as C) Sample selection of materials 12mm plywood 75mm expanded polystyrene 12mm plywood Building Science – Heat Transfer Be carful of units Sample selection of materials k =.036 12mm plywood 75mm expanded polystyrene (.075m ) 12mm plywood k tells us conductance ratio for a given material Need to convert into Conductance for a given thickness, in our case 75mm or.075m Units for formula formula Or C =.036 /.075 Building Science – Heat Transfer Best use a table for our calculations, it Sample selection of materials 12mm plywood will get complicated.075m expanded polystyrene 12mm plywood %R=(R1/Rtotal)*100 k =.036 – from chart delta T=(R1/Rtotal)*total delta T Element C = k/l R =1/C % R total delta T T ('C) outdoor air -5.0 f outside.075m expanded polystyrene.036/.075 =.48 1/.48 = 2.0833 100 -5 +22=27 f inside interior air +22.0 Total 2.08 27 Building Science – Heat Transfer Need to deal with two layers of plywood Sample selection of materials 12mm plywood k =.115 75mm expanded polystyrene k=.036 12 mm plywood k =.115 Building Science – Heat Transfer Sample selection of materials 12mm plywood 75mm expanded polystyrene k =.115 12mm plywood Element C = k/l R =1/C % R total delta T T ('C) outdoor air -5.0 f outside.012m plywood.115/.012 = 9.58 1/9.58 =.1044.1044/2.2921 =.045 or 4.5%.045 x 27 = 1.21'C -3.79.908 x 27 =.075m exp polysty.036/.075 =.48 1/.48 = 2.0833 2.083/2.2921 = 0.908 or 91% 24.516'C +20.73.012m plywood.115/.012 = 9.58 1/9.58 =.1044.1044/2.2921 =.045 or 4.5%.045 x 27 = 1.21'C f inside +21.94 interior air +22.0 Total 2.2921 27 Building Science – Heat Transfer So what we have learned is RT = R1 + R2 + R3 Element C = k/l R =1/C % R total delta T T ('C) outdoor air -5.0 f outside.012 plywood.115/.012 0.1044 4.5 1.21 -3.79.075m expanded polystyrene.036/.075 2.0833 91 24.52 +20.73.012 plywood.115/.012 0.1044 4.5 1.21 f inside +21.94 interior air +22.0 Total 2.2921 27 Building Science – Heat Transfer We have to account for two more items which Provide resistance to heat transfer, the film of still air on each surface f outside : C=34 (a constant) f inside : C=8.3 (a constant) Element C = k/l R =1/C % R total delta T T ('C) outdoor air -5.0 f outside 34.029 1.2.32.012 plywood.115/.012 0.104 4.3 1.16 -3.5.075m expanded polystyrene.036/.075 2.083 85 22.95 +19.4.012 plywood.115/.012 0.104 4.3 1.16 f inside 8.3.12 5.9 1.59 +22.2 interior air +22.0 Total 2.44 27 Graph of temperature Gradient Heating Degree-Days Zones https://natural-resources.canada.ca/energy-efficiency/homes/make-your-home-more-energy-efficient/keeping-the- heat/section-2-how-your-house-works/15630 Heating Degree-Days Zones  Heat Transfer in Gas The properties of construction materials are constant, we can use prepared charts Gas is always different; we will have to do our own calculations each and every time for C  Convective Heat Transfer We calculated Conductive Heat Transfer through solid materials What about gases? Conductive heat transfer through a gas is very inefficient because the molecules are far apart and the transfer of kinetic energy taking much longer than in solids A more efficient way to transport heat in a gas is to move the gas with the heat in it, called Convective Heat Transfer Convection ovens increase the speed of cooking Electric heater works better with a fan to distribute the warm air  Heat Transfer What are the scientific explanations of heat transfer? Convective Heat Transfer: Transfer Is theofheat heatexchange process that by movement occurs or flow ofonly in a fluid medium, molecules with a such change inas air orheat their liquid.content. Convection occurs inside wall cavities, between the studs. https://www.pbslearningmedia.org/resource/convection-heat-transfer-animation/unc-tv-science/ Occurs only in a fluid medium (air or liquid). Convection occurs inside wall cavities, between the studs Is strictly directional, It never transfers heat downward.  Natural Convection Architectural Technology Humber College  Convection in Wall Cavity Ensure that the insulation fills the space completely and evenly. Blank spots or corners allow convection currents to occur, sometimes letting heat bypass the insulation completely. Architectural Technology Humber College  Heat Transfer Radiant Heat Transfer https://www.pbslearningmedia.org/resource/therm al-radiation-heat-transfer-animation/unc-tv-science/ Radiant heat is transferred between two objects: NOT in contact NOT shielded from each other When you stand in the sun, you are experiencing heat transfer from the sun by radiation http://www.youtube.com/watch?v=58Oox90jRCQ&feature=related  Principles of Thermal Radiation All objects radiate to each other The wavelength of the radiation is based on the Temperature of the object. Hot bodies lose heat by radiation because they emit more energy than they absorb Warm things radiate infrared Since radiation is not effected by gravity, a body will radiate down as much as up. Architectural Technology Humber College Primary Ways Radiation Interacts With Materials 1. Transmittance: radiation passes through the material 2. Absorptance : radiation is converted into sensible heat 3. Reflectance : radiation is reflected by the surface 4. Emissivity : radiation is given off by the surface  Transmittance Materials with high transmittance, like clear glass, are transparent to certain wavelengths of radiation, allowing light or other radiation types to pass through with little attenuation.  Absorbance When radiation is absorbed, it heats the material, contributing to temperature increases.  Reflectance Mirrors or metallic surfaces, redirect radiation.  Emissivity ▪ Is the ability of a surface to emit radiation. ▪ Materials cool down by radiating energy away. ▪ High emissivity materials radiate heat effectively. ▪ Objects like stoves emit infrared radiation when heated. Radiation - Greenhouse Effect  Solar Radiation What will happed when solar radiation strikes?: – White washed walls – White clothing – Reflective sun glassless – Low emissivity glass – Fuselage of a commercial aircraft Reflective Materials: Lessons from Aerospace Design A/C system has weight! Top is white to reflect most solar short-wave radiation, what is absorbed is radiated back out of the skin by long wave radiation Underside is highly polished Aluminum, the runway absorbs short wave solar radiation, which then reflects long wave (Infrared) radiation to underside of plane, And the highly polished skin then reflects long wave radiation back Out-dated design. Now we use advanced composites Building Science – Heat Transfer Recap: Three main types of heat transfer mechanisms – Conduction – Convection – Radiation  Building Science – Heat Transfer A Rap version of the scientific explanations of heat transfer? http://www.teachertube.com/viewVideo.php?video_id=159713 My students' work: https://youtu.be/Eg9TXkOvgTc?si=6YdgnFD9IAorjyYX Would you like to do a similar work for your term project ?  In-Class activity Go to www.Kahoot.ir Then enter the ”Game Pin”  Questions? Architectural Technology Humber College  Thermal Thermal Density Conductanc Material Conductivity kg/m³ e (C) (k) W/(m·K) W/(m²·K) Metal siding, hollow backed Over board sheathing — — 9.0 Architectural glass — — 50 Wood Maple, oak, and similar hardwoods 720 0.16 — Fir, pine, and similar softwoods 510 0.12 — Metals Aluminum 2,740 220 — Brass, yellow 8,300 120 — Copper 8,900 390 — Lead 11,300 35 — Nickel 8,890 60 — Steel, mild 7,830 45 — Miscellaneous Glass, soda lime 2,470 1.0 — Air, still 1.2 0.025 — Water, still 1,000 0.60 — f f