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This 25 page Class Notes was uploaded by Maribel Abernathy on Wednesday September 23, 2015. The Class Notes belongs to AE544 at Drexel University taught by Staff in Fall. Since its upload, it has received 51 views. For similar materials see /class/212310/ae544-drexel-university in Architectural Engineering at Drexel University.
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Date Created: 09/23/15
V Thermal Design I Original Material by A Hamid Modified by J Mitchell 1003 Purpose of Thermal Design I Why do we undertake thermal design Write your own reasons We ll discuss them Thermal Design Obiective Control heat flow to 2 Design Proper HVAC systems 1 Maintain comfortable indoor conditions 2 Reduce heatingcooling loads which reduces operating costs 3 Understand Building Envelope Loads 1 Control vapor movementcondensation 2 Design to accommodate contractionexpansion of building materials and sealant joints 4 An Example I quotrv 513 23 HOW does Heat Transfer affect this building How we Control Heat Transfer I Roof Insulate amp Reflect I Walls Insulate Reflective seal gaps control vapor movement I Windows Insulate Emissivity seal gaps control vapor I Doors Insulate Seal gaps I Foundations Insulate avoid water i The Mechanisms of Heat Transfer I Now we ll look at how heat actually moves I This is almost all in Beall it should be a review How Heat Moves Temperature Heat ow Cooler area Pressure Mass ow Physical Mechanism I There are four ways heat moves I Define and explain them to a partner Conduction I Conduction direct transfer by contact of solid liquid or gas I QADeltaT R 32231 I Analog to Electrical Resistance 0mm 0 heat ow I Current VoltageR I Circuits follow directly I Linear if constant and in plane I Very complex if have time variance or complex shapes Convection I Convection transfer ofheatbythe movement of air or water Heat moves with the mass of fluid Warmer or colder replaces original I Complex Simplify E Interior Warmquot 1 39l nlfm l HO winder M wnll mm 5 ConductiOn M 85 Dhaka or air flow Radiation NonLinear Q B x T4 oHighly Complex We must simplify D0 so with equivalent temperatures empirically derived l ll 7 R I lt 5m I l N VT THEEM AL abN EXCHANGE W Tr i ATMDSQP EEE REFLECTED I AA RAD lATi 9N FROM VEG EIATION REFLECTED Figure 31 Radiant energy flow between a building and its environment TABLE 31 Di use Solar Radiation Incident on a Horizontal Sudace Ratio of actual direct Ratio nrrlil fuse to to maximum direct maximum direct Sky Condition radiatinn radiation Clear L00 01 2 Clear slightly hazy 080 025 llazy 060 035 40 055 Overcast SOURCE Watson and Labs 1953 Radiation Behavior Absorptance 025 gt 095 Reflectance 01 gt 095 Emittance 008 gt 095 Transmittance calculated Note that Reflection emittance transmittance is equal to 100 r if Evaporation I Phase change from solid or fluid to gas I Takes energy to do so thus cools the materials it s on I Complex calculation dependent on temperature Rh material properties air flow etc I Usually simplify look at long term Simplifications of Complexity I Simplify to linear behavior I Consider onedimensional situation I Ignore time variation I Use effective properties I Emperically derived simplifications Convective behavior converted to R Surface Temperature Air gap behavior More Complex I 3D complexity usually try to ignore it I Beall does deal with it slightly with ties analysis I Time effects I Daily Sun motion I Yearly Sun Path variation Shading variation from vegetation I Material Properties Thermal Mass Time variation degradation Temperature amp Humidity variation Calculations I Wall Gradient Calculator Spreadsheet Thermal gradient across a wall Saturation vapor pressure across a wall Actual vapor pressure across a wall Joint width necessary to address component movements and construction tolerances Thermal Bridging Wind pressure on a wall in both PSF and inches of water Factors affecting thermal performance continued Mass I Heat migrates through solid materials from the hot side to the cooler side The time of delay involving absorption of the heat is called thermal log I The amount of energy necessary to raise material temp is proportional to the wt of the material I Heavy materials like concrete and masonry absorb and store a significant amount of heat and substantially retard its migration This characteristic is called thermal storage capacity It affects the rate of conductive heat transfer and is a critical consideration in passive solar heating and cooling strategies Insulation I Low density material I Many types Beall Describes them l ajor Insulation Types Loose fibersohips fill insulation poured blown Flexible and semi rigid batt blanket Rigid wood fiberglass board Formedinplaoe urethane foam Effect of Moisture Content on Thermal resistance 100 I I g g 80 N u d 0 5 so g Cork g 76 4o Fertile E o 1 i5 D 20 39 quotquotquotquot m Fiberboard I l o 200 400 Moisture Content percent dry wt c Figure 312 Thermal resistance and moisture content of insulation From Tobiasson New Wetting Curves for Common Roof I nsulations Loss of Thermal Resistance It is recommended to have TRR be greater than 80 Less than 80 insulation is considered wet See table 313 in text Thermal Resistance Ratio TRR wet thermal resistivity dry thermal resistivity Recommended Minimum Thermal resistance of E in the US 94 Chapter Three 6 6 l 4 5 4 4 l l 3 v 2 3 2 0 2 4 i HAWAI I ALASKA RECOMMENDED MINIMUM THERMAL RESISTANCES R OF INSULATION ZONE CEILING WALL FLOOR 1 19 11 11 2 26 13 11 3 26 19 13 4 30 19 19 5 33 19 22 6 38 19 22 NOTE The minimum insulation R values recommended for various parts of the United States as delineated on the map of insulation zones Figure 311 Recommended minimum thermal resistances for ceilings walls and oors From Architectural Graphic Standards 9th ed Effects of Insulation Improve the thermal performances of building walls and roofs by reducing both conductive heat flow through the section and corrective heat flow in air spaces 2 Results in more comfortable indoor air temp and less fluctuation 3 Reduces coolingheating loads Thermal efficiency of insulation depends on 6 Thermal resistance R 7 Stability over time R value dimensional stability 8 Resistance to deterioration 9 Securing attachments Effects of Thermal Bridging A thermal bridge occurs when a subject of high thermal conductivity penetrates a material of low thermal conductivity insulation increasing the rate of heat flow at the penetration To account for thermal bridging correction factors lt10 should be used Example 1 Use table 39 in text for correction of R value 05 gt 038 Better yet calculate it Below Grade I Soil Temperature I Varies daily 12 I Varies yearly 2030 I Take 3D Effects into account I Water is critical issue
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