Download - Chapter 5 B1-1
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Heat Transmission in Building Structures
Chapter 5
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Basic Heat Transfer Modes
• Heat is transferred in buildings in the three known ways, which are
• conduction, • Convection • radiation
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Thermal Conduction
• It is a mechanism of heat transfer at the atomic level and it is given by the following equation:
1-D equation (steady state, 1-D) 3- D equation
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Flat wall case
•
In terms of thermal resistance…… Analogy to electrical system…. Same techniques can be applied to analyze wall or slab made up of two or more layers of dissimilar material.
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Composite flat wall
• Resistance in series
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Curved wall case • The temperature gradient is assumed to be uniform and
steady. The material is assumed to be homogeneous and have a constant value of thermal conductivity
• Heat transfer….
• Resistance….
• How to find thermal conductivity
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Thermal Convection • Convection is associated with fluids in motion • Heat transfer mechanism is complex and highly dependent on nature of
flow Convection equation… Types of convection… Convection in building structure How to find h….
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Combine Thermal Resistance
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Parallel thermal resistance
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• When the ration of the larger to the smaller of the thermal resistance is less than about 5 equation 5-18 gives reasonable approximation of the equivalent thermal resistance.
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Thermal bridge
• A large variation in the thermal resistance of parallel conduction paths is called a thermal bridge
• A thermal bridge as in ASHREA is an envelope area with significantly higher rate of heat transfer than the contiguous enclosure.
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Examples • Concrete balconies that extend the floor slab through
the building envelope are common examples of thermal bridging.
• In commercial construction, steel or concrete members incorporated in exterior wall or roof construction often form thermal bridges.
• Metal ties in cavity walls are another type of thermal bridge commonly found in masonry construction.
• Partially insulated, because thermal bridges are not considered by the calculation procedure, have been shown with actual thermal losses up to 35% higher than initially estimated.
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• Thermal bridges have two primary detrimental effects:
• they increase heat gain or loss, • they cause condensation inside or on the
envelope surface. • These effects can be significant in the
building’s energy cost or damage done to the building structure by moisture.
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• To overcome the effect of thermal bridging the following is suggested:
• a) use of lower-thermal-conductivity bridging material,
• b) changing the geometry or construction system,
• c) putting an insulating sheath around the bridge