thermal studies u vlaues
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Thermal studies U VlauesTRANSCRIPT
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Tools: For now you will need…. Tools: For now you will need….
PenPen
PaperPaper
CalculatorCalculator
Put bags under tablesPut bags under tablesContext…
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Salford City College
Construction&
Engineering
Nationals
In
Construction
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Construction science and materials After this next session is over you will be able to :
• Calculate the "U" value of a concrete roof slab 150 mm (0.15m) thick, asphalted 30 mm
(0.03m) for rainwater protection and the soffit faced with plasterboard 12 mm (0.012)thick on
50 mm softwood battens, the void filled with 50mm (0.05) Glass fibre quilting .
• K Concrete = 1.0 W/m² °CK Asphalt = 0.25 W/m² °C
K Plasterboard = 0.35 W/m² °CK Glass fibre Quilt = 0.035W/m² °C
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You will also…..
• Know about heat gains and losses in buildings
Where is heat lost in a dwelling?...
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Next – Heat Losses in Homes
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Heat Losses In
Homes
Coming Next – Construction Techniques
Give an example of steps you can take to save energy …
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Construction Techniques
DG
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Next note taking …Factors affecting energy use
Double galzing…
Next:Note taking…
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Name things which can increase or decrease energy usage …
• climate: • Location • solar radiation • Wind• degree of exposure
• Building Characteristics
• shape and size
• Usage: • the way that the property is
used including space and heating requirements
• internal temp causes :• domestic appliances• desired internal temp.• heating system and its
efficiency
Coming Next – the Total Heating Load
The Total Heating Load…
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The Total Heating Load
Boiler
Ventilation
Roof
Electrical appliances
Floor
Windows
Walls
Solar Gains
Fuel Supply
Flue losses
Hot water
Cooking
Next slide…FHL + VHL = SHL
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Total Heat Load• Gains
• What types of things heat a house up?
• • Solar effect
• Cooking
• Hot water
• Electrical appliances
• Losses
• How is heat lost from a
building?
• Fabric Heat Loss
• Ventilation Heat Loss
• Specific Heat Loss
• In what ways does heat flow? ...
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THERMAL STUDIES• HEAT FLOW
Radiated heat flow through air such as the rays from the sun
Convection as fluid, gas or liquid. It expands on heating becoming lighter and is forced upwards by cooler fluid taking its place and therefore creates circulation
conduction by passing heat from one molecule to another, this is the source of most heat loss in buildings.
• The current Building Regulations carry a section relating to the conservation of energy
• Someone has to know how to add up the amount of heat loss
• So who does it and how is it done?..
K…
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What is the standard unit of THERMAL CONDUCTANCE?
("K" value)• In physics, thermal conductivity, k, is the property of a
material that..
• …indicates its ability to conduct heat. It appears primarily in Fourier's Law for heat conduction.
Coming Next – values for common materials
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• K values vary with good insulators having a value of around 0.003 W/m² °C and up to 400 W/m² °C for good conductors.
• (Don’t try to remember these numbers they are set out in industry tables)….
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K values for
common building
materials: adapted from the CIBSE
chartered institute of building service engineers
guide
Material K value (W/m2 0C )
• Aluminium alloy 160 • Asbestos cement sheet 0.4• Asphalt roofing 0.8• Brickwork 0.84• Concrete (regular) 1.83• Concrete (lightweight) 0.19• Copper 160• Corkboard 0.042• Fibre insulating board 0.05• Glass 1.0• Glass wool (mat or fibre) 0.04• Hardboard 0.13• Mineral wool 0.039• Gypsum plaster 0.46• Plasterboard 0.16• Polystyrene (expanded) 0.033• Polystyrene (solid) 0.17• Polyurethane foam 0.026• PVC flooring 0.04• Carbon steel 150• Sandstone 1.3• Softwood timber 0.13• Hardwood timber 0.15• Woodwool slab 0.085 Reciprocal of K is R…
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Resistivity & Conductivity• K values are good but
so is the opposite of conductance,which is…
• Resistance
• to show how a material resists heat a Resistivity (r) value can be found. This is the reciprocal (opposite) of a K value
• How do you find resistance?
• Resistivity =1/K value• materials are not always
supplied in one-metre blocks, using the resistivity you can look at the thickness of the material and calculate the "resistance" (R)
What if you have lots of Rs …
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HEAT LOSS FROM BUILDINGS
• Finally to find how much heat energy is being conducted we use a thing called the unit of "Thermal Conductance" used …
• U = 1/total resistance expressed in Watts per metre²
What does the U-value represent?…..
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• The term ‘U’ represents overall thermal conductance from the outside to inside covering all modes of heat transfer.
• What use is this in relation to the whole?
• An average U-value can be calculated
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• The U-Value is an important concept in building design.
• It represents the air-to-air transmittance of an element (part or fabric). This refers to how well an element conducts heat from one side to the other, which makes it the reciprocal of its thermal resistance.
• So if you calculate the thermal resistance of an element, we can simply invert it to obtain the U-Value..
• U = ?
•U = 1/total resistance expressed in Watts per metre²
Lets work some out…
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Example • Calculate the "U" value of a brick wall, plastered one side
with 20 mm background and 3 mm of hardwall gypsum finish, where "K" values for Brickwork = 0.55 W/m² °C,
background = 0.6 W/m² °C, hardwall plaster = 0.9 W/m² °C.
Material L (Thickness) K Value R=(1/k) x L
Brickwork 0.103 0.55 0.187
Background 0.020 0.6 0.033
Gypsum 0.003 0.9 0.003
Total Resistance 0.223
U value = 1/Total Resistance = 1/0.233 = 4.484 W/m² °C
Coming Next – further examples…
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Example
• Calculate the "U" value of a concrete roof slab 150 mm (0.15m) thick, asphalted 30 mm (0.03) for rainwater protection and the soffit faced
with plasterboard 12 mm (0.012)thick on 50 mm softwood battens, the void filled with 50mm (0.05) Glass fibre quilting .
• K Concrete = 1.0 W/m² °CK Asphalt = 0.25 W/m² °C
K Plasterboard = 0.35 W/m² °CK Glass fibre Quilt = 0.035W/m² °C
• Work to 3 decimal places.
Material L K R=(1/k) x L
Concrete 0.15 1.0 0.15
Asphalt 0.03 0.25 0.12
Glass Fibre 0.05 0.035 1.426
Plasterboard 0.012 0.35 0.034U value = 1/Total Resistance = 0.58 W/m² °C
Total R
1.73
Coming Next – air layers & cavities
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BOUNDARY STILL AIR LAYER & CAVITIES
• In any construction between the fabric and the internal/external environment there will be a layer of air which is "trapped" by the surface roughness of the materials used
• This boundary still air layer has an insulating quality that can be included in the heat loss calculation.
• The exterior surfaces of buildings are usually rougher than the internal faces
Coming Next – thermal properties of cavities
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CAVITIES
• cavities are used to keep heat in • To increase the width of a cavity will not necessarily
produce any significant improvement in heat retention-meaning…
• You only need a small gap
Coming Next – standard values…
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BOUNDARY STILL AIR LAYER & CAVITIES
• Standard Internal Resistance value 0.123 W/m²
°C
• Standard External Resistance value 0.055
W/m² °C
• Standard Cavity Resistance value 0.18 W/m²
°C
Coming Next – further standard values
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HEAT AS A FORM OF ENERGY
• Total heat loss per second =
Area x "U" x Temperature difference.
• Calculations to determine the total
heat loss for a particular building for
a particular period for a particular
temperature difference can now be
made
This session completed…
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• Average U value = total exposed surface area / total exposed surface area
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Heat load calculator
• http://www.shophmac.com/info-center/hvac-calculators/heat-load-calculator.php
• STOP
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Material L (Thickness) K Value R=(1/k) x L
Brickwork 0.103 0.55 0.187
Background 0.020 0.6 0.033
Gypsum 0.003 0.9 0.003
Total Resistance 0.223
U value = 1/Total Resistance = U value = 1/Total Resistance = 1/0.233 = 4.484 W/m² °C
Coming Next – further examples…