Why do we talk about thermal Why do we talk about thermal insulation? insulation? Thermal insulation is the major is the major factor in factor in reducing the loss of heat from buildings. Chapter 3 Thermal Insula Chapter 3 Thermal Insula tion tion 建建建 建建 () 建建建 建建 ()
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Why do we talk about thermal insulation?Why do we talk about thermal insulation?
Thermal insulation is the major factor inis the major factor in reducing the loss of heat from buildings.
Benefits of good thermal Benefits of good thermal insulation ?insulation ?
(1) to maintain a constant temperature within a building, we (1) to maintain a constant temperature within a building, we
need to reduce the heat exchange with the surroundingsneed to reduce the heat exchange with the surroundings
(2) to save energy needed to run the cooling plant(2) to save energy needed to run the cooling plant
(3) to save energy needed to run the heating plant(3) to save energy needed to run the heating plant
(4) One of the benefits of good thermal insulation is that the risk of surface condensation is reduced because of the warmer internal surfaces.
(5) Good insulation can also reduce the time taken for a room to heat up to a comfortable temperature.
The relatively small cost of extra insulating materials is quickly paid for by the reduction in the size of the heating plant required and by the annual savings in the amount of fuel needed.
It is useful to remember that good thermal insulation will also reduce the flow of heat into a building, when the temperature outside is greater than the temperature inside.
A well-insulated structure and a A well-insulated structure and a poorly-insulated structure, which one poorly-insulated structure, which one will stay cooler in summer?will stay cooler in summer?
What is a thermal insulator What is a thermal insulator (隔热体)(隔热体) ?? A thermal insulator is a material which oppos
es (反对反对) the transfer of heat between areas at different temperatures.
what should a good insulator have?what should a good insulator have?
The best insulating materials have their atoms spaced well apart, so these materials will also tend to be porous (多孔多孔的的) and of low density.
Gas, which have the most widely-spaced atoms, are the best insulators against conduction.
Air, which is a mixture of gases, is the basis of insulators such as aerated lightweight concrete (轻型加气混凝土轻型加气混凝土) , expanded plastics (发泡塑料发泡塑料) , and cavities (空腔空腔) .
For air to act as an insulator it must be stationarystationary (静止(静止的)的) ,, otherwise moving air if allowed to move will transfer heat by convection.
When choosing materials for thermal insulation of buildings the physical properties of the material need to be considered.
Thermal insulation suitable for the purpose. Strength (强度强度) or rigidity (硬度硬度) suitable for the pur
pose. Moisture resistance. (防潮性能防潮性能) Fire resistance. (防火性能防火性能) Resistance to pests and fungi. (防虫和防霉防虫和防霉) Compatibility (兼容性(兼容性) with adjacent materials Harmless to humans and the environment.
3.1.2 Properties of insulators3.1.2 Properties of insulators
thermal conductivity Is a measure of the rate at which heat is conduction through a particular material under specified conditions
Unit: W/m K ( W/m )℃Referring to figure3.1 the general formula is: P39
Emissivity Emissivity is the fraction ofis the fraction of energy radiated by a body compared toto that radiated by a black body at the same temperatureAbsorptivity Absorptivity is the fraction ofis the fraction of ( ) by a body compared toto that ( ) by a black body
3.1.5 Clear sky radiation 3.1.5 Clear sky radiation 晴空辐射晴空辐射 At night time a building emits radiant heat to its surroundings an
d the rate of this heat loss from the roof will be increased if the night sky is clear and cloudless.
This radiant “suck”( 允吸允吸)occurs because a clear dark sky iscloser in form to a black body
The same radiant mechanism
causes dew( 露 )or ground frost (( 霜)霜)to occur during a clear
Standard U-values are calculated by making certain assumption
s about moisture contents of materials and about rates of heat t
ransfer at surfaces and in cavities.
not always agree exactly with U-values measured on site.
they are needed as a common basis for comparing the thermal
insulation of different types of structure and for predicting the he
at loss from buildings.
they are also used to specify the amount of thermal insulation r
equired by clients 客户 or by regulations.
the values for ( some common types of construction ) are giv
en in table 3.3
Figure 3.1 Maximum U-values for new dwellings
McMullan
3.2.3 Thermal resistance , R-value3.2.3 Thermal resistance , R-value 热阻 热阻 R-R- 值值 Is a measure of the opposition to heat transfer offered by a
particular component in a building element. Unit : m2K/w Higher thermal resistance gives better thermal insulation There are there general types of thermal resistance which
need to be determined , either by calculation or by seeking published standard values.
The thermal resistance of each layer of material in a
structure depends on the rate at which the material
conducts heat and the thickness of the material.
Assuming that a material is homogeneous, this type
of resistance can be calculated by the following can be calculated by the following
formulaformula.
kd /R
( 2 ) Surface resistances 表面热阻 The thermal resistance of an open surface depends upon t
he conduction, convection and radiation at that surface.
Some useful values are given in table 3.5**are given in table 3.5**
Factors which affect surface resistance are given beloware given below
Direction of heat flow: upward or downward
Climatic effects: sheltered or exposed
Surface properties: normal building materials with high emi
ssivity or polished metal with low emissivity.
( 3 ) Airspace resistances 空气间层热阻
The thermal resistance of an airspace
or empty cavity depends on the nature
of any conduction, convection and
radiation within the cavity. useful airspace resistances are given in table 3.5 P47 Factors affecting airspace resistance are given below Thickness of the airspace Flow of air in airspace ; ventilated or unventilated Lining (衬里 , 内层) of airspace ; normal surfaces
or reflective surfaces of low emissivity.
Total thermal resistances The thermal resistances of the consecutive (连续的) la
yers in a structural element, such as a wall or roof, can be likened to electrical resistances connected in seriesin series. (串联)
Thus the total thermal resistance is the sum of the thermal resistance of all components in a structural element.
3.2.4 Calculation of U-values3.2.4 Calculation of U-values
The thermal transmittance , or U-values, is the is the
reciprocal ofreciprocal of the total thermal resistance and can can
be calculated using the following formulabe calculated using the following formula
TR
1U
soa21si RR...RR
1U
R
Worked example 3.1
Calculate the U-value of a cavity wall (空心墙体空心墙体) with a 105mm(millimetre millimetre ) thick brick out leaf (外外砖墙砖墙) , a 75mm unventilated cavity containing 50mm of fibreglass quilt, then a 100mm lightweight concrete block inner leaf (内砖墙内砖墙) with a 15mm layer of lightweight plaster (轻质石膏板轻质石膏板) . The thermal conductivities in W/m K are: brickwork 0.84, lightweight concrete block 0.19, lightweight plaster 0.16, fibreglass 0.04. standard thermal resistances in m2K / W are: internal surface 0.123, external surface 0.055, cavity 0.18
Step 1:Step 1: sketch a figure indicating all parts of sketch a figure indicating all parts of the construction with surface layersthe construction with surface layers
kd /R
soa21si RR...RR
1U
R
Step 2:Step 2: tabulate all information and ,where tabulate all information and ,where necessary ,calculate thermal resistance using necessary ,calculate thermal resistance using
注意作图,分母是热阻, 不要带入导热率注意作图,分母是热阻, 不要带入导热率Work to a final accuracy of 2 decimal places结果精确到小数点后两位Lay the calculation out in a table
Step 3:Step 3: using using
3.2.5 U-values of floors3.2.5 U-values of floors
Heat losses through the floor are highest near the exposed edges of the floor,
Heat losses are lowest near the centre of the floor.
The calculation of the actual U-values for ground floors is complex and it is useful to use table or graphs to find the minimum thickness of insulation needed to meet a particular average standard required by regulations.
For some large buildings the standard may be achieved without the use of insulation because the ratio of exposed floors edges to total area is proportionally small.
3.2.6 Adjustments to U-values3.2.6 Adjustments to U-values
It is sometimes necessary to calculate the effect that
additional insulating material has upon a U-value,
or to calculate the thickness of material that is required
to produce a specified U-value.
Use the following guidelines to make adjustments
U-values can can not not be added together or subtracted be added together or subtracted
from one anotherfrom one another
Thermal resistances can be added and subtractedcan be added and subtracted.
Worked example 3.2 illustrates the technique.
A certain uninsulated cavity wall has a U-value of 0.91
W/m2K. If expanded polyurethane( 聚亚安酯 )board is i
ncluded in the construction what minimum thickness of
this materials is required to reduce the U-value to 0.45
W/m2K? Given that the thermal conductivity of the exp
anded polyurethane = 0.025W/mK.
Target U-value U2=0.45
Target total resistance(1/U) R2=1/0.45=2.222
Existing U-value U1=0.91
Existing total resistance(1/U) R1=1/0.91=1.099
Extra resistance required (所需的额外热阻) R2-R1=2.222-1.099=1.123
the k-value of proposed insulating material k=0.025
So using formula R=d/kSo using formula R=d/k
Thickness of material d=RXk=1.123X0.025=0.028metres=28mm
So minimum thickness of insulating board needed to give 0.45U-value=28mm
Pattern staining on a ceiling is the formation of a pattern , in dirt or dust, which outlines the hidden structure of the ceiling.
It is aIt is a particular result ofresult of thermal bridging and also depends upon the frequency of redecoration(再次装修) .
Pattern 模范 , 式样 , 图案 Staining 着色
Average U-valuesAverage U-values
If a wall, or other element , is composed of different constructions with different U-values then the overall insulation of the wall depends upon the relative areas of the different constructions.
The general formula is as follows
...AA
.....UAUAU(average)
21
2211
Worked example
A wall has a total area of 8 m2 of which 2m2 are windows. The U-values are 0.35 W/m2K for the cladding(覆层 )and 2.8W/m2K for the glazing. Calculate the average U-value for the wall.
...AA
.....UAUAU(average)
21
2211
96.08
8.2235.06U
3.4 Structural temperature 结构温度
The thermal insulation installed in a building affects
the rate at which the building loses heat energy
which is measured by the U-value.
The thermal performance of the building also
depends upon the thermal capacity of the insulating
material, which affects the times taken to heat or
cool the structure,
and the position of the insulation, which affects the
temperatures in the structural elements
3.4.1 Response times 响应时间
.
In general, lightweight structures respond more quickly to surrounding temperature changes than do heavyweight structure .This is because heavyweight materials have a higher thermal capacity and require more heat energy to produce given temperature changes
Figure 3.6
thermal response
A structure made up of different materials,
will have varying temperature gradients between the inside and
outside
The boundary temperatures between layers in a structural elem
ent can be determined from the thermal resistances which mak
e up the U-value of that element
The ratio of the temperature changes inside a structure is p
roportional to the ratio of the thermal resistance.
TT R
R
Worked example 3.5
A room has an external wall with a U-value of 1.5 W/m2K and contains air at 20 when the outside temperature is ℃5 . The internal surface resistance is 0.123 ℃ ㎡ K/W. calculate the boundary temperature on the internal surface of the wall.
inside temperature= 20 ℃ outside temperature= 5 ℃ resistance of that layer R= 0.123 ㎡ K/W
total resistance of the structure RT=1/U=1/1.5=0.667Using ⊿=2.77 So temperature on the inside surface=20-2.77=17.23 ℃
3.4 Structural Temperature3.4 Structural Temperature3.4.1 response time3.4.1 response time3.4.2 temperature gradients3.4.2 temperature gradients
That’s all for chapter 3 Since there’s time, please read the text
carefully and try to find the main points in chapter 3.
If you have questions, you can ask me.
some useful sentence patterns
影响 xxx 的因素如下:Factors which affect xxx are given below影响表面热阻的因素如下影响空气层热阻的因素如下
表 3.5给出了一些常用的 xxx 值Some useful xxx are given in table 2.5 表 3.5给出了一些有用的空气热阻值 表 3.5给出了一些常用结构的 U 值
xxx 是 yyy 的倒数xxx is the reciprocal of yyy 传热系数是总热阻的倒数
xxx 可用下式计算 xxx can be calculated using the following for
mula 传热系数可用下式计算
最终结果精确到小数点后 xx 位 Work to a final accuracy of xx decimal places最终结果精确到小数点后 2 位最终结果精确到小数点后 3 位列表计算 Lay the calculation out in a table
Exercises in class
1 Benefits of good thermal insulation ?( )1 Benefits of good thermal insulation ?( )A to maintain a constant temperature within a buildingA to maintain a constant temperature within a buildingB to reduce the heat exchange with the surroundingsB to reduce the heat exchange with the surroundingsC to save energy needed to run the cooling plantC to save energy needed to run the cooling plantD to save energy needed to run the heating plantD to save energy needed to run the heating plantE to reduce the risk of surface condensationE to reduce the risk of surface condensationF to reduce the time taken for a room to heat up to a F to reduce the time taken for a room to heat up to a
comfortable temperaturecomfortable temperatureG energy conversationG energy conversation
2 Rough black surfaces( )2 Rough black surfaces( )
4 ( ) is a measure of the overall rate of heat 4 ( ) is a measure of the overall rate of heat transfer ,by all mechanisms under standard condtransfer ,by all mechanisms under standard conditions, through a particular section of constructioitions, through a particular section of constructionn