VIEWS: 0 PAGES: 66 POSTED ON: 12/9/2012
Chapter 3 Thermal Insulation （建筑）隔热 Why do we talk about thermal insulation? Thermal insulation is the major factor in reducing the loss of heat from buildings. Benefits of good thermal insulation ? (1) to maintain a constant temperature within a building, we need to reduce the heat exchange with the surroundings (2) to save energy needed to run the cooling 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. A well-insulated structure and a poorly-insulated structure, which one will stay cooler in summer? 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. 《黄帝宅经》 治宅极宜壮实 four topics are discussed in this chapter 3.1 Insulating materials 绝热材料 3.2 U-values U-值 3.3 Thermal Bridging 热桥 3.4 Structural temperature 结构温度 3.1 Insulating materials 绝热材料 What is a thermal insulator （隔热体）? A thermal insulator is a material which opposes（反对） the transfer of heat between areas at different temperatures. what should a good insulator have? The best insulating materials tend to be porous（多 孔的） and of low density. Gas, 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）, expanded plastics（发泡塑料）, and cavities（空腔）. For air to act as an insulator it must be stationary （静止的）, otherwise moving air if allowed to move will transfer heat by convection. 3.1.1 Types of insulator (1) Rigid preformed materials: aerated concrete blocks 预制刚性材料 : 加气混凝土砖 以硅砂、水泥、石灰为主要原料，由经过防锈处理 的钢筋增强，经过高温、高压、蒸汽养护而成的多 气孔混凝土制品。可锯可刨，加工便捷，其施工效 率是传统红砖的4~5倍，可广泛用于建筑的内墙、 外墙。 (2) Flexible materials: fibreglass quilts 柔性材料 玻璃纤维布 (3) Loose fill materials: expanded polystyrene granules 疏松填充材料 膨胀聚苯乙烯颗粒 (4) Materials formed on site : foamed polyurethane 现场加工材料 泡沫聚亚安脂 (5) Reflective materials : aluminium foil 反射材料 铝箔 3.1.2 Properties of insulators 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）or rigidity（硬度） suitable for the purpose. Moisture resistance. （防潮性能） Fire resistance. （防火性能） Resistance to pests and fungi.（防虫和防霉） Compatibility（兼容性） with adjacent materials Harmless to humans and the environment. 3.1.3 Thermal conductivity , lamda-value 导热系数 lamda-值 thermal conductivity Unit: W/m K ( W/m℃) Referring to figure3.1 the general formula is: P39 H kA(1 2 ) t d Resistivity 热阻 Resistivity （r）is an alternative index of conduction is the reciprocal of（成反比） thermal conductivity, so that r = 1/lamda The thermal conductivities of some building materials are given in table 3.1. The thermal conductivity of practical building materials Varies with moisture content as the presence of water increases conduction. Variations in density have significant effects on the k-values of brickwork, concrete and stone and more comprehensive tables of thermal conductivities should be used to obtain specific values. 3.1.4 Emissivity and absorptivity 发射率 吸收率 the ability of a material to absorb or give off radiant heat is a property of the surface of the material. Rough black surfaces absorb most heat and emit most heat. Conversely, shiny slivered surfaces absorb least heat and emit least heat. black body 黑体：是指能吸收投入到其面上 的所有热辐射能的物体，是一种 科学假想的物体，现实生活中是 不存在的。但却可以人工制造出 近似的人工黑体。 黑体模型 Emissivity is the fraction of energy radiated by a body compared to that radiated by a black body at the same temperature Absorptivity is the fraction of ( ) by a body compared to that ( ) by a black body 3.1.5 Clear sky radiation 晴空辐射 At night time a building emits radiant heat to its surroundings and 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） occurs because a clear dark sky is closer in form to a black body The same radiant mechanism causes dew(露)or ground frost (霜） to occur during a clear starry (布满星星的) night. Now the next topic 3.2 U-values 3.2.1 Thermal transmittance ,U-value 传热系数，U-值 Unit : W/m2K (W/m2℃) The rate of heat flow in watts through 1m2 of a structure when there is a temperature difference across the structure of 1 ℃ Lower U-values provide better thermal insulation. 3.2.2 Elemental U-value （围护结构）各部分的U-值 Standard U-values are calculated by making certain assumptions about moisture contents of materials and about rates of heat transfer at surfaces and in cavities. not always agree exactly with U-values measured on site. the values for （some common types of construction）are given in table 3.3 Figure 3.1 Maximum U-values for new dwellings McMullan 3.2.3 Thermal resistance , R-value 热阻 R-值 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. （1）Material resistances 材料热阻 （2）Surface resistances 表面热阻 （3）Airspace resistances 空气间层热阻 （1）Material resistances材料热阻 Assuming that a material is homogeneous均匀的, this type of resistance can be calculated by the following formula. R d /k （2）Surface resistances表面热阻 The thermal resistance of an open surface depends upon the conduction, convection and radiation at that surface. Some useful values are given in table 3.5** Factors which affect surface resistance are given below Direction of heat flow: upward or downward Climatic effects: sheltered or exposed Surface properties: normal building materials with high emissivity 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 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-values The thermal transmittance , or U-values, is the reciprocal of the total thermal resistance and can be calculated using the following formula 1 1 U U RT Rsi R 1 R 2 ... R a R so Worked example 3.1 Calculate the U-value of a cavity wall（空心墙体） with a 105mm(millimetre ) thick brick out leaf（外砖 墙）, a 75mm unventilated cavity containing 50mm of fibreglass quilt, then a 100mm lightweight concrete block inner leaf （内砖墙）with a 15mm layer of lightweight plaster（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: sketch a figure indicating all parts of the construction with surface layers Step 2: tabulate all information and ,where necessary ,calculate thermal resistance using R d /k 1 Step 3: using U Rsi R 1 R 2 ... R a R so 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 3.2.5 U-values of floors 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-values 1）calculate the effect that additional insulating material has upon a U-value, 2） calculate the thickness of material that is required to produce a specified U-value. Use the following guidelines to make adjustments U-values can not be added together or subtracted from one another Thermal resistances can be added and subtracted. Worked example 3.2 illustrates the technique. A certain uninsulated cavity wall has a U-value of 0.91W/m2K. If expanded polyurethane(聚亚安酯)board is included 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 expanded 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/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 实例：墙角结露卧室像澡堂 设计方施工方各有说法 日期：2005-6-10 8:48:27 来源: 每到冬天，住在天天家园的崔女士就会为卧室墙角滴水的事犯 愁。昨天，天天家园开发商代表汪先生已答应尽快帮崔女士解决卧 室墙角结露的问题。 结露现象已有三年 记者在崔女士家的卧室内看到，屋顶墙角上布满了黄豆大小的 水滴，侧面墙壁上也出现了淡黄色的水痕，有的地方甚至发霉了。 崔女士说，她搬到这儿已经三年了，每年11月初到次年的2月都会 出现这种情况。 “我家现在不但不敢装修，就连柜子里的棉被都发霉了。”崔 女士告诉记者，自从前年冬天发现卧室墙角有结露现象后，她一直 用毛巾进行擦拭，“最严重的时候，柜子后边的墙壁都是湿的，跟 澡堂子似的。”崔女士怀疑是室内空气遇冷后在墙角凝结成水滴， “周围的邻居都没有出现过这种情况。” 3.3 Thermal bridges, cold bridges 热桥、冷桥 A thermal bridge is a portion of a structure whose high thermal conductivity lowers the overall thermal insulation of the structure. There is increased heat flow across the thermal bridge and the surfaces on the interior side of the bridge therefore become cooler, giving rise to the informal term of “cold bridge”. There is an increased risk of condensation and mould growth （发霉）on these internal surfaces around cold bridges. 房子"冒汗"-"冷桥"现象的处理与维护 热桥以往又称冷桥，现统一定名为热桥。热桥是指 处在外墙和屋面等围护结构中的钢筋混凝土或金属 梁、柱、肋等部位。因这些部位传热能力强，热流 较密集，内表面温度较低，故称为热桥。 由于热桥部位内表面温度较低，寒冬期间，该处温 度低于露点温度时，水蒸气就会凝结在其表面上， 形成结露。此后，空气中的灰尘容易沾上，逐渐变 黑，从而长菌发霉。热桥严重的部位，在寒冬时甚 至会淌水，对生活和健康影响很大。 通风\加热\保温\赔偿 石材内部产生的冷凝水积聚 石 材 内 部 产 生 的 冷 凝 水 积 聚 Pattern staining 暗斑 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 a particular result 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-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 A1U1 A 2 U 2 ..... U(average) A1 A 2 ... 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. A1U1 A 2 U 2 ..... U(average) A1 A 2 ... 6 0.35 2 2.8 U 0.96 8 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 . Figure 3.6 thermal response 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 A structure made up of different materials, will have varying temperature gradients between the inside and outside R T R T The boundary temperatures between layers in a structural element can be determined from the thermal resistances which make up the U-value of that element The ratio of the temperature changes inside a structure is proportional to the ratio of the thermal resistance. 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=0.667 Using R ⊿=2.77R T R T So temperature on the inside surface=20-2.77=17.23 ℃ T RT Chapter 3 Thermal Insulation 3.1 Insulating materials 绝热材料 3.1.1 Types of insulator 3.1.2 Properties of insulators 3.1.3 Thermal conductivity , k-value 3.1.4 Emissivity and absorptivity 3.1.5 Clear sky radiation 晴空辐射 3.2 U-values 3.2.1 Thermal transmittance ,U-value 3.2.2 Elemental U-values 3.2.3 Thermal resistances 3.2.4 Calculation of U-values 3.2.5 U-values for floors 3.2.6 Adjustments to U-values 3.3 Thermal Bridging 3.3.1 Thermal bridge 3.3.2 cold bridges 3.3.3 pattern staining 3.3.4 average U-values 3.4 Structural Temperature 3.4.1 response time 3.4.2 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 formula 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 ?( ) A to maintain a constant temperature within a building B to reduce the heat exchange with the surroundings C to save energy needed to run the cooling plant D to save energy needed to run the heating plant E to reduce the risk of surface condensation F to reduce the time taken for a room to heat up to a comfortable temperature G energy conversation 2 Rough black surfaces( ) A absorb most heat B emit most heat. C absorb least heat D emit least heat 3 Shiny silvered surfaces ( ) A absorb most heat B emit most heat. C absorb least heat D emit least heat 4( ) is a measure of the overall rate of heat transfer ,by all mechanisms under standard conditions, through a particular section of construction A a U-value B a r-value C a R-value D a lamda-value 5( ) gives better thermal insulation A higher thermal resistance B higher U-value C lower thermal resistance D lower U-value 6( ) be added together or subtracted from one another A U-values B R-values C r-values D lamda-values
"2 Thermal Insulation"