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Chapter 2 Principles of Heat. 建筑外环境. 人体对热湿环境的反应. A good thermal environment is a major aspect in the successful performance of a building. Both human beings and their buildings interact with the heat that surrounds them and they also contribute to this heat. 热湿环境. 空气质量环境. 光环境. - PowerPoint PPT Presentation
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Chapter 2 Principles of HeatChapter 2 Principles of Heat
A good thermal environmen
t is a major aspect in the su
ccessful performance of a b
uilding.
Both human beings and their
buildings interact with the h
eat that surrounds them and
they also contribute to this h
eat.
空气质空气质量环境量环境
热湿热湿环境环境
声环境声环境 光环境光环境
人体对热人体对热湿环境的湿环境的
反应反应
the requirements of human com
fort,
the types of heat loss and heat
gain by buildings ,and
the nature of moisture in the air.
空气质空气质量环境量环境
热湿热湿环境环境
声环境声环境 光环境光环境
Topics that are relevant to thermal design include:
Before these topics are considered, this chapter describes:
2.1 Nature of heat, its measurement and its effect
热的性质、量度和作用2.2 Heat transfer 热传递2.3 Gases and vapours, and their effects
气体和水蒸气,及其作用2.4 Refrigerators and heat pumps
制冷机及热泵
Let’s move on to the text.
The fist part:
2.1 Nature of heat 热的性质 2.1.1 Heat energy 热能Heat is a form of energy 热是能量的一种形式The joule is the standard SI unit of energy used for
measuring any other form of energy.
Other units of energy still found in use include the
following units.
Unit: calorie, where 1 cal 4.187 J
kilowatt hour, where 1kwh=3.6MJ
British Thermal Unit, where 1BTU=1.055kJ
英制热量单位
Heat energy is an internal molecular property of
a material.
Other forms of energy including mechanical energy,
electrical energy and chemical energy can all be
converted to thermal energy.
Can you give some examples ?
Thermal energy often forms an intermediate
stage in the production of other forms of energy.
now, I want someone to read the examples given in the
book
Power is a measure of the rate at which work is done, or at
which energy is converted from one form to another.
Unit : watt (W)
By definition, 1watt = 1 joule / second.
The watt is often used in the measurement of thermal
properties and it is useful to remember that it already
contains information about time and there is no need to
divided by seconds.
(t) time
H) (energy Heat (P)Power
2.1.2 Power 功率能力 , 力量 , 动力 , 权力 ,
Temperature is not the same thing as heat.
A red-hot spark 火花 is at a much higher temperature tha
n a pot of boiling water; yet the water has a much high
er heat “content” than the spark and is more damaging.
Temperature is the condition of a body that deter
mines whether heat shall flow from it.
Unit: kelvin (K)
2.1.3 Temperature 温度
Heat flows from objects at high temperature
to objects at low temperature.
When there is no net heat transfer between t
wo objects they are at the same temperatu
re.
The human body is sensitive to temperature but it is unreliable
for measuring temperature.
a metal surface and a plastic surface at the same a metal surface and a plastic surface at the same
temperature, which one feels colder?temperature, which one feels colder?
The brain tends to judge temperature by the rate of heat flow
in or out of the skin.
A thermometers is an instrument that measures temperature
by making use of some property of a material that changes
in a regular manner with changes in temperature.
2.1.4 Thermometers 2.1.4 Thermometers 温度计
Some of the more common types of thermometers are described below:
Mercury-in-glass thermometer 玻璃水银温度计 Alcohol-in-glass thermometer 玻璃酒精温度计 Thermoelectric thermometer 热电温度计 Resistance thermometer 电阻温度计 Optical pyrometer 光学高温计
How do these thermometers measure How do these thermometers measure temperature?temperature?
Thermoelectric thermometer 热电温度计
Use the electric current generated in a thermocoupl
e, 热电偶 which is made by joining two different me
tals such as iron and constantan alloy 铁 - 康铜合金 .
The current quickly varies with temperature and ca
n be incorporated in remote or automatic control s
ystems热电偶:用于准确测量温度的热电子元件,
由两种连在一起的不同金属组成,这样连结点间产生的电压变化就是两点间温度差异的量度
Resistance thermometer 电阻温度计
Use the change in electrical resistance which
occurs when a metal changes temperature.
Pure platinum is commonly used and the cha
nges in its resistance can be measured very
accurately by including the thermometer in an
electrical circuit 电路
Optical pyrometer 光学高温计
Measure high temperature by examining th
e brightness and colour of the light emitted f
rom objects at high temperatures.
The light varies with temperature and is co
mpared with a light from a filament 灯丝 at
a known temperature.
Celsius scale 摄氏温标 The Celsius temperature scale numbers the te
mperature of the melting point as 0, and the boiling poi
nt of water as 100
CELSIUS TEMPETATURE ( ) is a point on a te
mperature scale defined by reference to the meltin
g point of ice and the boiling point of water.
Unit : degree Celsius ( ) ℃
2.1.5 Temperature scales 2.1.5 Temperature scales 温标温标This is a proper non. Notice that the first letter must be capitalized
THERMODYNAMIC TEMPERATURE (T) is a point on a
temperature scale defined by reference to absolute zero
and to the triple point of water
Unit : degree kelvin (K)
The general relationship between the two temperature sc
ales is given by the following formula.
T= +273
Thermodynamic scale Thermodynamic scale 热力学温标热力学温标
The same mass of different materials can hold
different quantities of heat.
To produce the same rise in temperature ,
water and oil, which one should be supplied
more heat ?
Water has a greater heat capacity than oil.
2.1.6 Heat capacity 2.1.6 Heat capacity 热容热容
The SPECIFIC HEAT CAPACITY (C) 比热 of a materi
al is the quantity of heat energy required to raise the te
mperature of 1 kg of that material by 1 degree kelvin ( o
r 1 degree Celsius)
Unit: J/ kg. K (or J/kg )℃ Table 1.1 gives values for a variety of materials
What does 4190 J/ kg. K mean?
Water is a good medium for storing heat.
How to use the high heat capacity of water?
The same mass of different materials may occupy different
volumes of space, depending on their densities.
Unit: kilogram per cubic metre ( kg/m3)
Heavyweight masonry materials 石材 , such as brick, co
ncrete and stone, have high densities. This means that r
elatively small volumes of these materials have a large m
ass and therefore provide a relatively high heat capacity
within in a small volume.
(V) Volume
m) ( Mass)(Density
Next conceptionNext conception2.1.7 Density 2.1.7 Density 密度密度
There are three possible states of matter
Solid state
Liquid state
Gas state
2.1.8 Change of state 2.1.8 Change of state 相态变化相态变化
The state of a substance depends on the conditions of
temperature and pressure which act on the substance.
the absorption of heat byby a solid or a liquid can produce
the following changes of state
Solid → liquid → gas
the release of heat fromfrom a gas or a liquid can produce
the following changes of state
Gas → liquid → solid
How most substance behave with heating being supplied.it’s use
ful to consider Changes of state for water
Sensible heatSensible heat is the heat energy absorbed or released from a
substance during a change in temperature.
Latent heatLatent heat is the heat energy absorbed or released from a su
bstance during a change of state, with no change in temperatur
e.
A liquid may change to a gas without heating being supplied, by
evaporation for example. The latent heat required for this chang
e is taken from the surroundings and produces an important co
oling effect.
2.1.9 Sensible and latent heat2.1.9 Sensible and latent heat 显热和潜热显热和潜热
Figure 1.1
Enthalpy can be described an the total heat conte
nt of a sample, with reference to 0 .℃
焓可以描述为相对于 0℃ 所包含的总热能。
物理意义:⑴ H=U+pV 焓 = 流动内能 + 推动功
⑵ 焓表示流动工质所具有的能量中,取决于热力状态的那部分能量
2.1.10 Enthalpy 2.1.10 Enthalpy 焓焓
For the particular example of water shown in figure
1.1 , the steam at 100 has a much higher total ℃heat content than liquid water at 100 .℃
Steam at high temperature and pressure has a very
high enthalpy, which makes it useful for transferring
large amounts of energy such as from a boiler to a
turbine.
This steam is also very dangerous if it escapes.
Both sensible and latent heat are forms of
heat energy that are measured in joules,
although they are calculated in different
ways.
2.1.11 Calculation of heat quantities2.1.11 Calculation of heat quantities 热量计算热量计算
Sensible heat:
When a substance changes temperature the amount
of sensible heat absorbed or released is given by the
following formula
Worked example 1.1 ,page 13
mcHH= quantity of sensible heat (J)
m = mass of substance (kg)
c = specific heat capacity of that substance (J/kg K)
change re temperatu
Latent heat:
During a change of state in substance the
amount of latent heat absorbed or released is
given by the following formula. mlH H= quantity of latent heat (J)
m= mass of substance (kg)
l = specific latent heat for that change of state (j/kg)
Specific latent heat (l) Specific latent heat (l) 比潜热比潜热 is a measure o
f the latent heat absorbed or released from a
particular material for a given change of stat
e.
Unit :Unit : J/kg
Specific latent heat of ice=335kJ/kg
Specific latent heat of steam=2260kJ/kg
Worked example 1.2
2.1.12 Expansion 2.1.12 Expansion 膨胀膨胀
Most substance expand on heating and contrac
t on cooling.
If the natural expansion and contraction of a body is
restricted then very large forces may occur.
Different substances expand by different amounts
The coefficient of linear thermal expansion
线性热膨胀系数 is a measure of the relative chan
ge of length.
The coefficient of linear expansion for steel is
about 12x10-6/ . ℃ What does this mean?
Concrete expands at a similar rate to steel
The expansion of aluminium is about twice that ofis about twice that of steel
The expansion of plastic is up to ten times that ofis up to ten times that of steel
Allowance must be made in construction for the
effects of expansion. Particularly for concrete, metals and
plastics.
Sometimes we can use the expansion of solid
figure 1.2 simple thermostat 简易式温度调节器
1) Solids1) Solids
2) Liquids2) Liquids
Liquids tend to expand more than solids, for the same temperature
rise.
The expansion rates of different liquids vary and the expansion of
alcohol is about 5 times that ofis about 5 times that of water.
It is useful to notice the following questions.
Does any liquid contract upon cooling?
How to use the expansion of liquids?
——thermometers
What is the bad effect of liquids expand.
3) Gases3) Gases
The expansion of gases is hundreds of times greater hundreds of times greater
thanthan the liquids .This behaviour will be described in the
later section on gases and vapours .
The concept of an absolute zero of temperatureabsolute zero of temperature 绝对零度 waswas a result of imagining the effect of cooling an
ideal gas.
Starting at 0 , this ideal gas would shrink in size by 1/273℃for each drop in temperature of 1 . At -273 the volume ℃ ℃of the gas would therefore be zero, and matter would have
Disappeared.
Now let’s move on to the second topic
2.2 Heat transfer 传热 Heat energy always tends to transfer fromtransfer from h
igh temperature toto low temperature regions.
via three different modes of heat transfer.
1 Conduction 导热2 Convection 对流3 Radiation 辐射
2.2.1 Conduction 导热Conduction is the transfer of heat energy through a
material without the molecules of the material changing their basic positions.
Conduction can occur in solids, liquids and gases although the speed at which it occurs will vary.
Different materials conduct heat at different rates
and the measurement of thermal conductivitythermal conductivity
导热率导热率 is described in the next chapter.
Metals are the best conductors of heat, because o
f the free electrons that they possess.
Poor conductors are called insulatorsPoor conductors are called insulators 隔热体隔热体 ..
Since there is no Chinese, I will ask some questions to
see if you’ve understood.
Please answer in English, using your own words
1) When conduction happens, whether the molecules of
materials change their positions.
2) Can conduction occur in gases?
3) Can you give an example of heat transfer by
conduction?
Conduction depends on molecular activity and requires the presence of matter . Example is heat transfer through wall.Heat conduction heat transfeheat transfer rater rate (Qk) through a body, as shown in Figure at steady state is governed by Fourier's Law which is expressed as
Here is a picture of heat conduction
Translate this phrase into Chinese
Heated air rises, cools, thenfalls. Air near
heater isreplaced by cooler air, and the cy
cle repeats.
Convection 对流
Convection is the transfer of heat energy through a
material by the bodily movement of particles
Convection can occur in fluids ( liquids and gases) ,but
never in solids.
Convection heat transfer is of two typesis of two types:
(a) Natural / Free Convection
(b) Forced Convection uses a mechanical pump to
achieve a faster flow of fluid.
The basic equation for convection heat transfer isThe basic equation for convection heat transfer is
Newton's Law of Cooling
The second method of heat transfer
2.2.2 Convection 对流
Stack effect 烟囱效应
Stack effect describes the natural convection tha
t occurs in buildings causing warm air to flow fr
om the lower to the upper stories
主要作用是拔火拔烟,排走烟气,改善燃烧条件。
There are some pictures showing the damage of stack effect during a fire
Radiation is the transfer of heat energy by
electromagnetic waves.
Heat radiation occurs when the thermal energy of
surface atoms in a material generates
electromagnetic waves in the infra-red range of
wave length.
The last method of heat transfer
2.2.3 Radiation 辐射
Energy radiated per second: H = eAT4
e = emissivity (0-1) = Stefan-Boltzmann constant = 5.67 x 10-8 J/(s-m2-K4)A = surface area of objectT = Kelvin temperature
Ludwig Boltzmann
(1844-1906)
The governing equationThe governing equation for radiation
is is Stenfan-Boltzmann Law
Example: How much does the human body radiate?------------------------------------------------------------------------Body temperature = 37 C = 37 +273 = 310 K, Estimate surface area A = 1.5 m2 e = 0.70
H = eA T4
= (0.70)(5.67 x 10-8)(1.5 m2)(310)4
= 550 watts (5 light bulbs)------------------------------------------------------------------------The sun provides about 1000 watts per square meter at the Earth's surface. 30 % is reflected byhuman skin. 700 watts is absorbed per square meter.
Dull black surfaces 阴暗的黑色表面 have the hig
hest absorption and emission of radiation heat.
Shiny silver surfaces 光亮的银色表面 have the lo
west absorption and emission of radiation heat.
The rate at which a body emits heat increases with
the temperature of the body.
The greenhouse effect Is one result of the differing
properties of heat radiation when it is generated by
bodies at different temperatures.
As illustrated in figure
2.2.4 The greenhouse effect 温室效应温室效应
玻璃对不同波长的辐射有选择性玻璃对不同波长的辐射有选择性太阳短波辐射进如玻璃,太阳短波辐射进如玻璃,室内吸收后温度升高发出长波辐射透室内吸收后温度升高发出长波辐射透不过玻璃不过玻璃
That atmosphere surrounding the Earth also behaves
as a large “green house” around our world.
This particular green house effect therefore contribute
s to global warming.
After 2.2 heat transfer, we will talk about
2.3 Gases and vapours
2.3.1 Gases
Kinetic Theory:Kinetic Theory: 动力学理论动力学理论 the molecules of a
gas are always in motion and their velocity incre
ase with temperature.
The collisions of many molecules acting on a parti
cular area will be detected as pressure.
(A) Area
F) ( Force)( Pressure p
unit: pascal (Pa)
Is it a base unit in SI system?Is it a base unit in SI system?
By definition: 1 pascal = 1newton/ metre2 (1N/m2)
Other units which may be found in use include :
millibars (mb) , where 1 mb = 101 Pa
mm of mercury , where 1mm = 133Pa
Heating a gas increases the velocity and the
kinetic energy of the molecules.
The gas laws are an expression of the
relationship between the temperature, volume
and pressure of a constant mass of gas.
2.3.2 Gas laws 气体定律
Boyle’s Law 波义耳定律
Charle’s Law 查理定律
Pressure Law 压力定律
General Gas Law 气体普遍定律 tons tancT
pV
2
22
1
11
T
Vp
T
Vp
tonspV tanc
Ttons tancV
const/ tm
const/ pm
const/ VmTtons tancp
Dalton’s Law of Partial Pressures 道尔顿分压定律
Where there is a mixture of different gases each gas
exerts an individual partial pressure and has the
following features.
The partial pressure exerted by each gas is
independent of the pressure of the other gases
The total pressure of the mixture equals the sum of
the partial pressures.
Standard Temperature and pressure, STP 标准温度和压力
Standard temperature =0℃=273K
Standard pressure = 101.3kPa
= 760 mm of mercury
Worked example 1.3
A vapour is a material in a special form of the gas state
and has some different properties to those of a gas.
For example, when a vapour is compressed the press
ure increased until , at a certain point , the vapour con
dense to a liquid.
A vapour is a material in the gas state which can be liq
uefied by compression, without change in temperature.
2.3.3 Vapours 水蒸气
The critical temperature 临界温度 of a
substance is the temperature above whic
h a vapour is not able to exit.
Table 1.2 gives the critical temperatures
of some substances relevant to heating
and refrigeration
Air is a mixture of gases and has the following percent
age composition when it is clean and dry.
Nitrogen(N2) 78% Oxygen (O2) 21%
Carbon dioxide (CO2) 1% Other gases
The atmosphere is the collection of gases that surrou
nds the surface of the earth.
The absolute unit of pressure is the pascal but it is als
o convenient to refer to pressure in terms of height , s
uch as mm of mercury.
2.3.4 The atmosphere
Before next topic, we’d better to distinguish some conceptions.
Refrigerators & Heat Pump制冷机和热泵
Refrigeration: 制冷 The transfer of heat from lower
temperature regions to higher temperature is called
refrigeration.
Refrigerator: 制冷机 Devices that produces refrigeration are
called refrigerators.
Refrigerant: 制冷剂 The working fluid used in refrigerators
are called refrigerant.
Heat Pump: 热泵 Refrigerator used for the purpose of heating
a space by transferring heat from a cooler medium are called
heat pump.
2.4 Refrigerators and heat pumps 制冷机和热泵
A refrigeration cycle causes heat energy to be transferred from a c
ooler region to a warmer region. This is against the natural dire
ction of heat flow and can only be achieved by supplying energ
y to the cycle
2.4.1 The refrigeration cycle 制冷循环Can be divided into:
compression refrigerator and heat pump cyclecompression refrigerator and heat pump cycle
压缩式制冷机和热泵循环压缩式制冷机和热泵循环absorption refrigeration cycleabsorption refrigeration cycle
吸收式制冷循环吸收式制冷循环
compression refrigerator and heat pucompression refrigerator and heat pump cyclemp cycle
压缩式制冷机和热泵循环压缩式制冷机和热泵循环
the most important equipment in compression refrigerator cycle is the compressor
压缩机
Typical pressures and temperatures in a vapor-compression refrigerator.
Four main components of a kitchen refrigerator.
The operating principles of the absorption refrigeration cycle are shown as follows
Not every gas behaves in a way that is suitable for the cycle of vapour-liquid-vapour changes used in refrigeration.
A refrigerant must be below its critical temperature, or it will not liquefy, and it must be above its melting point, or else it will solidify.
in general a useful refrigerant should possess the following properties:
Low boiling point 低沸点 High latent heat of vaporisation 高的气化潜热 Easy liquefaction by compression 压缩后容易液化 Stable, non-toxic and non-corrosive 稳定、无毒无腐蚀 Environmental safety 对环境无害
2.4.2 Refrigerants : 制冷剂
Chlorofluorocarbon (CFC) is a mixture of organic
compounds contain carbon, chlorine, and fluorin
e which has been available in various commerci
al formulations such as freon 氟利昂 .
It is now accepted that CFCs help cause a reduc
tion of the ozone layer 臭氧层 in the upper atm
osphere of the earth and contribute to global war
ming of the World’s weather systems.
CFCs ) 氟氯碳化物类
CFC replacements 氟利昂替代物
Ammonia 氨 is an efficient refrigerant
but its toxicity and flammability require
s special safety measures.
Carbon dioxide 二氧化碳 is used in
some large refrigeration plants and re
quires high operating pressures.
Compression refrigeration cycle
压缩式制冷循环 Absorption refrigeration cycle
吸收式制冷循环
2.4.3 refrigerators 制冷机
A heat pump is a device which extracts heat from a
low temperature source and upgrades it to a higher
temperature.
For a heat pump, The condenser coils
are used to heat air or other medium
2.4.4 Heat pumps 热泵
The coefficient of performance (COPH) of a he
at pump 热泵的性能系数 is the ratio of heat ou
tput to the energy needed to operate the pum
p
21
1H inputenergy Pump
output energy Heat COP
TT
T
Heat pump efficiency
Heat sources for heat pumps Air Earth Water
A geothermal heat pump system.
useful sentence patterns in chapter 1
XX is the standard SI unit of YY 焦耳是国际单位制中能量的单位 开尔文是国际单位制中温度的单位
Table XX gives YY values for a variety of materials
表( 1.1 )给出了不同材料的(比容)值
xx will be discussed in later chapter.
这些问题在以后的章节中会讲到。 物质 / 体: matter material substance
They have the basic characteristics given belo
w.
它们具有以下特性
XX is given by the following formula.
( 潜热)可用下式计算 ( 显热)可用下式计算XX is about YY times that of ZZ
铝的膨胀系数是钢的 2 倍 塑料的膨胀系数是钢的 10 倍 酒精的膨胀系数是水的 5 倍
Expand on heating and contract on cooling
受热膨胀冷却收缩For each degree of temperature rise
温度每降低 1℃
For a fixed mass of gas at xx 一定温度下的定质量气体 一定压力下的定质量气体 一定体积下的定质量气体
xx is inversely proportional to yy
体积与压力成反比xx is directly proportional to yy
体积与热力学温度成正比 压力与热力学温度成正比
Temperature must always be in xx
温度应采用开氏温度 温度可采用摄氏温度
The sum of xx
各分压力之和Have already been described
已经讲过的xx is ( are ) shown schematically in figure yy
压缩循环的工作原理如图 1.5 所示
xx is the ratio of yy to zz
热泵的性能系数是(输出的热量)与(热泵运转所需能量)的比值
is of two typesis of two types:
对流有两种类型
Exercises in class
1) The following statements are related to the “Greenhouse effect”, which one is correct ( )
A short wavelength can pass through the atmosphere
B short wavelength can pass through the glass
C longer wavelength can pass through the atmosphere
D longer wavelength can pass through the glass
2) Among the following statements , which one is correct? ( )
A Arefrigeration cycle can make cooling effect only
B A refrigeration cycle can make heating effect only
C A refrigeration cycle can make the cooling effect and heat effect at the same time
D The principle of refrigerator is the same as that of heat pump
3) In compression refrigeration cycle, which component can make cooling effect ( )
A compressor
B evaporator
C expansion valve
D condenser
4) In compression refrigeration cycle, which component can make heating effect ( )
A compressor
B evaporator
C expansion valve
D condenser
5)the extraction coil (evaporator) of heat pump can absorb heat from ( )
A air
B earth
C water
D exhaust heat