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