Chapter 6. The property of Water and Vapor ( 第 6 章 . 水和水蒸气的性质 )

6.1 Basic conceptions ( 基本概念 )6.2 Phase change Process of Pure Substance ( 纯质的相变过程 )6.3 Property Tables ( 水蒸气性质表 ) 6.4 h-s diagrams for Water Vapor

( 水蒸气的 h-s 图 )6.5 Thermodynamic Processes of Water Vapor

( 水蒸气的热力过程 )

水和水蒸气是实际气体的代表

水蒸气在空气中含量极小，当作理想气体

一般情况下，为实际气体，使用图表18 世纪，蒸气机的发明，是唯一工质直到内燃机发明，才有燃气工质

目前仍是火力发电、核电、供暖、化工的工质 优点 : 便宜，易得，无毒， 膨胀性能好，传热性能好

是其它实际气体的代表

§ 6-1 纯物质的热力学面及相图

物质三种聚集状态：固态、液态、 气态水的三态： 冰、 水、 蒸汽

Pure substance

Solid Liquid Gas

Ice Water Steam

任何一种物质都可以经历下列所述的相变 . Any kind of substance may undergo various type

s of phase changes as following : 溶解 (Fusion/melting) = solid to liquid

凝固 (Freezing) = liquid to solid

气化 (Vaporization)= liquid to gas

凝结 (Condensation)= gas to liquid

升华 (Sublimation) = solid to gas

Phase changes ( 相变 ):

The sublimation line separates the solid and vapor regions;

The vaporization line separates the liquid and vapor regions; it ends at the critical point.

The melting line separates the solid and liquid regions;

The three lines meet at the triple point.

Triple point( 三相点）： all three phases coexist in equilibrium. ( 三种相态以平衡共存 )

Critical point ( 临界点 ): The point at which the saturated liquid and saturated vapor states are identical.

No distinction can be made between liquid and vapor phases above the critical point.

( 临界点就是气相与液相一致的状态点 , 温度高于临界点之上 , 气相与液相没有明显的区别 )

思考题

3. 有没有500ºC的水？

1. 溜冰冰刀

2. 北方冬天晾在外边的衣服，是否经过液相

4. 有没有-3 ℃ 的蒸汽？

5. 一密闭容器内有水的汽液混合物，对其 加热，是否一定能变成蒸汽？

饱和线、三相线和临界点

p

v

饱和气线

三相线

饱和液线

饱和固线T

临界点

汽相和液相

锅炉

汽轮机

发电机

给水泵

凝汽器

过热器 因固相不流动，更关心汽液两相

冰蓄冷

§6-2 汽化与饱和

气化 (Vaporization): 由液态变成气态的物理过程 ( 不涉及化学变化 )The Process ofThe Process of Changing from liquid into gas/vapor is Changing from liquid into gas/vapor is calledcalled vaporizationvaporization..

凝结 (Condensation) ： 由气态变成液态的物理过程 The process of changing from gas/vapor into liquid is cThe process of changing from gas/vapor into liquid is c

alledalled CondensationCondensation

6.2.1 汽化与凝结

The rate of condensation depends upon the density of vapor in the space above the liquid.

( 凝结的速率取决于液面上方气相空间气体分子的密度 , 即蒸气压力 )

Liquid molecule has to overcome the surface tension to become vapor, therefore vaporization consumes energy. ( 液相分子必须克服表面张力才能进入气相空间而气化 , 因此气化是要耗能的 )

The rate of Vaporization depends on the liquid temperature.( 气化的速率取决于液相的温度 )

)()( vc pfgw

)(1 Tgwv

凝结）(on condensati cv ww

气化）(zation vaporicv ww

平衡状态）( mequilibriu cv ww

The equilibrium is called Saturation State.( 饱和状态）

The properties at this state is called Saturation Properties. ( 饱和参数）

sT sp

6.2.2 饱和状态 Saturation state饱和状态：汽化与凝结的动态平衡At the equilibrium state, the pressure corresponding to only one temperature. That is, .

饱和温度 Ts

饱和压力 ps

一一对应

放掉一些水， Ts 不变， ps ？Ts

ps

Saturation temperature

Saturation pressure)(Tfp

ps=1.01325bar Ts=100 ℃

青藏 ps=0.6bar Ts=85.95 ℃

高压锅 ps=1.6bar Ts=113.32 ℃

There is a definite relation between There is a definite relation between .. )( ss Tfp

6.2.3 汽化 Vaporization

沸腾：表面和液体内部同时发生的汽化 It is an intensive vaporization phenomenon occurs in the liquid.

( 气体和液体均处在饱和状态下 )

汽化 : 由液态变成气态的物理过程 ( 不涉及化学变化 )

蒸发：汽液表面上的汽化 It It occurs above the free surface of liquidoccurs above the free surface of liquid

Boil

Evaporation

Evaporation occurs at any temperature and pressure.（蒸发可在任何温度和压力下发生．）

Evaporation rate depends on the free surface area, the temperature, the flow rate above,　 etc.

（蒸发的速度与自由液面表面积，液体温度，液面风速等有关．

Boiling can only occur as temperature reaches the saturation temperature corresponding to the specified pressure or the pressure drops below the saturation pressure corresponding to the specified temperature.

（沸腾只能当温度达到给定压力所对应的饱和温度或压力降到给定温度所对应的饱和压力时 , 才能发生 )

§6-3 水蒸气的定压发生过程

t < ts t = ts t = tst = ts t > ts

v < v’ v = v’ v = v’’v ’< v <v’’ v > v’’

未饱和水 饱和水 饱和湿蒸汽 饱和干蒸汽 过热蒸汽

h < h’ h = h’ h = h’’h ’< h <h’’ h > h’’s < s’ s = s’ s = s’’s ’< s <s’’ s > s’’

水预热 汽化 过热

6.3.1 P=constant ( 压力 p 是定值 )

Preheat Stage (预热阶段 )Compressed liquid to Saturated liquid.(未饱和液体 饱和液体 )Compressed liquid/subcooled liquid is not about to vap

orization.

As heat added, t slightly . When It becomes Saturated liquid. The liquid is about to vaporize. It is the state at which

it is still a liquid, but any heat addition will cause some of the liquid to vaporize.

)( pftt s

v

vstt

st t t 过冷

Vaporization Stage ( 气化阶段 )Saturated liquid to Saturated vapor ( 饱和液体 饱和蒸气 )

Saturated vapor is a vapor about to condense.

A substance at state between Saturatedliquid and Saturated Vapor is calledSaturated liquid-vapor mixture/wetvapor. Latent heat of Vaporization ( 气化潜热 ):The amount of energy

absorbed during vaporization

v

vstt

Superheat Stage ( 过热阶段 )Superheated vapor is the vapor that is not about to condense.

stt vt

st t t 过热

注意：水蒸气定压发生过程说明

(1)( )

Q U W U pdV

U p V U pV H

(2) f g 0S S S

(3) 理想气体 ( )h f T

实际气体汽化时， T＝ Ts 不变，但 h增加'' 'h h 汽化潜热

(4) 未饱和水 过冷度 st t t 过冷 过冷水过热蒸汽 过热度

st t t 过热

只有熵加热时永远增加

State of Liquid and vapor

未饱和液，过冷液

饱和液

饱和湿蒸气

饱和蒸气

过热蒸气

Saturated liquid

Saturated vapor

Saturated liquid-vapor mixture

Superheated vapor

Compressed liquid

Subcooled liquid

汽化潜热 Latent heat of Vaporization

压缩液

6.3.2 定压气化过程的 P-v 图和 T-s 图

P-v and T-s diagrams

cp

cT

cT

AB

C

A

C

B

cT T

cTT cpp

cpp

3a

3b 3d

3e

3c3a 3b 3d 3e3c

2c2a 2b 2d 2e2c

2a

2b2d

2e

1c1a 1b 1d 1e 1c

1a1b

1d

1e

T

s

p-v 图， T-s 图上的水蒸气定压加热过程

p

v

cp

一点，二线，三区，五态

Saturated liquid line, SLL is formed by connecting a series of boiling points.

saturated vapor line, SVL ： Connecting a series of points at dry saturated vapor builds a line, known as saturated vapor line, SVL.

Vaporization continues by further heat supply to the system until no liquid is left. This state is known as dry saturated vapor, e.g. point d. If the system is slightly cooled at this state, then droplets of liquid will begin to form.

The state of substance between saturated liquid and dry vapor is c

alled wet vapor.

Further heating of a dry saturated vapor at constant pressure causes a rise of vapor temperature and it becomes superheated vapor.

The state of substance is completely defined by its pressure and temperature if it is in liquid or superheated vapor phase.

v

One point ( 一点 ):critical point ( 临界点 )

Two lines ( 两线 ):SLL and SVL ( 饱和液体线和饱和蒸气线 )

Three regions( 三区 ): Subcooled liquid region (未饱和液体区 ) Saturated liquid-vapor mixture

region(湿蒸气区 ) Superheated vapor region ( 过热蒸气区 )

Five states(五态 ): Subcooled liquid(未饱和液体 ) Saturated liquid ( 饱和液体 ) Saturated liquid-vapor mixture (湿蒸气 ) Saturated vapor (干饱和蒸气 ) Superheated vapor ( 过热蒸气 )

6.4.1 Types of Property Tables ( 水蒸汽表的分类 ) 1. Saturated water Tables ( 饱和水和饱和蒸汽表 )

Saturated water---T Tables( 以温度 t 为独立变量排列 ) Saturated water---p Tables( 以压力 p 为独立变量排列 )

2. Subcooled water and Superheated vapor Table (未饱和水和过热水蒸汽表 )

P and t are dependent variables ( 以 p 和 t 为自变量 )

§6.4 §6.4 水蒸气性质表 水蒸气性质表 Property Tables of Water VaporProperty Tables of Water Vapor

2 、 Reference State and Reference Values ( 基准点的规定 )

For water, the saturated liquid at 0.01℃ is taken as reference state.

( 取水的三相点，即 0.01 oC 为基准点，规定在此温度下液态水的热力学能和熵为零。 )

The internal energy and entropy are assigned zero at this state.

kgkJu / 00 )/( 00 KkgkJs

3. Properties of Saturated liquid-vapor mixture at Pressure p

( 压力为 p 的湿饱和蒸汽 )

The state of wet vapor can not be defined by just pressure and temperature until one other property is given. The condition or quality of wet vapor is often defined by its dryness or wetness fraction.

定义干度 x 才可以确定湿蒸汽的状态。 We define dryness as

dryness fraction, x = the mass of dry vapor in 1 kg of the mixture,

wetness fraction, 1 - x = the mass of liquid in 1 kg of the mixture.

m

mx v

vxvxvx )1(

hxhxhx )1(

sxsxsx )1(

vxvxvx )1(

hxhxhx )1(

sxsxsx )1(

4. Exercise. Consulting property tables of water vapor to determine the state of each point and their h ， s ， x.(利用水蒸气表判断下列各点的状态，并确定其 h ， s ， x的值。 )

（ 1 ）由饱和水和饱和蒸汽表查得p=2MPa 时，

显然 ，可知该状态为过热蒸汽。 查未饱和水过热蒸汽表，得 p=2MPa ， t=3

00℃ 时

对于过热蒸汽，干度对于过热蒸汽，干度 xx 无意义。无意义。

（ 2 ） 查饱和表得 p=9MPa时

可见 ， 该状态为湿蒸汽，其干度为

又查饱和表得

=2489.0kJ/kg

按湿蒸汽的参数计算式得

                                                                                                                               

( 3 ) 显然，该状态为湿蒸汽状态。由已知参数查饱和水和饱和蒸汽表得

按湿蒸汽的参数计算公式得

（ 4 ）由饱和水和饱和蒸汽表查得 : 当 p=1.0MPa时， ts=179.9℃，显然 t<ts ，所以该状态为未饱和水。通常 t=175℃的状态参数可利用 t=170℃与 t=180℃的对应状态参数内插得到，但此处 t=170℃与 t=180℃跨越了未饱和表中的黑粗线，说明它们分别处于不同相区。应使内插在未饱和水区内进行，选取离最接近的 t=170℃与ts=179.9℃的未饱和水参数内插。查未饱和水和过热蒸汽表得

p=1.0MPa ， t=170℃时 p=1.0MPa ， t=179.9℃时

于是 t=175℃时

                                                                                                                                  

对于未饱和水干度 x 无意义。

（ 5 ） 时，饱和蒸汽比体积 ， 该状态为过热蒸汽。查过热蒸汽表得 t=260℃时， 　　　　　 , t=270℃时， 　 　 该状态的温度可由比体积值求得

讨论应该注意，在利用未饱和水与过热蒸汽作内插时，不允许跨越表中的粗折线，如遇这种情况，应选用更详细的表，或使内插计算在未饱和水（或过热蒸汽）区内进行。

§6.5 h-s diagram of water vapor ( 水蒸汽的 h-s 图 )

1. SLL and SVL ( 界限曲线， x=0、 x=1 两条曲线 )2. Constant Pressure Line and Constant Temperature Line ( 定压线和定温线 )In Saturated liquid-vapor region, constant pressure line and constant temperature line are identical. ( 在湿蒸汽区定压线与定温线重合。 )3. Constant Volume Line（定容线。 )4. Constant Dryness Line ( 定干度线。 ) Given Two Properties, all the other state properties can be determined from the Diagram.( 给定两个参数，即可查出其余全部参数 )

。 

Priorities ( 优点 ) ：Fast and convenient. 方便、快速。

Drawbacks (缺点 ) ：Not so accurate as Property tables 精确性稍差

。 

s (kJ/kg*K)

h (k

J/kg)

6.6.1 Characteristics of water vapor ( 水蒸汽的特点 )

1. Water vapor can not be treated as ideal gas ( 水蒸汽不是理想气体 )

pv=RT is not suitable. （理想气体状态方程不适用。）2. Analysis and Calculation on Water vapor can be solved by usi

ng Tables and Diagrams ( 水蒸汽的分析和计算采用水蒸汽图和表。 )

3. The First and the Second Laws must be obeyed during processes of water vapor ( 热力学第一定律和第二定律成立 )

§6.6 Thermodynamic Processes of Water Vapor

( 水蒸气的热力过程 )

wuq pvuh

For reversible processes, ( 对于可逆过程： )

2

1

pdvw 2

1

Tdsq

wzgchq 2

2

1

6.6.2 Procedures (步骤 )(1) Based on the given conditions, determine the initial state and its properties .

(根据已知条件，确定初始状态，查出其余参数。 )

(2) Based on the characteristics of the process and one of the properties of the final state, determine the final state and its properties.

( 根据过程特点和一个终态参数，确定终态，再查出其余终态参数。 )

(3) Based on the initial and final state, calculate the q 、 Δu 、 w during the process。

(根据初、终态参数，计算 q 、 Δu 、 w等。 )

0w)( 121212 ppvhhuuuq

6.6.3 Basic thermodynamic process of water vapor ( 水蒸气的基本热力过程 )(1) Constant Volume Process ( 定容过程 , v= 定值。 )

0wConstant volume Process ( 定容过程 )

)( 121212 ppvhhuuu

)( 12 vvpw

(2) Constant Pressure Process ( 定压过程 p= 定值 )

12 hhhq

Constant Pressure Process ( 定压过程 )

)( 12 ssTq

(3) Constant Temperature Process ( 定温过程 T= 定值 )

Isothermal Process ( 定温过程 )

uqw 0q uw

(4) Adiabatic Process ( 绝热过程 )

12 hhwt

Isentropic Process ( 定熵过程 )

Example: 1 kg water vapor, initially, ℃ undertakes an isentropic ex

pansion process and reaches the final state of Calculate the amount of work done by the water vapor during the process. Answer: According to the values of , the initial state 1 can be

determined. From water tables or h-s Diagram, we know that then

300,1 11 tMPap

MPap 1.02

11 , tp

kgmvkgkJh /26.0,/3050 311

kgkJvphu /279010*26.0*103050 36111

From point “1”, draw a vertical line, which will meet with the line of

at point “2”, the final state is obtained. Then from h-s diagram, we get to know that

300,1 11 tMPap

MPap 1.02

11 , tp

kgmvkgkJh /26.0,/3050 311

kgkJvphu /279010*26.0*103050 36111

MPap 1.02

kgmvkgkJh /6.1,/2600 322

kgkJvphu /244010*6.1*102600 352222

kgkJuuuw /3502440279021