1、Chapter 7. Moist Air (第章 湿空气）,7.1 Introduction (介绍)7.2 Properties of Moist Air （湿空气的参数）7.3 H-d Diagram of Moist Air （湿空气的H-d 图)7.4 Thermodynamic Process of Moist Air （湿空气的热力过程）,湿空气（干空气水蒸气）,空调、通风、烘干、冷却塔、储存,湿空气与一般理想混合气体的最大区别是：水蒸气的成分可能变化。,分压低,理想混合气体,Dry air,Atmospheric air,7.1 Introduction (介绍),Definit
2、ion of Atmospheric Air（湿空气的定义） Dry air + Water vapor (Moisture)=Atmospheric Air (含有水蒸汽的空气称为湿空气。) Air that contains no water vapor is called dry air. (完全不含水蒸汽的空气称为干空气。) 2. Characteristics of Moisture Air (湿空气的特点)Atmospheric air contains a very small amount of water vapor. The partial pressure of wate
3、r vapor is very low so that water vapor in the atmospheric air is always at superheated state. Thus, water vapor can be treated as ideal gas.(湿空气中水蒸汽含量很小，分压力很低，常处于过热状态，可以当作理想气体来处理。),Atmospheric air can be treated as a binary mixture (二元混合物） of ideal gases, dry air and moisture,Rda = 8314.41/28.9645
4、= 287.055 J/(kgK),Rw = 8314.41/18.01528 = 461.520 J/(kgK),Dry air: pdaV= mdaRdaT,Water vapor: pwV= mdaRwT,饱和蒸汽,(1)未饱和湿空气(Unsaturated Air): Dry air+ Superheated water vapor,过热蒸汽,水蒸气,干空气 过热水蒸气,pv ps(T),3. Saturated Air and Unsaturated Air (未饱和空气和饱和空气),未饱和湿空气,(2)饱和湿空气(Saturated air),pv ps(T),T,s,ps,温度一
5、定，不能再加入水蒸气,Saturated Air: Dry air+Saturated water vapor (饱和湿空气：干空气和饱和水蒸汽的混合物。),饱和湿空气,(3)从未饱和到饱和的途径,T,s,pv,ps,a,b,d,c,e,结露,Td 露点温度,7.2 湿空气的参数 Properties of Moist Air,The barometric pressure of atmospheric air vary considerably with altitude as well as with local geographic and weather conditions.,1.
6、Pressure of atmospheric air,The partial pressure of water vapor is also called vapor pressure. 2.干球温度 (Dry-bulb temperature)It is the temperature of moist air. The dry-bulb temperature of water vapor and dry air is the same. It is denoted as t. 3.露点温度(Dew-point Temperature)It is defined as the tempe
7、rature at which condensation begins when the air is cooled at constant pressure.In other words, it is the saturation temperature of water corresponding to the vapor pressure.,结露:A点表示未饱和湿空气中水蒸汽的状态，若保持空气中水蒸汽的含量（pv）不变，降低温度，使之达到pv对应的饱和温度ts，则湿空气成为饱和湿空气，为点B。若继续降温，水分析出，称为结露。露点:B点所对应的pv下的饱和温度成为露点，用td表示。,4.相
8、对湿度 (Relative Humidity of Air)定义：湿空气中实际包含的水蒸汽量与同温度下最多能包含的水蒸汽量的百分比。 The relative humidity is defined as the ratio of the amount of moisture in the air to the maximum amount of moisture the air can hold at the same temperature. If we use “a”、“v”、“s” denoting dry air, water vapor and saturated water va
9、por respectively, then (若用下标“a”、“v”、“s”表示干空气、水蒸汽和饱和水蒸汽的参数，则有：), 1 饱和湿空气 ( Saturated Moist Air),在相同的温度下：, 0 干空气 (dry air),0 1 未饱和湿空气,表明湿空气与同温下饱和湿空气的偏离程度,The lower the value of , the more water vapor it can absorb, and the drier it is. (越小，空气吸湿的能力越强，越干燥。)The higher the value of , the less water vapor
10、it can absorb, and the nearer it approaches saturated moist air.(越大，空气吸湿的能力越弱，越潮湿。),反映所含水蒸气的饱和程度,5.湿空气的含湿量(Humidity ratio)The mass of water vapor present in a unit mass of dry air is called humidity Ratio.含湿量d：以干空气为基准，包含1千克干空气的湿空气中中所含有的水蒸汽的质量。, is the ratio of the air humidity ratio d to the humidit
11、y ratio ds of saturated moist air at the same temperature and pressure(饱和度是湿空气的含湿量与同温度下的饱和湿空气的含湿量之比）,6. Degree of saturation (饱和度）,The density of a moist air mixture is the ratio of the total mass to the total volume:,. Density (密度),The density of moist air is always less than that of dry air. （湿空气的
12、密度总比干空气的密度小） And the higher the temperature, the less the density. （而且温度越高，密度越小）,.湿空气的焓(Enthalpy of Atmospheric Air )The enthalpy of atmospheric air is expressed per unit mass of dry air.含有1kg干空气的湿空气的焓 It is the sum of the enthalpies of dry air andwater vapor.,The atmospheric air is at . （）If is rel
13、ative humidity is equal to 60%, then calculate such properties as（）If , then calculate such properties asDiscussion: To determine the state of moist air, it needs three state properties at least.(为了确定湿空气的状态，至少需要个状态参数）,Example:,热力学湿球温度 (Thermodynamic wet-bulb temperature t* )t* is the temperature at
14、which water (liquid or solid), by evaporating into moist air at a given dry-bulb temperature t and humidity ratio d, can bring air to saturation adiabatically at the same temperature t* while the total pressure p is maintained constant.,湿球温度Wet-bulb Temp.,(2) 干、湿球温度法,图5-9 干湿球温度计,t = tw= ts(pv),球面上,蒸
15、发热=对流热,tw绝热饱和温度T2,干球温度,湿球温度与露点温度,图5-9 干湿球温度计,t = tw= td,tw,td,t,t tw td,Dry-bulb, Wet-bulb and Dew Point temperature,7.3 湿空气的焓湿图 The Psychrometric Chart (h-d图)Saturation Line (饱和湿空气线） In addition, there are other lines(还有下列曲线：） 1. Lines of constant enthalpy (定焓线，与y轴成45oC角向下) 2. Lines of constant hum
16、idity ratio (定含湿量线，垂直直线) 3.Lines of constant temperature (定温线) 4. Lines of constant volume ( 定容线) 5. Lines of constant relative humidity line( 定相对湿度线。) 6. Lines of constant pv和d. (水蒸汽分压力pv和d关系曲线 ),1.焓湿图的结构,d=0 干空气,d,h,135度,1.焓湿图的结构,d,h,正斜率的直线,等干球温度线,1.焓湿图的结构,d,h,(4) 线,ts=99.63oC,1.焓湿图的结构,d,h,1.焓湿图的结
17、构,d,h,(6)露点 td,pv下饱和湿空气,td,1.焓湿图的结构,d,h,(7) 湿球温度tw,绝热饱和温度,td,tw,t,(8) 等热湿比线 Process Line for Air Conditioned Space,The room process line, known also as room ratio line (RRL) is defined by the condition of supplied air and the desired air condition in the space.The RRL is dependent on both sensible a
18、nd latent heat load.,1.焓湿图的结构,d,h,热湿比,h-d图上为直线,:, :, :, :,角系数：过程方向与特征,d,h,1,2,已知初态1,过程斜率已知 可确定终态,tw,td,不同的B 不同的h-d图,7.4 湿空气的热力过程 （Thermodynamic Process of Moist Air ） 1. Sensible Heating Process (单纯加热过程),Sensible Heating Process is a process during which the dry-bulb temperature of air is increased.
19、 The process occurs at constant moisture content. The air passes over a hot and dry surface which might be pipe coil using steam or hot water, electrical resistance or an air-to-air heat recovery unit. The load on the heater is:,B,A,加热过程,2. Sensible Cooling Process at Constant Moisture Content (单纯冷却
20、过程),Sensible cooling process at constant moisture content is a process during which the dry-bulb temperature of air is decreased.,If the temperature of the cooling medium is still above its dew point temperature, then (若冷却介质温度没有降到露点以下，则过程前后)：,A,B,冷却过程,The air passes over a cooling coil which uses ch
21、illed water or direct expansion of some refrigerant into the pipe coil.,If the air is continuously cooled to a temperature which below its dew-point temperature, then water vapor will condense. (温度下降到露点以后，继续降温，析出水分，减小含湿量。),C,A,B,3. Sensible Cooling with Dehumidification (冷却去湿),4. Humidification Proc
22、ess (加湿过程),由于水的焓值很小，可以忽略不计，所以可以看作加湿之后焓不变，在h-d图上是沿着定焓线向d增大的方向进行（过程1-2）。,(1) Adiabatic Humidification Process (绝热加湿过程),(2) Isothermal Humidification Process (等温加湿),5. Mixing of Two Air Streams (混合过程),例题(Example),d,h,1,2,已知空气的t1=20oC，p1=0.1MPa 将其加热至t2=50oC，后送入干燥室，从干燥室排出时t3=30oC，求：1） 2）从干燥室每吸收1kg水分所需空气量和加热量,h2,h1,3,Solution:,d,h,1,2,h2,h1,3,根据t1=20oC，p1=0.1MPa，,根据d2=d1, t2=50oC,根据h3=h2, t3=30oC,d,h,1,2,h2,h1,3,含1kg干空气的湿空气所吸收水分,每吸收1kg水分所需干空气,每吸收1kg水分所需湿空气,每吸收1kg水分所需加热量,
