制冷技术(英文版)Ch9-090604.doc

Chapter 9. Psychrometry and Air Processes9-1) Composition and thermal properties of moist air (1) Atmospheric airAtmospheric air is not completely dry but a mixture of dry air and water vapor. In atmospheric air, the content water vapor varies from 0 to 3% by mass. The processes of air-conditioning and food refrigeration often involve removing water from the air (dehumidifying), and adding water to the air (humidifying).Psychrometry is the science of studying the thermodynamic properties of moist air and the use of these properties to analyze conditions and processes involving moist air.(2) Thermal parameters of dry air （干空气的热性质）由于压力已经确定，干空气的热物理性质就只是温度的单值函数Tab. 9-1 Thermal properties of dry air at atmospheric pressure (1.0133)TemperatureDensitySpecific volumeSpecific heat01.2940.7731.002101.2480.8011.002201.2050.8301.003301.1660.8581.003401.1290.8861.005501.0940.9141.006 For dry air at a fixed pressure (e.g., at atmosphere pressure, 1.0133), only one parameter, usually the temperature, is need to characterize a state. Table 9-1 lists some thermal properties of dry air at atmosphere pressure.(3) The thermal parameters of moist air由于压力已经确定，湿空气的热物理性质要由2个的参数决定The moist air at a fixed pressure requires two independent parameters to chracterize its state. Besides the temperature of air (it is also called dry bulb temperature), there must have another independent parameter to indicate the water content in the mixture. It may be one of the following parameters: the humidity ratio, the relative humidity, the wet bulb temperature, or the dew point temperatur and others.杯子中水的温度a) Dry bulb temperature : Dry bulb temperature is the temperature of the air, as measured by an ordinary thermometer. The temperature of water vapor is the same as that of the dry air in moist air. Such a thermometer is called a dry-bulb thermometer in psychrometry, because its bulb is dry. b) Wet bulb temperature: Wet bulb temperature is thermodynamic adiabatic temperatire in an adiabatic saturation process, and measured by a wet bulb thermometer. c) Dew point temperature :When the unsaturated moist air is cooled at constant vapor pressure or at constant humidity ratio, to a temperature, the moist air becomes saturated and the condensation of moisture starts, this temperature is called dew point temperature of the moist air.The relation of dry-bulb, wet-bulb, and dew point temperatures: 1. When the moist air is not saturated, the dewpoint temperature is lower than the dry-bulb temperature, and the wet-bulb temperature lies between them. That is .2. As the moisture in the air increases, the differences between the temperatures become smaller. 3. When the air is saturated, all three temperatures are the same. That is .d) Relative humidity: Relative humidity is defined as the ratio of the mole fraction of the water vapor in a given moist air to the mole fraction of water vapor in a saturated moist air at the same temperature and the same atmospheric pressure. Relative humidity is usually expressed in percentage (%). From the ideal gas relations, relative humidity can be expressed as (9-1)e) Degree of Saturation: Degree of saturation is defined as the ratio of the humidity ratio of moist air to the humidity ratio of saturated moist air at the same temperature and atmosheric pressure. (9-2)The difference between relative humidity and degree of saturation is usually less than 2%.f) Humidity ratio (Moisture Content): The humidity ratio is the massof water vapor interspersed in each of dry air. It should be noted that the mass of water refers only to the moisture in actual vapor state, and not to any moisture in the liquid state, such as dew, frost, fog or rain. The humididy ratio, like other several properties to be studied- enthalpy and specific volume-is based on 1of dry air. (9-3)湿空气的热物理性质是按照理想气体计算的In most applications of air conditioning and food refrigeration, both dry air and water vapor in moist air may be assumed as perfect gases.Dry air is assumed to be a perfect gas because its temperature is quite high relative to its saturation temperature, and water vapor is assumed to be a perfect gas because its pressur is quite low relative to its saturation pressure.1 湿空气的热物理性质是按照理想气体计算的 and (9-4) (9-5)g) Specific Volume/Moist Volume: Specific volume of moist air , is defined as the total volume of the moist air (dry air and water vapor mixture) per of dry air. , (9-8)h) Specific Enthalpy: Specific enthalpy of moist air () is defined as the total enthalpy of the dry air and water vapor mixture per kg of dry air. Enthalpy values are always based on some datum plane. Usually the zero value of the dry air is chosen as air at 0, and the zero value of the water vapor is the saturated liquid water at 0.9-2) Thermal properties of saturated water and saturated moist airThe thermal properties of saturated water are shown in Tab.9-2; and the thermal properties of saturated moist air at atmosphere of 101kPa are shown in Tab.9-3.Tab. 9-2 Thermal properties of saturated water vapor 2TemperatureSaturation pressureSpecific volume of saturated vaporLatent heat ofevaporation Enthapy of saturated vapor00.6106206.32501.42501.420.7060179.82496.12502.240.8135157.22491.32508.960.9353137.62486.62512.681.0728120.92481.82516.2101.2280106.32477.12519.9121.40393.742472.42523.6141.59982.822467.72527.2161.81873.312462.92530.9182.06465.022458.22534.5202.33957.772453.52538.2222.64551.432448.82541.8242.98545.872444.02545.5263.36340.992439.32549.1283.78236.682434.62552.7304.24632.892429.82556.4324.75929.542425.12560.0345.32426.572420.32563.6365.94723.942415.52567.2386.63221.602410.82570.8407.38419.522406.02574.4428.19817.692402.62577.9449.10016.042397.32581.54610.0914.562392.62585.14811.1613.232387.72588.65012.3412.052382.52592.2 Note: The data of saturation pressure, specific volume and latent heat of evaporation are from experiments. The data of enthaly are from calculation, and the zero value of the enthalpy is set for the saturated liquid water at 0.Tab. 9-3 Thermal properties of saturated air at atmosphere of 1.01 1Temperature, Saturation pressureof water vapor Humidity ratio，Specific volume, Enthapy of saturated air,00.610720.0037880.77819.47010.65660.004070.781311.20020.70550.004380.784512.97830.75750.004710.787814.80740.81300.005050.791116.69250.87190.005420.794418.63460.93470.005820.797820.63971.00130.006240.801222.70881.07220.006680.804624.84891.14740.007160.808127.059101.22720.007660.811629.348111.31190.008200.815231.716121.40170.008760.818834.172131.49690.009370.822536.719141.59770.010010.826239.362151.70440.010690.830042.105161.81730.011410.833844.955171.93670.012180.837747.918182.06300.012990.841750.998192.19640.013840.845754.205202.33730.014750.849857.544212.48610.015720.854061.021222.64310.016740.858364.646232.80860.017810.862668.425242.98320.018960.867172.366253.16710.020160.871676.481263.36090.021440.876380.777273.56490.022790.881185.263283.77970.024220.886089.952294.00550.025720.891094.851304.24310.027320.896199.977314.49280.029000.9014105.337324.75520.030780.9068110.946335.03080.032660.9124116.819345.32010.034640.9182122.968355.62370.036740.9241129.411365.94230.038950.9302136.161376.27640.041290.9365143.239386.62650.043760.9430150.660396.99350.046360.9497158.445407.37780.049110.9567166.615417.78030.052020.9639175.192428.20160.055090.9713184.200438.64240.058330.9790193.662449.10360.061760.9871203.610459.58560.065370.9954214.0674610.08960.069201.0040225.0684710.61610.073241.0130236.6434811.16590.077511.0224248.8284911.74020.082021.0322261.6675012.33970.086801.0424275.198Note: The data of saturation pressure are from experiments. The data of humidity ratio, specific volum are from calculation. The data of enthaly are calculated based on the properties experiment of air and water vapor. And the zero value of the dry air is chosen as air at 0, the zero value of the water vapor is the saturated liquid water at 0. 9-3) Adiabatic saturation process and wet bulb temperature (1) Adiabatic saturation process Fig.9-1, An adiabatic saturation processAn unsturated air, which has dry bulb temperature, humidity ratio and enthalpy , flows through a spray of water, as shown in Fig.9-1. The spray can provide enough surface area so that the air leaves the spray chamber in equilibrium with the water, with respect to both the temperature and the vapor pressue.In order to perpetuate the process, it is necessary to provide makeup water to compensate for the amount of water evaporated into the air. The temperature of the makeup water is the same as that in the sump.If the device is adiabatic, then the process is called adiabatic saturation process.(2) Thermodynamic wet bulb temperature In an adiabatic saturation process, the temperature of the water in the sump is called the thermodynamic wet bulb temperature of the air , or simply the wet bulb temperature of the air. An adiabatic saturation process is a constant wet bulb temperature process, the wet bulb temperatures of air at the inlet and the outlet are same, and they are equal to . That is (3) A constant wet bulb temperature process is nearly a constant enthaly process (4) Measurement of wet bulb temperatureA simple wet bulb thermometer used to measure the wet bulb temperature is based on the principle of the adiabatic saturation process. It is an ordinary thermometer being wrapt with a cloth sleeve of wool or flannel, around its bulb. The cloth sleeve should be clean and free of oil and thoroughly wet with clean fresh water. The water in the cloth sleeve evaporates as the air flows at a high velocity (). The evaporation, which takes the heat from the thermometer bulb, lowers the temperature of the bulb. The thermometer indicates approximately the wet bulb temperature .The difference between the dry-bulb and wet-bulb temperatures is called the wet-bulb depression. If the air is saturated, evaporation cannot take place, and the wet-bulb temperature is the same as the dry-bulb temperature; and the wet-bulb depression equals zero.9-4) Approximate calculation of the thermal properties of moist airIf two independent parameters which indicate the state of moist air are known, the thermal properties can be calculated by approximate methods. For example, if the temperature and relative humidity of a moist air are known at 27 and 50%, respectively, the other properties can be calculated as follows.(1) Partial pressure of vapor in moist air From Tab.9-2 or Tab.9-3, it can be found that the saturation pressure of water for 27is 3.5649.Alternatively, the saturation pressure over the range between 0C to +45C can also be estimated from the following formula: (9-10)where t is temperature in C ; in .For =27, the saturation pressure =3.55 calculated from Eq.(9-10).And from Eq. (9-1), = 3.56550%=1.78 .(2) Humidity ratio: From Eq.(9-5), (3) Dew point temperature :The dew point temperature is the temperature at which the moist air becomes saturated and starts to condense at constant vapor pressure or constant humidity ratio. Therefore, when the vapor pressure or constant humidity ratio is known, the dew point temperature can be found from Tab.9-3 in the corresponding saturation temperature column. For=1.78 or =0.0111, the dew point temperature is, therefore, =15.7(4) Specific Volume : From Eq.(9-8) (5) Specific enthalpy : As the enthalpy of moist air is the total enthalpy of the dry air and water vapor mixture per kg of dry air, and the enthalpy of dry air at 0is chosen as zero, the enthalpy of saturated liquid water at 0 is chosen as zero, therefore the specific enthalpy of moist air can be calculated from the following equation, (9-11)where is the specific heat of dry air , it is 1.003 for the temperatures from 0 to 50 as shown in Tab.9-1; is the humidity ratio.In the case of air-conditioning, the following approximation is valid (9-12)Where is the enthalpy of superheated vapor; and is the enthalpy of saturated water vapor at the same temperature. That is, the enthalpy of superheated vapor is nearly the same as the saturated vapor at same temperature.The enthalpy of saturated water vapor can also be approximately calculated from the following formula (9-13)By using equation (9-13), the enthalpy of moist air can be found from the formula below (9-14)For the moist air at = 27and =50%, its enthalpy is found from Eq.(9-14) : (6) Wet bulb temperature: Since a constant wet bulb temperature process is nearly a constant enthaly process, the wet bulb temperature can be found from the enthapy of 55.6 from Tab.9-3.This gives the wet bulb temperature = 19.43 .9-5) Psychrometric chart A psychrometric chart graphically represents the thermodynamic properties of moist air, as shown in Fig.9-2. It is very useful in presenting the air conditioning processes. The psychrometric chart is bounded by two perpendicular axes and a curved line: 1. The horizontal ordinate axis represents the dry bulb temperature line, in ; 2. The vertical ordinate axis represents the humidity ratio line, in ;3. The curved line shows the saturated air, it is corresponding to the relative humidity The psychrometric chart incorporates seven parameters and properties. They are dry bulb temperature, relative humidity, wet bulb temperature, dew point temperature, specific volume, humidity ratio w and enthalpy.1, Dry-bulb temperature is shown along the bottom axis of the psychrometric chart. The vertical lines extending upward from this axis are constant-temperature lines. 2, Relative humidity lines are shown on the chart as curved lines that move upward to the left in 10% increments. The line representing saturated air ( = 100%) is the uppermost curved line on the chart. And the line of = 0% is a horizontal ordinate axis itself. 3, Wet-bulb temperature: On the chart, the constant wet-bulb lines slope a little upward to the left, and the wet bulb temperature is read following a constant wet-bulb line from the state-point to the saturation line. 4, Dew point temperature: This temperature is read by following a horizontal line from the state-point to the saturation line. 5，Specific volume: It is shown from the constant-volume lines slanting upward to the left. 6, Humidity ratio w: it is indicated along the right-hand axis of the chart. 7, Enthalpy: It is read from where the constant enthalpy line crosses the diagonal scale above the saturation curve. The constant enthalpy lines, being slanted lines, are almost coincidental as the constant wet-bulb temperature lines.Fig.9-2, Psychrometric Chart (B=101.3 kPa)Only two properties are needed to characterize the moist air because the point of intersection of any two properties lines defines the state-point of air on a psychrometric chart. Once this point is located on the chart, the other air properties can be read directly.Exampleas shown in Fig.9-3: For a moist air with dry-bulb temperature and relative humidity , other parameters and properties found from the psychrometric chart are:Wet-bulb temperature: = 19.6 ;dew-point temperature: = 15.8 ;specific volume: ;khumidity ratio: ;（图上的数字也要改成0.0113）specific enthalpy: Fig.9-3, Determination of the properties from psychrometric chart .9-6) Air handing processes(1) The specifications for human comfort; relative humidity: ; air flow rate:.(2) Main air handing processes and their variations in propertiesa) Sensible cooling along a cooling coil, or sensible heating along a heating coilThe sensible cooling happens when the air is cooled without altering the specific humidity. The process is shown on the psychrometic chart by line 1a in Fig.9-4. During this process, the relative humidity of the air will increase. The sensible cooling can only take place under the condition when the temperature of the cooling coil is not below the dew point temperature of the air being processed.Fig.9-4, Main air handing processes and their variations in propertiesThe sensible heating is similar to sensible cooling, but with the dry bulb temperature increasing (line 1b in Fig.9-4). It should be noted that there should be no water within the heating system because the evaporation of the water will increase the specific humidity of the air.b) Adiabatic humidification and dehumidification using a humidifier or chemical dehumidifier The adiabatic humidification occurs when water vapor, of which temperature is near the wet bulb temperature of the moist air, is added to the air (line 2a in Fig.9-4). A humidifier performs this function by supplying the water vapor. During the adiabatic humidification process along the constant wet bulb temperature line, the spec