水和水蒸气的性质

单位质量的某种物质温度升高1°c吸收的热量叫做这种物质的比热容，简称比热。比热是通过比较单位质量的某种物质温升1C时吸收的热量，来表示各种物质的不同性质。水的比热最大。这就意味着，在同样受热或冷却的情况下，水的温度变化要小些。水的这个特征对气候的影响很大。在受太阳照射条件相同时，白天沿海地区比内陆地区温升慢，夜晚沿海地区温度降低也少。所以一天之中，沿海地区温度变化小，内陆地区温度变化大。在一年之中，夏季内陆比沿海炎热，冬季内陆比沿海寒冷。水比热大的特点，在生产、生活中也经常利用。如汽车发动机、发电机等机器，在工作时要发热，通常要用循环流动的水来冷却。冬季也常用热水取暖水的比热容是4.2*103焦/千克•摄氏度，蒸气的比热容是2.1*103焦/千克•摄氏度汽化热是一个物质的物理性质。其定义为：在标准大气压（101.325kPa）下，使一摩尔物质在其沸点蒸发所需要的热量。常用单位为千焦/摩尔（或称千焦耳/摩尔），千焦/千克亦有使用。其他仍在使用的单位包括Btu/lb（英制单位，Btu为BritishThermalUnit，lb为磅）。水的汽化热为40.8千焦/摩尔，相当于2260千焦/千克。一般地：使水在其沸点蒸发所需要的热量五倍于把等量水从一摄氏度加热到一百摄氏度所需要的热量1千瓦时=3.6x106焦=3600kj卡等于4.18焦耳,1千卡等于4.18千焦水和水蒸气的性质工业锅炉的介质是水和蒸汽，要了解锅炉的工作原理，须先认识水和水蒸气的基本性质。一、水的性质纯水是无色、无味、透明的液体，由氢和氧两种元素化合而成，其化学分子式为Hq0。2水的“三态”水的形态随温度变化而变化，会呈现出液态（水）、气态（水蒸气）和固态（冰），它们之间能够相互转化（图1—17）。锅炉中的水蒸气是由锅水吸收燃料燃烧放出的热量形成的。

水的体积与温度的关系在大气压力下，温度为4°C时，水的体积最小，密度最大［1克/厘米2（g/cm2）］。超过或低于4qC时，其体积均要膨胀变大，而密度减小［＜1克/厘米2（g/cm2）］。由此可见，水和其他物质不同，大多数物质的体积与温度成正比，而水是一种反常的液体，所以在冬季停用的锅炉和管道内不要存水，以免水结冰后体积膨胀，导致锅炉设备发生损坏。水平面水在连通器里，当水面上的压力（压强）相等时，水面始终保持相平（图1—18）。锅炉的水位表就是根据这个原理制成的，因为水位表与锅筒是一个连通器，所以水位表显示的水位就是锅筒的实际水位。水对容器的压力水在容器内对任何方向都有压力，且在相同高度时各方向的压力均相等。高度不同时，压力也不同。所以，在同一台锅炉上，如几只压力表安装的位置不同，其指示的压力值也不一样，高度降低1米（m），压力就增加9.8兆帕（kPa）［0.1千克/厘米2（kg/cm2）］。水的比热水的比热高于其他任何液体的比热。水的比热为4.1868千焦/千克・C（1千卡/千克・C），柴油的比热为2.093千焦/千克・C（0.50千卡/千克・C），即一定量的水，在温度升高时所吸收的热量或温度降低时所放出的热量，比其他液体在相同的情况下所吸收或放出的热量要多。因此，常常利用水作为吸热和冷却的介质。水的压缩系数水的压缩系数极小，因此，水在密闭容器内受到外加压力时，其体积几乎没有缩小，但是能够向各个方向均匀地传递压力。人们就是利用这个原理，对锅炉进行水压试验，以检验锅炉受压元、部件的强度和严密性。二、饱和水和水蒸气的性质饱和水、饱和蒸汽和过热蒸汽在一定的压力下，对水不断加热，水温将逐渐上升，最后水开始沸腾，温度不再上升，叫做水在一定压力下的饱和温度，这种具有饱和温度的水称为饱和水。饱和温度与压力有关：随着压力升高，饱和温度也相应升高。从本章附表1查得：在绝对压力为0.2兆帕（2.04千克力/厘米2）时，水的饱和温度为120.23C；在绝对压力为1.4兆帕（14.29千克力/厘米2）时，水的饱和温度为195.04C。在一定压力下，对饱和水继续加热，饱和温度保持不变，但水陆续地转化为蒸汽，这种具有饱和温度的水蒸气称为饱和蒸汽。通常饱和蒸汽中含有一定的水分，称为湿饱和蒸汽。如果湿饱和蒸汽再继续加热，其温度仍然不变，而是湿饱和蒸汽中的水分全部变为蒸汽，这种蒸汽称为干饱和蒸汽。

将干饱和蒸汽继续加热，此时压力不变而蒸汽温度升高，并超过了饱和蒸汽的温度，这种蒸汽称为过热蒸汽。饱和水、饱和蒸汽和过热蒸汽的性质见本章附表1和附表2。饱和水含热量、汽化热和干饱和蒸汽含热量（1） 饱和水含热量把1千克水在压力不变的情况下，从O°C加热到饱和温度所需要的热量，称为饱和水含热量（也称液体热或显热），它与压力有关，压力越高，饱和水的含热量越大，但它不能改变水的形态。其单位是千焦/千克（千卡/千克），用符号"kJ/kg（kcal/kg）”表示。（2） 汽化热把l千克（kg）饱和温度的水在压力不变的情况下，变为相同温度的干饱和蒸汽所需要的热量，称为汽化热（也称汽化潜热或蒸发热），它也与压力有关，但压力越高，汽化热则越小。汽化热的单位也是千焦/千克（千卡/千克），用符号"kJ/kg（kcal/kg）”表示。（3） 干饱和蒸汽含热量干饱和蒸汽的含热量等于饱和水含热量（液体热）与汽化热（汽化潜热）之和，压力越高，饱和蒸汽含热量越大。其单位为千焦/千克（千卡/千克），用符号"kJ/kg（kcal/kg）”表示。例如，锅炉的表压力为0.5兆帕（5.1千克力/厘米2）时，由本章附表1查得：饱和水含热量为670.5千焦/千克（160.15千卡/千克），汽化热为2086千焦/千克（498.23千卡/公斤），则干饱和蒸汽含热量为670.5+2086=2756.5千焦/千克或160.15+498.23=658.38千卡/千克若表压力在1.3兆帕（13.27千克力/厘米。）时，由本章附表1查得：饱和水含热量为830千焦/千克（198.24千卡/千克），汽化热为1960千焦/千克（468.14千卡/千克），则干饱和蒸汽含热量为830+1960=2790千焦/千克（kJ/kg）或198.24+468.14=666.38千卡/千克（kcal/kg）饱和水蒸气表

温度°C绝对压强蒸汽的密度(kg/m3)焓汽化热(kgf/cm2)(kPa)液体蒸汽(kcal/kg)(kJ/kg)(kcal/kg)(kJ/kg)(kcal/kg)(kJ/kg)00.00620.60820.00484005952491.15952491.150.00890.87310.006805.020.94597.32500.8592.32479.86100.01251.22620.0094010.041.87599.62510.4598.62468.53150.01741.70680.0128315.062.80602.02520.5587.02457.7200.02382.33460.0171920.083.74604.32530.1584.32446.3250.03233.16840.0230425.0104.67606.62539.7581.62435.0300.04334.24740.0303630.0125.60608.92549.3578.92423.7350.05735.62070.0396035.0146.54611.22559.0576.22412.4400.07527.37660.0511440.0167.47613.52568.6573.52401.1450.09779.58370.0654345.0188.41615.72577.8570.72389.4500.125812.3400.083050.0209.34618.02587.4568.02378.1550.160515.7430.104355.0230.27620.22596.7565.22366.4600.203119.9230.130160.0251.21622.52606.3562.52355.1650.255025.0140.161165.0272.14.624.72615.5559.72343.4700.317731.1640.197970.0293.08626.82624.3556.82331.2750.39338.5510.241675.0314.01629.02633.5554.02319.5800.48347.3790.292980.0334.94631.12642.3551.22307.8850.59057.8750.353185.0355.88633.22651.1548.22295.2900.71570.1360.422990.0376.81635.32659.9545.32283.1950.86284.5560.503995.0397.75637.42668.7542.42270.91001.033101.330.5970100.0418.68639.42677.0539.42258.41051.232120.850.7036105.1440.03641.32685.0536.32245.41101.461143.310.8254110.1460.97643.32693.4533.12232.01151.724169.110.9635115.2482.32645.22701.3530.02219.0

1202.025198.641.1199120.3503.67647.02708.9526.72205.21252.367232.191.296125.4525.02648.82716.4523.52291.81302.755270.251.494130.5546.38650.62723.9520.12177.61353.192313.111.715135.6567.73652.32731.0516.72163.31403.685361.471.962140.7589.08653.92737.7513.22148.71454.238415.722.238145.9610.85655.52744.4509.72134.01504.855476.242.543151.0632.21657.02750.7506.02118.51606.303618.283.252161.4675.75659.92762.9498.52087.11708.080792.594.113171.8719.29662.42773.3490.62054.018010.231003.55.145182.3763.25664.62782.5482.32019.319012.801255.66.378192.9807.64666.42790.1473.51982.420015.851554.777.840203.5852.01667.72795.5464.21943.521019.551917.729.567214.3897.23668.62799.3454.41902.522023.662320.8811.60225.1942.45669.02801.0443.91858.523028.532798.5913.98236.1988.50668.82800.1432.71811.624034.133347.9116.76247.11034.56668.02796.8420.81761.825040.553977.6720.01258.31081.45664.02790.1408.11708.626047.854693.7523.82269.61128.76664.22780.9394.51651.727056.115503.9928.27281.11176.91661.22768.3380.11591.428065.426417.2433.47292.71225.48657.32752.0364.61526.529075.887443.2939.60304.41274.46652.62732.3348.11457.430087.68592.9446.93316.61325.54646.82708.0330.21382.5310100.79877.9655.59329.31378.71640.12680.8310.81301.3320115.211300.365.95343.01436.07632.52648.2289.51212.1330131.312879.678.53357.51446.78623.52610.5266.61116.2340149.014615.893.98373.01562.93613.52568.6240.21005.7350168.616538.5113.2390.81636.20601.12516.7210.3880.5

360190.318667.1139.6413.01729.15583.42442.6170.3713.0370214.521040.9171.0451.01888.25549.82301.998.2411.137422522070.9322.6501.12098.0501.12098.000饱和水蒸汽表1.按温度排列温度t/°c绝对压强p/kPa水蒸汽的密度//kg-m-3焓H/kJ-kg-1汽化热r/kJ-kg-1液体 水蒸汽00.60820.0048402491.12491.150.8730.006820.942500.82479.86101.22620.009441.872510.42468.53151.70680.0128362.82520.52457.7202.33460.0171983.742530.12446.3253.16840.02304104.672539.72435304.24740.03036125.62549.32423.7355.62070.0396146.5425592412.1407.37660.05114167.472568.62401.1459.58370.06543188.412577.82389.45012.340.083209.342587.42378.15515.7430.1043230.272596.72366.46019.9230.1301251.212606.32355.16525.0140.1611272.142615.52343.17031.1640.1979293.082624.32331.27538.5510.2416314.012633.52319.58047.3790.2929334.942642.32307.88557.8750.3531355.882651.12295.29070.1360.4229376.812659.92283.19584.5560.5039397.752668.72270.5100101.330.597418.6826772258.4105120.850.7036440.0326852245.4110143.310.8254460.972693.42232115169.110.9635482.322701.32219120198.641.1199503.672708.92205.2125232.191.296525.022716.42191.8130270.251.494546.382723.92177.6135313.111.715567.7327312163.3140361.471.962589.082737.72148.7145415.722.238610.852744.42134150476.242.543632.212750.72118.5160618.283.252675.752762.92037.1170792.594.113719.292773.320541801003.55.145763.252782.52019.3

1901255.66.378807.642790.11982.42001554.777.84852.012795.51943.52101917.729.567897.232799.31902.52202320.8811.6942.4528011858.52302798.5913.98988.52800.11811.62403347.9116.761034.562796.81761.82503977.6720.011081.452790.11708.62604693.7523.821128.762780.91651.72705503.9928.271176.912768.31591.42806417.2433.471225.4827521526.52907443.2939.61274.462732.31457.43008592.9446.931325.5427081382.53109877.9655.591378.7126801301.332011300.365.951436.072648.21212.133012879.678.531446.782610.51116.234014615.893.981562.932568.61005.735016538.5113.21636.22516.7880.536018667.1139.61729.152442.671337021040.91711888.252301.9411.137422070.9322.62098209802.按压强排列绝对压强p/kPa温度t/°C水蒸汽的密度//kg-m-3焓汽化热r/kJ-kg-1液体水蒸气16.30.0077326.482503.12476.81.512.50.0113352.262515.324632170.0148671.212524.22452.92.520.90.0183687.452531.82444.3323.50.0217998.382536.82438.13.526.10.02523109.32541.82432.5428.70.02867120.232546.82426.64.530.80.032051292550.92421.9532.40.03537135.6925542416.3635.60.042149.062560.12411738.80.04864162.442566.32403.8841.30.05514172.7325712398.2943.30.06156181.162574.82393.61045.30.06798189.592578.52388.91553.50.09956224.03259423702060.10.13068251.512606.42354.93066.50.19093288.772622.42333.740750.24975315.932634.12312.25081.20.30799339.82644.32304.56085.60.36514358.212652.12393.9H/kJ-kg-17089.90.42229376.612659.82283.28093.20.47807390.082665.32275.39096.40.53384403.492670.82267.410099.60.58961416.92676.32259.5120104.50.6986843**12684.32246.8140109.20.80758457.672692.12234.41601130.82981473.882698.12224.2180116.61.0209489.322703.72214.3200120.21.1273493.712709.22204.6250127.21.3904534.392719.72185.4300133.31.6501560.382728.52168.1350138.81.9074583.762736.12152.3400143.42.1618603.612742.12138.5450147.72.4152622.422747.82125.4500151.72.6673639.592752.82113.2600158.73.1686676.222761.42091.