门窗结露的原理

1、o 四门窗玻璃中心部位结露温度的简易计算室内外空气都是含有一定水份的湿空气，可以视为干空气和水蒸气的混合物，在温度和压强一定的条件下，一定容积的干空气所能容纳的水蒸气量有一定限度，当水蒸气含量尚未达到这一限度时，该湿空气称为“未饱和”的，达到此限度就称为“饱和”的。达到饱和状态的水蒸气分压强称为饱和蒸气压。在标准大气压下，不同温度的饱和蒸气压ps值列表于附录3。由表中数据可知，温度越高，一定容积中所能容纳的水蒸气越多，饱和蒸气压也越高。当建筑物内的湿空气已经饱和，再继续向其供给水蒸气，则超量的水蒸气会凝结成水雾或在温度较低的墙面和顶棚结成水珠。当建筑物内湿空气未饱和，但温度下降到某一特定值时，

2、由于饱和蒸气压下降，使水蒸气达到饱和，再往下降温，空气中容纳不下的水蒸气就凝结成露水，这一特定温度称为“露点温度”，简称“露点”。显然“露点”和空气的压强和水蒸气含量有关。描述湿空气中水蒸气含量的物理量是“湿度”。绝对湿度为单位体积空气中所含水蒸气的重量，一般用 表示，饱和状态下的绝对湿度用饱和蒸气量 表示，单位均为g/m3。相对湿度为空气的绝对湿度 与同温同压下的饱和蒸气量 的百分比，一般用（%）表示，即： 在建筑热工计算中，由于涉及的温度变化范围不大，可近似认为 与p成正比， 与ps成正比，可用式（12）计算相对湿度： （12）式中p为空气中水蒸气分压强，单位paps为同温度下的水蒸气饱和

3、蒸气压，单位pa式（12）中，相对湿度可直接测出（如使用干湿球温度计）或由资料给出，不同温度下的饱和蒸气压ps可由附录3查出，由此可方便地进行有关露点的计算。计算例：例1用干湿球温度计测得某采暖居室气温为18，相对湿度=61.1%，试求该室空气的露点。 解由附录3查得气温18时，饱和蒸气压ps=2062.5pa由式（12）可得p=ps=1260 pa再由附录3查出对应于1260 pa的温度10.4，此温度即为该室空气的露点，也就是说如果该室温度降到10.4时就开始结露，也可以说如果该室玻璃的内表面温度降到10.4时，玻璃上就开始结露。例2如果例1的居室窗户安装的是本文三中例题所用的“真空+中空

4、”组合真空玻璃，当例1的居室的室外温度 =-20时，玻璃内表面会不会结露。解如果按我国冬季k值的标准条件计算，三中例题中算出组合真空玻璃的内表面温度 =15.30，高于例1中的结露温度10.4，所以不会结露。例3.某游泳馆设计冬季室内温度保持26，相对湿度 70%，该地区冬季室外最低温度为-10，问其窗玻璃的热阻至少需要多高。解由附录3查得气温26时，饱和蒸气压ps=3361pa由式（12）可得 p=ps=2353 pa再由附录3查出对应于2353pa的露点近似为20。也就是说，要玻璃内表面不结露，必须保证玻璃内表面温度高于20。由图9可写出上式中 =26 =-10 =20作为估算按标准值，

5、=1/8.7=0.115 w-1m2k， =1/23=0.0435 w-1m2k代入上式中可求出 =0.53 w-1m2k。本文表1所列的四种真空玻璃均可达到并超过此要求，而表7所列的中空玻璃中只有序号8的双膜充氩气中空玻璃可达到此要求。附录1. 本文中使用的传热学名词解释1热阻 r thermal resistance热量传递的规律可与电学中电量传递的规律“欧姆定律”类比。 欧姆定律为： 式中i是电流，代表电路传递的电量；u为电路两端的电压差，是电量传递的动力；r是电阻，代表电量传递的阻力，在平面导热过程中，类似的公式为： 式中为传递的热流量；t为两端的温度差，是热量传递的动力；r是热阻，代

6、表热量传递的阻力。热阻概念的建立对传热过程的分析和计算带来很大方便，比如可以类比使用串并联电路电阻的计算公式来计算传热过程的合成热阻，从而方便地再算出传热系数等参数。热阻的单位为w-1m2k2热导 c thermal conductance 热阻的倒数，即 单位为wm-2k-13导热系数（热导率） thermal conductivity在稳态条件下，1m厚的物体，两侧表面温差为1k时，单位时间内通过1m2面积传递的热量。我国法定单位为wm-1k-14传热系数 k coefficient of heat transfer (heat transfer coefficient) 在稳态条件下，围

7、护结构两侧空气温差为1k时，单位时间内通过1m2面积传递的热量。我国法定单位为wm-2k-1。5传热阻 r传 heat transfer resistance传热系数的倒数 单位 w-1m2k附录2. 传热系数的三种单位及其换算关系表1 换算关系我国法定计量单位工程单位英制单位w/(m2k)kcal/(m2h)btu/ft210.8598450.1761.16310.2055.684.8841表2 换算中涉及的单位符号量的名称汉文英文与基本单位的关系k热力学温度开尔文kelvin基本单位m长度米metre基本单位w功率瓦特watt1w=1j/skcal热量千卡路里kilo-calorie1ca

8、l=4.184jh时间小时hour1h=3600s(秒)摄氏温度摄氏度degree celsius1=1k（）+273.15=k 华氏温度华氏度degree fahrenhait1=1.8k()+459.675/9=k()=9/5（）+32ft英制长度英尺footfeet(pl)1 ft=0.3048mbtu热量英制热量单位british thermal unit1btu=1055.056j附录3 标准大气压下不同温度时的饱和水蒸气分压强ps 表1温度 0 - 40(与冰面接触) （单位：pa）t/0.00.10.20.30.40.50.60.70.80.9-0610.6605.3601.35

9、95.9590.6586.6581.3576.0572.0566.6-1562.6557.3553.3548.0544.0540.0534.6530.6526.6521.3-2517.3513.3509.3504.0500.0496.0492.0488.0484.0480.0-3476.0472.0468.0464.0460.0456.0452.0448.0445.3441.3-4437.3433.3429.3426.6422.6418.6416.0412.0408.0405.3-5401.3398.6394.6392.0388.0385.3381.3378.6374.6372.0-6368.

10、0365.3362.6358.6356.0353.3349.0346.6344.0341.3-7337.3334.6332.0329.3326.6324.0321.3318.6314.7312.0-8309.3306.6304.0301.3298.6296.0293.3292.0289.3286.6-9284.0281.3278.6276.0273.3272.0269.3266.6264.0262.6-10260.0257.3254.6253.3250.6248.0246.6244.0241.3240.0-11237.3236.0233.3232.0229.3226.6225.3222.622

11、1.3218.6-12217.3216.0213.3212.0209.3208.0205.3204.0202.6200.0-13198.6197.3194.7193.3192.0189.3188.0186.7184.0182.7-14181.3108.0177.3176.0174.7173.3172.0196.3168.0166.7-15165.3164.0162.7161.3160.0157.3156.0154.7153.3152.0-16150.7149.3148.0146.7145.3144.0142.7141.3140.0138.7-17137.3136.0134.7133.3132.

12、0130.7129.3128.0126.7126.0-18125.3124.0122.7121.3120.0118.7117.3116.6116.0114.7-19113.3112.0111.3110.7109.3108.0106.7106.0105.3104.0-20102.7102.0101.3100.099.398.797.396.095.394.7-2193.393.392.090.790.789.388.088.086.785.3-2285.384.084.082.781.381.380.080.078.777.3-2377.376.076.074.774.773.373.372.0

13、70.770.7-2470.769.368.068.066.766.765.365.364.064.0-2562.762.761.361.361.360.060.058.758.757.3-2657.357.356.056.054.753.353.353.353.352.0-2752.050.750.750.749.349.348.048.048.046.7-2846.746.745.345.345.344.044.044.042.742.7-2942.741.341.341.340.040.040.038.738.738.7-3037.337.337.337.336.036.036.034.

14、734.734.7-3134.733.333.333.333.332.032.032.032.030.7-3230.730.730.729.329.329.329.328.028.028.0-3328.028.026.726.726.726.725.325.325.325.3-3425.324.024.024.024.024.022.722.722.722.7-3522.722.721.321.321.321.321.320.020.020.2t/0.00.10.20.30.40.50.60.70.80.9-3620.020.020.018.718.718.718.718.718.718.7-

15、3717.317.317.317.317.317.317.316.016.016.0-3816.016.016.016.014.714.714.714.714.714.7-3914.714.713.313.313.313.313.313.313.313.3-4013.312.012.012.012.012.012.012.012.012.0 表2. 温度0 40（与水面接触） ( 单位：pa)t/0.00.10.20.30.40.50.60.70.80.90610.6615.9619.9623.9629.3633.3638.6642.6647.9651.91657.3611.3666.6670

16、.6675.9681.3685.3690.6695.9699.92705.3710.6715.9721.3726.6730.6735.9741.3746.6751.93757.3762.6767.9773.3779.9785.3790.6791.9801.3807.94813.3818.6823.9830.6835.9842.6847.9853.3859.9866.65871.9878.6883.9890.6897.3902.6909.3915.9921.3927.96934.6941.3947.9954.6961.3967.9974.6981.2987.9994.671001.21007.9

17、1014.61022.61029.21035.91043.91050.61057.21065.281071.91079.91086.61094.61101.21109.21117.21123.91131.91139.991147.91155.91162.61170.61178.61186.61194.61202.61210.61218.6101227.91235.91243.91251.91259.91269.21277.21286.61294.61303.9111311.91321.21329.21338.61347.91355.91365.21374.51383.91393.2121401

18、.21410.51419.91429.21438.51449.21458.51467.91477.21486.5131497.21506.51517.21526.51537.21546.51557.21566.51577.21587.9141597.21607.91618.51629.21639.91650.51661.21671.91682.51693.2151703.91715.91726.51737.21749.21759.91771.81782.51794.51805.2161817.21829.21841.21851.81863.81875.81887.81899.81911.819

19、25.2171937.21949.21961.21974.51986.51998.52011.82023.82037.22050.5182062.52075.82089.22102.52115.82129.22142.52155.82169.12182.5192195.82210.52223.82238.52251.82266.52279.82294.52309.12322.5202337.12351.82366.52381.12395.82410.52425.12441.12455.82470.5212486.52501.12517.12531.82547.82563.82579.82594

20、.42610.42626.4222642.42659.82675.82691.82707.82725.12741.12758.42774.42791.8232809.12825.12842.42859.82877.12894.42911.82930.42947.72965.1242983.73001.13019.73037.13055.73074.43091.73110.43129.13147.7253167.73186.43205.13223.73243.73262.43285.43301.13321.13341.0263361.03381.03401.03421.03441.03461.0

21、3482.43502.43523.73543.7273565.03586.43607.73627.73649.03670.43693.03714.43735.73757.0283779.73802.33823.73846.33869.03891.73914.33937.03959.73982.3294005.04029.04051.74075.74099.74122.34146.34170.34194.34218.3304243.64237.64291.64317.04341.04366.34391.74417.04442.34467.6t/0.00.10.20.30.40.50.60.70.

22、80.9314493.04518.34543.74570.34595.64622.34684.94675.64702.34728.9324755.64782.34808.94836.94863.64891.64918.24946.24974.25002.2335030.25059.65087.65115.65144.95174.25202.25231.65260.95290.2345319.55350.25379.55410.25439.55470.25500.95531.55562.25292.9355623.55655.55686.25718.25748.85780.85812.85844

23、.85876.85910.2365942.25975.56007.56040.86074.26107.56140.86174.16208.86242.1376276.86310.16344.86379.56414.16448.86484.86519.46555.46590.1386626.16662.16698.16734.16771.46807.46844.86882.16918.16955.4396999.17031.47068.77107.47144.77183.47222.17260.77299.47338.0407378.07416.77456.77496.77536.77576.7

24、7616.27658.07698.07739.3附录4.表1.中空玻璃热阻及传热系数（k值）玻璃1间隔玻璃2热阻k值玻璃1间隔玻璃2热阻k值玻璃1间隔玻璃2热阻k值46a40.143.3349a40.173.02412a40.192.8756a50.143.3259a50.173.02512a50.192.8766a60.143.3269a60.173.01612a60.192.8666al0.20.212.6869al0.20.292.24612al0.20.332.0466al0.170.222.6469al0.170.32.18612al0.170.351.9866al0.10.242.

25、5269al0.10.342.03612al0.10.391.8166al0.080.242.5269al0.080.342.02612al0.080.391.8166al0.050.252.4769al0.050.351.96612al0.050.411.75l0.26al0.20.232.55l0.29al0.20.322.07l0.212al0.20.381.87l0.176al0.170.242.51l0.179al0.170.332.03l0.1712al0.170.391.83l0.16al0.10.252.45l0.19al0.10.361.94l0.112al0.10.421.73l0.086al0.080.252.43l0.089al0.080.361.93l0.0812al0.080.421.7346ar40.173.0349ar40.22.78412ar40.212.6856ar50.173.0259ar50.22.79512ar50.212.6866ar60.173.0269ar60.22.78612ar60.222.6866arl0.20.292.2469arl0.20.381.87612arl0.20.411.7666arl0.170.32.1969arl0.