門窗結露的原理

1、o 四門窗玻璃中心部位結露溫度的簡易計算室內外空氣都是含有一定水份的濕空氣，可以視為干空氣和水蒸氣的混合物，在溫度和壓強一定的條件下，一定容積的干空氣所能容納的水蒸氣量有一定限度，當水蒸氣含量尚未達到這一限度時，該濕空氣稱為“未飽和”的，達到此限度就稱為“飽和”的。達到飽和狀態(tài)的水蒸氣分壓強稱為飽和蒸氣壓。在標準大氣壓下，不同溫度的飽和蒸氣壓ps值列表于附錄3。由表中數據可知，溫度越高，一定容積中所能容納的水蒸氣越多，飽和蒸氣壓也越高。當建筑物內的濕空氣已經飽和，再繼續(xù)向其供給水蒸氣，則超量的水蒸氣會凝結成水霧或在溫度較低的墻面和頂棚結成水珠。當建筑物內濕空氣未飽和，但溫度下降到某一特定值時，

2、由于飽和蒸氣壓下降，使水蒸氣達到飽和，再往下降溫，空氣中容納不下的水蒸氣就凝結成露水，這一特定溫度稱為“露點溫度”，簡稱“露點”。顯然“露點”和空氣的壓強和水蒸氣含量有關。描述濕空氣中水蒸氣含量的物理量是“濕度”。絕對濕度為單位體積空氣中所含水蒸氣的重量，一般用 表示，飽和狀態(tài)下的絕對濕度用飽和蒸氣量 表示，單位均為g/m3。相對濕度為空氣的絕對濕度 與同溫同壓下的飽和蒸氣量 的百分比，一般用（%）表示，即： 在建筑熱工計算中，由于涉及的溫度變化范圍不大，可近似認為 與p成正比， 與ps成正比，可用式（12）計算相對濕度： （12）式中p為空氣中水蒸氣分壓強，單位paps為同溫度下的水蒸氣飽和

3、蒸氣壓，單位pa式（12）中，相對濕度可直接測出（如使用干濕球溫度計）或由資料給出，不同溫度下的飽和蒸氣壓ps可由附錄3查出，由此可方便地進行有關露點的計算。計算例：例1用干濕球溫度計測得某采暖居室氣溫為18，相對濕度=61.1%，試求該室空氣的露點。 解由附錄3查得氣溫18時，飽和蒸氣壓ps=2062.5pa由式（12）可得p=ps=1260 pa再由附錄3查出對應于1260 pa的溫度10.4，此溫度即為該室空氣的露點，也就是說如果該室溫度降到10.4時就開始結露，也可以說如果該室玻璃的內表面溫度降到10.4時，玻璃上就開始結露。例2如果例1的居室窗戶安裝的是本文三中例題所用的“真空+中空

4、”組合真空玻璃，當例1的居室的室外溫度 =-20時，玻璃內表面會不會結露。解如果按我國冬季k值的標準條件計算，三中例題中算出組合真空玻璃的內表面溫度 =15.30，高于例1中的結露溫度10.4，所以不會結露。例3.某游泳館設計冬季室內溫度保持26，相對濕度 70%，該地區(qū)冬季室外最低溫度為-10，問其窗玻璃的熱阻至少需要多高。解由附錄3查得氣溫26時，飽和蒸氣壓ps=3361pa由式（12）可得 p=ps=2353 pa再由附錄3查出對應于2353pa的露點近似為20。也就是說，要玻璃內表面不結露，必須保證玻璃內表面溫度高于20。由圖9可寫出上式中 =26 =-10 =20作為估算按標準值，

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所列的四種真空玻璃均可達到并超過此要求，而表7所列的中空玻璃中只有序號8的雙膜充氬氣中空玻璃可達到此要求。附錄1. 本文中使用的傳熱學名詞解釋1熱阻 r thermal resistance熱量傳遞的規(guī)律可與電學中電量傳遞的規(guī)律“歐姆定律”類比。 歐姆定律為： 式中i是電流，代表電路傳遞的電量；u為電路兩端的電壓差，是電量傳遞的動力；r是電阻，代表電量傳遞的阻力，在平面導熱過程中，類似的公式為： 式中為傳遞的熱流量；t為兩端的溫度差，是熱量傳遞的動力；r是熱阻，代

6、表熱量傳遞的阻力。熱阻概念的建立對傳熱過程的分析和計算帶來很大方便，比如可以類比使用串并聯(lián)電路電阻的計算公式來計算傳熱過程的合成熱阻，從而方便地再算出傳熱系數等參數。熱阻的單位為w-1m2k2熱導 c thermal conductance 熱阻的倒數，即 單位為wm-2k-13導熱系數（熱導率） thermal conductivity在穩(wěn)態(tài)條件下，1m厚的物體，兩側表面溫差為1k時，單位時間內通過1m2面積傳遞的熱量。我國法定單位為wm-1k-14傳熱系數 k coefficient of heat transfer (heat transfer coefficient) 在穩(wěn)態(tài)條件下，圍

7、護結構兩側空氣溫差為1k時，單位時間內通過1m2面積傳遞的熱量。我國法定單位為wm-2k-1。待添加的隱藏文字內容35傳熱阻 r傳 heat transfer resistance傳熱系數的倒數 單位 w-1m2k附錄2. 傳熱系數的三種單位及其換算關系表1 換算關系我國法定計量單位工程單位英制單位w/(m2k)kcal/(m2h)btu/ft210.8598450.1761.16310.2055.684.8841表2 換算中涉及的單位符號量的名稱漢文英文與基本單位的關系k熱力學溫度開爾文kelvin基本單位m長度米metre基本單位w功率瓦特watt1w=1j/skcal熱量千卡路里kilo

8、-calorie1cal=4.184jh時間小時hour1h=3600s(秒)攝氏溫度攝氏度degree celsius1=1k（）+273.15=k 華氏溫度華氏度degree fahrenhait1=1.8k()+459.675/9=k()=9/5（）+32ft英制長度英尺footfeet(pl)1 ft=0.3048mbtu熱量英制熱量單位british thermal unit1btu=1055.056j附錄3 標準大氣壓下不同溫度時的飽和水蒸氣分壓強ps 表1溫度 0 - 40(與冰面接觸) （單位：pa）t/0.00.10.20.30.40.50.60.70.80.9-0610.6

9、605.3601.3595.9590.6586.6581.3576.0572.0566.6-1562.6557.3553.3548.0544.0540.0534.6530.6526.6521.3-2517.3513.3509.3504.0500.0496.0492.0488.0484.0480.0-3476.0472.0468.0464.0460.0456.0452.0448.0445.3441.3-4437.3433.3429.3426.6422.6418.6416.0412.0408.0405.3-5401.3398.6394.6392.0388.0385.3381.3378.6374.6

10、372.0-6368.0365.3362.6358.6356.0353.3349.0346.6344.0341.3-7337.3334.6332.0329.3326.6324.0321.3318.6314.7312.0-8309.3306.6304.0301.3298.6296.0293.3292.0289.3286.6-9284.0281.3278.6276.0273.3272.0269.3266.6264.0262.6-10260.0257.3254.6253.3250.6248.0246.6244.0241.3240.0-11237.3236.0233.3232.0229.3226.62

11、25.3222.6221.3218.6-12217.3216.0213.3212.0209.3208.0205.3204.0202.6200.0-13198.6197.3194.7193.3192.0189.3188.0186.7184.0182.7-14181.3108.0177.3176.0174.7173.3172.0196.3168.0166.7-15165.3164.0162.7161.3160.0157.3156.0154.7153.3152.0-16150.7149.3148.0146.7145.3144.0142.7141.3140.0138.7-17137.3136.0134

12、.7133.3132.0130.7129.3128.0126.7126.0-18125.3124.0122.7121.3120.0118.7117.3116.6116.0114.7-19113.3112.0111.3110.7109.3108.0106.7106.0105.3104.0-20102.7102.0101.3100.099.398.797.396.095.394.7-2193.393.392.090.790.789.388.088.086.785.3-2285.384.084.082.781.381.380.080.078.777.3-2377.376.076.074.774.77

13、3.373.372.070.770.7-2470.769.368.068.066.766.765.365.364.064.0-2562.762.761.361.361.360.060.058.758.757.3-2657.357.356.056.054.753.353.353.353.352.0-2752.050.750.750.749.349.348.048.048.046.7-2846.746.745.345.345.344.044.044.042.742.7-2942.741.341.341.340.040.040.038.738.738.7-3037.337.337.337.336.0

14、36.036.034.734.734.7-3134.733.333.333.333.332.032.032.032.030.7-3230.730.730.729.329.329.329.328.028.028.0-3328.028.026.726.726.726.725.325.325.325.3-3425.324.024.024.024.024.022.722.722.722.7-3522.722.721.321.321.321.321.320.020.020.2t/0.00.10.20.30.40.50.60.70.80.9-3620.020.020.018.718.718.718.718

15、.718.718.7-3717.317.317.317.317.317.317.316.016.016.0-3816.016.016.016.014.714.714.714.714.714.7-3914.714.713.313.313.313.313.313.313.313.3-4013.312.012.012.012.012.012.012.012.012.0 表2. 溫度0 40（與水面接觸） ( 單位：pa)t/0.00.10.20.30.40.50.60.70.80.90610.6615.9619.9623.9629.3633.3638.6642.6647.9651.91657.361

16、1.3666.6670.6675.9681.3685.3690.6695.9699.92705.3710.6715.9721.3726.6730.6735.9741.3746.6751.93757.3762.6767.9773.3779.9785.3790.6791.9801.3807.94813.3818.6823.9830.6835.9842.6847.9853.3859.9866.65871.9878.6883.9890.6897.3902.6909.3915.9921.3927.96934.6941.3947.9954.6961.3967.9974.6981.2987.9994.671

17、001.21007.91014.61022.61029.21035.91043.91050.61057.21065.281071.91079.91086.61094.61101.21109.21117.21123.91131.91139.991147.91155.91162.61170.61178.61186.61194.61202.61210.61218.6101227.91235.91243.91251.91259.91269.21277.21286.61294.61303.9111311.91321.21329.21338.61347.91355.91365.21374.51383.91

18、393.2121401.21410.51419.91429.21438.51449.21458.51467.91477.21486.5131497.21506.51517.21526.51537.21546.51557.21566.51577.21587.9141597.21607.91618.51629.21639.91650.51661.21671.91682.51693.2151703.91715.91726.51737.21749.21759.91771.81782.51794.51805.2161817.21829.21841.21851.81863.81875.81887.8189

19、9.81911.81925.2171937.21949.21961.21974.51986.51998.52011.82023.82037.22050.5182062.52075.82089.22102.52115.82129.22142.52155.82169.12182.5192195.82210.52223.82238.52251.82266.52279.82294.52309.12322.5202337.12351.82366.52381.12395.82410.52425.12441.12455.82470.5212486.52501.12517.12531.82547.82563.

20、82579.82594.42610.42626.4222642.42659.82675.82691.82707.82725.12741.12758.42774.42791.8232809.12825.12842.42859.82877.12894.42911.82930.42947.72965.1242983.73001.13019.73037.13055.73074.43091.73110.43129.13147.7253167.73186.43205.13223.73243.73262.43285.43301.13321.13341.0263361.03381.03401.03421.03

21、441.03461.03482.43502.43523.73543.7273565.03586.43607.73627.73649.03670.43693.03714.43735.73757.0283779.73802.33823.73846.33869.03891.73914.33937.03959.73982.3294005.04029.04051.74075.74099.74122.34146.34170.34194.34218.3304243.64237.64291.64317.04341.04366.34391.74417.04442.34467.6t/0.00.10.20.30.4

22、0.50.60.70.80.9314493.04518.34543.74570.34595.64622.34684.94675.64702.34728.9324755.64782.34808.94836.94863.64891.64918.24946.24974.25002.2335030.25059.65087.65115.65144.95174.25202.25231.65260.95290.2345319.55350.25379.55410.25439.55470.25500.95531.55562.25292.9355623.55655.55686.25718.25748.85780.

23、85812.85844.85876.85910.2365942.25975.56007.56040.86074.26107.56140.86174.16208.86242.1376276.86310.16344.86379.56414.16448.86484.86519.46555.46590.1386626.16662.16698.16734.16771.46807.46844.86882.16918.16955.4396999.17031.47068.77107.47144.77183.47222.17260.77299.47338.0407378.07416.77456.77496.77

24、536.77576.77616.27658.07698.07739.3附錄4.表1.中空玻璃熱阻及傳熱系數（k值）玻璃1間隔玻璃2熱阻k值玻璃1間隔玻璃2熱阻k值玻璃1間隔玻璃2熱阻k值46a40.143.3349a40.173.02412a40.192.8756a50.143.3259a50.173.02512a50.192.8766a60.143.3269a60.173.01612a60.192.8666al0.20.212.6869al0.20.292.24612al0.20.332.0466al0.170.222.6469al0.170.32.18612al0.170.351.9866

25、al0.10.242.5269al0.10.342.03612al0.10.391.8166al0.080.242.5269al0.080.342.02612al0.080.391.8166al0.050.252.4769al0.050.351.96612al0.050.411.75l0.26al0.20.232.55l0.29al0.20.322.07l0.212al0.20.381.87l0.176al0.170.242.51l0.179al0.170.332.03l0.1712al0.170.391.83l0.16al0.10.252.45l0.19al0.10.361.94l0.112al0.10.421.73l0.086al0.080.252.43l0.089al0.080.361.93l0.0812al0.080.421.7346ar40.173.0349ar40.22.78412ar40.212.6856ar50.173.0259ar50.22.79512ar50.212.6866ar60.173.0269ar60.22.78612ar60.222.6866arl0.20.292.2469arl0.20.381.87612arl0.20.411.7666arl0.170.32.1969arl0.