門窗結(jié)露的原理

1、o 四門窗玻璃中心部位結(jié)露溫度的簡易計(jì)算室內(nèi)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(qiáng)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(dāng)水蒸氣含量尚未達(dá)到這一限度時(shí)，該濕空氣稱為“未飽和”的，達(dá)到此限度就稱為“飽和”的。達(dá)到飽和狀態(tài)的水蒸氣分壓強(qiáng)稱為飽和蒸氣壓。在標(biāo)準(zhǔn)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(shù)據(jù)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(dāng)建筑物內(nèi)的濕空氣已經(jīng)飽和，再繼續(xù)向其供給水蒸氣，則超量的水蒸氣會(huì)凝結(jié)成水霧或在溫度較低的墻面和頂棚結(jié)成水珠。當(dāng)建筑物內(nèi)濕空氣未飽和，但溫度下降到某一特定值時(shí)，

2、由于飽和蒸氣壓下降，使水蒸氣達(dá)到飽和，再往下降溫，空氣中容納不下的水蒸氣就凝結(jié)成露水，這一特定溫度稱為“露點(diǎn)溫度”，簡稱“露點(diǎn)”。顯然“露點(diǎn)”和空氣的壓強(qiáng)和水蒸氣含量有關(guān)。描述濕空氣中水蒸氣含量的物理量是“濕度”。絕對(duì)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(duì)濕度用飽和蒸氣量 表示，單位均為g/m3。相對(duì)濕度為空氣的絕對(duì)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(jì)算中，由于涉及的溫度變化范圍不大，可近似認(rèn)為 與p成正比， 與ps成正比，可用式（12）計(jì)算相對(duì)濕度： （12）式中p為空氣中水蒸氣分壓強(qiáng)，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(duì)濕度可直接測出（如使用干濕球溫度計(jì)）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(jìn)行有關(guān)露點(diǎn)的計(jì)算。計(jì)算例：例1用干濕球溫度計(jì)測得某采暖居室氣溫為18，相對(duì)濕度=61.1%，試求該室空氣的露點(diǎn)。 解由附錄3查得氣溫18時(shí)，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(duì)應(yīng)于1260 pa的溫度10.4，此溫度即為該室空氣的露點(diǎn)，也就是說如果該室溫度降到10.4時(shí)就開始結(jié)露，也可以說如果該室玻璃的內(nèi)表面溫度降到10.4時(shí)，玻璃上就開始結(jié)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(dāng)例1的居室的室外溫度 =-20時(shí)，玻璃內(nèi)表面會(huì)不會(huì)結(jié)露。解如果按我國冬季k值的標(biāo)準(zhǔn)條件計(jì)算，三中例題中算出組合真空玻璃的內(nèi)表面溫度 =15.30，高于例1中的結(jié)露溫度10.4，所以不會(huì)結(jié)露。例3.某游泳館設(shè)計(jì)冬季室內(nèi)溫度保持26，相對(duì)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(shí)，飽和蒸氣壓ps=3361pa由式（12）可得 p=ps=2353 pa再由附錄3查出對(duì)應(yīng)于2353pa的露點(diǎn)近似為20。也就是說，要玻璃內(nèi)表面不結(jié)露，必須保證玻璃內(nèi)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(biāo)準(zhǔn)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(dá)到并超過此要求，而表7所列的中空玻璃中只有序號(hào)8的雙膜充氬氣中空玻璃可達(dá)到此要求。附錄1. 本文中使用的傳熱學(xué)名詞解釋1熱阻 r thermal resistance熱量傳遞的規(guī)律可與電學(xué)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(dòng)力；r是電阻，代表電量傳遞的阻力，在平面導(dǎo)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(dòng)力；r是熱阻，代

6、表熱量傳遞的阻力。熱阻概念的建立對(duì)傳熱過程的分析和計(jì)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(jì)算公式來計(jì)算傳熱過程的合成熱阻，從而方便地再算出傳熱系數(shù)等參數(shù)。熱阻的單位為w-1m2k2熱導(dǎo) c thermal conductance 熱阻的倒數(shù)，即 單位為wm-2k-13導(dǎo)熱系數(shù)（熱導(dǎo)率） thermal conductivity在穩(wěn)態(tài)條件下，1m厚的物體，兩側(cè)表面溫差為1k時(shí)，單位時(shí)間內(nèi)通過1m2面積傳遞的熱量。我國法定單位為wm-1k-14傳熱系數(shù) k coefficient of heat transfer (heat transfer coefficient) 在穩(wěn)態(tài)條件下，圍

7、護(hù)結(jié)構(gòu)兩側(cè)空氣溫差為1k時(shí)，單位時(shí)間內(nèi)通過1m2面積傳遞的熱量。我國法定單位為wm-2k-1。5傳熱阻 r傳 heat transfer resistance傳熱系數(shù)的倒數(shù) 單位 w-1m2k附錄2. 傳熱系數(shù)的三種單位及其換算關(guān)系表1 換算關(guān)系我國法定計(jì)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(hào)量的名稱漢文英文與基本單位的關(guān)系k熱力學(xué)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-calorie1ca

8、l=4.184jh時(shí)間小時(shí)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(biāo)準(zhǔn)大氣壓下不同溫度時(shí)的飽和水蒸氣分壓強(qiáng)ps 表1溫度 0 - 40(與冰面接觸) （單位：pa）t/0.00.10.20.30.40.50.60.70.80.9-0610.6605.3601.35

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. 溫度0 40（與水面接觸） ( 單位：pa)t/0.00.10.20.30.40.50.60.70.80.90610.6615.9619.9623.9629.3633.3638.6642.6647.9651.91657.3611.3666.6670

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附錄4.表1.中空玻璃熱阻及傳熱系數(shù)（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46a40.143.3349a40.173.02412a40.192.8756a50.143.3259a50.173.02512a50.192.8766a60.143.3269a60.173.01612a60.192.8666al0.20.212.6869al0.20.292.24612al0.20.332.0466al0.170.222.6469al0.170.32.18612al0.170.351.9866al0.10.242.

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.