psychrometric chart

今天星期日,天晴,這星期天氣預報說馬鞍颱風引進西南氣流造成南部下雨下到星期四.

上星期四晚上建日從歐洲度蜜月回來在貢寮"枋腳堡山產土雞城"補請一桌,與劉大哥,亮哥,彬哥,胡哥,安哥,明益,俊敏一起去沾喜氣.這枋腳堡的環境清幽老闆人很客氣做菜的材料也都是自己養的種的很新鮮,還吃到鱘龍魚一魚三吃,讚!

上上上星期與改善組才富施工處配管組林大哥品質組周大哥一起到二號機Q級管路包的某侷限空間執行銲道油漆前溫度溼度檢驗.依程序書的規定物體表面的露點(dew point)溫度須大於大氣溫度3度以上才能油漆;據工業配管專家富琪兄說這樣是為了怕物體內含有過多水份在油漆乾後汽化爆出,也可以用烘乾方式來達到要求. 說到這乾球(dry bulb),濕球(wet bulb)及露點溫度就要由psychrometric chart(濕度特性圖)來了解最簡單,附圖如下:

由乾球溫度及相對濕度找到狀態點後1.沿著等濕球溫度線往左上(adiabatic humidification)交至飽和溫度線即得到濕球溫度 2.沿著等濕度比線往左(cooling)交至飽和溫度線即得露點溫度;除了在飽和溫度時三者會相同一般同一位置的乾球溫度>濕球溫度>露點溫度. 另外就理論上:relative humidity(phi)=existing partial pressure of water vapor/saturation pressure of pure water at same temperature;humidity ratio(w)=kg of water vapor/kg of dry air=0.622*ps/(pt-ps) where ps:vapor pressure of water at surface temperature,pt:atmospheric pressure;enthaply(h)=cp*t+w*hg where cp:specific heat of dry air at const. pressure,t:temperature of air-vapor mixture,hg:enthalpy of saturated steam at temperature of air-vapor mixture;specific volume(v)=Ra*T/(pt-ps) where Ra:gas const. of dry air,T:absolute temperature of air-vapor mixture. 有很多基本的process都可以很容易的在圖上表示,e.g.sensible heating or cooling,adiabatic and nonadiabatic humidification,cooling and dehumidification,chemical dehumidification,mixing,etc.
最重要的是這裡transfer of sensible and latent heat with a wetted surface:
(1)if there is a difference in temperature between the air ta and the wetted surface ti,heat will be transferred.i.e.dqs=hc*dA*(ti-ta) where qs:rate of sensible heat transfer,hc:convection coefficient,A:area. (2)if there is a difference in the partial pressure of water vapor in the air ps,a and that of water ps,i,there will be transfer of mass(water).i.e.dql=hd*dA*(wi-wa)*hfg where ql:rate of latent heat transfer,hd:proportionality const.,wi:humidity ratio of saturated air at the wetted surface temperature,wa:humidity ratio of air in the free stream,hfg:latent heat of water at ti. (3)if (1)+(2),dqt=hc*dA/cpm*(hi-ha) where qt;enthalpy potential,cpm:specific heat of moist air,hi:enthalpy of saturated air at the wetted surface temperature,ha:enthalpy of air in the free stream. 所以流汗時還會覺得熱(dqt<0)並不一定是空氣的溫度較高(ta>ti)或是空氣的濕度較大(wa>wi),不過一定是空氣的焓較大(ha>hi).