冷却塔

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英格蘭劍橋郡雙曲面結構冷卻塔
圖為英國一座自然抽風式(Natural draft)雙曲面結構的冷卻塔近底部位置。

冷卻塔(Cooling tower),火力發電廠核能发电厂、大型空調設備製冷設備的循环水冷却装置。根据其通风方式,可以分为自然通风冷却塔和机力通风冷却塔。

結構[编辑]

大多數發電廠的冷卻塔結構都是單葉雙曲面形狀。由於單葉雙曲面是一種雙重直紋曲面(Ruled surface),它可以用直的建造。這樣既可減少阻力,又可以用最少的材料來維持結構的完整。

以熱傳導方式分類[编辑]

若以其熱傳導方式分類,冷却塔大致有 :

  • 濕式冷卻塔(Wet cooling towers) (or open circuit cooling towers) operate on the principle of evaporative cooling. The working fluid and the evaporated fluid (usually water) are one and the same.
  • 閉合迴路冷卻塔(Closed circuit cooling towers) (or fluid coolers) pass the working fluid through a tube bundle, upon which clean water is sprayed and a fan-induced draft applied. The resulting heat transfer performance is close to that of a wet cooling tower, with the advantage of protecting the working fluid from environmental exposure and contamination.
  • 乾燥式冷卻塔(Dry cooling towers) are closed circuit cooling towers which operate by heat transfer through a surface that separates the working fluid from ambient air, such as in a tube to air heat exchanger, utilizing convective heat transfer. They do not use evaporation.
  • 混合式冷卻塔(Hybrid cooling towers) are closed circuit cooling towers that can switch between wet and dry operation. This helps balance water and energy savings across a variety of weather conditions.

In a wet cooling tower (or open circuit cooling tower), the warm water can be cooled to a temperature lower than the ambient air dry-bulb temperature, if the air is relatively dry (see dew point and psychrometrics). As ambient air is drawn past a flow of water, a small portion of the water evaporates, and the energy required to evaporate that portion of the water is taken from the remaining mass of water, thus reducing its temperature. Approximately 970 BTU of heat energy is absorbed for each pound of evaporated water (2 MJ/kg). Evaporation results in saturated air conditions, lowering the temperature of the water processed by the tower to a value close to wet-bulb temperature, which is lower than the ambient dry-bulb temperature, the difference determined by the initial humidity of the ambient air.

To achieve better performance (more cooling), a medium called fill is used to increase the surface area and the time of contact between the air and water flows. Splash fill consists of material placed to interrupt the water flow causing splashing. Film fill is composed of thin sheets of material (usually PVC) upon which the water flows. Both methods create increased surface area and time of contact between the fluid (water) and the gas (air), to improve heat transfer.

以空氣流動方式分類[编辑]

把空氣流動通過冷却塔的方法大致有三種 :

  • 自然抽風(Natural draft) — 通過煙囪產生的上揚力。由於在相等的壓力中,大氣中的空氣相對較為乾燥及低溫並與排放氣體產生密度上的差距,造成冷却塔內溫暖而潮濕的空氣「自然地」向上流動。
  • 機械式抽風(Mechanical draft) — 利用大型風扇製造氣流並把冷却塔內的潮濕蒸發氣體抽走。
    • 引風(Induced draft) — 在煙囪的排氣口位置(大部份位於上方)裝設大型風扇並把空氣向上抽。風扇會把冷却塔內的潮濕空氣導出排氣口。此方式會產生較低的進入空氣流動速度及較高的排出空氣流動速度,並減低已排走氣體回流至進氣口處的可能性。
    • 鼓風(Forced draft) — 在煙囪的引風 / 吸風口位置裝設大型風扇或鼓風機(Blower)。風扇強制把外在空氣「推入」冷却塔內,並營造較高的進入空氣流動速度及較低的排出空氣流動速度。 低排出空氣流動速度較容易受回流空氣易受影響。 With the fan on the air intake, the fan is more susceptible to complications due to freezing conditions. Another disadvantage is that a forced draft design typically requires more motor horsepower than an equivalent induced draft design. The benefit of the forced draft design is its ability to work with high static pressure. Such setups can be installed in more-confined spaces and even in some indoor situations. This fan/fin geometry is also known as blow-through.
  • 機力輔助式自然通風(Fan assisted natural draft) — 一種混合式構型,以自然抽風式為基礎,輔以風扇引導氣流

由於有良好的結構強度及可以節省較多建造物料,雙曲面結構(Hyperboloid structure)冷卻塔所有自然抽風形式冷卻塔的標準設計。雙曲面亦可以增加向上對流空氣的流動速度並增加冷卻效率。此種設計的冷卻塔亦特別廣為不靠近海邊或大量水源的核能發電廠燃煤發電廠所用。而混合型冷卻塔的熱處理效能則可高達92%。[1]

  1. ^ Gul, S. Optimizing the performance of Hybrid: Induced-Forced Draft Cooling Tower. Journal of the Pakistan Institute of Chemical Engineers. 2015-06-18, 43 (2). ISSN 1813-4092 (英语).