對數平均溫差法在相變化熱交換器上的應用研究

Study of Logarithmic Mean Temperature Difference Method Applied to Heat Exchangers with Phase-Change

傳統殼管式熱交換器之熱傳量，在設計與計算上多使用對數平均溫差（Log Mean Temperature Difference，LMTD）法來作為該熱交換器之計算基礎理論。但一般的熱交換器中，通常熱流體在經冷卻後，並不發生相變化之情形，所以在熱交換器中熱流體進、出口均為液體或汽體。然而，在某些場合熱交換器於冷凝器或蒸發器的應用上會遇到相變化的問題，此時僅藉傳統對數平均溫差法的公式來計算是不恰當的。而本文條藉著模擬冷卻水與Ammonia、R-290、R-134a、R-22等冷媒作為冷熱交換的兩流體，計算相變他之熱交換器中的熱傳量。文中假設冷媒與冷卻水之比熱、總體熱傳係數及總熱傳面積為己知，並以冷流體之質量流率為變化且冷媒在飽和溫度不變的情況下，將本論文所提出三種模式求得之熱傳量與Stoecker and Jones理論相比較。本論文所提模式所預測之冷凝器熱傳量較準確。期藉著本研究可於未來冷凝器之設計上提出一快速且簡便之設計方法。 The majority of the designs of traditional shell-and-tube heat exchangers are evaluated by logarithmic mean temperature difference method (LMTD). The phenomenon of phase change does not occur for most cases. Subsequently, both the hot and cold fluids enter and exit the heat exchanger in liquid or vapor phase. However, in some situations, some heat exchanger design problems involve phase change such as condenser and evaporator. The LMTD method will no longer be appropriate means for accurate design. In this study, an approximate LMTD method is proposed to the heat transfer design of a condenser. Refrigerant of ammonia, R-290, R-134a, and R-22 are used as hot fluids and are cooled and condensed by cooling water. It is assumed that the specific heats of refrigerant and cooling water, overall heat transfer coefficient and total heat transfer of heat exchangers are known. The heat dissipations of refrigerants were obtained for fixed saturated temperatures with various cooling water flow rates while condensing. Three different models are presented to predict the heat transfer rate of the condensers. The results are compared with that proposed by Stoecker and Jones. It is shown that the calculated data from the models of this study are mostly much more accurate than that from Stoecker and Jones. Through this work, it is expected that a fast and precise evaluation method can be developed for prospective condenser designs.