殼管式 (Shell-and-Tube) 熱交換器是工業中最普遍被採用的熱交換器形式。此類熱交換器之熱換管與端板的接合，通常於管內壁施力擴張，形成類似Expansion Joint之作用。由於擴管過程中所施加之外力，足以達到材料之塑性範圍，熱換管與端板接合處必存有殘留應力。在高溫高壓之流動液體環境及應力之加成作用下，更增強腐蝕損害。本篇論文即針對擴管過程進行分析研究，找出最容易造成應力腐蝕之處，使設計者在設計之初即納入考量，而使用者在使用時能預先防範應力腐蝕破裂。 首先我們對單一熱交換管與端板接合問題進行分析，其次再分析多熱交換管依續擴管問題。在分析過程中，以非線性有限元素法的觀念為基礎，使用有限元素分析軟體ABAQUS，對準二維的軸對稱模型及實際的三維模型進行應力分析。 經過模型的建立到軟體的分析，我們可以發現最大殘留張應力會發生在熱交換管中接近端板底部附近的元素，此處即為最容易發生應力腐蝕的地方，因為應力腐蝕作用只會發生在殘留張應力存在之處。這點與實際上觀察熱交換管應力腐蝕破裂的情形是一致的。而由實際的三維模型分析結果發現，最先擴管完成的熱交換管，在管與管間以正三角形方式排列的模型中，其最大殘留張應力值受到鄰近管擴管的影響較大。另外，腐蝕環境也是造成應力腐蝕破裂的重要因素，在進行熱交換管與端板的擴管接合設計時，必須同時考慮到最大殘留張應力可能發生的區域及所用金屬容易發生應力腐蝕破裂的環境，以避免熱交換管因為應力腐蝕的產生而比預估的壽命提早破裂。 In numbers of kinds of heat exchanger, the shell-and-tube heat exchanger is the most commonly used type of heat exchanger in the industry. In this kind of heat exchanger, the connection of a heat exchanger tube and a tube sheet is usually made by dilating the tube. However, due to plastic deformation after the connection, the residual stresses are always existent. In the environment of moving high temperature fluid, the stress corrosion easily appears when residual stresses exist. Then, a crack will propagate and make a heat exchanger fail. In this study, we analyze the tubes which are dilated and will find the part of the tube possessing maximum residual stress. In the problem with the connection of heat exchanger tubes and a tube sheet, we first analyze residual stresses of the tube connecting to the tube sheet. Then, we analyze that of the tube after the others are dilated one by one. The residual stress in the joints of exchanger tubes and a tube sheet will be investigated using non-linear finite element method and the finite element package “ABAQUS” is applied in this study. The results will be used to predict where the stress corrosion phenomenon occurs. From the results of analysis, we can find that the maximum residual stress appears at the parts of the tube which are near the bottom of the tube sheet. There is the place where the stress corrosion cracking (SCC) occurs , because SCC appears in the elements with residual tensile stresses only. This result is very consistent with the real observations. In the results of analyzing problems with the 3D models, it is more clear that the maximum value of residual tensile stresses in the tube which was dilated first is affected by dilating that around in the model which tubes are arranged as equilateral triangle. In addition, it is important that the corrosion environments are factors of SCC, and they should be considered when heat exchangers are designed.
