U-tube designs while offering the best answer for differential thermal expansion have some drawbacks.
Individual tubes can be difficult of expensive to replace, especially for interior tubes. Also, the tube interior cannot be effectively cleaned in the u-bends. Erosion damage is also frequently seen in the u-bends in high tube side velocity applications. In large diameter
shells, the long length of unsupported tube in the u-bends of outer tubes can lead to vibration induced damage.