Tube Bundle Heat Exchangers U Tube bundle Heat Exchangers can duplicate any existing bundle to include dimensions, materials and performance. A heat-exchanger system consisting of a bundle of U tubes hairpin tubes surrounded by a shell outer vessel. One fluid flows through the tubes, and the other fluid flows through the shell, around the tubes. One of the most common applications is the cooling of Hydraulic Fluid. And oil in engines, transmissions and hydraulic power packs. With the right choice of materials they can used to cool or heat other mediums. Such swimming pool water or charge air.