A tube bundle is one of the most common heat transfer surfaces, especially in shell-and-tube heat exchangers. It usually consists of a small number of rows of tubes housed in a rectangular frame, which may be mounted vertically, horizontally, or in an inclined position. The two most common tube arrangements are staggered and in-line. In-line tubes form rectangular with centers of their cross sections, where staggered tubes form isosceles triangles. This is characterized by the cylinder diameter.