Tube sheets have holes drilled to accept a series of tubes inside of an enclosed tubular pressure vessel. These pressure vessels, commonly referred to as shell and tube heat exchangers, are the most common heat exchangers used in oil refineries and large chemical plants. The tube sheet itself serves as a supporting element in heat exchangers and boilers. The tube sheet is called this because the tubes–which are carefully arranged in a specific pattern–penetrate the tube sheet in multiple places to allow the fluid within these tubes to flow in and out of the heat exchanger. The fluid flowing through the tubes within the shell exchanges heat with the fluid that flows into the shell but flows past the outside of the tubes exchanging the heat with the tube fluid.