Heat transfer is the process of transferring heat from a high temperature object or system to a lower temperature one. It occurs due to a temperature difference and can happen through conduction, convection, or radiation. Heat transfer is important in many pharmaceutical processes like evaporation, distillation, drying, sterilization, and crystallization. Heat exchangers and heat interchangers are devices that facilitate heat transfer between fluids through conduction across a metal wall, allowing the transfer of heat from one liquid or gas to another. Tubular heat exchangers in particular provide a large surface area for heat transfer in a compact volume.