Heat exchangers are devices that transfer heat between fluids without mixing or blending them. The fluids are separated by a wall that has high thermal conductivity. The thickness of the wall is designed to prevent the mixing of the fluids or the possibility of direct contact between them. Included in the process is a working media that rejects or absorbs heat from the liquid being processed. The resulting outcome of the process is the cooling or heating of the fluid stream. There are an endless number of heat exchangers, with new ones developed each year as technology improves and the properties of various metals advance.
The transfer in a heat exchanger involves the use of convection in fluids and thermal conduction. The discussion of the design of heat exchangers begins with the heat transfer coefficient, known as the U factor, which is an expression of Newton’s law of cooling. Additionally, engineers use the mean temperature difference (LMTD) to decide on the temperature driving force for heat transfer. Fluids may have the same or different phases (such as liquid-to-liquid or vapor-to-liquid), that are also considered.
The hot and cold fluids may be separated by a wall with high thermal conductivity (usually made of steel, copper, stainless steel tube), or they may have direct contact with each other.