Heat transfer equipments are found to be useful in a wide range of industries. They can be broadly defined as equipments used for transfer of heat from a hot medium to cold medium. Heat exchanger design and structure of these equipments varies a lot depending on the application where they are used and the heat transfer media involved.
Some fundamental parameters governing the efficiency of heat transfer.
1. Temperature difference - High temperature difference between the involved media ensures efficient heat transfer by increasing the heat flux.
2. Nature of fluid - Fluid properties such as viscosity, conductivity, density play critical role in heat transfer efficiency and determination of heat transfer coefficient.
3. Type of flow - Usually, the media involved in heat transfer are in continuous flow. These flows can either be parallel to each other (co-current flow), in opposite directions to each other (counter-current flow) or approximately perpendicular to each other (cross flow). The temperature difference between the two media varies with location along the length of the flow. Counter current flows offer the best efficiency by ensuring high overall temperature difference (measured by LMTD).
4. Surface area - High surface area available for transfer of heat energy means more efficient transfer of heat energy for the same heat flux. Enhancement in the heat transfer area can be achieved by using fins on the heat transfer surface.
5. Turbulence - High turbulence means easier heat transfer through convection in a fluid. As heat transfer convection is more efficient way of heat transfer than conduction, turbulence can be used to enhance the heat transfer efficiency.
There are some other factors as well, which are not related to heat transfer but still govern the design of heat exchangers.
1. Capital costs - Large heat exchangers can perform heat transfer for even very small temperature differences, but they come at a higher cost.
2. Operating costs - In shell and tube exchangers, small diameter tubes can be used to increase the surface area available for heat transfer. But, small diameter also means higher pressure drop across the tubes leading to higher operating costs.
3. Materials - Depending on the type of fluid and its corrosiveness, a suitable material has to be chosen for construction of a heat exchanger, even if does not assure maximum heat transfer.
4. Fluid phase - Different types of exchangers are used for different phases of fluid and even phase changes e.g. condensers, boilers, air cooled exchangers etc.