Intensification of Heat Transfer Processes

Abstract

New challenges in efficient heat recuperation arise when integrating renewables, polygeneration and combined heat and power (CHP) units with traditional sources of heat in industry and the communal sector, as it is shown by Klemeš et al. (2010). Heat transfer enhancement is an efficient technique to increase energy saving when retrofitting heat exchangers or designing a new heat transfer system. By implementing intensified techniques in existing exchangers, higher heat transfer coefficients can be achieved, leading to higher heat exchange duties allowing a reduced size of heat transfer equipment and the associated benefits (especially improving heat transfer performance). Intensification techniques provide: (i) Reduction in size of a heat exchanger for a given duty; (ii) Increase in capacity of an existing heat exchanger; (iii) Reduction in approach temperature difference; or (iv) Reduction in pumping power. Conventional enhancement techniques include tube-side enhancements (i.e. enhanced surface tubes, internal tube fins, coatings, fluid additives, mechanical mixing devices, twisted-tape inserts, coiled-wire inserts, etc.); shell-side enhancements (i.e. externally enhanced surface tubes, external tube fins, coatings, fluid additives, helical baffles, etc.). The compact heat exchangers such as tube-fin, plate-fin and plate heat exchangers are using heat transfer intensification and offer significant reduction in size, weight and cost of heat recuperation equipment. Developments in mini- and micro- channel heat exchangers are offering new possibilities of heat transfer intensification in channels of very small hydraulic diameters. Recently such intensification has been widely studied in the process industry from the point of view of individual heat exchangers. Combining several enhancement techniques can achieve higher energy savings when compare to implementing a single technique. It is difficult to identify which intensification technique is more suitable in a certain design, or which combinations of enhancement techniques are expected to contribute the most in compound augmentation applications. This work will survey current practices and review recent advances in enhancement techniques from an economic and performance standpoint.