The need to decrease the sizes and masses of heat exchangers while preserving their performance has stipulated the development in compact heat exchangers (CHEs). It is supported by the additional push from process industries for increased recuperation of heat energy, facilitating better energy efficiency in process plants with strict limitations for space, material and cost. The adequate substitution of conventional heat exchangers by CHE in the same process conditions requires maintaining the same heat load not exceeding the allowable pressure losses. The different ways to increase the compactness of CHE are analysed, including the change of hydraulic diameter of heat exchanger channels, and the use of various methods of heat transfer intensification by changing channel geometry and flow structure. The Nusselt numbers and friction factors correlations for plane tubes, enhanced tubes and channels of plate heat exchangers are compared based on available literature data. A newer form of the core velocity equation is developed, which allows a comparison of the performance of CHE heating surfaces with different enhancement techniques and varying scales in specific process conditions. The results of the calculations illustrate the influence of the channel's hydraulic diameter and length on CHE thermal and hydraulic performance for channels with heat transfer intensification. The recommendations on choosing the best channel geometry and size, depending on specified process conditions and stream nature, are formulated.

The need to decrease the sizes and masses of heat exchangers while preserving their performance has stipulated the development of compact heat exchangers with mini and micro channels (MCHE). It is supported by the need for increased recuperation of heat energy, facilitating better energy efficiency with strict limitations for space, material and cost. The adequate substitution of conventional heat exchangers with MCHE requires maintaining the same heat load, not exceeding the allowable pressure losses. The different ways to increase the compactness are analysed, including the change of hydraulic diameter, and the use of various methods of heat transfer intensification by changing the channel geometry and flow structure. The Nusselt numbers and friction factors correlations for plane tubes and criss-cross flow channels of plate heat exchangers are compared. A newer form of the core velocity equation has been developed, which allows comparison of the performance of MCHE heating surfaces with different enhancement techniques in specific process conditions. The results of the calculations illustrate the influence of the channel's hydraulic diameter and length on the MCHE performance for channels with the considered methods of heat transfer intensification. The ways to decrease channel size to mini and micro scales are determined. The recommendations on choosing the best channel geometry and size, depending on specified process conditions and stream nature, are formulated.

The efficient use of energy is a prerequisite for the sustainable development of modern society. It requires increasing heat recuperation in various energy-reliant systems, which is possible with heat transfer devices of intensive action, operating in conditions of limited space for installation and material availability for their production. This is achieved by heat exchangers with mini- and micro-channels, regarded as the next generation of heat transfer equipment. A survey of publications on heat transfer and pressure losses in mini- and microchannels is presented, with focus on their thermal-hydraulic performance. It includes straight channels of various cross-sectional forms, channels with enhanced heat transfer for electronic cooling, additively manufactured microchannels, crisscross flow channels of microturbine recuperators, and plate heat exchangers. A novel Micro Heat Factor for the comparison of mini- and microchannels thermal-hydraulic performance is derived. For the detailed estimation of channel performance in specified process conditions, accounting for the differences in hydraulic diameters, the equation for optimal fluid velocity is proposed. The comparison of thermal-hydraulic performance for different types of mini- and micro channels is performed, and the possibilities of their use in heat exchangers at specific applications are discussed, followed by directions of future studies.