Various heat exchangers, such as evaporators, condensers, air-to-air and water-to-air heat exchangers, are commonly used for heat-transfer applications in the air-conditioning and refrigeration industry. The design criteria for most of these heat exchangers were developed at a time when energy was "cheap" and "abundant". The current energy crisis and soaring energy costs have made it necessary to reassess and improve the performance and design criteria for these heat exchangers.