Heat transfer analysis of air-to-carbon dioxide two-phase heat absorption and supercritical heat rejection

Regulations restricting the present and future use of fluorocarbon-based refrigerants have prompted a worldwide investigation of new refrigerants and refrigeration cycles. In particular, carbon dioxide is a fluid with a history of being used as a refrigerant that has recently been considered for certain applications that may take advantage of its unique thermophysical properties. The high volumetric heat capacity of carbon dioxide is a particular feature that could make a carbon-dioxide-based refrigerating/air-conditioning device appropriate for a weight or space-limited application. This study presents a comparison of heat-exchanger models of the hear absorption (evaporation) and heat rejection process for both carbon dioxide and R-22 air-conditioning cycles. The heat exchangers were modeled as single straight tubes with outside fins of constant thickness and spacing. Diameters for each tube were based on operating pressure and allowable power loss. The required length, weight, smd volume for the heat exchangers were determined and compared.