DEVELOPMENT OF COMPACT HEAT-EXCHANGERS FOR CO2 AIR-CONDITIONING SYSTEMS

Compact and lightweight heat exchangers are needed for motor vehicle a ir-conditioning systems and for several types of unitary equipment. Th e high-pressure natural refrigerant CO:! is now being evaluated for us e in such applications, and efficient heat exchangers are being develo ped and investigated. Carbon dioxide heat exchangers are designed for high refrigerant mass flux and use small-diameter tubes or extruded fl at microchannel tubes. Refrigerant-side heat transfer coefficients are higher-than with fluorocarbons, and reduced internal surface areas ca n therefore be tolerated. Both small-diameter mechanically expanded ro und-tube heat exchangers and brazed microchannel-type units have been built and tested successfully. Results show that compact heat exchange rs optimized for CO2 are very competitive with baseline HFC/HCFC units in terms of physical dimensions, exchanger mass and thermal performan ce. Smaller tube and manifold dimensions can give reduced size compare d with HFC-134a equipment. The temperature approach between air inlet and refrigerant outlet is much lower in CO2 gas coolers than in baseli ne system condensers of equal size and capacity, and the reduced refri gerant exit temperature has a marked influence on the coefficient of p erformance, Microchannel heat exchangers give the best overall efficie ncy. Refrigerant distribution in multiport manifolds and heat transfer tubes does not seem to be a problem. (C) 1998 Elsevier Science Ltd an d IIR. All rights reserved.