Development of compact heat exchangers for CO2 air-conditioning systems☆

Abstract Compact and lightweight heat exchangers are needed for motor vehicle air-conditioning systems and for several types of unitary equipment. The high-pressure natural refrigerant CO2 is now being evaluated for use in such applications, and efficient heat exchangers are being developed and investigated. Carbon dioxide heat exchangers are designed for high refrigerant mass flux and use small-diameter tubes or extruded flat microchannel tubes. Refrigerant-side heat transfer coefficients are higher than with fluorocarbons, and reduced internal surface areas can therefore be tolerated. Both small-diameter mechanically expanded round-tube heat exchangers and brazed microchannel-type units have been built and tested successfully. Results show that compact heat exchangers optimized for CO2 are very competitive with baseline HFC/HCFC units in terms of physical dimensions, exchanger mass and thermal performance. Smaller tube and manifold dimensions can give reduced size compared with HFC-134a equipment. The temperature approach between air inlet and refrigerant outlet is much lower in CO2 gas coolers than in baseline system condensers of equal size and capacity, and the reduced refrigerant exit temperature has a marked influence on the coefficient of performance, Microchannel heat exchangers give the best overall efficiency. Refrigerant distribution in multiport manifolds and heat transfer tubes does not seem to be a problem.