On the suitability of carbon dioxide in forced circulation-type secondary loops

A recent spurt in the universal interest to study alternatives to the conventional DX refrigeration systems and use of natural refrigerants as secondary fluids has been visible. This investigation presents a comparative analysis between carbon dioxide and other working fluids for use in forced circulation-type secondary loops suitable for various refrigeration and air conditioning applications. The comparison is made on the basis of equal heat transfer rate, frictional pressure drop per unit length, temperature drop/rise of secondary fluid and mean temperature difference between the secondary fluid and heat exchanger tube wall. Employing friction factor and heat transfer correlations, equations are obtained for diameter ratio, area ratio, mass ratio and pumping power ratio. Results show that employing carbon dioxide in place of other conventional secondary fluids except water leads to compact and lightweight heat exchangers, despite the high-pressure operation of carbon dioxide-based loops. The pumping power required is much smaller as well in case of carbon dioxide compared with all other fluids for the wide range of operating temperatures. Though results are presented here for the case of cooling only, calculations indicate similar trends for the heating configuration as well. Carbon dioxide appears to be an excellent secondary fluid for a wide range of refrigeration and air conditioning applications and it could turn out to be a disruptive technology for such applications in view of its benign environmental footprint.