Carbon dioxide as a secondary fluid in natural circulation loops

Abstract

In recent years there is growing interest in the use of carbon dioxide as a secondary fluid in various cooling and heating applications. In the present work a comparative study is made between carbon dioxide and conventional secondary fluids that can be used in natural circulation loops for various refrigeration and air conditioning applications. For the same heat input and total loop length, the results are presented in terms of the ratios of pipe diameter required when a conventional fluid is used to the required pipe diameter with carbon dioxide as the secondary fluid. Results are presented for both laminar and turbulent flow conditions. It is observed that due to its excellent thermo-physical and transport properties and near critical point operation, carbon dioxide gives rise to very compact systems when compared to most of the conventional secondary fluids. The difference in size is much greater for laminar flow conditions compared to turbulent flow conditions. Also, for a given system size, the temperature rise/drop across the heat exchangers is much smaller in case of carbon dioxide in comparison to other fluids.