Steady-state analysis of CO2 based natural circulation loops with end heat exchangers

In the present work, results obtained from the steady-state analysis of single-phase rectangular natural circulation loop with parallel flow, tube-in-tube type heat exchangers are presented. Due to certain favorable properties, CO2 is chosen as the secondary fluid in the loop. Water is used as the external fluid in both hot and cold heat exchangers. Using a simple analytical expression, it is first shown that use of CO2 in place of water can result in a compact loop. The results obtained are found to match with the results obtained using the analytical expressions, which are generally used as first approximations for the steady-state behaviour of natural circulation loops [Y. Zvirin, A review of natural circulation loops in pressurized water reactors and other systems, Nucl. Eng. Des. 67 (1981) 203–225]. However, unlike the classic expressions, the method suggested here is general as it considers the heat exchanger configuration, non-symmetric geometry and variable properties, so that it can be used for optimal system design. The results obtained on a rectangular loop show that for a given input data there exist optimum values for heat exchanger length and tube diameters. It is also seen that the external fluid flow rate does not influence the loop performance significantly beyond a certain value. However, the temperatures of external fluids affect the loop performance strongly.