Empirical correlation for the time lag of double pipe heat exchanger operating with variable flow rates

The behavior of a heat exchanger in variable regime can be described by a two parameter model with a time lag and a time constant. In many studies, the analytical calculation based on the energy balance permitted to express the time constant in various configurations of the device operating. However, the time lag is only experimentally determined. We propose in this paper an empirical method for the prediction of this parameter when a double pipe heat exchanger is submitted to a flow rate step at the entrance. Experimental data are used for developing correlations of both hot and cold fluids.