In this paper we try to characterise the behaviour of a double pipe or shell and tube heat exchanger in transient phase when a sudden change of the flow rate is imposed at the entrance of one of the two inlets. A simplified model is proposed : a two parameter model with a time lag land a time constant. An analytical expression of the time constant is obtained by using the energy balance equation. Experimental investigations show a good agreement with theoretical results. This method can be generalised to describe the response of any variation of the flow rates at the entrance of the heat exchanger.