Transient regime of heat exchangers

The unsteady operation of heat exchangers of all kinds is a daily reality in industry, and indeed for everyone. The challenge is to gain a better understanding of transient behavior, with a view to appropriate control and monitoring, and, beforehand, better sizing. This article is an indispensable addition to the current availability of Techniques de l'Ingénieur.

In this article, the focus is on the HEX (Heat Exchanger) component, which in itself represents a very vast world. The heat exchanger's relationship with other system components is a natural extension of this article, but is beyond its scope.

The article therefore introduces the notion of the transient regime and its privileged relationship with the additional variable of time. A number of approaches appear, centered essentially on physical mechanisms, or on the opposite, on a systemic vision.

The main types of stresses and disturbances encountered in heat exchangers lead to the various models available to account for and study the transient behavior of a heat exchanger. Moving from the mechanism to the system, we pass from the continuous medium (3D) approach to the 0D systemic model. The fundamental concept of time constant emerges, complemented by the notion of time delay (or phase shift).

The literature cites specific in-depth tools. To date, these have been the subject of a great deal of research, which is discussed in this article in five successive sections.

The usefulness of this study is demonstrated through examples, including the oldest one, cowpers, which remains current, but also for :

• solar thermal collectors in connection with heat storage ;

• thermal storage ;

• exchanger fouling, which remains a major economic concern.

A more specific example deals with heat pipe transients.