Heat exchangers – Thermal rating and sizing.

The tools available to the engineer for sizing a heat exchanger or evaluating the performance of an existing exchanger are very numerous, and the choice may seem very difficult. In fact, they all complement each other and can be used successively when sizing a unit.

Analytical methods are all that's needed to size a heat exchanger, or to evaluate the performance of a heat exchanger already defined within the framework of an industrial process already well mastered and a range of conventional equipment already used on a similar process. This is the type of method on which manufacturers' tools are based. The advantage they offer is that they draw on databases of performance, pressure losses and exchange coefficients, based on measurements carried out on their exchangers. The two main methods described in this article are the DTML (logarithmic mean temperature difference) method, which is better suited to sizing, and the – NUT (number of transfer units) efficiency method, which is better suited to evaluating the heat transfer of an already defined exchanger.

Numerical simulation comes later, when we move away from conventional configurations, particularly for fluids and operating regimes, and when problems may arise in terms of distribution or local heat exchange. Simulations can then provide much more detailed information, with 1D, 2D or 3D representation of flows and heat transfers.

A table of notations and symbols is included at the end of the article.